- Email: [email protected]
Thermomagnetic analysis of co-existing nickel-iron metal phases in iron meteorites and the thermal histories of the meteorites
Geochimica et Cosmochimica Acta, 1062,Vol. 26, pp. 361 to 382. Pergamon PressLtd. Printedin Northern Ireland
Thermomagnetic analysis of co-existing nickel-iron metal phases in iron meteorites and the thermal histories of the meteorites* J. F. LOVERING~ (Received
and L. G. November
PARRY~
1960)
Abstract-The nickel contents of co-existing kamacite, taenite and p&site in all major structure classes of iron meteorites have been determined by thermomagnetic analysis. The results are in substantial agreement with preliminary data available on the electron probe microanalysis of octahedrites and are consistent with the view that the metal phases in iron meteorites have formed during cooling to temperatures of 300 to 370°C while the meteorites were subject to pressures up to about 25,000 atmospheres. There is some evidence that iron meteorites with relatively low nickel contents (i.e. hexahedrites and coarse octahedrites) formed at pressures slightly higher than those with higher nickel contents (i.e. fine octahedrites and nickel-rich ataxites). It is also suggested that iron meteorites with nickel contents greater than about 30 per cent (e.g. Santa Catherina) may have been composed of two y-phase alloys while they still formed part of the core within a parent meteorite body.
I. INTRODUCTION RECEKT work (UHLIG, 1954; LOVERING, 1957b) has shown that the nickel contents of the various co-existing nickel-iron phases in iron meteorites may be used to estimate the temperatures and pressures operating on the iron meteorites while they still formed portion of a parent meteorite body. FARRINGTON (1915) summarized the data available at that time and reported that kamacite (u-phase nickel-iron) values varied from 5.26 to 6.81 per cent nickel and taenite (y-phase nickel-iron) values varied from 13.0 to 48.0 per cent nickel so that it is important that more precise data should be obtained. NICHIPORUK (1958) studied the nickel contents of co-existing kamacite and taenite in a number of iron meteorites by a process of selective leaching of the kamacite phases in very dilute acid solutions. Kamacite values from 3.3 to 5.8 per cent nickel and taenite values from 30-S to 53.7 per cent nickel were reported (Table 1) but these results are likely to be in error as a result of the solution of other phases during the selective leaching’process. The most promising technique at the present time is electron probe X-ray analysis and several preliminary studies have already been made (MARINGER et al. 1959; AGRELL and LONG, 1959; WITTRY, 1959). Kamacite values reported ranged from 4.7 to 7.5 & 2 per cent nickel while taenite varies from about 30 to 40 per cent at the outer rim to about 15 per cent in the plessite regions situated in the centres of the taenite bands.
* The above paper was fist submitted to the Editor for Cosmochemistry in November, 1960. Owing to his serious illness at that time he was unable to deal with it and the subsequent combination of circumstances which led to a long delay in the publication of the paper is greatly regretted. t Department of Geophysics, Australian National University, Canberra. A.C.T. $ School of Physics, University of New South Wales, Kensington, N.S.W., Australia. 361 1
This is a very powerful technique but’ at the present8 stage if’ its (development, it does not seem possible to determine the nickel concentration in t’he kamacite phase with sufficient precision for the st’ud\rof the detailed t’hermal and pressure historirs of the iron meteorites. In this work the technique of thermomagnetic analg-sis has been used which, in favourable circumstances, enables the nickel content of cc-phase alloys to be determined within -j-0*5 per cent nickel and y-.phase alloys to
Mrteoritr
within -+:! per cent nickel. l’he overall nickel contents of plcssitic areas in iron meteorites can also be determined accurately by this method. Other workers (SMITH, 2908: PESCHARD, 1925: KASJC, 1915: SMITH and Yorrx(:: 1939) have used thermomagnetic analysis to study the kamacite and taenite in iron meteorites but, in general, their methods have not been as sensitive as those described and used in this work. A summary of the results of previous work on the thermomagnetic analysis of iron meteorites is given in 'I':tl)le 2.
tive
Thermomagnetic analysis is a well estabilshed technique ~wtl in the quantitaestimation of ferromagnetic components in metal samples and has been
Thermomagnetic
analysis of co-existing
nickel-iron
metal phases in iron meteorites
363
described by HOSELITZ (1952). It is convenient to display the results graphically by plotting the ratio of the saturation magnetic moment at temperature T”C (Us) to the saturation magnetic moment at 20°C (Go,,) as a function of the temperature T”C. Points of inflection on these curves indicate the characteristic temperatures at which components in a sample become non-magnetic and this characteristic temperature is then used to identify the component. From the shape of the curve and a knowledge of the magnetic properties of the suspected component an estimate is made of the amount of this material present. Table Meteorite
2. Previous
Classification
Butler
Of
Chinautla
Om
Sacramento
Om
thermomagnetic Kamacite (wt. % Ni)
analyses
of iron meteorites.
Taenite (wt. % Ni)
Reference
and YOUNG
(1939)
-50
SMITH
6.9 -7.1
-32.0
PESCHARD (1925b)
6%
~27%
SMITH (1908)
7%
-25%
KASE (1925)
The curves were obtained
in this case with a pendulum magnetometer (Boz10 and 100 mgm contained in a sealed silica tube evacuated initially to a pressure less than O-1 mm of mercury. The temperature was controlled by a tubular furnace. The magnetometer and electromagnet were calibrated by using nickel powder and the Curie points of pure iron, nickel and magnetite agreed to -&Y’C with values given by BOZORTH (1951). In some cases the sample was not completely saturated and consequently the absolute value of the magnetic moments are reliable to only f5 per cent. For any one sample the ratio u,/a,,.was reproducible to better than 0.5 per cent. In the interpretation of the thermomagnetic curves for the iron meteorites, two main problems arise: (1) The Curie temperatures of some characteristic magnetic transformations, which were used to identify the magnetic phases, were ambiguous in the temperature range 300°C to 600°C. (2) The heating cycle, which was made by steps at an average rate of 10°C per minute, may have induced chemical changes since diffusion may take place across and within grain boundaries. Because considerable information is available from other sources about the material studied it is possible to resolve many of the ambiguities in the thermomagnetic evidence. The possibility of diffusion of nickel atoms is more serious since the sample is normally heated from 500°C to 750°C in a period of about 25 minutes. The relatively small bodies of y-phase alloy are modified by this heating but usually they have passed beyond their Curie points and have been identified before the high temperature is reached. The large bodies of a-phase alloy were not modified seriously during the first heating cycle and in most samples measured the characteristic temperature of the a-phase was changed only by small amounts by ORTH, 1951) using samples of mass between
J. E’. Lov~~rsc
364
ancl L.
(i. I’ARHY
repeated cycles of heating. The estimate of the quantity of a particular phase present in a meteorite is inaccurate due principally to sampling uncertainties and the difficulty of ensuring complete magnetic saturation. Iron meteorites are composed of kamacite, taenite, and plessite regions \\-ith minor amounts of troilite and schreibersite so that their thermomagnetic curves
\ 0.C
\
-_
-
T
\ \
/ 04
\--2
\
k
/ i
(3)
I
b-
(i
I
-.i_
0.4
I
I1
I 0.2
.L
I_-!!..
f-\
\ \. C
zoo
4
--- --0
Temperature,
600
b
I
“C
Fig. 1. Thermomagnetic curves of im irreversible cc-phase iron alloy (Kamacite) containing 6 ~NX cent nickel [heating enwe (1) and subsequent, cooling curve (2)] and sohreibwsite from t)he Mungindi No. 2 line octahrdrite [heding cur\.e, (3)].
are composite CUWCS. First of all it is necessary to understand t’he characteristic thermomagnetic behaviour of each of these components before the complex curves of the iron meteorites can be interpreted. (1) Kamacite (a-phase nickel iron alloy) : In most iron meteorites studied the kamacite occurred in large grains and no rapid compositional change was found during heating cycles. This alloy is irreversible and is characterised by two sharp phase transitions involving a change in magnet,ic moment. On heating, the alloy transforms from magnetic a-phase to non-magnetic y-phase, reaching completion at a characteristic temperature ‘II; and on cooling, the non-magnetic y-phase begins to transform to the magnetic cc-phase at a characteristic temperature T2, Fig. 1.
Thermomagnetic
analysis of co-existing
nickel-iron
metal phases in iron meteorites
365
Curves 1 and 2 illustrate these transformations. PESCHARD (1925) and HOSELITZ (1952) and others have confirmed that these temperatures are characteristic of the particular nickel content of the original u-phase for heating cycles of duration In alloys with less than 6 per cent comparable to those used in these experiments. nickel the Curie temperature may be below the transformation temperature and by observing the extent to which the transformation has occurred at 77O“C an It should be pointed out that the estimate can be made of the composition. magnetic transformations observed at these heating and cooling rates are not equilibrium processes (JONES and PUMPHREY, 1949) and consequently have no bearing on the equilibrium phase diagram. Using the data of PESCHARD (1925), BOZORTH (1951) and HOSELITZ (1952) for these characteristic temperatures the composition of the a-phase can, in favourable circumstances, be estimated to ho.5 per cent and changes induced by heating are within this range of uncertainty (Table 3). (2) Taenite (y-phase nickel iron alloy): Taenite is a reversible alloy with a Curie temperature, depending on the nickel content, within the range 100°C to 600°C. Taenite with a composition between 27 per cent to 33 per cent of nickel has been found in small quantities in most of the samples examined. The presence of this alloy has been confirmed by noting that when cooled in liquid air it is converted to cc-phase of the same nickel content (HOSELITZ, 1952) then when heated beyond the temperature of the u to y transition the appropriate Curie point inflection reappears on cooling (see Fig. 2). For these alloys it is possible to detect about 1 per cent of taenite in kamacite. When taenite is richer in nickel its identification is more difficult because of the less favourable shape of the curve and because schreibersite and meteoritic iron sulphide also affect the shape of the curve. In a few samples an alloy of about 40 per cent nickel was identified. The taenite composition varies from sample to sample in any one meteorite and an average figure has been given in Table 3. (3) Plessite: Plessite is an intergrowth of kamacite and taenite which occurs within the taenite lamellae of iron meteorites and as the dark etching ground-mass UHLIG (1954) has suggested that certain plessite of many nickel-rich ataxites. areas may also contain an ancestral acicular as-phase which OWEP~(1940) has called ‘cmetakamacite.” Thermomagnetic analysis indicates that plessite is unstable during the heating cycle and undergoes rapid compositional changes. Between 5OO’C and 600°C the shape of the initial heating curve (Fig. 3) depends on the rate of heating and probably reflects the rate of reaction between the various constituents of the plessite. However, after a heating cycle the plessite becomes stabilized as a characteristic E-phase of definite composition having the same overall nickel content of the parent phase of the plessite and characterised by a sharp transition temperature (Table 3). (4) Phosphide phases: Iron meteorites contain two generations of phosphide phase with the general composition (PeNi),P. The first generation phosphide is schreibersite and is found either as isolat’ed thick prismatic crystals or as rims around sulphide nodules so that it must have separated before the metal phase
2
Ataxites
Nickel-rich
Anomalous Ataxite
-
Fine Octahedrites
-.
Octahedrites
Medium
4
3
Santa Catherina
4
Tawallah Valley
Wedderburn
2
5
4
2
3
1
Mount Magnet
Arltunga
Mungindi No. 2
TOlUCa,
Kyancutta
Henbury
5
10
10
10
90 & 10
140 *
200 & 5
190 *
212 + 3
200 +
180 + 20
210 *
195 & 20 10
Ni y
20 20 700 5
30 10 10
120 5 550 * 590 *
10 320 570 & 10 750 * 10
& 20 & 10 ~ 680 & 5
120 *
130 320 600 755
150 * 300 + 470 = 750%
20 20 320 550 - 680 730 + 10
110 + 240 *
130 & 20 500 - 670 745 * 10
220 5 20
100 * 10 630 & 10
600 ~+ 10
590 & 20
250 * 30 590 + 10
Xi y 24 + 2”,/0 Ni (P) 55 l 2%Niy
28 _C 2%
troilite (?) 23 l 1% Ni (P) 6-5 & 0.596 Ni c(
28 + 1Of ,o Ni y
27 & 3% Ni y schreibersite 16 ,i 1% Ni (P) 7.0 & 1% Ni t(
29 & l%Niy schreibersite 15 & 3% Ni (P) 7 *2%Nicr
27 f 1 o/o Ni 1, 32 & l%Niy schreibersite 13% Ni (P) 7.5&l%Nicr
28 *2%Niy 19 + 2% Ni (P) 7.0 + 1% Ni a
40 -+ 3% Niy 7.5 k 1.0% Ni a
2%
410 & 10 745 & 50 10
l%Niy 1% Ni a
29 &- 2% Ni y schreibersite 17 _C 2% Ni (P) 7.5 + 1.0% Ni a
30 + 7.0 +
32 i
585 *
270 & 30 590 * 10
600 i
230 h 20
20 350 620 & 20 745 l 50
130 i_
170 & 20 750 & 5
10
40 _c 4 20 + 5
5*3
4+ 2 Trace 50 * 10 15& 5
412 Trace about SO 15 i_ 5
5* 2 Trace 70 * 10 20* 2
3&l 311 Trace 14 + 2 75 * 5
3xtl 30* 5 70 * 10
4+2 85 *
6xk2
5*2 Trace 15* 5 80 * 10
4rt2 92 * 5
33.60
22.2
18.21
14.72
10.08
12.36
8.31
8.28
7.66
This work
LOVERIN~ et al. (1957)
LOVERING et al. (1957)
LOVERIN~ et al. (1957)
LOVERIN~ et al. (1957)
LOVERING et az. (1957)
NICHIPORUK (1958)
NICHIPORUK (1958)
LOVERING et al. (1957)
solidified. The second generation phosphide (rhabdite) occurs as acicular crystals regularly oriented in the kamacite phase of iron meteorites and is a result of exsolution from kamacite (GOLDSCHMIDT,1954). Little is known of the distribution of nickel in schreibersite or rhabdite but HEX.DERSOK and PERRY (1958) found 19.58 per cent nickel in rhabdite and 12.03 per cent in schreibersite in the Tombigbee hexahedrit’e. Pure Fe,P is ferromagnetic with a C’urie point of 4?O”C (l%)ZOKTH. 1951) but) the addition of nickel is likely to depress the (‘uric t’emperature. A sample of
I ‘3
1
?CO
400 Temperature,
600
F 10
“C
bchaviow of reversible Fig. 2. Thermomagnetic lf-~)hase iroll alloy (taenito) containing approximately 29 per cent nickel in the Toluca meciium octahedrite. Curve (1) original sample heated to 400°C showing characterist,ic Curie temperat.ure of taenite at 160°C. Curve (2) original sample cooled to 180°C' for severa. Curve (3): hours and then heated to 450°C’. Transition at 150°C not tlrtr~c~l~wl. cooling curve from (2). Taenite detected by inflexion at 150°C.
schreibersite inclusion was drilled from Mungindi No. :! fine octahedrite. The thermomagnetic curve is reproduced in Fig. 1 curve 3. The (‘uric temperature is 310 i_ 10°C and the material is not affected much by the heating cycle. (5) Iron sulphide phases (FeS, i .r) : NICHIFORUR and CHol)os (1969) concluded from X-ray studies of iron sulphide nodules from a number of iron meteorites that they are composed of troilite (z -= 0) and pyrrhotite (0 ,: .c ” 1) with some daubreelite, chrompicotite, schreibersite and kamacite. Troilite is non-magnetic, pyrrhotite .c -: 0.15) is ferroand pyrrhotite (047 (0 < 5 < 0.07) is antiferromagnetic magnetic wit,h a Curie point of npprosimately 3W(’ (PEKTEIEL. 1960). SAITO (1956) has measured a number of terrestrial pyrrh0tit.e samples and finds Curie points from 310~320°C. Samples have been taken from nodules in Mungindi No. 2. ‘The behaviour of these samples was similar t,o Moonbi and Delegate meteorites. that observed by SAITO (1056). The Mungindi No. 2 sample had a very low maguetic moment (0.2s e.m.u. g-l) and the curves had a peak at about WO’Y’ (see curves 5 and 1 of Fig. 4) which givesavalueof xintherangeO.0’7~O~lOforthismaterialaccording ~~PERTHEL( 1960). During the heating in a sealed tube some highly magnetic material having a Curie point at 580°C was formed. This appears tjo be magnet,ite since PERTHEL (1960) finds that oxygen entering the sulphide lattice does not affect the (‘uric point8 greatly.
Thermomagnctic
analysis
of co-existing
nickel-iron
metal
phases
in iron meteorites
369
When this sample was reheated in air ferromagnetic pyrrhotite and magnetite were formed. This material had a magnetic moment of 5.3 e.m.u. g-l and was similar in thermomagnetic behaviour to the sample taken from Moonbi. Material drilled from the Moonbi meteorite had a magnetic moment of 20
0
200
\
400
Temperature,
630
10
‘C
Fig. 3. Thermomagnetic analysis of plessita in iron meteoritos. Curve (1) is the initial heating CUPVOfor a sample of the Mt. Magnet nickel-rich ataxite which contains 90 per cent plessite. Curve (2) is a reheating curve for the came sample after it had been cooled from 750°C. Curve (4) is the initial hoating curve for a sample of the Wedderburn nickel-rich ataxit.e largely composed of plesaito with subsequent cooling curve (5) after heating t.o 620°C. Curves (3) and (6) are heating and coooling curves for material which has been previously heatod to 620°C.
e.m.u. g-l. The thermomagnetic curves are given in Fig. 4 curves 3 and 2. Both magnetic pyrrhotite and magnetite are present but this nodule is near the A&ace and may have been partly oxidised during ablation of the meteorite in the atmosphere. The third sample from the Delegate nodule had an initial magnetic moment of O-6 e.m.u. g-l and on heating above 4OO’C a highly magnetic component wa8 formed with a Curie point at 580°C. On cooling the magnetic moment had increased to 30 e.m.u. g-l but there was no evidence of magnetic pyrrh0tit.e. X-ray
J. 8. LOVERING
370
and L. G. PARRY
photographs showed that the initial material was mainly troilite with some pyrrhotite and the after-heating was a mixture of ~~yrrhotite~ troilite and magnetite with the magnetite apparently forming from dhc troilite.
Temperature,
OC
%‘ig. 4. Thermomagnek
curw
(I),
analyses of’ iron sulphide phases from Ximgindi [heating cooling curve (5)], Moonbi cheating (3), cooling (Z)i, and Delegate [heating (6), cooling (4)] octahectrites.
When troilite is present its variable behaviour when heated leads to difficulty in identifying any component with a Curie point in the range 320 to 580°C) but an increase in cPI‘in this range indicates that troilite is being converted to magnetite. However it is very disrupt to see how any signi~cant formation of magnetite could take place during the heatings tts these experiments are carried out under very reduced atmospheric pressure (i.e. less than O-I mm of mercury). (6) Daubreelite (FeS. Cr2S3): Daubreelite is rare in most iron meteorites and,
Thermomagnetic
analysis of co-existing nickel-iron metal phases in iron meteorites
371
according to PERRY (1944), it is not ferromagnetic and even if present in our samples it would not be detected. (7) Cohenite (FeNi)&: The iron carbide phase in iron meteorites apparently contains about 2 per cent nickel but it is comparatively rare and only known from a few coarse octahedrites (PERRY, 1944). Pure iron carbide (cementite) has a Curie temperature of 215°C (BOZORTH, 1951) so that the small amount of nickel in cohenite should depress its Curie temperature a little below 215°C. However, no samples studied in this work were found to contain cohenite. 3. RESULTS A collection of 14 iron meteorites of all major structural types have been studied in this work and the results are summarized in Table 3. The range of experimental error assigned to the various characteristic temperatures in Table 3 take in to account differences within each sample and also between different samples of the same meteorite. The errors assigned to the nickel contents of the metal phases identified are related to the experimental errors of the characteristic temperatures. This procedure would tend to mask small but real variations in taenite composition but it should not affect the identification of the kamacite phases. (1) Hexahedrites: In both hexahedrites studied (Coya Norte and Sikhote-Alin) the major phase (greater than 80 per cent) present is u-phase alloy containing 5.5 f O-5 per cent nickel. Usually some y-phase was detected but because the structure of hexahedrites was so coarse it was difficult to make a reliable estimate of the total amount present. The nickel content is 30 f 3 per cent. A very small amount of plessite containing 14 to 17 per cent nickel was also detected along with minor amounts of schreibersite. Fig. 5 curves 1, 2 are of Sikhote-Alin except that no y-phase alloy was detected in this particular sample. NICHIPORUK (1958) recently examined the Coya Norte hexahedrite and found kamacite phase only with a nickel content of 4.78 per cent which is in reasonable agreement with the results of the present study. (2) Nickel-poor ataxites: The only nickel-poor ataxite examined was Bingara No. 2 and the results are very similar to those for the hexahedrites. The main phase present (about 96 per cent of the sample) is an irreversible u-phase alloy with 5.5 * O-5 per cent nickel and minor y-phase (31 * 2 per cent nickel) and schreibersite. (3) Coarse octahedrites: Three coarse octahedrites, Canyon Diablo, Cranbourne (Beaconsfield mass) and Mount Tabby were studied and the results were significantly different from the hexahedrites and nickel-poor ataxites in that over 80 per cent of all samples was made up of 7-O f 0.5 per cent a-phase alloy. Some taenite with 30 & 2 per cent nickel was always present and in Canyon Diablo there was evidence of 41 per cent nickel y-phase. All coarse octahedrites examined showed the presence of around 2 per cent of plessite containing from 18 to 26 per cent nickel (see Fig. 6). Canyon Diablo has also been examined recently by chemical and electron probe microanalysis techniques and these results are compared with the present results in Table 2. These data indicate that both electron probe and thermomagnetic techniques give very similar results, but that the chemical methods used by
NICHIP~RUK (Table 1) give quite different results. NICHIPOIWK (195X) has also studied the Mount Tabby iron (Table 1) but although the taenite composition is in good agreement with the thermomagnetic results, the kamacite results are significantly lower than the thermomagnetic analyses. (4) Medium and fine octahedrites: The medium and fine octahedrites examined
T
I
-----A-
.I
1’
t
Fig. 5. Thermomagnetic analyses of’ a nickel-l)oor at,;txite (Sikhote-Alin) and hcxahedrito (Hingara p\i’o.2). Curve (1) is the initial treating CIII’\-V for Sikhot,c41in hexahedrite
(:ur\.(~ (3) is t,ho heating while curve (2) is the cooling cwvp. curve for Ringara No. 2 nickel-poor at.axitc.
showed rather similar properties so they will he discussed together. The kamacite phase in these iron meteorites shows a tendency to slightly higher nickel contents (‘7.5 & 1% nickel) than those of the coarse octahedrites. ‘I’arnite is virtuall) identical within the range of experimental error (30 :t :!O,, nickel) hut Toluca showed evidence of a small amount of y-phase with 40 per cent nickel. Plessite with nickel content about 18 Ik 2:: was detected in both Kyancut,ta and Mungindi No. 2 (Fig. 6, curves 1 and 5; 3 and 6). The Henbury and Toluca irons have also been studied by XICHIPOHIJIC(Table l), who found nickel contents in taenite much the same as the thermomagnetic results, while kamacite was considerab1.v lower.
~he~oma~etic
analysis of co-existing nickel-ironmetal phases in iron me~or~tes
373
(5) Anomalous
is that hedral a thin overall
Ataxites: The macro-structure of the Arltunga iron meteorite of a nickel-rich ataxite but the micro-structure shows a well-marked octaarrangement of fine kamacite plates. Each kamacite plate is surrounded by zone of taenite while the interstitial material is apparently plessite. The nickel content (i.e. 10.1 per cent) is less than that of many octahedrites and 1
0
0
10
Temperature,
OC
Fig. 6. The~o~~etic analyses of coarse, medium and fine octahedrites. Cra&ourne coarse octahedrite: heating curve (2), cooling curve (4). Kyancutta medium ootahedrite: heating curve (Z), cooling curve (5). Mungindi No. 2 fine octahedrite: heating curve (S), cooling curve (6). LOVERINC et al. (1957)
have also reported anomalous gallium and germanium concentrations in this iron, In view of all these pecularities, Arltunga has been classified as an anomalous nickel-rich ataxite. Thermomagnetic analysis indicates kamacite with 7.5 f 1 y0 nickel, two taenite phases with 27 -& 1% and 32 4 1% nickel respectively, and plessite containing about 13 per cent nickel. (6) Nickel-rich Ataxites: The four nickel-rich ataxites studied can be divided into two groups, those whose saturation magnetic moments lie within the range HO-220 e.m.u. g-l, as do all other types of iron meteorites, and those with considerably lower moments (W-140 e.m.u. g-l).
374
The two nickel-rich ataxites with normal saturation magnet,ic moments (Mount Magnet and Ta~~al~ah Valley) both contain kamacite with a nickel content of 7 5 1% as do the octahedrites but the proportion of this phase in the nickel-rich ataxites is much lower (i.e. about 1.3y/U)1 Small amount,s of taenite cont,aining about 28 & 1 o/o of nickel are also observed but the major phase present is IJessite which in Mount’ Magnet stabilizes around 18 3: 30{, nickel and in ‘I’awallah Valley
0
Temperature,
OC
Fig. 7. Thern~omagnotic analysis of t,he Tawalltth Valley nickel-ric:tl ntaxito sl~oaing heating (1) and tiooling (3) curves for previously unheated material anc.lhoitting (2) SXXIcooling (4) e~rve~ fcs material previously heated to 730°C’ for 6% mkbes.
around 16 i lo/A nickel after heating (see Fig. 7) beyond the transition temperature (i.e. :::,680YY). Of the Lwo nickel-rich ataxites with abnormally Iow saturation moments (Wedderburn and Santa Catherina) only the Wedderburn shows any similarity with the other group. The Wedderburn ataxite contains some kamacite (7 & 1% nickel) and taenite (28 4: IO/, nickel) but the major ~lagnetic phase is plessite averaging about 23 $I lo/, nickel. The thermomagnetic curve of the Santa Catherina is quite different from all other iron meteorites and is difficult to interpret due t’o instability during the heating cycle (Fig. 8). However, the most
~e~omagnetic
analysis of co-existing
nickel-iron
metal phases in iron metsorites
375
reasonable interpretation of the curve would suggest this meteorite is composed of an irreversible phase which behaves like plessite with an overall 24 &- 2% nickel and a reversible y-phase containing 55 _L 2% nickel. Some samples suggested that, a small amount of a y-phase containing 28 f 2 yO nickel might also be present. Santa Catherine was the only iron meteorite studied which did not contain a low nickel content kamacite phase.
I
jO(
Temperature, Fig.
8. The~om~~etie
ac)O
“C
analysis of the Santa Cbtherina (heating and cooling curves).
nickel-rich
ataxite
4. D~scussro~ General The main results of ‘the thermomagnetic study of the iron meteorites which have direct bearing on the problem of the evolution of these meteorites may be summarized as follows: (1) Representatives of all the major structural classes of iron meteorites (i.e. hexahedrites, octahedrites and nickel-rich ataxites) were found to contain a y-phase alloy containing about 29 per cent nickel while certain octahedrites also showed small amounts of another y-phase alloy containing about 40 per cent nickel.
376
,J. F. LOVEJUNG and L. G. PARSY
(2) Plessite was detected in all major structure types and became increasingly prominent as the overall nickel contents of the meteorite increased. The average composition of the plessite detected varied from 13 to 26 per cent nickel, with a mean nickel content of about 15 per cent. There does not seem to be any close relationship between the composition of the plessite and the overall nickel content of the meteorite. (8) The kamacite which occurs in any particular structural t,ype of iron meteorite has a constant nickel content but there are significant differences between kamacites occurring in (i) hexahedrites and nickel-iron ataxites, (ii) coarse octahedrites, (iii) medium and fine octahedrites. (iv) most nickel-rich ataxites. (4) The Santa Catherina, an extreme type of nickel-rich at,axite with about 34 per cent nickel, does not have a low-nickel content kamacite but seems to have originally consisted of two y-phases with nickel contents of about, 24 per cent and 5.5 per cent, respectively. In discussing the thermal history of iron meteorites it, is generally assumed that the effect of impurity elements. other t’han nickel, on the 3’ x transition ill iron meteorites is negligible. ltecently CLARK and Kl:LLERI-I) (1!)59) have given reasons for believing that the effect of impurity-met3allic clomcnts is very small but that non-metallic elements could have larger effects. However: they concluded that the available analyses of non-metallic elements in iroll meteorites x~ould indicate that the carbon content (about 0.11 per cent) is enough to dr~prcss the y -+ x transition by lOO“(’ or so, and the phosphorus content (about 0.22O/~) is enough to raise it by the same amount. The sulphur content- (( 1.169:) should have litt,le effect’. so that the overall e’ffect of both meOallic and non-mrt8allic c1ement.s appears t’o cause little change in tShe)1 -+ x transition in iron I11et,coritos. It should be emphasized that before more definit#e conclusions can I)e rctached there is considerable need of modern analyses of non-metallic elements itI t,he metal phases of iron meteorites. l
Tamite md plessite Micro-st,ructural electron probe and tbermomagnetic studies have shown that teanite and plessite areas are intimately associated in iron meteorites. Generally, thin rims of taenite surround plessite areas and are in direct, contact with the kamacite grains. DERGE and KOMXEL (1937) suggested that plessite areas within t’aenite formed as a result of a later transformation of first-formed low-nickel regions within the taenit,e but Owen- (1940) later proposed that plessite resulted from the transformation of an acicular cc,-phase (“metakamacite”). UHMG (1954) agreed with OWES that the parent phase of plessite was metakamacite which he suggested formed from certain y-phase alloys in iron meteorites as a result of the sudden pressure decrease during the disruption of the parent. meteorite body. Although t’he origin of plessite is still obscure, there is general agreement that the ancestral phase was a relatively low nickel content y-phase alloy. The nature of this ancestral y-phase would have an important bearing on the Ohermal histories of the iron meteorites. The thermomagnetic results suggest that in virtually all the major structure types of iron meteorites (i.e. hexahedrites, octahedrites, nickel-rich ataxites) the
Thermomagnetic
analysis of co-existing nickel-iron metal phases in iron meteorites
377
plessite areas and taenite phases show remarkably little variation in composition, On the other hand, the kamacite in the same iron meteorites varies significantly in nickel content as the structure of the meteorite changes. This behaviour does not seem to be explained by the equilibrium iron-nickel phase diagram determined by OWEN and LIU (1949) (see Fig. 9). It is virtually impossible to calculate the overall composition of the y-phase which would have been in equilibrium with the observed u-phase alloys in iron-meteorites, because the original y-phase areas were so inhomogeneous. Presumably if the nickel contents and relative proportions of
Nickel,
w+
%
Fig. 9. The cr-y equilibrium phase diagram determined by OWENand LIU (1949) for iron-nickel alloys at one atmosphere (full curve). The broken curve is the estimated diagram at 25,000 atmospheres.
the various taenite and plessite areas could be determined accurately, then the overall nickel content of the original y-phase alloy could be calculated. However, at the present time neither the thermomagnetic nor the electron probe microanalysis methods are sufficiently precise to enable such a calculation to be made. The inhomogeneity of the original y-phase in the iron meteorites is itself an indication of the thermal history of the meteorites. The central zone of broadly constant, though relatively low, nickel content (now plessite with about 18 per cent nickel) represents y-phase which was presumably in equilibrium with an a-phase alloy at a time when temperatures were high enough so that relatively rapid diffusion of nickel was possible in the y-phase. From the zero pressure equilibrium iron-nickel phase diagram (Fig. 9) y-phase alloys containing 18 per cent 2
378
.J. B’.
Lovmtr~c
and
L. G.
I'.~RNY
nickel would be in equilibrium with an a-phase containing 3.7 per cent nickel at 59O‘X’. The rapidly increasing nickel content towards the margins of the original y-phase indicates that, with further cooling and “J --+ cc transformation, the nickel atoms rejected from the rapidly growing a-phase areas banked up just inside the y-phase boundaries. At the time the parent body1 was disrupted it, would appear that temperatures within the metal core were low enough to restrict diffusion of these nickel atoms into the rest of the y-phase alloy. If it is assumed that the 30 per cent nickel y-phase at Dhe boundaries represents an eyuilibrium y-phase alloy then the one atmosphere l)hast’ diagram would indicate equilibrium temperatures around 4SW’. However. electron-probe analyses (WITTRY, 1959: MARZS(:W it al., 1!)59) indicate that the high nickel 5 content margins of the taenit,c lamellae are only about, 10 microiis across so that steep concentration gradients exist’ in the outer margins of t’aenite lamellae. It is exbremely difficult to see how such steep concentration gradients could have existed for long periods of time in the parent meteorite body core if temperatures were 406°C’ or higher. These steep gradients are, however, more in keeping with UHLIG’S (1964) conclusion that the iron meteorites must have cooled to temperatures considerably less than 400”(’ within t,he l)arent body. Under these circumsta,nces, it would seem that the composition of the y-phase alloys in iron meteorites is not consistent with the equilibrium nickel-iron phase diagram at zero pressure but that pressures high enough to depress transition temperatures l)y 100 to bOO”(’ must be considered to have operated within the core of’ a parent body. Kamacitc ill hexahedrites
and
octa,hedrites
The thermomagnetic dat,a have shown that the kamacite phase occurring in the various structure types of iron meteorites is not constant, in composition as was formerly thought, to be the case (UHLIG, 1954). In t,he hexahedrites and nickel-poor ataxites the kamacite contains about 6.5 per cent, nickel. The zero pressure phase diagram indicates that kamacite of this composition would be in equilibrium with a 33 per cent nickel y-phase alloy at 450°C: (Fig. 9). However, thermomagnetic analyses of the rare y-phase alloy in these irons indicate that probably the outer rims of the taenite phase may contain 30 per cent nickel but that the taenite is extremely inhomogeneous and the overall nickel content would be less than 30 per cent. From the previous discussion of the significance of the taenite phase in iron meteorites, it would seem that the taenite phase in the hexahedrites had cooled to temperatures considerably less than the 450°C indicated by the kamacite composition and probably more in the region of 300 to 4OO’C. The effect of pressure on the y -+ cc transformation in nickel-iron alloys has been discussed by UHLIG (1854) who concluded that in the absence of contradictory evidence it is reasonable to assume that the rate of depression of t’he y --) Mtransformation Oemperature with pressure (dT/dP) in nickel-iron alloys is not greatly different from that of pure iron. In nickel-iron alloys the volume change between the 01-and y-phases is somewhat larger than that in pure iron but this effect would be offset by a lower transformation temperature and a higher AH value. Recently STROKG (1959) has measured dT/dP for the y --f M transformation in pure iron to pressures of 96,000 atmospheres and his data show that at 30,000 atmospheres the
Thermomagnetic
analysis of co-existing
nickel-iron
metal phases in iron meteorites
379
transformation temperature is depressed by 16O”C, at 20,000 atmospheres it is depressed by 135’C and at 10,000 atmospheres it is depressed by 95°C. In the absence of a, better model for the behaviour of the iron-nickel phase diagram at high pressures, it will be assumed that at pressures of the order of 25,000 atmospheres the sub-solidus iron-nickel phase diagram would move symmetrically down the temperature axis by about 140°C, as is shown in Fig. 9. From this new diagram the a-phase of hexahedrites would represent an equilibrium phase at 320°C. This low temperature is in agreement with the observation that the associated y-phase in hexahedrites is inhomogeneous. The coarse octahedrites contain kamacite with an average nickel content of 7-O per cent which, on the basis of the zero pressure phase diagram, should be in equilibrium with a y-phase containing 51 per cent nickel at 320°C. The thermomagnetic and electron-probe evidence indicates that the y-phase in coarse octahedrites is extremely inhomogeneous and also that its over-all nickel content must be less than 40 per cent nickel and probably closer to 30 per cent. The observed composition of the LX-and y-phase in coarse octahedrites more closely approximate those calculated on the assumed phase diagram at about 25,000 atmospheres (Fig. 9), with equilibrium temperatures between 200°C and 320°C. The medium and fine octahedrites contain kamacite with an average nickel content of 7.5 per cent while the original y-phase appears to have had much the same composition as that found in the coarse octahedrites. It is apparent from Fig. 9 that the composition of the coexisting metal phases in these iron meteorites also cannot be explained on the zero pressure phase diagram but that pressures somewhat less than those for the hexahedrites and coarse octahedrites are indicated with equilibrium temperatures between 200°C and 300°C (see Fig. 9). In summary, then, it may be concluded that the kamacite and taenite phases co-existing in hexahedrites, nickel-poor ataxites and coarse, medium and fine octahedrites are not in equilibrium on the basis of the iron-nickel phase diagram determined at zero pressure. To explain their observed compositions it seems necessary to assume that the phases were formed at temperatures around 320°C under pressures up to 25,000 atmospheres. There is also evidence that the hexahedrites formed under maximum pressure and temperature and perhaps that both pressure and temperature steadily decreased as the structures of the meteorites changed from coarse to medium and fine octahedrites. Nickel-rich
ataxites
UHLW (1954) has shown how the observed structures of nickel-rich ataxites cannot be explained on the basis of the nickel-iron phase diagram at zero pressure and that considerably higher pressures must be assumed to have operated in the metal core of the parent meteorite body. The results of thermomagnetic analyses of nickel-rich ataxites also suggest that significantly high pressures must be assumed. One very puzzling fact is the relatively sharp boundary between meteorites showing a well-developed Widmanstatten pattern (i.e. fine octahedrites) and those without such a well-developed structure (i.e. nickel-rich ataxites) when the nickel content of the meteorite reaches about 13 or 15 per cent. On the zero pressure
iron-nickel phase diagram alloys containing Id per cent nickel should hegin the y - cc transformation at 620°C. Now UFLL~C(1954) has pointed out that alloys transforming at or below 460°C may be expected to resist complete transformation under any conditions, perhaps including long periods of t,ime below the transformation temperature, whereas alloys transforming above 460”~~ do so readily. Of course, once the Widmanstatten pattern has formed it) may continue to grow below 460°C since its formation depends on the growth of phases already nucleated. On t,his evidence, then, it would appear that the iron meteorites must have cooled under considerable pressures so that those with nickel contents in t.he range 15 to about 27 per cent would not have entered the M -;- y field until their tem~era,tures had dropped to about 460°C at which temperatures, the amounts of transfor~~ation of the y-phase even over times of the order of 10” years, would have been very small. From the estimated iron-nickel phase diagram at %.(100 atmospheres (Fig. 9) it would seem that the M 1. y and y-phase boundary for 13 per cent nickel alloy would be depressed to 510”(! whereas that for 27 per cent alloy would be depressed to 360°C which would certainly explain the formation of the nickel-rich ataxites. Within the nickel-rich ataxites themselves, UNLIG (1!)54) has shown t,hat those with nickel contents less than about 27 per cent contain some kamacite while those with nickel contents greater than 27 per cent do not contain kamacite. During the present work it was found that Wedderburn (22 per cent nicke1) eontaine~~ kamacite but Santa C~atherina (34 per cent nickel) did not. These observations agree with UHLIG'S conclusion that all iron l~~eteorites with nickel contents equal to, or greater tohan, 27 per cent must have existed above the a ,+ y region of the phase diagram in the parent body. However, from the zero pressure iron-nickel phase diagram (Fig. 9) it can be seen that alloys containing 27 per cent nickel would lie just on the boundary of the a ..(. y and y-phase regions at a temperature of 5OOT. Subsequent cooling from this temperature should have allowed to have taken place in these iron meteorites considerable y --)c a transformation but in general no transformation is observed. Once again the reconstructed phase diagram at 25,000 atmospheres (Fi,.0 9) would explain the absence of a-phase alloy in meteorites with more than 27 per cent nickel because under these pressures the alloy would still be entirely in the y-phase down to temperatures of 36O”C, which is well below that at which s~lbsequent transformation would take place of further cooling of the meteorite. One surprising result of the present study is that iron meteorites containing more than about 30 per cent nickel are not composed simply of a single untransformed y-phase as UHLIG (1964) has suggested. ‘The Santa Catherina ataxite seems to consist of a 24 per cent nickel plessite (or metakamacite) and a 55 per cent nickel y-phase alloy. It is difficult to see how, on the basis of the zero pressure iron-nickel phase diagram, these phases could have formed, but it is possible that within the parent body the Santa Catherina sample may have been composed of two distinct y-phase alloys, containing about, 24 per cent and 56 per cent nickel respectively, which represented sub-solidus equilibrium phases for a 34 per cent nickel alloy cooling under these conditions. Subsequently the low nickel y-phase (24 per cent nickel) may have partly transformed to plessite or ~netakamacite, so
Thcrmomagnetic
analysis
of co-exist.@
nickal-iron
metal phases in iron meteorites
381
that the meteorite would now consist of t.wo major phases-24 per cent nickel plessite or metakamacit.e and a 65 per cent nickel y-phase. It is important that electron probe studies should be carried out on the Santa Catherina ataxite to establish that these various phases are present.. .4ckncnde&emen~~-The iron mctcorite .samplcs studied in this work were generously provided by Mr. R. 0. ("HALMERS, Australian Musellm, Sydney and ?uIr. \v. SIcHIPOR~l& California lnxtitute of Technology, I’u*iadonu.
REPEREXCES A(:RJXLI. S. 0. and LONG J'.V. P. (1959) The application of tho scanning S-ray microanalyser to mineralogy, I’roc. Second Int. Symposium on X-ra!y Microclcop?j and S-ray Microanal~y8i.9, Stockholm 1959, (to be published). HOZoRTH )1. M. (1951) Ferromagnetism, Van Nostrand, New York, 968 pp. CT.ARK S. P. and KI:LLERUD G. (1959) Iron meteorites, Ann. Rept. Geophye. Lab. 1958-1959, Curnegie In&. Wwhington Year Hook 58 167-170. VERGE 0. and KOMMEL A. It. (1937) The structure of meteoric irons, Amer. J. Sci. 234 203-214. FARRIS~TON 0. C. (1915) Meteorites, Chicago, 233 pp. (;OLDS~HWDT V. M. (1954) Geo&mtitq (edited by A. Muir), Clarendon Press, Oxford, 730 pp. HENDBRSOS E. I’. and PJZRRY S. H. (1958) Studies of seven siderites, Proc. U.S. ,Vat. 3fu.q. 107, 339-403. HOSELJTZ K. (1952) Ferromagnetic properties of ,metals and alloys, Clarendon Press, Oxford, 317 pp. .JOSES F. W. and Z’UWJHRES \V. I. (1949) Free encqy and motastable states in the iron-nicks1 and iron-manganese systems, J. Iron Steel Inst. 103, 121-131. KASE T. (1925) On tho LVidmanstatten st,ructuro in iron-carbon and iron-nickel alloys and in metcorit,es, Science Reps. Ser. 1, Tohoku Imperial University 14, 537-558. KRIS~V E. L. (1956) I>er Eisenmeteoritonregen van Sichote-Alin, C’hemie der f+%e 18, 3338. LOVERIX .J. F. (1955a) IXffcrentiation in the iron-nickel core of a parent meteorite body, Geochim. et Coamochim. Acta 12, 238-252. LOVERIXC ,J. P. (1957b) Temperatures and pre?lruras within a typical paront mcteorito body, Gsochim. et Cosmochim. Acta 12, 253-261. LWERISG .J. F., NICHIPOR~~K W., CHODoS A. A. and BROWS H. (1957) Tho distribution of Kallium, germanium, cobalt, chromium, and copper in iron and stony-iron mrt.eoritcs in relation to nickel content and structure, Geochim. et Cosmochim. Actu 11, 263-278. MARJSCER Ii. E., RICHARD S. A. and AI:STIX A. E. (1959) Microbeam analysis of 1jTidmanstrattcn structure in meteoric iron, Trans. Met. Sot. AIME 215, 56-58. NICHIPORUH \V. (1958) Variat,ions in the content, of nickel, gallium, germanium, cobalt, copper and chromium in t.he kamacit.e and taenitc phares of iron metiorites, Geochim. et Coemochim. Acta 13, 233-247. ~ICHIPoRlX LV. and CHODOV A. A. (1959) The concentration of vanadium, chromium, iron, cobalt, nickel, copper, zinc, and arsenic in the meteoritic iron sulphide nodules, .I. Geoph?~.~ RPS. 184, 2451-2463. (hvEN E. A. (1940) The structure of meteoritic iron, I’hil. Msg. 29, 553-567. OWEN E. A. and LNJ Y. H. (1949) Further X-ray study of the equilibrium diagram of the ironnickel system, J. Iron Steel Inst. 163, 132-137. PERRY S. H. (1944) The metallography of meteoric iron, T1.S. A’at. J1u.v. Bull. l&1, 206 pp. PERTHF.L R. (1960) Uber den Ferrimapotismus nicht stiichiometrischer Eiscn sulphide, Ann. J’hys. (Leipzig), v. 5 (series 7), 273-295. PEWHARD M. (1925a) Contribution & l’dtude des ferro-nickels. Part II, Rev. Metall. 22, 581-609. I’~SCHARD M. (1925b) Contribution B 1’6tude des Ferro-nickels. Part III, Zlec. Met&Z. 22, 663-685. SAITO 'l'.(1956) On tho mapetic propert.ios of natural pyrrhotitc, Bull. Geol. SW-V. *Japan 7, 3, pp. 39-46.
J. (1908) The thermomagnctic analysis of mekoric itntt iirtilis:iill nickel-iron alloys, Truns. 22my. A’oc. (London) 208 (series A), 21-109. SMITH 8. IV. -7. and Yorsc: .I. (1939) The Widmanstat~tcn struct IN of oc~ahtvlri-11 Illttt,eoric iron, Snture, T,,on.tE.143,384-385. STRONG H. M. (1959) ‘l’hc expcri~wntal fitsion cnrv~ of iron I,O R~i.OOOa~ IIIOH~~~IC~WS. ./. CGeoph,+s. ZZes. 64 , 1’53-659. k, UHLIG H. H. (1954) C!ont,ribut,ion of’ metallurgy to t)he origin oi nwtcwrites. I’art I. Structun~ of’ metallic meteorites, their composition and t,he effk& of’ press~~re, ~:cwc~hrnl. r:t C!oarrmchi?rr. =Icta 6, 282X301. MTITTRY Il. 13. (19.59) JIetIallurgical apl)lications of alvctron 1)rol~* ,~li~.l.c,illlill~si~. (~mpul~lishrtl report, Applied Research Laboratories, (Glendale, (‘alif’orni~~). SMITH
Phil.
8. IV.
Thermomagnetic analysis of co-existing nickel-iron metal phases in iron meteorites and the thermal histories of the meteorites* J. F. LOVERING~ (Received
and L. G. November
PARRY~
1960)
Abstract-The nickel contents of co-existing kamacite, taenite and p&site in all major structure classes of iron meteorites have been determined by thermomagnetic analysis. The results are in substantial agreement with preliminary data available on the electron probe microanalysis of octahedrites and are consistent with the view that the metal phases in iron meteorites have formed during cooling to temperatures of 300 to 370°C while the meteorites were subject to pressures up to about 25,000 atmospheres. There is some evidence that iron meteorites with relatively low nickel contents (i.e. hexahedrites and coarse octahedrites) formed at pressures slightly higher than those with higher nickel contents (i.e. fine octahedrites and nickel-rich ataxites). It is also suggested that iron meteorites with nickel contents greater than about 30 per cent (e.g. Santa Catherina) may have been composed of two y-phase alloys while they still formed part of the core within a parent meteorite body.
I. INTRODUCTION RECEKT work (UHLIG, 1954; LOVERING, 1957b) has shown that the nickel contents of the various co-existing nickel-iron phases in iron meteorites may be used to estimate the temperatures and pressures operating on the iron meteorites while they still formed portion of a parent meteorite body. FARRINGTON (1915) summarized the data available at that time and reported that kamacite (u-phase nickel-iron) values varied from 5.26 to 6.81 per cent nickel and taenite (y-phase nickel-iron) values varied from 13.0 to 48.0 per cent nickel so that it is important that more precise data should be obtained. NICHIPORUK (1958) studied the nickel contents of co-existing kamacite and taenite in a number of iron meteorites by a process of selective leaching of the kamacite phases in very dilute acid solutions. Kamacite values from 3.3 to 5.8 per cent nickel and taenite values from 30-S to 53.7 per cent nickel were reported (Table 1) but these results are likely to be in error as a result of the solution of other phases during the selective leaching’process. The most promising technique at the present time is electron probe X-ray analysis and several preliminary studies have already been made (MARINGER et al. 1959; AGRELL and LONG, 1959; WITTRY, 1959). Kamacite values reported ranged from 4.7 to 7.5 & 2 per cent nickel while taenite varies from about 30 to 40 per cent at the outer rim to about 15 per cent in the plessite regions situated in the centres of the taenite bands.
* The above paper was fist submitted to the Editor for Cosmochemistry in November, 1960. Owing to his serious illness at that time he was unable to deal with it and the subsequent combination of circumstances which led to a long delay in the publication of the paper is greatly regretted. t Department of Geophysics, Australian National University, Canberra. A.C.T. $ School of Physics, University of New South Wales, Kensington, N.S.W., Australia. 361 1
This is a very powerful technique but’ at the present8 stage if’ its (development, it does not seem possible to determine the nickel concentration in t’he kamacite phase with sufficient precision for the st’ud\rof the detailed t’hermal and pressure historirs of the iron meteorites. In this work the technique of thermomagnetic analg-sis has been used which, in favourable circumstances, enables the nickel content of cc-phase alloys to be determined within -j-0*5 per cent nickel and y-.phase alloys to
Mrteoritr
within -+:! per cent nickel. l’he overall nickel contents of plcssitic areas in iron meteorites can also be determined accurately by this method. Other workers (SMITH, 2908: PESCHARD, 1925: KASJC, 1915: SMITH and Yorrx(:: 1939) have used thermomagnetic analysis to study the kamacite and taenite in iron meteorites but, in general, their methods have not been as sensitive as those described and used in this work. A summary of the results of previous work on the thermomagnetic analysis of iron meteorites is given in 'I':tl)le 2.
tive
Thermomagnetic analysis is a well estabilshed technique ~wtl in the quantitaestimation of ferromagnetic components in metal samples and has been
Thermomagnetic
analysis of co-existing
nickel-iron
metal phases in iron meteorites
363
described by HOSELITZ (1952). It is convenient to display the results graphically by plotting the ratio of the saturation magnetic moment at temperature T”C (Us) to the saturation magnetic moment at 20°C (Go,,) as a function of the temperature T”C. Points of inflection on these curves indicate the characteristic temperatures at which components in a sample become non-magnetic and this characteristic temperature is then used to identify the component. From the shape of the curve and a knowledge of the magnetic properties of the suspected component an estimate is made of the amount of this material present. Table Meteorite
2. Previous
Classification
Butler
Of
Chinautla
Om
Sacramento
Om
thermomagnetic Kamacite (wt. % Ni)
analyses
of iron meteorites.
Taenite (wt. % Ni)
Reference
and YOUNG
(1939)
-50
SMITH
6.9 -7.1
-32.0
PESCHARD (1925b)
6%
~27%
SMITH (1908)
7%
-25%
KASE (1925)
The curves were obtained
in this case with a pendulum magnetometer (Boz10 and 100 mgm contained in a sealed silica tube evacuated initially to a pressure less than O-1 mm of mercury. The temperature was controlled by a tubular furnace. The magnetometer and electromagnet were calibrated by using nickel powder and the Curie points of pure iron, nickel and magnetite agreed to -&Y’C with values given by BOZORTH (1951). In some cases the sample was not completely saturated and consequently the absolute value of the magnetic moments are reliable to only f5 per cent. For any one sample the ratio u,/a,,.was reproducible to better than 0.5 per cent. In the interpretation of the thermomagnetic curves for the iron meteorites, two main problems arise: (1) The Curie temperatures of some characteristic magnetic transformations, which were used to identify the magnetic phases, were ambiguous in the temperature range 300°C to 600°C. (2) The heating cycle, which was made by steps at an average rate of 10°C per minute, may have induced chemical changes since diffusion may take place across and within grain boundaries. Because considerable information is available from other sources about the material studied it is possible to resolve many of the ambiguities in the thermomagnetic evidence. The possibility of diffusion of nickel atoms is more serious since the sample is normally heated from 500°C to 750°C in a period of about 25 minutes. The relatively small bodies of y-phase alloy are modified by this heating but usually they have passed beyond their Curie points and have been identified before the high temperature is reached. The large bodies of a-phase alloy were not modified seriously during the first heating cycle and in most samples measured the characteristic temperature of the a-phase was changed only by small amounts by ORTH, 1951) using samples of mass between
J. E’. Lov~~rsc
364
ancl L.
(i. I’ARHY
repeated cycles of heating. The estimate of the quantity of a particular phase present in a meteorite is inaccurate due principally to sampling uncertainties and the difficulty of ensuring complete magnetic saturation. Iron meteorites are composed of kamacite, taenite, and plessite regions \\-ith minor amounts of troilite and schreibersite so that their thermomagnetic curves
\ 0.C
\
-_
-
T
\ \
/ 04
\--2
\
k
/ i
(3)
I
b-
(i
I
-.i_
0.4
I
I1
I 0.2
.L
I_-!!..
f-\
\ \. C
zoo
4
--- --0
Temperature,
600
b
I
“C
Fig. 1. Thermomagnetic curves of im irreversible cc-phase iron alloy (Kamacite) containing 6 ~NX cent nickel [heating enwe (1) and subsequent, cooling curve (2)] and sohreibwsite from t)he Mungindi No. 2 line octahrdrite [heding cur\.e, (3)].
are composite CUWCS. First of all it is necessary to understand t’he characteristic thermomagnetic behaviour of each of these components before the complex curves of the iron meteorites can be interpreted. (1) Kamacite (a-phase nickel iron alloy) : In most iron meteorites studied the kamacite occurred in large grains and no rapid compositional change was found during heating cycles. This alloy is irreversible and is characterised by two sharp phase transitions involving a change in magnet,ic moment. On heating, the alloy transforms from magnetic a-phase to non-magnetic y-phase, reaching completion at a characteristic temperature ‘II; and on cooling, the non-magnetic y-phase begins to transform to the magnetic cc-phase at a characteristic temperature T2, Fig. 1.
Thermomagnetic
analysis of co-existing
nickel-iron
metal phases in iron meteorites
365
Curves 1 and 2 illustrate these transformations. PESCHARD (1925) and HOSELITZ (1952) and others have confirmed that these temperatures are characteristic of the particular nickel content of the original u-phase for heating cycles of duration In alloys with less than 6 per cent comparable to those used in these experiments. nickel the Curie temperature may be below the transformation temperature and by observing the extent to which the transformation has occurred at 77O“C an It should be pointed out that the estimate can be made of the composition. magnetic transformations observed at these heating and cooling rates are not equilibrium processes (JONES and PUMPHREY, 1949) and consequently have no bearing on the equilibrium phase diagram. Using the data of PESCHARD (1925), BOZORTH (1951) and HOSELITZ (1952) for these characteristic temperatures the composition of the a-phase can, in favourable circumstances, be estimated to ho.5 per cent and changes induced by heating are within this range of uncertainty (Table 3). (2) Taenite (y-phase nickel iron alloy): Taenite is a reversible alloy with a Curie temperature, depending on the nickel content, within the range 100°C to 600°C. Taenite with a composition between 27 per cent to 33 per cent of nickel has been found in small quantities in most of the samples examined. The presence of this alloy has been confirmed by noting that when cooled in liquid air it is converted to cc-phase of the same nickel content (HOSELITZ, 1952) then when heated beyond the temperature of the u to y transition the appropriate Curie point inflection reappears on cooling (see Fig. 2). For these alloys it is possible to detect about 1 per cent of taenite in kamacite. When taenite is richer in nickel its identification is more difficult because of the less favourable shape of the curve and because schreibersite and meteoritic iron sulphide also affect the shape of the curve. In a few samples an alloy of about 40 per cent nickel was identified. The taenite composition varies from sample to sample in any one meteorite and an average figure has been given in Table 3. (3) Plessite: Plessite is an intergrowth of kamacite and taenite which occurs within the taenite lamellae of iron meteorites and as the dark etching ground-mass UHLIG (1954) has suggested that certain plessite of many nickel-rich ataxites. areas may also contain an ancestral acicular as-phase which OWEP~(1940) has called ‘cmetakamacite.” Thermomagnetic analysis indicates that plessite is unstable during the heating cycle and undergoes rapid compositional changes. Between 5OO’C and 600°C the shape of the initial heating curve (Fig. 3) depends on the rate of heating and probably reflects the rate of reaction between the various constituents of the plessite. However, after a heating cycle the plessite becomes stabilized as a characteristic E-phase of definite composition having the same overall nickel content of the parent phase of the plessite and characterised by a sharp transition temperature (Table 3). (4) Phosphide phases: Iron meteorites contain two generations of phosphide phase with the general composition (PeNi),P. The first generation phosphide is schreibersite and is found either as isolat’ed thick prismatic crystals or as rims around sulphide nodules so that it must have separated before the metal phase
2
Ataxites
Nickel-rich
Anomalous Ataxite
-
Fine Octahedrites
-.
Octahedrites
Medium
4
3
Santa Catherina
4
Tawallah Valley
Wedderburn
2
5
4
2
3
1
Mount Magnet
Arltunga
Mungindi No. 2
TOlUCa,
Kyancutta
Henbury
5
10
10
10
90 & 10
140 *
200 & 5
190 *
212 + 3
200 +
180 + 20
210 *
195 & 20 10
Ni y
20 20 700 5
30 10 10
120 5 550 * 590 *
10 320 570 & 10 750 * 10
& 20 & 10 ~ 680 & 5
120 *
130 320 600 755
150 * 300 + 470 = 750%
20 20 320 550 - 680 730 + 10
110 + 240 *
130 & 20 500 - 670 745 * 10
220 5 20
100 * 10 630 & 10
600 ~+ 10
590 & 20
250 * 30 590 + 10
Xi y 24 + 2”,/0 Ni (P) 55 l 2%Niy
28 _C 2%
troilite (?) 23 l 1% Ni (P) 6-5 & 0.596 Ni c(
28 + 1Of ,o Ni y
27 & 3% Ni y schreibersite 16 ,i 1% Ni (P) 7.0 & 1% Ni t(
29 & l%Niy schreibersite 15 & 3% Ni (P) 7 *2%Nicr
27 f 1 o/o Ni 1, 32 & l%Niy schreibersite 13% Ni (P) 7.5&l%Nicr
28 *2%Niy 19 + 2% Ni (P) 7.0 + 1% Ni a
40 -+ 3% Niy 7.5 k 1.0% Ni a
2%
410 & 10 745 & 50 10
l%Niy 1% Ni a
29 &- 2% Ni y schreibersite 17 _C 2% Ni (P) 7.5 + 1.0% Ni a
30 + 7.0 +
32 i
585 *
270 & 30 590 * 10
600 i
230 h 20
20 350 620 & 20 745 l 50
130 i_
170 & 20 750 & 5
10
40 _c 4 20 + 5
5*3
4+ 2 Trace 50 * 10 15& 5
412 Trace about SO 15 i_ 5
5* 2 Trace 70 * 10 20* 2
3&l 311 Trace 14 + 2 75 * 5
3xtl 30* 5 70 * 10
4+2 85 *
6xk2
5*2 Trace 15* 5 80 * 10
4rt2 92 * 5
33.60
22.2
18.21
14.72
10.08
12.36
8.31
8.28
7.66
This work
LOVERIN~ et al. (1957)
LOVERING et al. (1957)
LOVERIN~ et al. (1957)
LOVERIN~ et al. (1957)
LOVERING et az. (1957)
NICHIPORUK (1958)
NICHIPORUK (1958)
LOVERING et al. (1957)
solidified. The second generation phosphide (rhabdite) occurs as acicular crystals regularly oriented in the kamacite phase of iron meteorites and is a result of exsolution from kamacite (GOLDSCHMIDT,1954). Little is known of the distribution of nickel in schreibersite or rhabdite but HEX.DERSOK and PERRY (1958) found 19.58 per cent nickel in rhabdite and 12.03 per cent in schreibersite in the Tombigbee hexahedrit’e. Pure Fe,P is ferromagnetic with a C’urie point of 4?O”C (l%)ZOKTH. 1951) but) the addition of nickel is likely to depress the (‘uric t’emperature. A sample of
I ‘3
1
?CO
400 Temperature,
600
F 10
“C
bchaviow of reversible Fig. 2. Thermomagnetic lf-~)hase iroll alloy (taenito) containing approximately 29 per cent nickel in the Toluca meciium octahedrite. Curve (1) original sample heated to 400°C showing characterist,ic Curie temperat.ure of taenite at 160°C. Curve (2) original sample cooled to 180°C' for severa. Curve (3): hours and then heated to 450°C’. Transition at 150°C not tlrtr~c~l~wl. cooling curve from (2). Taenite detected by inflexion at 150°C.
schreibersite inclusion was drilled from Mungindi No. :! fine octahedrite. The thermomagnetic curve is reproduced in Fig. 1 curve 3. The (‘uric temperature is 310 i_ 10°C and the material is not affected much by the heating cycle. (5) Iron sulphide phases (FeS, i .r) : NICHIFORUR and CHol)os (1969) concluded from X-ray studies of iron sulphide nodules from a number of iron meteorites that they are composed of troilite (z -= 0) and pyrrhotite (0 ,: .c ” 1) with some daubreelite, chrompicotite, schreibersite and kamacite. Troilite is non-magnetic, pyrrhotite .c -: 0.15) is ferroand pyrrhotite (047 (0 < 5 < 0.07) is antiferromagnetic magnetic wit,h a Curie point of npprosimately 3W(’ (PEKTEIEL. 1960). SAITO (1956) has measured a number of terrestrial pyrrh0tit.e samples and finds Curie points from 310~320°C. Samples have been taken from nodules in Mungindi No. 2. ‘The behaviour of these samples was similar t,o Moonbi and Delegate meteorites. that observed by SAITO (1056). The Mungindi No. 2 sample had a very low maguetic moment (0.2s e.m.u. g-l) and the curves had a peak at about WO’Y’ (see curves 5 and 1 of Fig. 4) which givesavalueof xintherangeO.0’7~O~lOforthismaterialaccording ~~PERTHEL( 1960). During the heating in a sealed tube some highly magnetic material having a Curie point at 580°C was formed. This appears tjo be magnet,ite since PERTHEL (1960) finds that oxygen entering the sulphide lattice does not affect the (‘uric point8 greatly.
Thermomagnctic
analysis
of co-existing
nickel-iron
metal
phases
in iron meteorites
369
When this sample was reheated in air ferromagnetic pyrrhotite and magnetite were formed. This material had a magnetic moment of 5.3 e.m.u. g-l and was similar in thermomagnetic behaviour to the sample taken from Moonbi. Material drilled from the Moonbi meteorite had a magnetic moment of 20
0
200
\
400
Temperature,
630
10
‘C
Fig. 3. Thermomagnetic analysis of plessita in iron meteoritos. Curve (1) is the initial heating CUPVOfor a sample of the Mt. Magnet nickel-rich ataxite which contains 90 per cent plessite. Curve (2) is a reheating curve for the came sample after it had been cooled from 750°C. Curve (4) is the initial hoating curve for a sample of the Wedderburn nickel-rich ataxit.e largely composed of plesaito with subsequent cooling curve (5) after heating t.o 620°C. Curves (3) and (6) are heating and coooling curves for material which has been previously heatod to 620°C.
e.m.u. g-l. The thermomagnetic curves are given in Fig. 4 curves 3 and 2. Both magnetic pyrrhotite and magnetite are present but this nodule is near the A&ace and may have been partly oxidised during ablation of the meteorite in the atmosphere. The third sample from the Delegate nodule had an initial magnetic moment of O-6 e.m.u. g-l and on heating above 4OO’C a highly magnetic component wa8 formed with a Curie point at 580°C. On cooling the magnetic moment had increased to 30 e.m.u. g-l but there was no evidence of magnetic pyrrh0tit.e. X-ray
J. 8. LOVERING
370
and L. G. PARRY
photographs showed that the initial material was mainly troilite with some pyrrhotite and the after-heating was a mixture of ~~yrrhotite~ troilite and magnetite with the magnetite apparently forming from dhc troilite.
Temperature,
OC
%‘ig. 4. Thermomagnek
curw
(I),
analyses of’ iron sulphide phases from Ximgindi [heating cooling curve (5)], Moonbi cheating (3), cooling (Z)i, and Delegate [heating (6), cooling (4)] octahectrites.
When troilite is present its variable behaviour when heated leads to difficulty in identifying any component with a Curie point in the range 320 to 580°C) but an increase in cPI‘in this range indicates that troilite is being converted to magnetite. However it is very disrupt to see how any signi~cant formation of magnetite could take place during the heatings tts these experiments are carried out under very reduced atmospheric pressure (i.e. less than O-I mm of mercury). (6) Daubreelite (FeS. Cr2S3): Daubreelite is rare in most iron meteorites and,
Thermomagnetic
analysis of co-existing nickel-iron metal phases in iron meteorites
371
according to PERRY (1944), it is not ferromagnetic and even if present in our samples it would not be detected. (7) Cohenite (FeNi)&: The iron carbide phase in iron meteorites apparently contains about 2 per cent nickel but it is comparatively rare and only known from a few coarse octahedrites (PERRY, 1944). Pure iron carbide (cementite) has a Curie temperature of 215°C (BOZORTH, 1951) so that the small amount of nickel in cohenite should depress its Curie temperature a little below 215°C. However, no samples studied in this work were found to contain cohenite. 3. RESULTS A collection of 14 iron meteorites of all major structural types have been studied in this work and the results are summarized in Table 3. The range of experimental error assigned to the various characteristic temperatures in Table 3 take in to account differences within each sample and also between different samples of the same meteorite. The errors assigned to the nickel contents of the metal phases identified are related to the experimental errors of the characteristic temperatures. This procedure would tend to mask small but real variations in taenite composition but it should not affect the identification of the kamacite phases. (1) Hexahedrites: In both hexahedrites studied (Coya Norte and Sikhote-Alin) the major phase (greater than 80 per cent) present is u-phase alloy containing 5.5 f O-5 per cent nickel. Usually some y-phase was detected but because the structure of hexahedrites was so coarse it was difficult to make a reliable estimate of the total amount present. The nickel content is 30 f 3 per cent. A very small amount of plessite containing 14 to 17 per cent nickel was also detected along with minor amounts of schreibersite. Fig. 5 curves 1, 2 are of Sikhote-Alin except that no y-phase alloy was detected in this particular sample. NICHIPORUK (1958) recently examined the Coya Norte hexahedrite and found kamacite phase only with a nickel content of 4.78 per cent which is in reasonable agreement with the results of the present study. (2) Nickel-poor ataxites: The only nickel-poor ataxite examined was Bingara No. 2 and the results are very similar to those for the hexahedrites. The main phase present (about 96 per cent of the sample) is an irreversible u-phase alloy with 5.5 * O-5 per cent nickel and minor y-phase (31 * 2 per cent nickel) and schreibersite. (3) Coarse octahedrites: Three coarse octahedrites, Canyon Diablo, Cranbourne (Beaconsfield mass) and Mount Tabby were studied and the results were significantly different from the hexahedrites and nickel-poor ataxites in that over 80 per cent of all samples was made up of 7-O f 0.5 per cent a-phase alloy. Some taenite with 30 & 2 per cent nickel was always present and in Canyon Diablo there was evidence of 41 per cent nickel y-phase. All coarse octahedrites examined showed the presence of around 2 per cent of plessite containing from 18 to 26 per cent nickel (see Fig. 6). Canyon Diablo has also been examined recently by chemical and electron probe microanalysis techniques and these results are compared with the present results in Table 2. These data indicate that both electron probe and thermomagnetic techniques give very similar results, but that the chemical methods used by
NICHIP~RUK (Table 1) give quite different results. NICHIPOIWK (195X) has also studied the Mount Tabby iron (Table 1) but although the taenite composition is in good agreement with the thermomagnetic results, the kamacite results are significantly lower than the thermomagnetic analyses. (4) Medium and fine octahedrites: The medium and fine octahedrites examined
T
I
-----A-
.I
1’
t
Fig. 5. Thermomagnetic analyses of’ a nickel-l)oor at,;txite (Sikhote-Alin) and hcxahedrito (Hingara p\i’o.2). Curve (1) is the initial treating CIII’\-V for Sikhot,c41in hexahedrite
(:ur\.(~ (3) is t,ho heating while curve (2) is the cooling cwvp. curve for Ringara No. 2 nickel-poor at.axitc.
showed rather similar properties so they will he discussed together. The kamacite phase in these iron meteorites shows a tendency to slightly higher nickel contents (‘7.5 & 1% nickel) than those of the coarse octahedrites. ‘I’arnite is virtuall) identical within the range of experimental error (30 :t :!O,, nickel) hut Toluca showed evidence of a small amount of y-phase with 40 per cent nickel. Plessite with nickel content about 18 Ik 2:: was detected in both Kyancut,ta and Mungindi No. 2 (Fig. 6, curves 1 and 5; 3 and 6). The Henbury and Toluca irons have also been studied by XICHIPOHIJIC(Table l), who found nickel contents in taenite much the same as the thermomagnetic results, while kamacite was considerab1.v lower.
~he~oma~etic
analysis of co-existing nickel-ironmetal phases in iron me~or~tes
373
(5) Anomalous
is that hedral a thin overall
Ataxites: The macro-structure of the Arltunga iron meteorite of a nickel-rich ataxite but the micro-structure shows a well-marked octaarrangement of fine kamacite plates. Each kamacite plate is surrounded by zone of taenite while the interstitial material is apparently plessite. The nickel content (i.e. 10.1 per cent) is less than that of many octahedrites and 1
0
0
10
Temperature,
OC
Fig. 6. The~o~~etic analyses of coarse, medium and fine octahedrites. Cra&ourne coarse octahedrite: heating curve (2), cooling curve (4). Kyancutta medium ootahedrite: heating curve (Z), cooling curve (5). Mungindi No. 2 fine octahedrite: heating curve (S), cooling curve (6). LOVERINC et al. (1957)
have also reported anomalous gallium and germanium concentrations in this iron, In view of all these pecularities, Arltunga has been classified as an anomalous nickel-rich ataxite. Thermomagnetic analysis indicates kamacite with 7.5 f 1 y0 nickel, two taenite phases with 27 -& 1% and 32 4 1% nickel respectively, and plessite containing about 13 per cent nickel. (6) Nickel-rich Ataxites: The four nickel-rich ataxites studied can be divided into two groups, those whose saturation magnetic moments lie within the range HO-220 e.m.u. g-l, as do all other types of iron meteorites, and those with considerably lower moments (W-140 e.m.u. g-l).
374
The two nickel-rich ataxites with normal saturation magnet,ic moments (Mount Magnet and Ta~~al~ah Valley) both contain kamacite with a nickel content of 7 5 1% as do the octahedrites but the proportion of this phase in the nickel-rich ataxites is much lower (i.e. about 1.3y/U)1 Small amount,s of taenite cont,aining about 28 & 1 o/o of nickel are also observed but the major phase present is IJessite which in Mount’ Magnet stabilizes around 18 3: 30{, nickel and in ‘I’awallah Valley
0
Temperature,
OC
Fig. 7. Thern~omagnotic analysis of t,he Tawalltth Valley nickel-ric:tl ntaxito sl~oaing heating (1) and tiooling (3) curves for previously unheated material anc.lhoitting (2) SXXIcooling (4) e~rve~ fcs material previously heated to 730°C’ for 6% mkbes.
around 16 i lo/A nickel after heating (see Fig. 7) beyond the transition temperature (i.e. :::,680YY). Of the Lwo nickel-rich ataxites with abnormally Iow saturation moments (Wedderburn and Santa Catherina) only the Wedderburn shows any similarity with the other group. The Wedderburn ataxite contains some kamacite (7 & 1% nickel) and taenite (28 4: IO/, nickel) but the major ~lagnetic phase is plessite averaging about 23 $I lo/, nickel. The thermomagnetic curve of the Santa Catherina is quite different from all other iron meteorites and is difficult to interpret due t’o instability during the heating cycle (Fig. 8). However, the most
~e~omagnetic
analysis of co-existing
nickel-iron
metal phases in iron metsorites
375
reasonable interpretation of the curve would suggest this meteorite is composed of an irreversible phase which behaves like plessite with an overall 24 &- 2% nickel and a reversible y-phase containing 55 _L 2% nickel. Some samples suggested that, a small amount of a y-phase containing 28 f 2 yO nickel might also be present. Santa Catherine was the only iron meteorite studied which did not contain a low nickel content kamacite phase.
I
jO(
Temperature, Fig.
8. The~om~~etie
ac)O
“C
analysis of the Santa Cbtherina (heating and cooling curves).
nickel-rich
ataxite
4. D~scussro~ General The main results of ‘the thermomagnetic study of the iron meteorites which have direct bearing on the problem of the evolution of these meteorites may be summarized as follows: (1) Representatives of all the major structural classes of iron meteorites (i.e. hexahedrites, octahedrites and nickel-rich ataxites) were found to contain a y-phase alloy containing about 29 per cent nickel while certain octahedrites also showed small amounts of another y-phase alloy containing about 40 per cent nickel.
376
,J. F. LOVEJUNG and L. G. PARSY
(2) Plessite was detected in all major structure types and became increasingly prominent as the overall nickel contents of the meteorite increased. The average composition of the plessite detected varied from 13 to 26 per cent nickel, with a mean nickel content of about 15 per cent. There does not seem to be any close relationship between the composition of the plessite and the overall nickel content of the meteorite. (8) The kamacite which occurs in any particular structural t,ype of iron meteorite has a constant nickel content but there are significant differences between kamacites occurring in (i) hexahedrites and nickel-iron ataxites, (ii) coarse octahedrites, (iii) medium and fine octahedrites. (iv) most nickel-rich ataxites. (4) The Santa Catherina, an extreme type of nickel-rich at,axite with about 34 per cent nickel, does not have a low-nickel content kamacite but seems to have originally consisted of two y-phases with nickel contents of about, 24 per cent and 5.5 per cent, respectively. In discussing the thermal history of iron meteorites it, is generally assumed that the effect of impurity elements. other t’han nickel, on the 3’ x transition ill iron meteorites is negligible. ltecently CLARK and Kl:LLERI-I) (1!)59) have given reasons for believing that the effect of impurity-met3allic clomcnts is very small but that non-metallic elements could have larger effects. However: they concluded that the available analyses of non-metallic elements in iroll meteorites x~ould indicate that the carbon content (about 0.11 per cent) is enough to dr~prcss the y -+ x transition by lOO“(’ or so, and the phosphorus content (about 0.22O/~) is enough to raise it by the same amount. The sulphur content- (( 1.169:) should have litt,le effect’. so that the overall e’ffect of both meOallic and non-mrt8allic c1ement.s appears t’o cause little change in tShe)1 -+ x transition in iron I11et,coritos. It should be emphasized that before more definit#e conclusions can I)e rctached there is considerable need of modern analyses of non-metallic elements itI t,he metal phases of iron meteorites. l
Tamite md plessite Micro-st,ructural electron probe and tbermomagnetic studies have shown that teanite and plessite areas are intimately associated in iron meteorites. Generally, thin rims of taenite surround plessite areas and are in direct, contact with the kamacite grains. DERGE and KOMXEL (1937) suggested that plessite areas within t’aenite formed as a result of a later transformation of first-formed low-nickel regions within the taenit,e but Owen- (1940) later proposed that plessite resulted from the transformation of an acicular cc,-phase (“metakamacite”). UHMG (1954) agreed with OWES that the parent phase of plessite was metakamacite which he suggested formed from certain y-phase alloys in iron meteorites as a result of the sudden pressure decrease during the disruption of the parent. meteorite body. Although t’he origin of plessite is still obscure, there is general agreement that the ancestral phase was a relatively low nickel content y-phase alloy. The nature of this ancestral y-phase would have an important bearing on the Ohermal histories of the iron meteorites. The thermomagnetic results suggest that in virtually all the major structure types of iron meteorites (i.e. hexahedrites, octahedrites, nickel-rich ataxites) the
Thermomagnetic
analysis of co-existing nickel-iron metal phases in iron meteorites
377
plessite areas and taenite phases show remarkably little variation in composition, On the other hand, the kamacite in the same iron meteorites varies significantly in nickel content as the structure of the meteorite changes. This behaviour does not seem to be explained by the equilibrium iron-nickel phase diagram determined by OWEN and LIU (1949) (see Fig. 9). It is virtually impossible to calculate the overall composition of the y-phase which would have been in equilibrium with the observed u-phase alloys in iron-meteorites, because the original y-phase areas were so inhomogeneous. Presumably if the nickel contents and relative proportions of
Nickel,
w+
%
Fig. 9. The cr-y equilibrium phase diagram determined by OWENand LIU (1949) for iron-nickel alloys at one atmosphere (full curve). The broken curve is the estimated diagram at 25,000 atmospheres.
the various taenite and plessite areas could be determined accurately, then the overall nickel content of the original y-phase alloy could be calculated. However, at the present time neither the thermomagnetic nor the electron probe microanalysis methods are sufficiently precise to enable such a calculation to be made. The inhomogeneity of the original y-phase in the iron meteorites is itself an indication of the thermal history of the meteorites. The central zone of broadly constant, though relatively low, nickel content (now plessite with about 18 per cent nickel) represents y-phase which was presumably in equilibrium with an a-phase alloy at a time when temperatures were high enough so that relatively rapid diffusion of nickel was possible in the y-phase. From the zero pressure equilibrium iron-nickel phase diagram (Fig. 9) y-phase alloys containing 18 per cent 2
378
.J. B’.
Lovmtr~c
and
L. G.
I'.~RNY
nickel would be in equilibrium with an a-phase containing 3.7 per cent nickel at 59O‘X’. The rapidly increasing nickel content towards the margins of the original y-phase indicates that, with further cooling and “J --+ cc transformation, the nickel atoms rejected from the rapidly growing a-phase areas banked up just inside the y-phase boundaries. At the time the parent body1 was disrupted it, would appear that temperatures within the metal core were low enough to restrict diffusion of these nickel atoms into the rest of the y-phase alloy. If it is assumed that the 30 per cent nickel y-phase at Dhe boundaries represents an eyuilibrium y-phase alloy then the one atmosphere l)hast’ diagram would indicate equilibrium temperatures around 4SW’. However. electron-probe analyses (WITTRY, 1959: MARZS(:W it al., 1!)59) indicate that the high nickel 5 content margins of the taenit,c lamellae are only about, 10 microiis across so that steep concentration gradients exist’ in the outer margins of t’aenite lamellae. It is exbremely difficult to see how such steep concentration gradients could have existed for long periods of time in the parent meteorite body core if temperatures were 406°C’ or higher. These steep gradients are, however, more in keeping with UHLIG’S (1964) conclusion that the iron meteorites must have cooled to temperatures considerably less than 400”(’ within t,he l)arent body. Under these circumsta,nces, it would seem that the composition of the y-phase alloys in iron meteorites is not consistent with the equilibrium nickel-iron phase diagram at zero pressure but that pressures high enough to depress transition temperatures l)y 100 to bOO”(’ must be considered to have operated within the core of’ a parent body. Kamacitc ill hexahedrites
and
octa,hedrites
The thermomagnetic dat,a have shown that the kamacite phase occurring in the various structure types of iron meteorites is not constant, in composition as was formerly thought, to be the case (UHLIG, 1954). In t,he hexahedrites and nickel-poor ataxites the kamacite contains about 6.5 per cent, nickel. The zero pressure phase diagram indicates that kamacite of this composition would be in equilibrium with a 33 per cent nickel y-phase alloy at 450°C: (Fig. 9). However, thermomagnetic analyses of the rare y-phase alloy in these irons indicate that probably the outer rims of the taenite phase may contain 30 per cent nickel but that the taenite is extremely inhomogeneous and the overall nickel content would be less than 30 per cent. From the previous discussion of the significance of the taenite phase in iron meteorites, it would seem that the taenite phase in the hexahedrites had cooled to temperatures considerably less than the 450°C indicated by the kamacite composition and probably more in the region of 300 to 4OO’C. The effect of pressure on the y -+ cc transformation in nickel-iron alloys has been discussed by UHLIG (1854) who concluded that in the absence of contradictory evidence it is reasonable to assume that the rate of depression of t’he y --) Mtransformation Oemperature with pressure (dT/dP) in nickel-iron alloys is not greatly different from that of pure iron. In nickel-iron alloys the volume change between the 01-and y-phases is somewhat larger than that in pure iron but this effect would be offset by a lower transformation temperature and a higher AH value. Recently STROKG (1959) has measured dT/dP for the y --f M transformation in pure iron to pressures of 96,000 atmospheres and his data show that at 30,000 atmospheres the
Thermomagnetic
analysis of co-existing
nickel-iron
metal phases in iron meteorites
379
transformation temperature is depressed by 16O”C, at 20,000 atmospheres it is depressed by 135’C and at 10,000 atmospheres it is depressed by 95°C. In the absence of a, better model for the behaviour of the iron-nickel phase diagram at high pressures, it will be assumed that at pressures of the order of 25,000 atmospheres the sub-solidus iron-nickel phase diagram would move symmetrically down the temperature axis by about 140°C, as is shown in Fig. 9. From this new diagram the a-phase of hexahedrites would represent an equilibrium phase at 320°C. This low temperature is in agreement with the observation that the associated y-phase in hexahedrites is inhomogeneous. The coarse octahedrites contain kamacite with an average nickel content of 7-O per cent which, on the basis of the zero pressure phase diagram, should be in equilibrium with a y-phase containing 51 per cent nickel at 320°C. The thermomagnetic and electron-probe evidence indicates that the y-phase in coarse octahedrites is extremely inhomogeneous and also that its over-all nickel content must be less than 40 per cent nickel and probably closer to 30 per cent. The observed composition of the LX-and y-phase in coarse octahedrites more closely approximate those calculated on the assumed phase diagram at about 25,000 atmospheres (Fig. 9), with equilibrium temperatures between 200°C and 320°C. The medium and fine octahedrites contain kamacite with an average nickel content of 7.5 per cent while the original y-phase appears to have had much the same composition as that found in the coarse octahedrites. It is apparent from Fig. 9 that the composition of the coexisting metal phases in these iron meteorites also cannot be explained on the zero pressure phase diagram but that pressures somewhat less than those for the hexahedrites and coarse octahedrites are indicated with equilibrium temperatures between 200°C and 300°C (see Fig. 9). In summary, then, it may be concluded that the kamacite and taenite phases co-existing in hexahedrites, nickel-poor ataxites and coarse, medium and fine octahedrites are not in equilibrium on the basis of the iron-nickel phase diagram determined at zero pressure. To explain their observed compositions it seems necessary to assume that the phases were formed at temperatures around 320°C under pressures up to 25,000 atmospheres. There is also evidence that the hexahedrites formed under maximum pressure and temperature and perhaps that both pressure and temperature steadily decreased as the structures of the meteorites changed from coarse to medium and fine octahedrites. Nickel-rich
ataxites
UHLW (1954) has shown how the observed structures of nickel-rich ataxites cannot be explained on the basis of the nickel-iron phase diagram at zero pressure and that considerably higher pressures must be assumed to have operated in the metal core of the parent meteorite body. The results of thermomagnetic analyses of nickel-rich ataxites also suggest that significantly high pressures must be assumed. One very puzzling fact is the relatively sharp boundary between meteorites showing a well-developed Widmanstatten pattern (i.e. fine octahedrites) and those without such a well-developed structure (i.e. nickel-rich ataxites) when the nickel content of the meteorite reaches about 13 or 15 per cent. On the zero pressure
iron-nickel phase diagram alloys containing Id per cent nickel should hegin the y - cc transformation at 620°C. Now UFLL~C(1954) has pointed out that alloys transforming at or below 460°C may be expected to resist complete transformation under any conditions, perhaps including long periods of t,ime below the transformation temperature, whereas alloys transforming above 460”~~ do so readily. Of course, once the Widmanstatten pattern has formed it) may continue to grow below 460°C since its formation depends on the growth of phases already nucleated. On t,his evidence, then, it would appear that the iron meteorites must have cooled under considerable pressures so that those with nickel contents in t.he range 15 to about 27 per cent would not have entered the M -;- y field until their tem~era,tures had dropped to about 460°C at which temperatures, the amounts of transfor~~ation of the y-phase even over times of the order of 10” years, would have been very small. From the estimated iron-nickel phase diagram at %.(100 atmospheres (Fig. 9) it would seem that the M 1. y and y-phase boundary for 13 per cent nickel alloy would be depressed to 510”(! whereas that for 27 per cent alloy would be depressed to 360°C which would certainly explain the formation of the nickel-rich ataxites. Within the nickel-rich ataxites themselves, UNLIG (1!)54) has shown t,hat those with nickel contents less than about 27 per cent contain some kamacite while those with nickel contents greater than 27 per cent do not contain kamacite. During the present work it was found that Wedderburn (22 per cent nicke1) eontaine~~ kamacite but Santa C~atherina (34 per cent nickel) did not. These observations agree with UHLIG'S conclusion that all iron l~~eteorites with nickel contents equal to, or greater tohan, 27 per cent must have existed above the a ,+ y region of the phase diagram in the parent body. However, from the zero pressure iron-nickel phase diagram (Fig. 9) it can be seen that alloys containing 27 per cent nickel would lie just on the boundary of the a ..(. y and y-phase regions at a temperature of 5OOT. Subsequent cooling from this temperature should have allowed to have taken place in these iron meteorites considerable y --)c a transformation but in general no transformation is observed. Once again the reconstructed phase diagram at 25,000 atmospheres (Fi,.0 9) would explain the absence of a-phase alloy in meteorites with more than 27 per cent nickel because under these pressures the alloy would still be entirely in the y-phase down to temperatures of 36O”C, which is well below that at which s~lbsequent transformation would take place of further cooling of the meteorite. One surprising result of the present study is that iron meteorites containing more than about 30 per cent nickel are not composed simply of a single untransformed y-phase as UHLIG (1964) has suggested. ‘The Santa Catherina ataxite seems to consist of a 24 per cent nickel plessite (or metakamacite) and a 55 per cent nickel y-phase alloy. It is difficult to see how, on the basis of the zero pressure iron-nickel phase diagram, these phases could have formed, but it is possible that within the parent body the Santa Catherina sample may have been composed of two distinct y-phase alloys, containing about, 24 per cent and 56 per cent nickel respectively, which represented sub-solidus equilibrium phases for a 34 per cent nickel alloy cooling under these conditions. Subsequently the low nickel y-phase (24 per cent nickel) may have partly transformed to plessite or ~netakamacite, so
Thcrmomagnetic
analysis
of co-exist.@
nickal-iron
metal phases in iron meteorites
381
that the meteorite would now consist of t.wo major phases-24 per cent nickel plessite or metakamacit.e and a 65 per cent nickel y-phase. It is important that electron probe studies should be carried out on the Santa Catherina ataxite to establish that these various phases are present.. .4ckncnde&emen~~-The iron mctcorite .samplcs studied in this work were generously provided by Mr. R. 0. ("HALMERS, Australian Musellm, Sydney and ?uIr. \v. SIcHIPOR~l& California lnxtitute of Technology, I’u*iadonu.
REPEREXCES A(:RJXLI. S. 0. and LONG J'.V. P. (1959) The application of tho scanning S-ray microanalyser to mineralogy, I’roc. Second Int. Symposium on X-ra!y Microclcop?j and S-ray Microanal~y8i.9, Stockholm 1959, (to be published). HOZoRTH )1. M. (1951) Ferromagnetism, Van Nostrand, New York, 968 pp. CT.ARK S. P. and KI:LLERUD G. (1959) Iron meteorites, Ann. Rept. Geophye. Lab. 1958-1959, Curnegie In&. Wwhington Year Hook 58 167-170. VERGE 0. and KOMMEL A. It. (1937) The structure of meteoric irons, Amer. J. Sci. 234 203-214. FARRIS~TON 0. C. (1915) Meteorites, Chicago, 233 pp. (;OLDS~HWDT V. M. (1954) Geo&mtitq (edited by A. Muir), Clarendon Press, Oxford, 730 pp. HENDBRSOS E. I’. and PJZRRY S. H. (1958) Studies of seven siderites, Proc. U.S. ,Vat. 3fu.q. 107, 339-403. HOSELJTZ K. (1952) Ferromagnetic properties of ,metals and alloys, Clarendon Press, Oxford, 317 pp. .JOSES F. W. and Z’UWJHRES \V. I. (1949) Free encqy and motastable states in the iron-nicks1 and iron-manganese systems, J. Iron Steel Inst. 103, 121-131. KASE T. (1925) On tho LVidmanstatten st,ructuro in iron-carbon and iron-nickel alloys and in metcorit,es, Science Reps. Ser. 1, Tohoku Imperial University 14, 537-558. KRIS~V E. L. (1956) I>er Eisenmeteoritonregen van Sichote-Alin, C’hemie der f+%e 18, 3338. LOVERIX .J. F. (1955a) IXffcrentiation in the iron-nickel core of a parent meteorite body, Geochim. et Coamochim. Acta 12, 238-252. LOVERIXC ,J. P. (1957b) Temperatures and pre?lruras within a typical paront mcteorito body, Gsochim. et Cosmochim. Acta 12, 253-261. LWERISG .J. F., NICHIPOR~~K W., CHODoS A. A. and BROWS H. (1957) Tho distribution of Kallium, germanium, cobalt, chromium, and copper in iron and stony-iron mrt.eoritcs in relation to nickel content and structure, Geochim. et Cosmochim. Actu 11, 263-278. MARJSCER Ii. E., RICHARD S. A. and AI:STIX A. E. (1959) Microbeam analysis of 1jTidmanstrattcn structure in meteoric iron, Trans. Met. Sot. AIME 215, 56-58. NICHIPORUH \V. (1958) Variat,ions in the content, of nickel, gallium, germanium, cobalt, copper and chromium in t.he kamacit.e and taenitc phares of iron metiorites, Geochim. et Coemochim. Acta 13, 233-247. ~ICHIPoRlX LV. and CHODOV A. A. (1959) The concentration of vanadium, chromium, iron, cobalt, nickel, copper, zinc, and arsenic in the meteoritic iron sulphide nodules, .I. Geoph?~.~ RPS. 184, 2451-2463. (hvEN E. A. (1940) The structure of meteoritic iron, I’hil. Msg. 29, 553-567. OWEN E. A. and LNJ Y. H. (1949) Further X-ray study of the equilibrium diagram of the ironnickel system, J. Iron Steel Inst. 163, 132-137. PERRY S. H. (1944) The metallography of meteoric iron, T1.S. A’at. J1u.v. Bull. l&1, 206 pp. PERTHF.L R. (1960) Uber den Ferrimapotismus nicht stiichiometrischer Eiscn sulphide, Ann. J’hys. (Leipzig), v. 5 (series 7), 273-295. PEWHARD M. (1925a) Contribution & l’dtude des ferro-nickels. Part II, Rev. Metall. 22, 581-609. I’~SCHARD M. (1925b) Contribution B 1’6tude des Ferro-nickels. Part III, Zlec. Met&Z. 22, 663-685. SAITO 'l'.(1956) On tho mapetic propert.ios of natural pyrrhotitc, Bull. Geol. SW-V. *Japan 7, 3, pp. 39-46.
J. (1908) The thermomagnctic analysis of mekoric itntt iirtilis:iill nickel-iron alloys, Truns. 22my. A’oc. (London) 208 (series A), 21-109. SMITH 8. IV. -7. and Yorsc: .I. (1939) The Widmanstat~tcn struct IN of oc~ahtvlri-11 Illttt,eoric iron, Snture, T,,on.tE.143,384-385. STRONG H. M. (1959) ‘l’hc expcri~wntal fitsion cnrv~ of iron I,O R~i.OOOa~ IIIOH~~~IC~WS. ./. CGeoph,+s. ZZes. 64 , 1’53-659. k, UHLIG H. H. (1954) C!ont,ribut,ion of’ metallurgy to t)he origin oi nwtcwrites. I’art I. Structun~ of’ metallic meteorites, their composition and t,he effk& of’ press~~re, ~:cwc~hrnl. r:t C!oarrmchi?rr. =Icta 6, 282X301. MTITTRY Il. 13. (19.59) JIetIallurgical apl)lications of alvctron 1)rol~* ,~li~.l.c,illlill~si~. (~mpul~lishrtl report, Applied Research Laboratories, (Glendale, (‘alif’orni~~). SMITH
Phil.
8. IV.