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Chemical remanent magnetization of the FeOOH, Fe2O3 system
1968, Phys. Earth Planet. Interiors 1, 103—121. North-Holland Publishing Company, Amsterdam
CHEMICAL REMANENT MAGNEI1ZATION OF THE FeOOH, Fe203 SYSTEM
I. G. HEDLEY Department of Geophysics and Planetary Physics, The University, Newcastle upon Tyne, England Received 30 June 1967 The Chemical Remanent Magnetization (CRM) produced by dehydrating the four isomers of ferric oxyhydroxide has been investigated using X-ray and thermoanalytical as well as thermomagnetic techniques. Synthetic powders were used set in a plaster of Paris matrix and the CRM acquired after heating in the Earth’s field was measured using an astatic magnetometer. The CRM of an artificial chalybeate sandstone was also studied. The nature of the resultant CRM was dependent on the parent oxyhydroxide and the remanence of very fine hematite particles
was unexpectedly large in view ofprevious work on the magnetic stability of such fine grains. The reversal of the remanence in the case of y-FeOOH was particularly striking and could be of some paleomagnetic importance. The CRM carried by oc-Fe,03 derived from FeOOH was seen not only to reflect the crystal structure ofthe parentmaterial but also the temperature to which the oxide had been heated.
1. Introduction
ate solution) on grains of sand to match the colour of an Old Red Sandstone from Dartmouth. The nature of this precipitate was investigated by HOFER and WELLER (1947) using X-ray analysis. The brown skin formed on the surface of the solution was found to be y-FeOOH. Rayleigh’s red coating, produced by drying out chalybeate water on a glass plate at temperatues well below 100 °C,was found by HOFER et a!. (1946) to be mainly cc-Fe203 with some of the highly magnetic y-Fe203 which would certainly give rise to an appreciable remanence.
Although a great deal of paleomagnetic evidence is based on measurements on red sandstones, the manner in which these rocks gained their remanence is still uncertain. COLLINSON (1965), after reviewing the geological and magnetic evidence, concludes that chemical magnetization (CRM) is the most likely mechanism for many formations. Possible chemical reactions would include, 1. Fe304 y-Fe203 or cc-Fe203, 2. Precipitation from aqueous solution of some form of iron oxyhydroxide, 3. The dehydration of iron oxyhydroxide, FeOOH Fe203 + H20. KOBAYASHI (1959) oxidized synthetic magnetite at 270 °Cin a stream of gaseous oxygen to a mixture of cc-Fe203 and a spinel intermediate in composition between y-Fe203 and Fe304. This material carried a stable CRM, although the direction and intensity of its moment are not mentioned. Reaction in the reverse direction is unlikely due to the oxidizing environment present when the red beds were formed (VAN HOUTEN, 1961). In view of the sedimentary origin of red sandstones, the presence of iron oxyhydroxides is highly probable and their subsequent dehydration to iron oxide could produce a remanent moment as well as the characteristic red colour. This was first noticed by RAYLEIGH (1946) who dried chalybeate water (ferrous bicarbon-+
-+
2. Chalybeate sandstone 2. 1. Production of chalybeate sandstones Artificial red sandstones were prepared using chalybeate solution and the remanence and other physical properties of the resultant pigment investigated. The chalybeate solution was produced by bubbling CO2 from a gas cylinder through a Quickfit 700 cc flask filled with distilled water and some hydrogen reduced iron powder (98 % pure). The chalybeate solution, which contained 0.4—0.9 g/l of iron after ten days reaction time, was filtered through a sintered glass disc to remove any solid particles >100/1 before dripping into the self-flushing applicator(fig. 1). This delivered 10 ccs of solution to the sample. Although it was initially
103
104
I. 0. HEDLEY
D A
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JUL11 —
~fTh~E L\\\\\V4
-
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C
H°
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—
Fig. 1. Apparatus for producing chalybeate sandstone. A: CO 2 from gas cylinder; B: reaction vessel; C: iron powder; Dr line filter; E: self-flushing applicator; F: orientated specimen; U: hot plate; H: pressurization bottle; —÷: CO8 gas flow; -*~: chalybeate solution flow.
hoped to make the application of solution automatic, manual control was employed as it proved to control the flow rate due to furring upvery of difficult the line
surement at 3a of concentration of chalybeate matrix, the remanence did notsolid inC = 0.06 g/cm crease in proportion to the amount ofpigment present.
filter with chalybeate solid. The sample was heated on an orientated copper plate heated by a non-magnetic Bunsen burner, which despite the poor temperature control available was more satisfactory than an electric hot plate with its large stray a.c. and d.c. magnetic fields. A high concentration of ferrous ion was achieved by pressurizing the reaction container with 2 wash tubes to 108 cm water, to give up to 1 g/l of iron compared to 0.3 g/l for natural water from Langenswalbach (quoted by Rayleigh). Gentle heating of the solution had the opposite effect and halved the Fe2 concentration. Initially the coating was applied to white sand but due to the possibility of the sand grains moving during wetting and dehydration of the solution, a rigid matrix was chosen, namely a sintered glass disc 9 cm diameter, 0.5 cm thick, of porosity 100 p. The disc was orientated in the earth’s field using a fiducial line drawn on its upper surface. The average temperature of the plate near the glass disc over the 11-month period of deposition was 52 °C varying ±20 °Cand once accidentally reaching 110 °C.
Table 1 shows that for a twenty fold increase in deposit the remanence did not increase appreciably and for concentrations above 0.09 g/cm3 it actually decreased.
+
2.2. CRM of chalybeate sandstone The remanence of the sintered glass disc was measured several times as the deposition progressed using a sensitive astatic magnetometer. After the first mea-
TABLE
1
Progress of remanence of uncut coarse sintered glass disc Deposition time
Weight ofchalybeate solid
Total intensity
(days)
deposited (g)
(gauss cm8)
10
0.2
26
0.24
2.6 x 10-s 4.02x 106
257
2.9
3.87 x 10—s
308
2.9
2.9 X 10~
It is not possible to say anything about the change in direction of the remanence during deposition as the disc was too large and too weakly magnetized for meaningful measurements to be made with an astatic magnetometer. At the end of the experiment, however, the four discs cored from the large sintered glass disc gave a reasonable grouping around the ambient field direction (fig. 2). The remanence of chalybeate sandstones has been studied by GREENEWALT (1960) who dried chalybeate solution at temperatures mainly less than 100 °Con a variety of matrices and in the earth’s magnetic field. He found that the direction of CRM was close to that of the ambient field with an inclination error ofperhaps
CHEMICAL REMANENT MAGNETIZATION OF THE
FeOOH, Fe203
105
SYSTEM
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10
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S Fig. 2.
CRM of chalybeate sandstone.
5° and as far as one can infer, with a specific intensity 3/g of red material*. However, some of Greea,. = l0 samples (36—43) were too weak to be measured, newalt’s although they had a large concentration of deposit C = 0.1 g/cm3, whilst some of his other firebrick samples (26—29) which had a typical amount of coating C = 0.026 g/cm3 gave scattered NE directions. Apart from their high intensity Greenewalt’s isotropic samples gave a similar grouping around the applied field direction to the sandstones prepared in this study. The low intensity of CRM attained, approximately i0~ of Greenewalt’s values as well as the change in moment as the experiment proceeded, suggest that 3/g the
nature of the chalybeate solid is highly variable and that the CRM is possibly unstable. It is well known that the oxidation of Fe2~ in aqueous solution is very sensitive to the experimental conditions (Grn~&RD, 1935) and this could well explain the divergence of the results in terms of an uncontrolled quality of the deposit. The colour of the discs was brown and X-ray analysis on the pigment from an unconsolidated sand matrix indicated x-FeOOH with some p-FeOOH, although any y-Fe 2O3 present in a quantity necessary to explain the observed remanence would be undetectable by powder diffraction techniques. An estimate of the possible amount of maghemite
otherwise the disc, units of remanence are gauss ~ ofUnless magnetic materialstated in each
present by extrapolating the Arrhenius plot to 50 °C
*
106
i. G. HEDLEY
(using the thermogravimetricdata) indicates that 30mg y-Fe203 might be present after 10 months. However such a calculation is not really valid as the Arrhenius plot is known to deviate from a straight line at low temperatures (LIMA DE FARIA, 1963) and allowing for this the estimate will be very much lower. In view of the lack of data one can only suggest that the conditions in Greenewalt’s and Hofer and Weller’s experiments encouraged the production of y-FeOOH and its subsequent dehydration to y-Fe203 at a low ternperature. If the thermal stability test applied by Greenewalt means that no secondary chemical magnetization is produced on heating then it would appear that any y-Fe203 had already been formed during the deposition of his samples. Examination of a polished section of the sintered disc used in this study by N. PETERSEN (1966) revealed a few bright red specks of hematite set in a groundmass of iron oxyhydroxide. Thus even at 110 °Csome dehydration does occur over a long time interval. The remaining possibility, that Ix-FeOOH carries the chalybeate remanence cannot be disregarded, particularly as the hematite is present at a concentration of less than 1 % and goethite can be weakly ferromagnetic (HEDLEY, 1967).
ments on chalybeate material prepared by Nairn and the peak due to the reactions y-FeOOH —+ y-Fe203 -+ ~-Fe2O3 is shown in fig. 3b, together with the DTG and CRM curve obtained with the Chalybeate material described in this investigation. 3. CRM of the iron oxyhydroxides
3. 1. Dehydration of synthetic oxyhydroxides Instead of working with a chalybeate solid of somewhat arbitrary composition, it was decided to use synthetic samples of the oxyhydroxides which had previously been examined by X-ray and thermomagnetic techniques. Mixtures of the synthetic FeOOH and Analar plaster of Paris were made ranging from 10—50% and the paste moulded into discs 1 x 2.5 cm diameter. The initial remanence was measured both of the samples and of some plaster of Paris blanks. The plaster matrix was considered to be non-magnetic as discs of the material heated to 630 °Cin the earth’s field and then cooled in zero field gave remanent intensities of the order of 10-v gauss. The discs were orientated with their planes horizontal and with a fiducial line marked on them orientated in the direction ofthe earth’s magnetic field. They were then heated in steps from 220°to over 600 °C 2.3. Heating experiments on chalybeate sandstone using a thermal demagnetization furnace (STEPHENSON, The four sintered glass discs impregnated with chaly- 1967) and then cooled in zero field, so that the CRM beate solid were subjected to the thermal treatment induced was free from any partial thermoremanence discussed in section 3. 1 and the resultant change in (PTRM). In practice a residual field of possibly 100 remanence was measured. Briefly this involved heating gamma was sometimes found to ‘be present at the end the discs in the earth’s magnetic field and cooling in of the experiment due to instrumental drift. Although zero field so that only a CRM was produced. this might be considered excessive, it did not constitute By 273 °Cthe moment had risen to 10-2 due to a serious disturbance. After each heating and cooling dehydration (see DTG trace—fig. 3b) of the y-FeOOH cycle, the discs were measured at room temperature present in the chalybeate solid. The direction was that using a conventional astatic magnetometer. of the ambient field but with a shallower dip of 63° The phase changes involved in the dehydration and compared to 72°in the laboratory. However the radius subsequent alteration of iron oxyhydroxide were folof the cone of 95% confidence of the mean is 11°so lowed by heating samples of the synthetic powders. that this is not significant. Differential Thermal Analysis (DTA) was carried out The intensity decreased as thetemperature of heating on a Netzsche thermoanalyser belonging to the Univerwas raised and like the other specimens the directions be- sity Geology Department. Thermogravimetric (TGA) came more scattered. No sharp dropinmoment was visi- and eventually Differential ThermogravimetricAnalysis ble at the y ~ transition although the measurements (DTG) were performed using a Stanton MF-H5 therwere made at discrete temperatures and not contin- mobalance, whilst thermomagnetic measurements were uously. made by the Faraday method. CHAMALAUN (1963) made susceptibility measureThe phases involved were identified by X-ray powder —+
CHEMICAL REMANENT MAGNETIZATION OF THE
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techniques and their grain sizes measured by DebyeScherrer line broadening and examination by Electron Microscopy.
FeOOH, Fe203
SYSTEM
111
y-Fe203 and so explain the larger and stable CRM of discs made from this sample. However, no evidence of y-Fe203 is seen in the thermomagnetic curve of G1 and a more probable explanation would seem to be in the defect nature of synthetic goethites (VAN O0sTERHOUT (1965). This defect which was proposed to explain the variation in thermal and magnetic properties of goethite would seem to influence the magnetic nature of the resultant hematite. This could also explain the CRM-values reported by, KAWAI et a!. (1959) although considerable grain growth could have occurred over the 11 hours heating under pressure to 450 °C.
3.2. CRM, Ix-FeOOH —i’ c~-Fe~O3 As this is the naturally occurring form, and therefore of greater importance paleomagnetically, several samples of goethite were used. These were all synthetic materials, three being well crystallized, the fourth being fine grained. Only one of the goethites, G1, followed the ambient field from dehydration to 450 °C.The other two sets, made from goethites (87) and MAPICO Yellow LL 100 (a commercial paint pigment) gave rather scattered 3.3. CRM, fine grain cc-FeOOH ~x-Fe~O3 directions and were weaker, 8 x 10—6 compared to The pigment chosen was sample (82) prepared by 20 x 10-6 for G1, although all three were made of a adding sodium hydroxide to molar ferric chloride solu20% mix. The Mapico pigment was kindly supplied by tion. The alkaline precipitate produced was then aged the Columbian Carbon Co., New York. at room temperature for a week before washing and It is not known why G1 should behave differently drying. The BOHM (1925)—KRAUSE (1932) ~-FeOOH from the others as it had almost the same grain size obtained was of uncertain grain size. Electron micro(5000 A x 800 A), initial magnetic susceptibility (30 x scope photographs show a few poorly defined acicular 10-6) and decomposition temperature (290 °C), the grains 2000 x 650 A against a background of 250 A only physical difference being in the well marked double particles. If the colour is any guide, the deep brown of peak in the DTG record, corresponding more to VAN the ground powder would indicate a particle size OosmRI-iouT’s (1965) type A goethite. KAWAI, et a!. ~ 800 A (HEDLEY 1967). The X-ray powder pattern (1959) also found that the direction of remanent mo- showed no back reflections yet the forward reflections, ment closely followed the applied field on dehydrating although weak, were reasonably sharp. This could be a goethite, which was possibly prepared in the same explained by the presence of acicular grains diluted in way as G1. an amorphous matrix. A 50% mix of pigment with The discs made up with the Mapico pigment behaved Analar plaster of Paris was used. inconsistently. The scatter of directions around the The four discs measured gave surprisingly high reinitial remanence suggests that the crystalline aniso- manences, greater than the previous goethite samples, tropy of the goethite in the cold disc plays a rOle in even before heating. The ultra-fine hematite produced determining the CRM of the dehydration product of by dehydration of (82) carried a remanence an order of this pigment. magnitude greater than that of the fine hematite disDue to the different heating rates employed, it is cussed in the previous section. The variation of remadifficult to compare precisely the thermal analysis and nence with temperature is completely unlike any of the thermomagnetic curves with the change of CRM with other sets of samples in that two maxima are present, temperature, the dehydration temperature being higher one at 270 °C,the other at 470 °C(see fig. 7). for higher heating rates. Nevertheless, it would appear The directions of the CRM are scattered around the (figs. 4b, 5b and 6b) that the CRM reaches a maximum ambient field direction at 273°to 335° but show no as soon as the goethite has decomposed and not, as grouping at other temperatures. might be supposed from HAIGH’S (1958) treatment of To try to understand these peculiar results, the CRM, when the hematite grains had grown sufficiently dehydration of(82) was studied usingthermal and X-ray to achieve a stable magnetization. techniques. Some of the synthetic powder was heated It could bethat G1 contains a small amount of in air at temperatures up to 800 °Cfor periods of half y-FeOOH which would dehydrate to the magnetic an hour and the product X-rayed. —÷
112
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At 270 °Conly broad lines of cc-Fe203 were seen which sharpened as the temperature was increased with a few unidentified lines appearing at 600 °Cand the lattice shrinking by several per cent to that of pure hematite. The maximum in the CRM curve would thus seem to be due to very fine c~-Fe2O3,about 100 A in size as the DTG plot indicates that (82) has practically completely decomposed by this temperature. The second peak in the DTA and DTG curves is that associated with the chemical decomposition of (82) 2 c~-FeOOH c~-Fe2O3+ H2O but calculation shows that the amount of water liberated is only 7% not the stoichiometric 10%. Other fine grain c~-FeOOHsamples studied have between 9.8 % and 13 % of chemical water. The appearance of very fine hematite with its high susceptibility is not apparent in the thermomagnetic record (fig. 6b). It is interesting, however, to compare the isothermal remanence of a series of fine hematite samples, HEDLEY (1967), prepared by heating BohmKrause type goethite to increasing temperatures with —+
100 IRM
FeOOH, Fe203
SYSTEM
113
the observed behaviour of the CRM of (82). The IRM is displayed in fig. 8 and the remanence of ultra fine c~-Fe2O3at room temperature is quite surprising. The weak ferromagnetism seen in such small antiferromagnetic particles is explained by NEEL (1961) in terms of uncompensated spins between the two sublattices. However the fact that an appreciable fraction of the particles are not superparamagnetic but exhibit a remanence is not to be expected. It may be that particle interactions play a role in the acquisition of a remanence. The second maximum at 450 °Cin the CRM curve (fig. 7b) is probably caused by grain growth of the ultra fine hematite into the 100—200 A region where the IRM has been shown to increase rapidly (fig. 8). The texture of the material is also unusual as the initial brown colour of the discs was preserved up to 690 °C although heating the synthetic powder alone produced the hematite “red” at 700 °Cafter the slight exotherm in the DTA record. This exotherm is presumably linked with recrystallization of the fine particles. Although it is difficult to account for some of the features of the resultant CRM, the unusually strong and possibly stable moment produced at moderate temperatures by dehydrating Bohm-Krause goethite is highly interesting and needs further investigation using the normal stability tests and employing different starting materials. 3.4. CRM, /3-FeOOH
-~
cc-Fe203
A well crystallized sample of fl-FeOOH ofgrain size,
1
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4500 x 900 A obtained by the hydrolysis of0.1 m Ferric
•
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chloride solution was made up into a 1: 1 mixture with plaster of Paris. The thermal and X-ray evidence show that this sample, (84), has practically completely dehydrated to ~-Fe2O3 by 300 °Cand that the sharp exotherm in the DTA record shortly after this temperature is due to recrystallization and not to the formation of a magnetic spinel phase as found by MACKAY (1960). It is surprising therefore, in comparison with the goethite synthetic rocks previously mentioned, that the (84) series was nonmagnetic below 520 °C.Above this
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200
600
800
1200
temperature remanence appeared and rose to a maximum of a, = lOOx lO_6 °C.Although the moment dropped sharply to at 7 x630 10—6 at 694 °Cthere was still a reasonable grouping of directions around the
114
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0 0
101
l~I I I H
FeOOH, Fe2O3 0 0
0 0
(0
.~.U.
‘0’O
CCC’
I
~
I I I
115
SYSTEM
I
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‘9
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C
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116
I. G. HEDLEY
earth’s field direction suggesting that the Curie point of the “Fe203” was slightly above 700 °C. However, the thermomagnetic curve showed a normal Curie point at 675 °Cand this anomalous high temperature remanence is thought to be due to the cooling of the magnetic “Fe2O3” in a small residual field. A CRM can still be distinguished from a PTRM however, as any residual field is most unlikely to be parallel to the earth’s magnetic field. The fact that the substance at these temperatures is crystallographically c~-Fe2O3would seem hard to reconcile with the magnetic data. However, such hematite is known to have different catalytic properties in that it has no activity in the Fischer-Tropsch reaction (HOFER et a!., 1946) and there is some evidence, albeit confficting, as to the existence above 500 °C of a magnetic ferric oxide (FINCH-SINHA, 1957). The large remanence observed on heating/3-FeOOH above 500 °Cis due to a more magnetic Fe203 formed from the initially paramagnetic dehydration product. 3.5. CRM, y-FeOOH —+ y-Fe~O3—p o-Fe203 Two samples of synthetic lepidocrocite were used to make up the artificial rock discs and as they gave entirely different results they will be treated separately. 3.5. 1. Tabular ‘y-FeOOH. Specimen (71) cd, made by the HAHN and HI1RTICH (1923) method, consisted of “cornflake”-like particles 2500 x 150 A although the broad lines on the X-ray powder photograph indicate dimensions ~ 100 A. No cc-FeOOH was detected. A 50% mix with Analar plaster of Paris was used and 5 discs were employed in the heating experiments. By 224 °Cthe remanence had risen to i0-~(fig. lOb) and although all specimens had a positive dip, the declinations were well scattered (fig. lOa). The specimens were inhomogeneously magnetized because a sin U curve was not obtained when the disc was rotated below the lower magnet of the astatic magnetometer (CoLuNsoN et a!., 1957). “0” is the declination of the fiducial line on the disc with respect to the index mark on the magnetometer declination circle. The magnetic susceptibility of this sample was very high giving a ratio of CRM to induced magnetization in the earth’s field Q = 0.06 at 335 °C.
The remanence increased to
10-2
at 392 °Cbut the
directions of magnetization remained scattered and the “0” versus deflection curve which had become more complex on heating now gave a better sin 0 dependence. This behaviour is clearly due to the production of y-Fe203 and judging from the X-ray powder pattern, superparamagnetic maghemite of size about 100 A is present which would explain the instability ofthe discs. This instability is illustrated by the magnetic parameters as measured on 3 different magnetometers of a disc heated to 455 °C. Measured twice on a sensitive astatic instrument, once on an igneous astatic and once on a spinner magnetometer, the intensity, declination and dip, a,, 0, ~ were 2160x lO_6, 6, +79; 400x 10—6, 16, —1; 5200 x 10-6, 163, not meastirable; and 760 x 10_6, 23, —29, respectively. By 522 °Cthe remanence had fallen to l0 6 due to the y c.~-Fe2O3transition, although one would expect the magnetization to have decreased much earlier judging from the thermal data. Instead of a purely superparamagnetic behaviour, the discs now showed a viscous component when placed under the magnetometer with a time constant of the order of ten seconds. Some of the discs exhibited a negative dip although due to the wide scatter in declination, it is not possible to say whether or not this is a true self reversal. It is significant, however, that all the dips were positive by 630 °C.After heating to 630 °Cthere was no trace of a viscous behaviour although it should be pointed out that the specimens were practically demagnetized with moments 10- 6~ —‘
3.5.2. Acicular y-FeOOH. Needle-shaped particles of y-FeOOH (2500 x 150 A) were produced by the acidic oxidation of Fe(OH)2 together with as much as 50% cc-FeOOH. A 20% mix of this specimen (86) with Analar plaster of Paris was used to make only one disc. A high remanence a,. = 0.3 and a direction closely following that of the ambient field was observed after heating to 224 °Cdue to the production of y-Fe203 (see fig. 11). This apparent stability was matched by a much lower susceptibility than the (71) series, with the ratio of CRM to induced magnetization, Q = 140 at 335 °C.The remanence decreased above 400 °Cdue to the y-cc phase change, the direction of the moment being that of the applied field at all times. At 522°the
FeOOH, Fe2O3
CHEMICAL REMANENT MAGNETIZATION OF THE
H1~ 1~
H
A
117
SYSTEM
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118
I.G. HEDLEY 0
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CHEMICAL REMANENT MAGNETIZATION OF THE
moment was found to be exactly reversed, the direction being reproducible to within a few degrees on repeating the measurement. The possibility of such a mechanism operating in nature makes the y —~cc transition highly interesting, and BANERJEE (1963) has considered this to be a possible example of self reversal by exchange anisotropy. Unfortunately no further magnetic evidence is available to decide on the mechanism causing the reversal but exchange interaction is thought to be more likely than a magnetostatic interaction. This reversed moment of x-Fe203, however, reverses again and becomes normal on heating to 630 °Cwith an intensity lower by a factor of 10 at 2 x 10- 6~ It might be thought that the remanent moment of the oc-Fe2O3 should be at right angles to that of the parent maghemite due to the antiparallel spin arrangement produced by the superexchange interaction between the y and cc-phases, but this was not the case. Clearly furtherinvestigationof this reversal is needed, based as it is on only one disc, before any conclusions can be reached on what could be an important mechanism in the acquisition of a reversed NRM. 3.6. CRM of 45-FeOOH —~cc-Fe203 The oxyhydroxide used, sample (83), consisted of 350 A hexagonal platelets and the plaster of Paris discs had a high remanence even before heating. The initial declinations well scattered but which all thewere discsmade had positive dips were similar to the (82) series from fine cc-FeOOH. The first measurement after heating was at 225 °C when the intensity of magnetization was practically unchanged (6 x 10-i) but the directions were now closely grouped about the ambient field. This state of affairs continued up to 630 °Cwhen the intensity now much reduced to 10-s and the scatter of directions was much larger. Thermomagnetic measurements showthat ~-FeOOH loses its magnetization before 200 °Cin the transformadon .~ cc-FeOOH andfinally to cc-Fe203. It is surprising that the CRM is so large at 224 °Cfor X-ray evidence would suggest that only fine cc-Fe203 is present at these temperatures, the dehydration being completed by 280 °C. Such ultra fine cc-Fe203 (i~ 100 A in size) obtained from 5-FeOOH is seen to possess a considerable remanence which decreases gradually on heating. This is —~
FeOOH, Fe2O3
119
SYSTEM
probably partly due to sintering, and partly because the technique used to produce the CRM is equivalent to thermal demagnetization at higher temperatures. This surprisingly large remanence illustrates the influence that the parent oxyhydroxide has on the magnetic and structural nature of the resultant hematite. 4. Theoretical considerations 4. 1. Stability of CRM N~EL’Ssingle domain theory (1949, 1955) gives the relationship between the relaxation time ‘r0 and the spontaneous magnetization J5 of a uniaxial grain of volume v and critical field H11 in the absence of an external field. 1 / vH J \ = C exp ~, 2kT 3), (1) —
—
where C can be taken as constant. ‘to can be regarded as the decay time of the remanent magnetization and it can be seen to depend critically on v for any given T removed from the Ned point. In the presence of an external field h parallel to the grain’s axis two relaxation times exist, ‘r(O, it) and ‘r(ir, 0) corresponding to the magnetic moment moving antiparallel to h and parallel to h respectively. ‘t(O,1
it)
i. ~ O\
=..~
2 K exp — vJ0(H11 2H + h) 11kT vJ5(H11 11)2 K exp 2H kT ‘ —
=
(2
—
(3)
/
where Kis afunction of the critical field H11, the external field h and the physical constants of the magnetic grains. Even for h << H11, t(0, it) is greater than ‘r(x, 0) so that magnetization in the direction of the applied field is the more stable state. ‘r varies rapidly with v so that as the grains grow it varies from seconds to many years over a very narrow interval of grain size to produce a stable remanence. H11 also increases with ~‘ for very small particles to improve the stability of the CRM. CREER (1961) has calculated the critical grain size of hematite using Néel’s theory and finds relaxation times of 0.9 sec to 2.18 x 1010 years for particles of diameter 1200 A and 1700 A respectively. This is to be contrasted with the grain size of hematite produced in this
120
1.0. HEDLEY
study. Although no direct measurements are available’ surements was the great variety in thermal behaviour on the material in the discs themselves, comparison ~ of the CRM of cc-Fe203 produced by dehydrating the with control samples suggests sizes of only a few hundred A. Even allowing for possible sintering, as the discs were heated in discrete temperature steps, the sizes are at least a factor of two less than Creer’s critical values, It could be that the values used for H11 and .1~(1000 Oe and 2 gauss respectively) in the calculation are in error. CHEVALLIER and MATHIEIJ (1943) give a coercive force of 400 Oe for an assembly of particles of average size 1 Magnetic measurements on particles several hundred A in size (HEDLEY, 1967) indicate coercive forces of the order iO~Oe so that the value taken by CREER (1961) is not unreasonable. Some synthetic specimens of cc-Fe203 (HEDLEY 1967) suggest moments = 0.5 gauss. These parameters would suggest that the critical size is even greater than that given by CREER (1961). ~.
4.2. Magnitude ofobserved CRM STACEY (1963) using single domain theory has calculated that the CRM carried by cc-Fe203 in a field of 0.5 Oe should be about 10-~gauss per 1 % hematite by volume. This is very much greater than the magnetizations produced in this investigation and also for those of red sandstones. Measurements by COLLINSON (1966) on a red bed in which the red pigment is the carrier of the remanence suggest a value of 10-i to 10 ~ for this parameter, whilst for another formation in which the majority of the remanence is attributed to the specular grains of hematite (COLLINSON, 1965) the ratio is even lower, about 2x 10- 6 The deviation of the initial susceptibility of an assembly of superparamagnetic particles from the theoretical estimate has been explained by KNELLER (1965) and BROWN (1967) in terms of particle interactions which can also reduce the coercive force (M0RRIsH and WATT, 1957). According to STACEY (1963) such particle interactions could lower the expected remanence. However, no theory as yet deals with the effect of magnetostatic interactions on the stability of an assembly of superparamagnetic hematite particles. 5. Summary of Results The most striking feature to emerge from the mea-
four isomers of FeOOH. 1. The three acicular samples of cc-FeOOH gave rather varying results in that only one of them, viz. G1, acquired a remanent direction close to that of the applied field, with an intensity a, = 20 x 1o 6~ This CRM was at a maximum immediately after dehydration when the hematite crystallites are probably only 400 A in size and possibly disordered. Note that on Haigh’s theory of CRM one would expect a remanence only after the particles had grown sufficiently to carry a stable moment. In the light of these measurements, CREER’s (1961) estimate of critical grain size would seem to be too small by a factor of two but there may be other factors involved which could affect the stability such as the degree of order or disorder in the crystallites. Further measurements are needed on other pigments with a careful monitoring of the grain size of cc-Fe203 produced before any definite conclusion can be reached. 2. The remarkably high remanence produced on the dehydration of fine x-FeOOH is due to ultra fine x-Fe203 and this fact, unexpected on Néel’s theory, is in agreement with measurements of the IRM of powdered cc-Fe203 prepared from fine cc-FeOOH. Above 350 °C,however, the remanence decreases and the directions become scattered possibly due to sintering. As cc-FeOOH is the naturally stable isomer, the dehydration of amorphous goetbite could well play an important role in the magnetization of certain red beds. 3. The /J-FeOOH samples had to be heated to 600°C before an appreciable moment was produced and this is thought to be due to an Fe203 which is slightly different both chemically and magnetically from ccFe203 produced from goethite. The remanence was high, a, 100 x 106 and in the ambient field direction. 4. The two lepidocrocite series had very different magnetic behaviours. The instability of (71) (flaky habit) was due to the production of superparamagnetic y-Fe203 whilst the acicular (86) followed the ambient field with a high moment until it reversed at 500 °C. This solitary reversal may be due to exchange anisotropy between y and cc-Fe203. It is not stable on further heating. 5. The very fine 1x-Fe203 obtained by heating ö-FeOOH gave an even higher remanence than that ‘
CHEMICAL REMANENT MAGNETIZATION OF THE
derived from fine cc-FeOOH. The direction was stable at all temperatures up to 630 °C.As the grain sizes of the cc-Fe203 derived from the samples of cc- and c5-FeOOH are not very different, the difference in magnetic properties must be structural. 6. The low moment a, = 10-s of the chalybeate sandstone compared to Greenewalt’s results can only be attributed to the presence of y-Fe203 in the majority of his specimens. The brown coating laid down on the sintered glass matrix in this experiment was identifled as cc+ y-FeOOH and the change in CRM with temperature is clearly due to the dominance of stable y-Fe203.
FeOOH, Fe203,
121
SYSTEM
References S. K. (1963), Ph. D. Thesis, University of Cambridge. Bom4, J. (1925), Z. Anorg. Aligem. Chem. 149, 203. BRowN, W. F. (1967), J. App!. Phys. 38 1017. CHAMALAUN, F. H. (1963), Ph. D. Thesis, University of Durham. BANERJEE,
CHEVALLIER, R. and S. MATHIEU (1943), Ann. Phys. Paris 18, 258. C0LLIN50N, D. W., K. M. CREER, E. IRVING and S. K. RUNCORN
(1957), Phil. Trans. Roy. Soc. London, Ser. A 250, 73 C0LLINs0N, D. W. (1965a), Geophys. 5. 9, 203. COLLINSON, COLLINSON,
D. W. (1965b), Geophys. J. 10, 105. D. W. (1966), Geophys. 5. 11, 337.
CREER, K. M. (1961), Geophys. J. 5, 16.
FINCH, G. I. and K. P. SINHA (1957), Proc. Roy. Soc. London, Ser. A. 241, 1. GIRARD, A. (1935), Thesis Ing. Doc. Universit6 GREENEWALT, D. (1960), Ph. D. Thesis, M.I.T.
de Lille.
HAHN, F. L. and M. HERTICH (1923), Chem. Ber. 56, 1729.
6. Relevance to paleomagnetism
HAIGH, 6. (1958), Phil. Mag. 3, 267. HEDLEY, I. G. (1967), Ph. D. Thesis,
The observed reversal of magnetization for the transition y cc-Fe203 could be of importance in interpreting the magnetization of baked maghemite bearing rocks (WILsoN, 1961), although this self reversal was found in only one of the three samples initially contaming y-FeOOH. However the most interesting result to emerge from this study was the large and possible stable remanence of ultra fine cc-Fe203. This could be of considerable significance in the magnetization of the red cement in red sandstones.
HOFER,
Acknowledgements
NEEL, L. (1961), Low Temperature Physics, pp. 413—442, Théorie des Proprietes Magnetiques des grains fins Antiferromagnetique: Superparamagnetisme et Superantiferromagnetisme. PETERSEN, N. (1966), Private Communication. LORD RAYLEIGH, (1946), Proc. Roy. Soc. London, Ser. A 186, 411. STACEY, F. D. (1963), Advan. Phys. 12, 45.
—+
This work was carried out as part of a research programme on superexchange interaction financed by N.E.R.C. and under the supervision of Professor K. M. Creer. The author gratefully acknowledges the tenure of a research associateship under N.E.R.C. and he would like to thank members of the Department of Geophysics and Planetary Physics who helped with the measurements, and in particular Dr. D. W. Collinson for his interest in this work from its earliest stage.
L. J. E., W. C.
PEEBLES and
University of Newcastle. W. E. DIETER (1946), J. Am.
Chem. Soc. 68, (1953). HOFER, L. J. E. and S. WELLER (1947), Science 106, 470. KAWAI, N., H. ITO, K. YASKAWA and S. Kusm (1959), Mem. Coil. Sci. Univ. Kyoto, Ser B. 26, 235.
mt. Con!. on Magnetism Nottingham 1964, p. 174, The Institute of Physics and The Physical Society, London. KOBAYASHI, K. (1959), 5. Geomag. Geoelect. 10, 99. KNELLER, E. (1965), in: Proc.
KRAUSE, A., H. LAKOSCIUKOWNA and J. CICIiowsKI (1932), Z.
Anorg. Allgem. Chem. 208, 282.
LIMA DE FARIA, 5. (1963), Z. Krist. 119, 176. MACKAY, A. L. (1960), Mining Mag. 35, 545. MORRISH, A. H. and L. A. K. WATT (1957), Phys. Rev. 105, 1476. NEEL, L. (1949), Ann. Geophys. 5, 99. NEEL, L. (1955), Phil. Mag. Suppi. 4, 191.
STEPHENSON,
A. (1967) in: Methods in Falaeomagnetism, D. W.
Collinson, K. M. Creer and S. K. Runcorn (Ed.), Elsevier. VAN HOurEN, F. B. (1961), in: Descriptive Palaeoclimatology,
A. E. M. Nairn (Ed.), Interscience, New York. OosmRHou’r, U. W. (1965), in: Proc. mt. Conf. on Magnetism, Nottingham, 1964, The Institute of Physics and The Phy-
VAN
sical Society, London.
WILSoN, R. L. (1961), Geophys. J. 5, 45.
CHEMICAL REMANENT MAGNEI1ZATION OF THE FeOOH, Fe203 SYSTEM
I. G. HEDLEY Department of Geophysics and Planetary Physics, The University, Newcastle upon Tyne, England Received 30 June 1967 The Chemical Remanent Magnetization (CRM) produced by dehydrating the four isomers of ferric oxyhydroxide has been investigated using X-ray and thermoanalytical as well as thermomagnetic techniques. Synthetic powders were used set in a plaster of Paris matrix and the CRM acquired after heating in the Earth’s field was measured using an astatic magnetometer. The CRM of an artificial chalybeate sandstone was also studied. The nature of the resultant CRM was dependent on the parent oxyhydroxide and the remanence of very fine hematite particles
was unexpectedly large in view ofprevious work on the magnetic stability of such fine grains. The reversal of the remanence in the case of y-FeOOH was particularly striking and could be of some paleomagnetic importance. The CRM carried by oc-Fe,03 derived from FeOOH was seen not only to reflect the crystal structure ofthe parentmaterial but also the temperature to which the oxide had been heated.
1. Introduction
ate solution) on grains of sand to match the colour of an Old Red Sandstone from Dartmouth. The nature of this precipitate was investigated by HOFER and WELLER (1947) using X-ray analysis. The brown skin formed on the surface of the solution was found to be y-FeOOH. Rayleigh’s red coating, produced by drying out chalybeate water on a glass plate at temperatues well below 100 °C,was found by HOFER et a!. (1946) to be mainly cc-Fe203 with some of the highly magnetic y-Fe203 which would certainly give rise to an appreciable remanence.
Although a great deal of paleomagnetic evidence is based on measurements on red sandstones, the manner in which these rocks gained their remanence is still uncertain. COLLINSON (1965), after reviewing the geological and magnetic evidence, concludes that chemical magnetization (CRM) is the most likely mechanism for many formations. Possible chemical reactions would include, 1. Fe304 y-Fe203 or cc-Fe203, 2. Precipitation from aqueous solution of some form of iron oxyhydroxide, 3. The dehydration of iron oxyhydroxide, FeOOH Fe203 + H20. KOBAYASHI (1959) oxidized synthetic magnetite at 270 °Cin a stream of gaseous oxygen to a mixture of cc-Fe203 and a spinel intermediate in composition between y-Fe203 and Fe304. This material carried a stable CRM, although the direction and intensity of its moment are not mentioned. Reaction in the reverse direction is unlikely due to the oxidizing environment present when the red beds were formed (VAN HOUTEN, 1961). In view of the sedimentary origin of red sandstones, the presence of iron oxyhydroxides is highly probable and their subsequent dehydration to iron oxide could produce a remanent moment as well as the characteristic red colour. This was first noticed by RAYLEIGH (1946) who dried chalybeate water (ferrous bicarbon-+
-+
2. Chalybeate sandstone 2. 1. Production of chalybeate sandstones Artificial red sandstones were prepared using chalybeate solution and the remanence and other physical properties of the resultant pigment investigated. The chalybeate solution was produced by bubbling CO2 from a gas cylinder through a Quickfit 700 cc flask filled with distilled water and some hydrogen reduced iron powder (98 % pure). The chalybeate solution, which contained 0.4—0.9 g/l of iron after ten days reaction time, was filtered through a sintered glass disc to remove any solid particles >100/1 before dripping into the self-flushing applicator(fig. 1). This delivered 10 ccs of solution to the sample. Although it was initially
103
104
I. 0. HEDLEY
D A
-~
——
JUL11 —
~fTh~E L\\\\\V4
-
3
__
C
H°
-
~
—
Fig. 1. Apparatus for producing chalybeate sandstone. A: CO 2 from gas cylinder; B: reaction vessel; C: iron powder; Dr line filter; E: self-flushing applicator; F: orientated specimen; U: hot plate; H: pressurization bottle; —÷: CO8 gas flow; -*~: chalybeate solution flow.
hoped to make the application of solution automatic, manual control was employed as it proved to control the flow rate due to furring upvery of difficult the line
surement at 3a of concentration of chalybeate matrix, the remanence did notsolid inC = 0.06 g/cm crease in proportion to the amount ofpigment present.
filter with chalybeate solid. The sample was heated on an orientated copper plate heated by a non-magnetic Bunsen burner, which despite the poor temperature control available was more satisfactory than an electric hot plate with its large stray a.c. and d.c. magnetic fields. A high concentration of ferrous ion was achieved by pressurizing the reaction container with 2 wash tubes to 108 cm water, to give up to 1 g/l of iron compared to 0.3 g/l for natural water from Langenswalbach (quoted by Rayleigh). Gentle heating of the solution had the opposite effect and halved the Fe2 concentration. Initially the coating was applied to white sand but due to the possibility of the sand grains moving during wetting and dehydration of the solution, a rigid matrix was chosen, namely a sintered glass disc 9 cm diameter, 0.5 cm thick, of porosity 100 p. The disc was orientated in the earth’s field using a fiducial line drawn on its upper surface. The average temperature of the plate near the glass disc over the 11-month period of deposition was 52 °C varying ±20 °Cand once accidentally reaching 110 °C.
Table 1 shows that for a twenty fold increase in deposit the remanence did not increase appreciably and for concentrations above 0.09 g/cm3 it actually decreased.
+
2.2. CRM of chalybeate sandstone The remanence of the sintered glass disc was measured several times as the deposition progressed using a sensitive astatic magnetometer. After the first mea-
TABLE
1
Progress of remanence of uncut coarse sintered glass disc Deposition time
Weight ofchalybeate solid
Total intensity
(days)
deposited (g)
(gauss cm8)
10
0.2
26
0.24
2.6 x 10-s 4.02x 106
257
2.9
3.87 x 10—s
308
2.9
2.9 X 10~
It is not possible to say anything about the change in direction of the remanence during deposition as the disc was too large and too weakly magnetized for meaningful measurements to be made with an astatic magnetometer. At the end of the experiment, however, the four discs cored from the large sintered glass disc gave a reasonable grouping around the ambient field direction (fig. 2). The remanence of chalybeate sandstones has been studied by GREENEWALT (1960) who dried chalybeate solution at temperatures mainly less than 100 °Con a variety of matrices and in the earth’s magnetic field. He found that the direction of CRM was close to that of the ambient field with an inclination error ofperhaps
CHEMICAL REMANENT MAGNETIZATION OF THE
FeOOH, Fe203
105
SYSTEM
N 350
10
340
0
330
30
320
40
310
50
300
60
290
70
—
~:III;I::~
280
W I
I
I
I
I
I
260
+
I
I
I
~1\\8O
E
I
00
—
250
110
—
240
120
230
30
220
40 210
50 200
60 ISO
170
S Fig. 2.
CRM of chalybeate sandstone.
5° and as far as one can infer, with a specific intensity 3/g of red material*. However, some of Greea,. = l0 samples (36—43) were too weak to be measured, newalt’s although they had a large concentration of deposit C = 0.1 g/cm3, whilst some of his other firebrick samples (26—29) which had a typical amount of coating C = 0.026 g/cm3 gave scattered NE directions. Apart from their high intensity Greenewalt’s isotropic samples gave a similar grouping around the applied field direction to the sandstones prepared in this study. The low intensity of CRM attained, approximately i0~ of Greenewalt’s values as well as the change in moment as the experiment proceeded, suggest that 3/g the
nature of the chalybeate solid is highly variable and that the CRM is possibly unstable. It is well known that the oxidation of Fe2~ in aqueous solution is very sensitive to the experimental conditions (Grn~&RD, 1935) and this could well explain the divergence of the results in terms of an uncontrolled quality of the deposit. The colour of the discs was brown and X-ray analysis on the pigment from an unconsolidated sand matrix indicated x-FeOOH with some p-FeOOH, although any y-Fe 2O3 present in a quantity necessary to explain the observed remanence would be undetectable by powder diffraction techniques. An estimate of the possible amount of maghemite
otherwise the disc, units of remanence are gauss ~ ofUnless magnetic materialstated in each
present by extrapolating the Arrhenius plot to 50 °C
*
106
i. G. HEDLEY
(using the thermogravimetricdata) indicates that 30mg y-Fe203 might be present after 10 months. However such a calculation is not really valid as the Arrhenius plot is known to deviate from a straight line at low temperatures (LIMA DE FARIA, 1963) and allowing for this the estimate will be very much lower. In view of the lack of data one can only suggest that the conditions in Greenewalt’s and Hofer and Weller’s experiments encouraged the production of y-FeOOH and its subsequent dehydration to y-Fe203 at a low ternperature. If the thermal stability test applied by Greenewalt means that no secondary chemical magnetization is produced on heating then it would appear that any y-Fe203 had already been formed during the deposition of his samples. Examination of a polished section of the sintered disc used in this study by N. PETERSEN (1966) revealed a few bright red specks of hematite set in a groundmass of iron oxyhydroxide. Thus even at 110 °Csome dehydration does occur over a long time interval. The remaining possibility, that Ix-FeOOH carries the chalybeate remanence cannot be disregarded, particularly as the hematite is present at a concentration of less than 1 % and goethite can be weakly ferromagnetic (HEDLEY, 1967).
ments on chalybeate material prepared by Nairn and the peak due to the reactions y-FeOOH —+ y-Fe203 -+ ~-Fe2O3 is shown in fig. 3b, together with the DTG and CRM curve obtained with the Chalybeate material described in this investigation. 3. CRM of the iron oxyhydroxides
3. 1. Dehydration of synthetic oxyhydroxides Instead of working with a chalybeate solid of somewhat arbitrary composition, it was decided to use synthetic samples of the oxyhydroxides which had previously been examined by X-ray and thermomagnetic techniques. Mixtures of the synthetic FeOOH and Analar plaster of Paris were made ranging from 10—50% and the paste moulded into discs 1 x 2.5 cm diameter. The initial remanence was measured both of the samples and of some plaster of Paris blanks. The plaster matrix was considered to be non-magnetic as discs of the material heated to 630 °Cin the earth’s field and then cooled in zero field gave remanent intensities of the order of 10-v gauss. The discs were orientated with their planes horizontal and with a fiducial line marked on them orientated in the direction ofthe earth’s magnetic field. They were then heated in steps from 220°to over 600 °C 2.3. Heating experiments on chalybeate sandstone using a thermal demagnetization furnace (STEPHENSON, The four sintered glass discs impregnated with chaly- 1967) and then cooled in zero field, so that the CRM beate solid were subjected to the thermal treatment induced was free from any partial thermoremanence discussed in section 3. 1 and the resultant change in (PTRM). In practice a residual field of possibly 100 remanence was measured. Briefly this involved heating gamma was sometimes found to ‘be present at the end the discs in the earth’s magnetic field and cooling in of the experiment due to instrumental drift. Although zero field so that only a CRM was produced. this might be considered excessive, it did not constitute By 273 °Cthe moment had risen to 10-2 due to a serious disturbance. After each heating and cooling dehydration (see DTG trace—fig. 3b) of the y-FeOOH cycle, the discs were measured at room temperature present in the chalybeate solid. The direction was that using a conventional astatic magnetometer. of the ambient field but with a shallower dip of 63° The phase changes involved in the dehydration and compared to 72°in the laboratory. However the radius subsequent alteration of iron oxyhydroxide were folof the cone of 95% confidence of the mean is 11°so lowed by heating samples of the synthetic powders. that this is not significant. Differential Thermal Analysis (DTA) was carried out The intensity decreased as thetemperature of heating on a Netzsche thermoanalyser belonging to the Univerwas raised and like the other specimens the directions be- sity Geology Department. Thermogravimetric (TGA) came more scattered. No sharp dropinmoment was visi- and eventually Differential ThermogravimetricAnalysis ble at the y ~ transition although the measurements (DTG) were performed using a Stanton MF-H5 therwere made at discrete temperatures and not contin- mobalance, whilst thermomagnetic measurements were uously. made by the Faraday method. CHAMALAUN (1963) made susceptibility measureThe phases involved were identified by X-ray powder —+
CHEMICAL REMANENT MAGNETIZATION OF THE
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techniques and their grain sizes measured by DebyeScherrer line broadening and examination by Electron Microscopy.
FeOOH, Fe203
SYSTEM
111
y-Fe203 and so explain the larger and stable CRM of discs made from this sample. However, no evidence of y-Fe203 is seen in the thermomagnetic curve of G1 and a more probable explanation would seem to be in the defect nature of synthetic goethites (VAN O0sTERHOUT (1965). This defect which was proposed to explain the variation in thermal and magnetic properties of goethite would seem to influence the magnetic nature of the resultant hematite. This could also explain the CRM-values reported by, KAWAI et a!. (1959) although considerable grain growth could have occurred over the 11 hours heating under pressure to 450 °C.
3.2. CRM, Ix-FeOOH —i’ c~-Fe~O3 As this is the naturally occurring form, and therefore of greater importance paleomagnetically, several samples of goethite were used. These were all synthetic materials, three being well crystallized, the fourth being fine grained. Only one of the goethites, G1, followed the ambient field from dehydration to 450 °C.The other two sets, made from goethites (87) and MAPICO Yellow LL 100 (a commercial paint pigment) gave rather scattered 3.3. CRM, fine grain cc-FeOOH ~x-Fe~O3 directions and were weaker, 8 x 10—6 compared to The pigment chosen was sample (82) prepared by 20 x 10-6 for G1, although all three were made of a adding sodium hydroxide to molar ferric chloride solu20% mix. The Mapico pigment was kindly supplied by tion. The alkaline precipitate produced was then aged the Columbian Carbon Co., New York. at room temperature for a week before washing and It is not known why G1 should behave differently drying. The BOHM (1925)—KRAUSE (1932) ~-FeOOH from the others as it had almost the same grain size obtained was of uncertain grain size. Electron micro(5000 A x 800 A), initial magnetic susceptibility (30 x scope photographs show a few poorly defined acicular 10-6) and decomposition temperature (290 °C), the grains 2000 x 650 A against a background of 250 A only physical difference being in the well marked double particles. If the colour is any guide, the deep brown of peak in the DTG record, corresponding more to VAN the ground powder would indicate a particle size OosmRI-iouT’s (1965) type A goethite. KAWAI, et a!. ~ 800 A (HEDLEY 1967). The X-ray powder pattern (1959) also found that the direction of remanent mo- showed no back reflections yet the forward reflections, ment closely followed the applied field on dehydrating although weak, were reasonably sharp. This could be a goethite, which was possibly prepared in the same explained by the presence of acicular grains diluted in way as G1. an amorphous matrix. A 50% mix of pigment with The discs made up with the Mapico pigment behaved Analar plaster of Paris was used. inconsistently. The scatter of directions around the The four discs measured gave surprisingly high reinitial remanence suggests that the crystalline aniso- manences, greater than the previous goethite samples, tropy of the goethite in the cold disc plays a rOle in even before heating. The ultra-fine hematite produced determining the CRM of the dehydration product of by dehydration of (82) carried a remanence an order of this pigment. magnitude greater than that of the fine hematite disDue to the different heating rates employed, it is cussed in the previous section. The variation of remadifficult to compare precisely the thermal analysis and nence with temperature is completely unlike any of the thermomagnetic curves with the change of CRM with other sets of samples in that two maxima are present, temperature, the dehydration temperature being higher one at 270 °C,the other at 470 °C(see fig. 7). for higher heating rates. Nevertheless, it would appear The directions of the CRM are scattered around the (figs. 4b, 5b and 6b) that the CRM reaches a maximum ambient field direction at 273°to 335° but show no as soon as the goethite has decomposed and not, as grouping at other temperatures. might be supposed from HAIGH’S (1958) treatment of To try to understand these peculiar results, the CRM, when the hematite grains had grown sufficiently dehydration of(82) was studied usingthermal and X-ray to achieve a stable magnetization. techniques. Some of the synthetic powder was heated It could bethat G1 contains a small amount of in air at temperatures up to 800 °Cfor periods of half y-FeOOH which would dehydrate to the magnetic an hour and the product X-rayed. —÷
112
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At 270 °Conly broad lines of cc-Fe203 were seen which sharpened as the temperature was increased with a few unidentified lines appearing at 600 °Cand the lattice shrinking by several per cent to that of pure hematite. The maximum in the CRM curve would thus seem to be due to very fine c~-Fe2O3,about 100 A in size as the DTG plot indicates that (82) has practically completely decomposed by this temperature. The second peak in the DTA and DTG curves is that associated with the chemical decomposition of (82) 2 c~-FeOOH c~-Fe2O3+ H2O but calculation shows that the amount of water liberated is only 7% not the stoichiometric 10%. Other fine grain c~-FeOOHsamples studied have between 9.8 % and 13 % of chemical water. The appearance of very fine hematite with its high susceptibility is not apparent in the thermomagnetic record (fig. 6b). It is interesting, however, to compare the isothermal remanence of a series of fine hematite samples, HEDLEY (1967), prepared by heating BohmKrause type goethite to increasing temperatures with —+
100 IRM
FeOOH, Fe203
SYSTEM
113
the observed behaviour of the CRM of (82). The IRM is displayed in fig. 8 and the remanence of ultra fine c~-Fe2O3at room temperature is quite surprising. The weak ferromagnetism seen in such small antiferromagnetic particles is explained by NEEL (1961) in terms of uncompensated spins between the two sublattices. However the fact that an appreciable fraction of the particles are not superparamagnetic but exhibit a remanence is not to be expected. It may be that particle interactions play a role in the acquisition of a remanence. The second maximum at 450 °Cin the CRM curve (fig. 7b) is probably caused by grain growth of the ultra fine hematite into the 100—200 A region where the IRM has been shown to increase rapidly (fig. 8). The texture of the material is also unusual as the initial brown colour of the discs was preserved up to 690 °C although heating the synthetic powder alone produced the hematite “red” at 700 °Cafter the slight exotherm in the DTA record. This exotherm is presumably linked with recrystallization of the fine particles. Although it is difficult to account for some of the features of the resultant CRM, the unusually strong and possibly stable moment produced at moderate temperatures by dehydrating Bohm-Krause goethite is highly interesting and needs further investigation using the normal stability tests and employing different starting materials. 3.4. CRM, /3-FeOOH
-~
cc-Fe203
A well crystallized sample of fl-FeOOH ofgrain size,
1
o
10-’)
4500 x 900 A obtained by the hydrolysis of0.1 m Ferric
•
10_a
chloride solution was made up into a 1: 1 mixture with plaster of Paris. The thermal and X-ray evidence show that this sample, (84), has practically completely dehydrated to ~-Fe2O3 by 300 °Cand that the sharp exotherm in the DTA record shortly after this temperature is due to recrystallization and not to the formation of a magnetic spinel phase as found by MACKAY (1960). It is surprising therefore, in comparison with the goethite synthetic rocks previously mentioned, that the (84) series was nonmagnetic below 520 °C.Above this
LI. 0 0
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~ (A)
1400 T(~C) 8/g after 4 kOe; Fig.= 8.Mean IRMparticle of fine size oc-Fe5O3 IRM in gauss. cm (A); T = annealing temperature (°C).
200
600
800
1200
temperature remanence appeared and rose to a maximum of a, = lOOx lO_6 °C.Although the moment dropped sharply to at 7 x630 10—6 at 694 °Cthere was still a reasonable grouping of directions around the
114
I. U. HEDLEY
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CHEMICAL REMANENT MAGNETIZATION OF THE
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115
SYSTEM
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116
I. G. HEDLEY
earth’s field direction suggesting that the Curie point of the “Fe203” was slightly above 700 °C. However, the thermomagnetic curve showed a normal Curie point at 675 °Cand this anomalous high temperature remanence is thought to be due to the cooling of the magnetic “Fe2O3” in a small residual field. A CRM can still be distinguished from a PTRM however, as any residual field is most unlikely to be parallel to the earth’s magnetic field. The fact that the substance at these temperatures is crystallographically c~-Fe2O3would seem hard to reconcile with the magnetic data. However, such hematite is known to have different catalytic properties in that it has no activity in the Fischer-Tropsch reaction (HOFER et a!., 1946) and there is some evidence, albeit confficting, as to the existence above 500 °C of a magnetic ferric oxide (FINCH-SINHA, 1957). The large remanence observed on heating/3-FeOOH above 500 °Cis due to a more magnetic Fe203 formed from the initially paramagnetic dehydration product. 3.5. CRM, y-FeOOH —+ y-Fe~O3—p o-Fe203 Two samples of synthetic lepidocrocite were used to make up the artificial rock discs and as they gave entirely different results they will be treated separately. 3.5. 1. Tabular ‘y-FeOOH. Specimen (71) cd, made by the HAHN and HI1RTICH (1923) method, consisted of “cornflake”-like particles 2500 x 150 A although the broad lines on the X-ray powder photograph indicate dimensions ~ 100 A. No cc-FeOOH was detected. A 50% mix with Analar plaster of Paris was used and 5 discs were employed in the heating experiments. By 224 °Cthe remanence had risen to i0-~(fig. lOb) and although all specimens had a positive dip, the declinations were well scattered (fig. lOa). The specimens were inhomogeneously magnetized because a sin U curve was not obtained when the disc was rotated below the lower magnet of the astatic magnetometer (CoLuNsoN et a!., 1957). “0” is the declination of the fiducial line on the disc with respect to the index mark on the magnetometer declination circle. The magnetic susceptibility of this sample was very high giving a ratio of CRM to induced magnetization in the earth’s field Q = 0.06 at 335 °C.
The remanence increased to
10-2
at 392 °Cbut the
directions of magnetization remained scattered and the “0” versus deflection curve which had become more complex on heating now gave a better sin 0 dependence. This behaviour is clearly due to the production of y-Fe203 and judging from the X-ray powder pattern, superparamagnetic maghemite of size about 100 A is present which would explain the instability ofthe discs. This instability is illustrated by the magnetic parameters as measured on 3 different magnetometers of a disc heated to 455 °C. Measured twice on a sensitive astatic instrument, once on an igneous astatic and once on a spinner magnetometer, the intensity, declination and dip, a,, 0, ~ were 2160x lO_6, 6, +79; 400x 10—6, 16, —1; 5200 x 10-6, 163, not meastirable; and 760 x 10_6, 23, —29, respectively. By 522 °Cthe remanence had fallen to l0 6 due to the y c.~-Fe2O3transition, although one would expect the magnetization to have decreased much earlier judging from the thermal data. Instead of a purely superparamagnetic behaviour, the discs now showed a viscous component when placed under the magnetometer with a time constant of the order of ten seconds. Some of the discs exhibited a negative dip although due to the wide scatter in declination, it is not possible to say whether or not this is a true self reversal. It is significant, however, that all the dips were positive by 630 °C.After heating to 630 °Cthere was no trace of a viscous behaviour although it should be pointed out that the specimens were practically demagnetized with moments 10- 6~ —‘
3.5.2. Acicular y-FeOOH. Needle-shaped particles of y-FeOOH (2500 x 150 A) were produced by the acidic oxidation of Fe(OH)2 together with as much as 50% cc-FeOOH. A 20% mix of this specimen (86) with Analar plaster of Paris was used to make only one disc. A high remanence a,. = 0.3 and a direction closely following that of the ambient field was observed after heating to 224 °Cdue to the production of y-Fe203 (see fig. 11). This apparent stability was matched by a much lower susceptibility than the (71) series, with the ratio of CRM to induced magnetization, Q = 140 at 335 °C.The remanence decreased above 400 °Cdue to the y-cc phase change, the direction of the moment being that of the applied field at all times. At 522°the
FeOOH, Fe2O3
CHEMICAL REMANENT MAGNETIZATION OF THE
H1~ 1~
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A
117
SYSTEM
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CHEMICAL REMANENT MAGNETIZATION OF THE
moment was found to be exactly reversed, the direction being reproducible to within a few degrees on repeating the measurement. The possibility of such a mechanism operating in nature makes the y —~cc transition highly interesting, and BANERJEE (1963) has considered this to be a possible example of self reversal by exchange anisotropy. Unfortunately no further magnetic evidence is available to decide on the mechanism causing the reversal but exchange interaction is thought to be more likely than a magnetostatic interaction. This reversed moment of x-Fe203, however, reverses again and becomes normal on heating to 630 °Cwith an intensity lower by a factor of 10 at 2 x 10- 6~ It might be thought that the remanent moment of the oc-Fe2O3 should be at right angles to that of the parent maghemite due to the antiparallel spin arrangement produced by the superexchange interaction between the y and cc-phases, but this was not the case. Clearly furtherinvestigationof this reversal is needed, based as it is on only one disc, before any conclusions can be reached on what could be an important mechanism in the acquisition of a reversed NRM. 3.6. CRM of 45-FeOOH —~cc-Fe203 The oxyhydroxide used, sample (83), consisted of 350 A hexagonal platelets and the plaster of Paris discs had a high remanence even before heating. The initial declinations well scattered but which all thewere discsmade had positive dips were similar to the (82) series from fine cc-FeOOH. The first measurement after heating was at 225 °C when the intensity of magnetization was practically unchanged (6 x 10-i) but the directions were now closely grouped about the ambient field. This state of affairs continued up to 630 °Cwhen the intensity now much reduced to 10-s and the scatter of directions was much larger. Thermomagnetic measurements showthat ~-FeOOH loses its magnetization before 200 °Cin the transformadon .~ cc-FeOOH andfinally to cc-Fe203. It is surprising that the CRM is so large at 224 °Cfor X-ray evidence would suggest that only fine cc-Fe203 is present at these temperatures, the dehydration being completed by 280 °C. Such ultra fine cc-Fe203 (i~ 100 A in size) obtained from 5-FeOOH is seen to possess a considerable remanence which decreases gradually on heating. This is —~
FeOOH, Fe2O3
119
SYSTEM
probably partly due to sintering, and partly because the technique used to produce the CRM is equivalent to thermal demagnetization at higher temperatures. This surprisingly large remanence illustrates the influence that the parent oxyhydroxide has on the magnetic and structural nature of the resultant hematite. 4. Theoretical considerations 4. 1. Stability of CRM N~EL’Ssingle domain theory (1949, 1955) gives the relationship between the relaxation time ‘r0 and the spontaneous magnetization J5 of a uniaxial grain of volume v and critical field H11 in the absence of an external field. 1 / vH J \ = C exp ~, 2kT 3), (1) —
—
where C can be taken as constant. ‘to can be regarded as the decay time of the remanent magnetization and it can be seen to depend critically on v for any given T removed from the Ned point. In the presence of an external field h parallel to the grain’s axis two relaxation times exist, ‘r(O, it) and ‘r(ir, 0) corresponding to the magnetic moment moving antiparallel to h and parallel to h respectively. ‘t(O,1
it)
i. ~ O\
=..~
2 K exp — vJ0(H11 2H + h) 11kT vJ5(H11 11)2 K exp 2H kT ‘ —
=
(2
—
(3)
/
where Kis afunction of the critical field H11, the external field h and the physical constants of the magnetic grains. Even for h << H11, t(0, it) is greater than ‘r(x, 0) so that magnetization in the direction of the applied field is the more stable state. ‘r varies rapidly with v so that as the grains grow it varies from seconds to many years over a very narrow interval of grain size to produce a stable remanence. H11 also increases with ~‘ for very small particles to improve the stability of the CRM. CREER (1961) has calculated the critical grain size of hematite using Néel’s theory and finds relaxation times of 0.9 sec to 2.18 x 1010 years for particles of diameter 1200 A and 1700 A respectively. This is to be contrasted with the grain size of hematite produced in this
120
1.0. HEDLEY
study. Although no direct measurements are available’ surements was the great variety in thermal behaviour on the material in the discs themselves, comparison ~ of the CRM of cc-Fe203 produced by dehydrating the with control samples suggests sizes of only a few hundred A. Even allowing for possible sintering, as the discs were heated in discrete temperature steps, the sizes are at least a factor of two less than Creer’s critical values, It could be that the values used for H11 and .1~(1000 Oe and 2 gauss respectively) in the calculation are in error. CHEVALLIER and MATHIEIJ (1943) give a coercive force of 400 Oe for an assembly of particles of average size 1 Magnetic measurements on particles several hundred A in size (HEDLEY, 1967) indicate coercive forces of the order iO~Oe so that the value taken by CREER (1961) is not unreasonable. Some synthetic specimens of cc-Fe203 (HEDLEY 1967) suggest moments = 0.5 gauss. These parameters would suggest that the critical size is even greater than that given by CREER (1961). ~.
4.2. Magnitude ofobserved CRM STACEY (1963) using single domain theory has calculated that the CRM carried by cc-Fe203 in a field of 0.5 Oe should be about 10-~gauss per 1 % hematite by volume. This is very much greater than the magnetizations produced in this investigation and also for those of red sandstones. Measurements by COLLINSON (1966) on a red bed in which the red pigment is the carrier of the remanence suggest a value of 10-i to 10 ~ for this parameter, whilst for another formation in which the majority of the remanence is attributed to the specular grains of hematite (COLLINSON, 1965) the ratio is even lower, about 2x 10- 6 The deviation of the initial susceptibility of an assembly of superparamagnetic particles from the theoretical estimate has been explained by KNELLER (1965) and BROWN (1967) in terms of particle interactions which can also reduce the coercive force (M0RRIsH and WATT, 1957). According to STACEY (1963) such particle interactions could lower the expected remanence. However, no theory as yet deals with the effect of magnetostatic interactions on the stability of an assembly of superparamagnetic hematite particles. 5. Summary of Results The most striking feature to emerge from the mea-
four isomers of FeOOH. 1. The three acicular samples of cc-FeOOH gave rather varying results in that only one of them, viz. G1, acquired a remanent direction close to that of the applied field, with an intensity a, = 20 x 1o 6~ This CRM was at a maximum immediately after dehydration when the hematite crystallites are probably only 400 A in size and possibly disordered. Note that on Haigh’s theory of CRM one would expect a remanence only after the particles had grown sufficiently to carry a stable moment. In the light of these measurements, CREER’s (1961) estimate of critical grain size would seem to be too small by a factor of two but there may be other factors involved which could affect the stability such as the degree of order or disorder in the crystallites. Further measurements are needed on other pigments with a careful monitoring of the grain size of cc-Fe203 produced before any definite conclusion can be reached. 2. The remarkably high remanence produced on the dehydration of fine x-FeOOH is due to ultra fine x-Fe203 and this fact, unexpected on Néel’s theory, is in agreement with measurements of the IRM of powdered cc-Fe203 prepared from fine cc-FeOOH. Above 350 °C,however, the remanence decreases and the directions become scattered possibly due to sintering. As cc-FeOOH is the naturally stable isomer, the dehydration of amorphous goetbite could well play an important role in the magnetization of certain red beds. 3. The /J-FeOOH samples had to be heated to 600°C before an appreciable moment was produced and this is thought to be due to an Fe203 which is slightly different both chemically and magnetically from ccFe203 produced from goethite. The remanence was high, a, 100 x 106 and in the ambient field direction. 4. The two lepidocrocite series had very different magnetic behaviours. The instability of (71) (flaky habit) was due to the production of superparamagnetic y-Fe203 whilst the acicular (86) followed the ambient field with a high moment until it reversed at 500 °C. This solitary reversal may be due to exchange anisotropy between y and cc-Fe203. It is not stable on further heating. 5. The very fine 1x-Fe203 obtained by heating ö-FeOOH gave an even higher remanence than that ‘
CHEMICAL REMANENT MAGNETIZATION OF THE
derived from fine cc-FeOOH. The direction was stable at all temperatures up to 630 °C.As the grain sizes of the cc-Fe203 derived from the samples of cc- and c5-FeOOH are not very different, the difference in magnetic properties must be structural. 6. The low moment a, = 10-s of the chalybeate sandstone compared to Greenewalt’s results can only be attributed to the presence of y-Fe203 in the majority of his specimens. The brown coating laid down on the sintered glass matrix in this experiment was identifled as cc+ y-FeOOH and the change in CRM with temperature is clearly due to the dominance of stable y-Fe203.
FeOOH, Fe203,
121
SYSTEM
References S. K. (1963), Ph. D. Thesis, University of Cambridge. Bom4, J. (1925), Z. Anorg. Aligem. Chem. 149, 203. BRowN, W. F. (1967), J. App!. Phys. 38 1017. CHAMALAUN, F. H. (1963), Ph. D. Thesis, University of Durham. BANERJEE,
CHEVALLIER, R. and S. MATHIEU (1943), Ann. Phys. Paris 18, 258. C0LLIN50N, D. W., K. M. CREER, E. IRVING and S. K. RUNCORN
(1957), Phil. Trans. Roy. Soc. London, Ser. A 250, 73 C0LLINs0N, D. W. (1965a), Geophys. 5. 9, 203. COLLINSON, COLLINSON,
D. W. (1965b), Geophys. J. 10, 105. D. W. (1966), Geophys. 5. 11, 337.
CREER, K. M. (1961), Geophys. J. 5, 16.
FINCH, G. I. and K. P. SINHA (1957), Proc. Roy. Soc. London, Ser. A. 241, 1. GIRARD, A. (1935), Thesis Ing. Doc. Universit6 GREENEWALT, D. (1960), Ph. D. Thesis, M.I.T.
de Lille.
HAHN, F. L. and M. HERTICH (1923), Chem. Ber. 56, 1729.
6. Relevance to paleomagnetism
HAIGH, 6. (1958), Phil. Mag. 3, 267. HEDLEY, I. G. (1967), Ph. D. Thesis,
The observed reversal of magnetization for the transition y cc-Fe203 could be of importance in interpreting the magnetization of baked maghemite bearing rocks (WILsoN, 1961), although this self reversal was found in only one of the three samples initially contaming y-FeOOH. However the most interesting result to emerge from this study was the large and possible stable remanence of ultra fine cc-Fe203. This could be of considerable significance in the magnetization of the red cement in red sandstones.
HOFER,
Acknowledgements
NEEL, L. (1961), Low Temperature Physics, pp. 413—442, Théorie des Proprietes Magnetiques des grains fins Antiferromagnetique: Superparamagnetisme et Superantiferromagnetisme. PETERSEN, N. (1966), Private Communication. LORD RAYLEIGH, (1946), Proc. Roy. Soc. London, Ser. A 186, 411. STACEY, F. D. (1963), Advan. Phys. 12, 45.
—+
This work was carried out as part of a research programme on superexchange interaction financed by N.E.R.C. and under the supervision of Professor K. M. Creer. The author gratefully acknowledges the tenure of a research associateship under N.E.R.C. and he would like to thank members of the Department of Geophysics and Planetary Physics who helped with the measurements, and in particular Dr. D. W. Collinson for his interest in this work from its earliest stage.
L. J. E., W. C.
PEEBLES and
University of Newcastle. W. E. DIETER (1946), J. Am.
Chem. Soc. 68, (1953). HOFER, L. J. E. and S. WELLER (1947), Science 106, 470. KAWAI, N., H. ITO, K. YASKAWA and S. Kusm (1959), Mem. Coil. Sci. Univ. Kyoto, Ser B. 26, 235.
mt. Con!. on Magnetism Nottingham 1964, p. 174, The Institute of Physics and The Physical Society, London. KOBAYASHI, K. (1959), 5. Geomag. Geoelect. 10, 99. KNELLER, E. (1965), in: Proc.
KRAUSE, A., H. LAKOSCIUKOWNA and J. CICIiowsKI (1932), Z.
Anorg. Allgem. Chem. 208, 282.
LIMA DE FARIA, 5. (1963), Z. Krist. 119, 176. MACKAY, A. L. (1960), Mining Mag. 35, 545. MORRISH, A. H. and L. A. K. WATT (1957), Phys. Rev. 105, 1476. NEEL, L. (1949), Ann. Geophys. 5, 99. NEEL, L. (1955), Phil. Mag. Suppi. 4, 191.
STEPHENSON,
A. (1967) in: Methods in Falaeomagnetism, D. W.
Collinson, K. M. Creer and S. K. Runcorn (Ed.), Elsevier. VAN HOurEN, F. B. (1961), in: Descriptive Palaeoclimatology,
A. E. M. Nairn (Ed.), Interscience, New York. OosmRHou’r, U. W. (1965), in: Proc. mt. Conf. on Magnetism, Nottingham, 1964, The Institute of Physics and The Phy-
VAN
sical Society, London.
WILSoN, R. L. (1961), Geophys. J. 5, 45.