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Comparison of the hysteresis characteristics of synthetic samples with different magnetite and haematite contents
Physics of the Earth and Planetary Interiors, 46 (1987) 93—99 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands
93
Comparison of the hysteresis characteristics of synthetic samples with different magnetite and haematite contents H. Mauritsch and M. Becke Institute of Geophysics, Montanuniversität, Leoben (Austria)
V. Kropâ~ek,T. Zelinka and P. Hejda Geophysical Institute, Czechoslovak Academy of Sciences, Prague (Czechoslovakia) (Received December 23, 1985; revision accepted November 20, 1986)
Mauritsch, H., Becke, M., Kropá~ek,V., Zelinka, T. and Hejda, P., 1987. Comparison of the hysteresis characteristics of synthetic samples with different magnetite and haematite contents. Phys. Earth Planet. Inter., 46: 93—99. The values of the initial magnetic susceptibility Ic were measured with different instruments. The values of saturated magnetic polarization J~,the coercive force H~,the alternating current demagnetization curves ~rs/~rso = F(H) and the hysteresis curves J = J(H) were determined for synthetic samples containing synthetic magnetite with grain sizes of <40 ~&m,40—80 ,~m,80—120 ~m and 120—160 ~cm,and concentrations of 0.1—5.0% by volume. Analogous parameters were also determined for synthetic samples containing natural and synthetic haematite with a grain size of 40—80 ~m and concentration of 1.0—10.0% by volume. The results indicate a significant effect of the magnetic interactions between the grains of the magnetic minerals in the samples and the ways of determining the grain size and concentration of magnetite or haematite in rock samples.
1. Introduction
ent instruments (astatic magnetometer, various types of a.c. bridges).
The magnetic properties of the majority of rocks depend on the content and size of the grains of the present ferrimagnetic minerals (magnetite, titanomagnetite, maghaemite, pyrrhotine) or of minerals with weak ferromagnetism (haematite). The knowledge of how these properties depend on the minerals the rocks contain helps in studying natural rocks and enables the observed phenomena to be interpreted. The purpose of this study was to determine the dependence of the initial magnetic susceptibility ic on the content of magnetite and its grain size in the samples, and on the content of haematite of a given grain size in the samples, as well as the comparison of the values of the initial magnetic susceptibility K determined with differ0031-9201/87/$03.50
© 1987 Elsevier Science Publishers By.
2. Methods and instruments used The samples to be measured were prepared from synthetic magnetite, synthetic haematite and natural haematite (locality Hradi~t~ near Kadah, Czechoslovakia). These samples were used to produce the separate grain-size fractions by crushing in a vibration crusher (in acetone) and subsequent screening on vibration screens. Screens with meshes of 160 ~tm, 125 ~sm (grain-size fraction 120—160 ~tm), 112, ~sm,90 ~&m(grain-size fraction 80—120 ~sm), 71 ~sm, 50 ~sm (grain-size fraction 40—80 ~sm), 36 ~sm, 10 ~&m(grain-size fraction
94
<40 rim) were used. Most of the grain-size fractions thus had a grain-size corresponding to the mean values, i.e., 140, 100, 60 and 20 ~tm. These
determined with a DRON 2.0 X-ray diffractometer (Coa-radiation). The lattice parameters were determined as follows: a (Fe304) = 839.2 ±0.2
grain-size fractions of magnetite (only the fraction 40—80 ~.tmwas used for haematite) were used to produce samples of plaster of Paris with the appropriate content of the magnetic component to yield concentratiojis p 0.1, 0.5, 1.0, 2.0 and 5.0% by volume for the samples with magnetite and p 1.0, 2.0, 5.0, 7.0 and 10.0% by volume for the samples with haematite. The purity and parameters of the fractions were
pm, synthetic a Fe203 a = 541.0 ±0.2 pm, a = 55°17’,natural a Fe203 a = 541.8 ±0.3 pm, a = 55°16’. The samples used in the experiments were cylindrical 25.4 mm in diameter and 18 mm in height, and those measured with the vibratingsample magnetometer were 8 mm in diameter, 10 mm in height. The following instruments were used to mea—
—
—
=
=
—
TABLE 1 Comparison of the values of the initial magnetic susceptibility SamP pie [vol.%] synthetic Fe 304 2 0.1 7 0.1 12 0.1 17 0.1 3 0.5 8 0.5 13 0.5 18 0.5 4 1.0 9 1.0 14 1.0 19 1.0 5 2.0 10 2.0 15 2.0 20 2.0 6 5.0 11 5.0 16a 5.0 16b 5.0 21 5.0 flat. a-Fe203 22 1.0 23 2.0 24 5.0 25 7.0 26 10.0 synth. a-Fe203 27 1.0 28 2.0 29 5.0 30 7.0 31 10.0
K,
determined by means of different instruments
Granularity 0 (sm)
KLY-2 a, d
KLY-2 b, d
KLY-2 a, e
KL-1 a
LAM 24 a
LAM 14 b
Digico c
i~ 6 [x10
160—120 120— 80 80— 40 <40 160—120 120— 80 80— 40 <40 160—120 120— 80 80— 40 <40 160—120 120— 80 80— 40 <40 160—120 120— 80 80— 40 80— 40 <40
3788 2185 1971 1758 10983 7688 6092 5989 13369 13454 11669 10194 30339 28320 25672 19659 71022 63753 48949 54732 50956
3801 2214 1990 1772 11024 7766 6073 5989 13375 13590 11713 10262 30813 28266 25860 19764 71092 63824 49103 54593 52566
3949 2147 1982 1761 11148 7815 6133 6013 13365 13562 11644 10341 30730 28361 25758 19729 70895 63471 48913 54988 51215
3516 2001 1810 1630 10092 7097 5694 5609 13658 12850 10921 9713 29366 26276 23806 18488 65512 60500 45210 50929 47934
3462 1924 1553 1418 9923 7351 5481 5302 13414 12410 1b811 9615 28956 27083 23701 18704 65548 58900 45410 50971 48032
3239 1814 1466 1413 9985 7207 5188 5117 12451 12359 10768 9286 28116 28788 23286 18115 65462 58796 45342 50902 47513
3292 1759 1696 1495 9111 6384 5202 4776 11033 11800 10128 8910 26389 23600 21338 17002 56536 59451 38930 46810 42336
3678 2006 1781 1607 10324 7329 5695 5542 12952 12861 11093 9760 29244 27242 24203 18780 66581 66581 45980 51989 48650
SI]
± 272 ± 182 ± 215 ± 163 ± 753 ± 503 ± 416 ± 492 ± 928 ± 702 ± 601 ± 540 ±1601 ±1829 ±1672 ±1028 ±5212 ±2351 ±3610 ±2985 ±3398
80— 80— 80— 80— 80—
40 40 40 40 40
79.6 141.1 340.9 465.3 644.9
79.1 141.5 343.8 466.4 646.8
78.7 141.0 342.7 466.1 644.9
73.1 138.0 346.5 474.2 640.9
71.5 155.5 396.6 498.1 612.3
80.3 177.3 395.9 496.0 656.3
71.3 126.8 299.0 411.2 556.1
76.2± 145.9± 352.2± 468.2± 628.9±
80— 80— 80— 80— 80—
40 40 40 40 40
17.3 34.9 108.6 189.1 259.3
17.1 34.9 109.0 190.6 260.2
16.7 35.4 107.9 188.3 262.2
17.4 38.9 115.9 195.2 266.9
22.2 34.1 125.2 198.4 281.2
21.1 30.9 122.2 207.8 271.2
14.8 32.0 100.3 161.7 224.1
18.1± 2.6 34.4± 11.4 112.7± 8.8 190.2± 14.2 260.7± 17.9
(a) LAB GI, (b) LAB G, (c) LAB LG, (d) small coils, (e) large coils.
4.2 16.2 34.2 28.8 34.9
95
sure the magnetic susceptibility: a LAM-24 digital astatic magnetometer (Pe~ina,1968), a.c. bridges KL-1, KLY-2 with small coils and KLY-2 with large coils (Jelinek, 1973) (all in the laboratory of the Geophysical Institute, CSAS = LAB GI), a LAM 14 digital astatic magnetometer (Pesina, 1968), a KLY-2 a.c. bridge (Laboratory of Geofyzika, N.C., Prühonice-LAB G) and a Digico a.c. bridge (Leoben-Gams Laboratory-LAB IG). The demagnetization curves were determined using the equipment in Lab LG and a Schonstedt demagnetizer (LAB GI), the hysteresis curves were determined with the vibrating-sample magnetometer ‘Vibromicro’ (LAB GI) (Zelinka et aL, 1984), and the magnetization curves J J(H) with a Petersen balance (LAB LG). =
3. Experimental results The values of the magnetic susceptibility of the samples studied in the initial state measured with the various instruments are given in Table 1. The values in this table indicate systematic differences, but the maximum differences do not exceed 20%. For example, if the Digico a.c. bridge is taken as the reference instrument, the susceptibility values
determined with the LAM 24 and LAM 14 astatic magnetometers are approximately 10% higher, those determined with the KL-1 bridge (in a field of 64 A m 1) 11% higher and those determined with the KLY-2 bridges (in a field of 320 A m 1) approximately 15% higher. However, these data are linear over the whole range of measured values. Table 1 compareds the values of magnetic susceptibility determined with the Digico bridge and the KLY-2 bridge and shows the systematic shift of the values as well as the linearity of the pattern (probably during the difference in the used magnetic fields). The experiments carried out with these samples have shown that the value of the initial magnetic susceptibility depends not only on the magnetite content (p), but also on its grain size. The character of the x = f( p) curves is very similar for all grain sizes, but does not conform to the curve described by the relation * ‘1 N * x x /~+ x where x is the susceptibility of the sample, x * the susceptibility of magnetite, N the demagnetization factor and p the concentration of magnetite in the sample. The samples containing magnetite with a grain size of 120—160 ~sm display the —
—
JrS~rSO 1.0
o5\\ç~\~\j:~
100
300
500
3/4r(AImJ 200
700 10 —~
400
600
800 — —H
Fig. 1. Curves of a.c. demagnetization of saturated remanent magnetic polarization of samples containing magnetite and haematite. (a) Leoben (Gams) laboratory. (b) Laboratory of the Geophysical Institute in Prühonice.
96 J ( mT)
I
2 I
230
Fig. 2. Hysteresis curves for samples with a magnetite content p
highest susceptibility values, the samples containing magnetite with a grain size of less than 40 ~tm display the lowest values, The effect of the different grain sizes of the magnetite particles in the samples is also reflected in the demagnetization curves in an a.c. magnetic field, J,~/J~ F( H), shown in Fig. la, b. Figure la shows the average a.c. curves obtained in LAB =
H~
[Oe]
H(kA/m)
=
2.0% by volume and various grain sizes of the magnetite.
LG, Fig. lb shows the curves obtained for the same samples in LAB GI. Standard deviations, which were obtained by measuring five samples, are given at the individual points. The results of the two laboratories agree very well and show that the samples with the magnetite with a grain size of less than 40 ~tm display the highest stability and samples with the magnetite with a grain size of
H~ [kA/mi
150
12
100
8
50
4
1~~ ~--~ ~
40
80
k
120
~[,.um]
Fig. 3. The coercive force H~as a function of the mean grain size of magnetite.
97
120—160 ~&mdisplay the lowest stability. Figure lb also shows the curves of a.c. demagnetization ~r5 for samples containing natural and synthetic haematite; the synthetic haematite displays a much higher stability than the natural haematite. Moreover, the grain size is the same (40—80 ism) and the natural haematite is very pure with no magnetite admixtures (the value of the initial magnetic
:~29
susceptibility of the sample containing natural haematite in the concentration p 10% by volume is 628 x 106 SI, whereas the sample containing 0.1% by volume of magnetite has a susceptibility of 3580 x 10—6 SI—cf. Table 1). Examples of the hysteresis curves obtained for the samples with magnetite are in Fig. 2 (p 2.0% by volume). These figures also prove that the =
=
~=31
~O
0.32 0.64 1.0 MA/m 16 48 80 kA/m
0.32 0.64 tO MAIm 16 48 80 kAIm -
0.8 t
2
7
12
17
04
0.08
32
Kb..K~.
0.24 0.4 1.2 MAIm 64 kA/m J
-
0~~8
0.08
32
0.24
64
0.4 1.2 MAIm kAIm
Fig. 4. The magnetic polarization of samples containing magnetite (haematite) as a function of the magnetic field; a.c. demagnetization.
98 J (ml)
:~
..~
0.1 /~/~
-
~ —
— -—
—~
——
— ~-
~
-
—
—
—-
Fig. 5. Hysteresis curves for samples with different contents of synthetic haematite (p
1, 2, 3, 5, 7 and 10% by volume).
coercive force varies with grain size. The increase of the value of the coercive force H~with decreasing grain size for the four grain-size fractions being considered is shown in Fig. 3. These results agree very well with the results obtained by a.c. demagnetization. Figure 4 show the curves representing the mag-
~.
netic polarization as a function of the magnetic field H for some samples containing magnetite and for the samples containing haematite. One can see that the samples containing magnetite become saturated in fields of roughly 80 kA m 1, and the samples with synthetic haematite in fields of 1.2 MA m The curves of a.c. demagnetiza021
J
9[nT]
—
5
______
_____
______
_____
~ ~
~‘/o~/
io~
27
10
/~3~4
22 —
I
_____
29~y25~2,
__
3 i0
______
I
______
11111111
5
102
I
______ sample rmormlber
—~~—
I
11—11111
5
~
I
I
1111111
5
I
1111111
iU~ 5 kISIxlOt)
iO~
Fig. 6. cpmp~rispnol values of the ipagnetic sttscepPbility (Di~icq,LAP J.Q~~pdsaturat~4gpagI~eticpolarization .J~(in ~ fjel4 of 1.2 MA m_t) for samples with different contents and grain size of magnetite and for samples with different contents of natural and synthetic haematite.
99
tion for the separate samples (LAB LG) are also shown. The hysteresis curves for the samples containing synthetic haematite are shown in Fig. 5 which indicates that only the values of the saturated remanent magnetic polarization and of the saturated magnetic polarization change with the concentration of haematite (p = 1, 2, $7 and 10% by volume); no change of the coercive force is observed. A significant relation was observed between the values of the saturated magnetic polarization J, and the values of the initial magnetic susceptibility 1~.These relations are plotted on a logarithmic scale in Fig. 6. It was found that samples with different contents of magnetite or haematite but the same grain size lie on straight lines which differ for the separate grain sizes. This means that not only the content of the magnetite or haematite in the rock, but also the approximate grain size of magnetite could be roughly determined with the aid of this graph and the curve of a.c. demagnetization of the saturated magnetic polarization J,.
Conclusions
(1) It was proved that the values of the initial magnetic susceptibility measured with different
instruments differ. However, the difference does not exceed 20% of the measured value and has a systematic character. (2) The magnetic susceptibility as a function of concentration and grain size, the coercive force as a function of grain size, and the character of the demagnetization curves in dependence on grain size were determined for the samples containing various concentrations of synthetic magnetite. (3) A dependence between the saturated magnetic polarization J, and magnetic susceptibility was observed in the samples containing magnetite or haematite of various grain sizes.
References Jelinek, V., 1973. Precision A.C. bridge set for measuring magnetic susceptibility of rocks and its anisotropy. Stud. Geophys. Geod., 17: 36-48. PeSina, B., 1968. A multiple-purpose astatic magnetometer. Trav. Geopys. No. 278, Academia, Praha, pp. 355-365. Zelinka, T., Hejda, P. and Kroptiek, V., 1984. Vibrating-sample magnetometer for measuring magnetically weak materials. Tesla Electronics. 17: 35-43.
93
Comparison of the hysteresis characteristics of synthetic samples with different magnetite and haematite contents H. Mauritsch and M. Becke Institute of Geophysics, Montanuniversität, Leoben (Austria)
V. Kropâ~ek,T. Zelinka and P. Hejda Geophysical Institute, Czechoslovak Academy of Sciences, Prague (Czechoslovakia) (Received December 23, 1985; revision accepted November 20, 1986)
Mauritsch, H., Becke, M., Kropá~ek,V., Zelinka, T. and Hejda, P., 1987. Comparison of the hysteresis characteristics of synthetic samples with different magnetite and haematite contents. Phys. Earth Planet. Inter., 46: 93—99. The values of the initial magnetic susceptibility Ic were measured with different instruments. The values of saturated magnetic polarization J~,the coercive force H~,the alternating current demagnetization curves ~rs/~rso = F(H) and the hysteresis curves J = J(H) were determined for synthetic samples containing synthetic magnetite with grain sizes of <40 ~&m,40—80 ,~m,80—120 ~m and 120—160 ~cm,and concentrations of 0.1—5.0% by volume. Analogous parameters were also determined for synthetic samples containing natural and synthetic haematite with a grain size of 40—80 ~m and concentration of 1.0—10.0% by volume. The results indicate a significant effect of the magnetic interactions between the grains of the magnetic minerals in the samples and the ways of determining the grain size and concentration of magnetite or haematite in rock samples.
1. Introduction
ent instruments (astatic magnetometer, various types of a.c. bridges).
The magnetic properties of the majority of rocks depend on the content and size of the grains of the present ferrimagnetic minerals (magnetite, titanomagnetite, maghaemite, pyrrhotine) or of minerals with weak ferromagnetism (haematite). The knowledge of how these properties depend on the minerals the rocks contain helps in studying natural rocks and enables the observed phenomena to be interpreted. The purpose of this study was to determine the dependence of the initial magnetic susceptibility ic on the content of magnetite and its grain size in the samples, and on the content of haematite of a given grain size in the samples, as well as the comparison of the values of the initial magnetic susceptibility K determined with differ0031-9201/87/$03.50
© 1987 Elsevier Science Publishers By.
2. Methods and instruments used The samples to be measured were prepared from synthetic magnetite, synthetic haematite and natural haematite (locality Hradi~t~ near Kadah, Czechoslovakia). These samples were used to produce the separate grain-size fractions by crushing in a vibration crusher (in acetone) and subsequent screening on vibration screens. Screens with meshes of 160 ~tm, 125 ~sm (grain-size fraction 120—160 ~tm), 112, ~sm,90 ~&m(grain-size fraction 80—120 ~sm), 71 ~sm, 50 ~sm (grain-size fraction 40—80 ~sm), 36 ~sm, 10 ~&m(grain-size fraction
94
<40 rim) were used. Most of the grain-size fractions thus had a grain-size corresponding to the mean values, i.e., 140, 100, 60 and 20 ~tm. These
determined with a DRON 2.0 X-ray diffractometer (Coa-radiation). The lattice parameters were determined as follows: a (Fe304) = 839.2 ±0.2
grain-size fractions of magnetite (only the fraction 40—80 ~.tmwas used for haematite) were used to produce samples of plaster of Paris with the appropriate content of the magnetic component to yield concentratiojis p 0.1, 0.5, 1.0, 2.0 and 5.0% by volume for the samples with magnetite and p 1.0, 2.0, 5.0, 7.0 and 10.0% by volume for the samples with haematite. The purity and parameters of the fractions were
pm, synthetic a Fe203 a = 541.0 ±0.2 pm, a = 55°17’,natural a Fe203 a = 541.8 ±0.3 pm, a = 55°16’. The samples used in the experiments were cylindrical 25.4 mm in diameter and 18 mm in height, and those measured with the vibratingsample magnetometer were 8 mm in diameter, 10 mm in height. The following instruments were used to mea—
—
—
=
=
—
TABLE 1 Comparison of the values of the initial magnetic susceptibility SamP pie [vol.%] synthetic Fe 304 2 0.1 7 0.1 12 0.1 17 0.1 3 0.5 8 0.5 13 0.5 18 0.5 4 1.0 9 1.0 14 1.0 19 1.0 5 2.0 10 2.0 15 2.0 20 2.0 6 5.0 11 5.0 16a 5.0 16b 5.0 21 5.0 flat. a-Fe203 22 1.0 23 2.0 24 5.0 25 7.0 26 10.0 synth. a-Fe203 27 1.0 28 2.0 29 5.0 30 7.0 31 10.0
K,
determined by means of different instruments
Granularity 0 (sm)
KLY-2 a, d
KLY-2 b, d
KLY-2 a, e
KL-1 a
LAM 24 a
LAM 14 b
Digico c
i~ 6 [x10
160—120 120— 80 80— 40 <40 160—120 120— 80 80— 40 <40 160—120 120— 80 80— 40 <40 160—120 120— 80 80— 40 <40 160—120 120— 80 80— 40 80— 40 <40
3788 2185 1971 1758 10983 7688 6092 5989 13369 13454 11669 10194 30339 28320 25672 19659 71022 63753 48949 54732 50956
3801 2214 1990 1772 11024 7766 6073 5989 13375 13590 11713 10262 30813 28266 25860 19764 71092 63824 49103 54593 52566
3949 2147 1982 1761 11148 7815 6133 6013 13365 13562 11644 10341 30730 28361 25758 19729 70895 63471 48913 54988 51215
3516 2001 1810 1630 10092 7097 5694 5609 13658 12850 10921 9713 29366 26276 23806 18488 65512 60500 45210 50929 47934
3462 1924 1553 1418 9923 7351 5481 5302 13414 12410 1b811 9615 28956 27083 23701 18704 65548 58900 45410 50971 48032
3239 1814 1466 1413 9985 7207 5188 5117 12451 12359 10768 9286 28116 28788 23286 18115 65462 58796 45342 50902 47513
3292 1759 1696 1495 9111 6384 5202 4776 11033 11800 10128 8910 26389 23600 21338 17002 56536 59451 38930 46810 42336
3678 2006 1781 1607 10324 7329 5695 5542 12952 12861 11093 9760 29244 27242 24203 18780 66581 66581 45980 51989 48650
SI]
± 272 ± 182 ± 215 ± 163 ± 753 ± 503 ± 416 ± 492 ± 928 ± 702 ± 601 ± 540 ±1601 ±1829 ±1672 ±1028 ±5212 ±2351 ±3610 ±2985 ±3398
80— 80— 80— 80— 80—
40 40 40 40 40
79.6 141.1 340.9 465.3 644.9
79.1 141.5 343.8 466.4 646.8
78.7 141.0 342.7 466.1 644.9
73.1 138.0 346.5 474.2 640.9
71.5 155.5 396.6 498.1 612.3
80.3 177.3 395.9 496.0 656.3
71.3 126.8 299.0 411.2 556.1
76.2± 145.9± 352.2± 468.2± 628.9±
80— 80— 80— 80— 80—
40 40 40 40 40
17.3 34.9 108.6 189.1 259.3
17.1 34.9 109.0 190.6 260.2
16.7 35.4 107.9 188.3 262.2
17.4 38.9 115.9 195.2 266.9
22.2 34.1 125.2 198.4 281.2
21.1 30.9 122.2 207.8 271.2
14.8 32.0 100.3 161.7 224.1
18.1± 2.6 34.4± 11.4 112.7± 8.8 190.2± 14.2 260.7± 17.9
(a) LAB GI, (b) LAB G, (c) LAB LG, (d) small coils, (e) large coils.
4.2 16.2 34.2 28.8 34.9
95
sure the magnetic susceptibility: a LAM-24 digital astatic magnetometer (Pe~ina,1968), a.c. bridges KL-1, KLY-2 with small coils and KLY-2 with large coils (Jelinek, 1973) (all in the laboratory of the Geophysical Institute, CSAS = LAB GI), a LAM 14 digital astatic magnetometer (Pesina, 1968), a KLY-2 a.c. bridge (Laboratory of Geofyzika, N.C., Prühonice-LAB G) and a Digico a.c. bridge (Leoben-Gams Laboratory-LAB IG). The demagnetization curves were determined using the equipment in Lab LG and a Schonstedt demagnetizer (LAB GI), the hysteresis curves were determined with the vibrating-sample magnetometer ‘Vibromicro’ (LAB GI) (Zelinka et aL, 1984), and the magnetization curves J J(H) with a Petersen balance (LAB LG). =
3. Experimental results The values of the magnetic susceptibility of the samples studied in the initial state measured with the various instruments are given in Table 1. The values in this table indicate systematic differences, but the maximum differences do not exceed 20%. For example, if the Digico a.c. bridge is taken as the reference instrument, the susceptibility values
determined with the LAM 24 and LAM 14 astatic magnetometers are approximately 10% higher, those determined with the KL-1 bridge (in a field of 64 A m 1) 11% higher and those determined with the KLY-2 bridges (in a field of 320 A m 1) approximately 15% higher. However, these data are linear over the whole range of measured values. Table 1 compareds the values of magnetic susceptibility determined with the Digico bridge and the KLY-2 bridge and shows the systematic shift of the values as well as the linearity of the pattern (probably during the difference in the used magnetic fields). The experiments carried out with these samples have shown that the value of the initial magnetic susceptibility depends not only on the magnetite content (p), but also on its grain size. The character of the x = f( p) curves is very similar for all grain sizes, but does not conform to the curve described by the relation * ‘1 N * x x /~+ x where x is the susceptibility of the sample, x * the susceptibility of magnetite, N the demagnetization factor and p the concentration of magnetite in the sample. The samples containing magnetite with a grain size of 120—160 ~sm display the —
—
JrS~rSO 1.0
o5\\ç~\~\j:~
100
300
500
3/4r(AImJ 200
700 10 —~
400
600
800 — —H
Fig. 1. Curves of a.c. demagnetization of saturated remanent magnetic polarization of samples containing magnetite and haematite. (a) Leoben (Gams) laboratory. (b) Laboratory of the Geophysical Institute in Prühonice.
96 J ( mT)
I
2 I
230
Fig. 2. Hysteresis curves for samples with a magnetite content p
highest susceptibility values, the samples containing magnetite with a grain size of less than 40 ~tm display the lowest values, The effect of the different grain sizes of the magnetite particles in the samples is also reflected in the demagnetization curves in an a.c. magnetic field, J,~/J~ F( H), shown in Fig. la, b. Figure la shows the average a.c. curves obtained in LAB =
H~
[Oe]
H(kA/m)
=
2.0% by volume and various grain sizes of the magnetite.
LG, Fig. lb shows the curves obtained for the same samples in LAB GI. Standard deviations, which were obtained by measuring five samples, are given at the individual points. The results of the two laboratories agree very well and show that the samples with the magnetite with a grain size of less than 40 ~tm display the highest stability and samples with the magnetite with a grain size of
H~ [kA/mi
150
12
100
8
50
4
1~~ ~--~ ~
40
80
k
120
~[,.um]
Fig. 3. The coercive force H~as a function of the mean grain size of magnetite.
97
120—160 ~&mdisplay the lowest stability. Figure lb also shows the curves of a.c. demagnetization ~r5 for samples containing natural and synthetic haematite; the synthetic haematite displays a much higher stability than the natural haematite. Moreover, the grain size is the same (40—80 ism) and the natural haematite is very pure with no magnetite admixtures (the value of the initial magnetic
:~29
susceptibility of the sample containing natural haematite in the concentration p 10% by volume is 628 x 106 SI, whereas the sample containing 0.1% by volume of magnetite has a susceptibility of 3580 x 10—6 SI—cf. Table 1). Examples of the hysteresis curves obtained for the samples with magnetite are in Fig. 2 (p 2.0% by volume). These figures also prove that the =
=
~=31
~O
0.32 0.64 1.0 MA/m 16 48 80 kA/m
0.32 0.64 tO MAIm 16 48 80 kAIm -
0.8 t
2
7
12
17
04
0.08
32
Kb..K~.
0.24 0.4 1.2 MAIm 64 kA/m J
-
0~~8
0.08
32
0.24
64
0.4 1.2 MAIm kAIm
Fig. 4. The magnetic polarization of samples containing magnetite (haematite) as a function of the magnetic field; a.c. demagnetization.
98 J (ml)
:~
..~
0.1 /~/~
-
~ —
— -—
—~
——
— ~-
~
-
—
—
—-
Fig. 5. Hysteresis curves for samples with different contents of synthetic haematite (p
1, 2, 3, 5, 7 and 10% by volume).
coercive force varies with grain size. The increase of the value of the coercive force H~with decreasing grain size for the four grain-size fractions being considered is shown in Fig. 3. These results agree very well with the results obtained by a.c. demagnetization. Figure 4 show the curves representing the mag-
~.
netic polarization as a function of the magnetic field H for some samples containing magnetite and for the samples containing haematite. One can see that the samples containing magnetite become saturated in fields of roughly 80 kA m 1, and the samples with synthetic haematite in fields of 1.2 MA m The curves of a.c. demagnetiza021
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Fig. 6. cpmp~rispnol values of the ipagnetic sttscepPbility (Di~icq,LAP J.Q~~pdsaturat~4gpagI~eticpolarization .J~(in ~ fjel4 of 1.2 MA m_t) for samples with different contents and grain size of magnetite and for samples with different contents of natural and synthetic haematite.
99
tion for the separate samples (LAB LG) are also shown. The hysteresis curves for the samples containing synthetic haematite are shown in Fig. 5 which indicates that only the values of the saturated remanent magnetic polarization and of the saturated magnetic polarization change with the concentration of haematite (p = 1, 2, $7 and 10% by volume); no change of the coercive force is observed. A significant relation was observed between the values of the saturated magnetic polarization J, and the values of the initial magnetic susceptibility 1~.These relations are plotted on a logarithmic scale in Fig. 6. It was found that samples with different contents of magnetite or haematite but the same grain size lie on straight lines which differ for the separate grain sizes. This means that not only the content of the magnetite or haematite in the rock, but also the approximate grain size of magnetite could be roughly determined with the aid of this graph and the curve of a.c. demagnetization of the saturated magnetic polarization J,.
Conclusions
(1) It was proved that the values of the initial magnetic susceptibility measured with different
instruments differ. However, the difference does not exceed 20% of the measured value and has a systematic character. (2) The magnetic susceptibility as a function of concentration and grain size, the coercive force as a function of grain size, and the character of the demagnetization curves in dependence on grain size were determined for the samples containing various concentrations of synthetic magnetite. (3) A dependence between the saturated magnetic polarization J, and magnetic susceptibility was observed in the samples containing magnetite or haematite of various grain sizes.
References Jelinek, V., 1973. Precision A.C. bridge set for measuring magnetic susceptibility of rocks and its anisotropy. Stud. Geophys. Geod., 17: 36-48. PeSina, B., 1968. A multiple-purpose astatic magnetometer. Trav. Geopys. No. 278, Academia, Praha, pp. 355-365. Zelinka, T., Hejda, P. and Kroptiek, V., 1984. Vibrating-sample magnetometer for measuring magnetically weak materials. Tesla Electronics. 17: 35-43.