Thermo-Magnetic Study of Ferromagnetic Minerals Contained in Igneous Rocks

Transcription

1 Thermo-Magnetic Study of Ferromagnetic Minerals Contained in Igneous Rocks By Syun-iti AKIMOTO (Geophysical Institute, Tokyo University) Abstract The change with temperature in intensity of magnetization of ferromagnetic mineral grains separated from the igneous rocks was measured in a strong magnetic field by means of a magnetic iahance. The intensity of saturation magnetization and the Curie point of ferromagnetic minerals of titanomagnetite series, which are commonly found in natural igneous rocks as the solid solution of TiFe20. 1 and Fe304, were examined in taking their chemical and crystallographic properties into consideration. Linear relations between each two of the chemical composition, the lattice parameter, the Curie point and the intensity of saturation magnetization were found as the first approximation. Besides, it was found that there exist natural ferromagnetic minerals in ilmenite--hematite series having the peculiar type of their thereto-magnetic curve. S1. Introduction. The magnetic properties of rocks and minerals have been studied by a number of investigators, especially in connection with the local geomagnetic anomalies and their variation with time. C1 In recent years, the phenomenon of the magnetic polarization of rocks in situ has attracted the attention of many geophysicists, since it may be used as a certain kind of fossil which can represent the ancient geomagnetic field. In other words, the magnetization of rocks could be used in order to find the secular variation in the geomagnetic field throughout the long geologic time. It will be obvious that the magnetic properties of rocks are almost due to the magnetic properties of the ferromagnetic minerals contained in them. f-fence it seems that the most fundamental way of clarifying the magnetic properties of rocks, on the basis of physics of matters, is to examine the magnetic properties of ferromagnetic minerals by taking their crystallographic and chemical properties into consideration. If this could be done, various magnetic properties of rocks could be reduced to those of the rock-forming ferromagnetic minerals and then the former would be described fairly systematically. The studies of the magnetic properties of rocks made so far have mainly dealt * Contribution from Division of Geopietism and Gcrlectricity, Gephysical Institute, Tokyo University, Series II, No. 43.

2 2 S. AKINIOTO with those in a weak magnetic field such as the geomagnetic field, since the result of such studies may be directly connected to the magnetic behaviours of rocks in situ. However, such magnetic properties in a weak magnetic field are the structure-sensitive properties, upon which small stress or grain size of the ferromagnetic minerals have rather large influence. At the present stage of knowledge of magnetism of rocks, it seems that the first thing to be done in order to establish a general catalogue of rock-magnetism will be to find the general magnetic properties of ferromagnetic minerals where chemical composition and crystal structure are given. For this purpose, the structureinsensitive magnetic properties such as the saturation magnetization and the Curie point, which are mostly independent of internal stress and grain size, etc., and dependent only on chemical composition and crystal structure, will represent the primary magnetic character of the rock-forming ferromagnetic minerals. Several studies along this line have already been reported. (2) Although they are very useful, they are still insufficient for getting a general view of the fundamental properties of rock-magnetism. In the present work, the author attempted to describle as systematically as possible the structure-insensitive magnetic properties of the ferromagnetic minerals separated from a number of typical igneous rocks in relation to their chemical composition and crystal structure. It is a well-known fact that the ferromagnetic minerals in rocks are t'he metallic oxides which are chiefly composed of FeO, Fe20;; and Ti02. The magneto-chemical study of these ternary system has already been performed in fair detail by E. Pouillard by examining artificially synthesized specimens. (3) In the present study, the experimental results obtained from the natural ferromagnetic minerals'are examined in reference to this result obtained by E. Pouillard. S2. Specimens. (A) Titanomagnetite series. The specimens examined here are 15 in total number and each of them pis the ferromagnetic mineral separated from the typical igneous rocks, being called "magnetite " petrologically in the broad sense of the word. Their magnetic properties in a weak magnetic field, such as magnetic susceptibility, have already been described in the previous paper. (4) In Table I, the chemical composition of these ferromagnetic minerals which were analysed by T. Katsura are summarized together with their locality, petrological description and crystallographic parameter. Plotting the chemical composition of these specimens on the FeO-Fe20-Ti02 ternary system shown in Fig. 1, we find that they are distributed around the TiFe204-Fe3O4 line. According to Pouillard, the equilibrium among FeO, Fe203 and Ti02 at the room temperature takes place at the points, D, E, F and G and the complete line EG and the limited line between C and D in the diagram in Fig. 1. In this figure, the line EG represents the complete solid solution between TiFe204 and Fe304 and the limited line CD represents the solid solution between TiFeO3 and Fe2O3. The former and the latter will be called here the titanomagnetite series and the ilmenite-hematite series

3 Tiermo-Magnetic Study of Ferromagnetic Minerals Contained in Igneous Rocks 3

4 respectively. The X-ray analyses of these specimens were carried out by means of a wide-range X-ray spectrometer, " Norelco," with Fe Ka radiation. The results are summarized also in Table I. It was found that the crystallographic structure of all the specimens belongs to the spinel structure, the lattice parameter, a, varying from about 8. 42A to 8. 45A. From these chemical and crystallographic data, it may be concluded that these 15 specimens chiefly consist of the minerals belonging to the titanomagnetite series; i. e. the solid solution of magnetite with TiFe2O4. (B) Itmenite-hematite series. It seems likely from the above result that the principal ferromagnetic minerals in rocks are generally the titanomagnetite. There are however several cases that a fair amount of minerals of ilmenite-hematite series are contained in rocks. The specimens examined here are six in total number; their chemical composition, petrological description and crystallographic characteristics are summarized in Table II, where No. 1' specimens is the ilmenite ore minerals and No. 2', No. 3', No. 4' specimens are the ilmenites, petrologically so-called, separated from the typical igneous rocks, while No. 5' and No. 6' specimens are the peculiar ferromagnetic minerals separated thermomagnetically from those rocks which show the phenomenon of the reverse thermo-remanent magnetism. (5) Fig. 1. Chemical composition of ferromagnetic minerals separated from igneous rocks, represented on a FeO-Fe203-Ti02 diagram in Mol percent. Representing the results of the chemical analysis, v hick was performed on No. 2', No. 3', No. 4' and No. 6' specimens by T. Katsura and J. Osaka, as the points on the diagram of the ternary system shown in Fig. 1, it was found that these Table II. Chemical composition and crystallographic and magnetic properties of the ferro-

5 Thermo-Magnetic Study of Ferromagnetic Minerals Contained in Igneous Rocks 5 specimens are distributed around the TiFe03-Fe203 line. The results of the X-ray analysis by " Norelco " with Fe target are given in Table II. It was found that these specimens have the rhombohedral structure and their crystallographic parameters, ar, and a, are distributed between those of ilmenite and hematite. 3. Apparatus for magnetic measurement. Since the content of ferromagnetic mineral in igneous rocks is usually less than several percents, it is rather laborious to separate out a great amount of ferromagnetic minerals from the mother igneous rocks. For the sake of practical convenience an apparatus which can be used with a sufficient accuracy for measuring the magnetic properties of a small number of specimens was especially deviced. The apparatus is the same in principle as the magnetic balance which is generally used for the measurement of the magnetic susceptibility of paramagnetic or diamagnetic substances. However, the magnetic balance was improved in some points so as to be suitable for the measurement of the ferromagnetic substance. The apparatus is illustrated schematically in Fig. 2. The specimens, S, which is inserted in the fused silica bascket, is suspended by a quartz spring, Q, in the middle of the pole-pieces of the electromagnet. When the inhomogeneous magnetic field was produced by the electromagnet, the magnetic force. F, acting on the specimen is expressed by magnetic minerals of iimenite-hematite series. Fig. 2. Schematic view of measuring apparatus. Q...quartz spring S...specimen E. F...electric furnace E. M...electro-magnet T. J...thermo-junction * The numerical values were calculated in taking the presence of some amounts of hypersthene into consideration. **H represents heating process. C represents cooling process.

6 6 S. AKIMOT0 F=m.SAKIMOT (1) Q=x.H (2) where a m and H are the specific magnetization of the specimen, the specific susceptibility of the specimen, the mass of the specimen and the magnetic field acting on the specimen respectively. The saturation magnetization of the specimen can be measured provided that the applied magnetic field is strong enough to saturate magnetization of the specimen. This magnetic force acting on the specimen is determined approximately by measuring the amount of the elongation of the length of the quartz spring with the aid of a microscope., it was ascertained that the error: expected by the measurement of the elongation is less than 5% in the present study, provided that the elongation of the quartz spring is not so large, i. e. about 1mm. Hence it seems that this apparatus is sufficiently useful for the thermo-magnetic study, of ferromagnetic minerals. Thus, the change of the magnetization of the specimens with temperature in a strong magnetic field can be measured easily by putting the specimen into the electric furnace, shown in Fig. 2. In the present study, the measurement was made under the atmospheric pressure. The absolute intensity of magnetization of specimens could be determined by calibrating the instrumental constants. The intensity of the magnetic field acting on the specimen in thermo-magneticc study is e. for all specimens. When the demagnetizing factor owing to the shape of the specimen is taking into consideration, this intensity of the field willl be sufficient for saturating the magnetization of specimens. Hence the thermomagnetic curve measured by the present apparatus may be considered to represent approximately the change of the spontaneous magnetization with temperature. Since the spncimen is very small, + the uniformity of the temperature within it will be almost perfect, but, an error owing to discrepancy of temperature of the sample from that of the thermo-junction amounts to a few degrees. This error may be within the limit allowable for the present purpose of study. As mentioned above, this simple apparatus could be used for the study of the thermomagnetic analysis of ferromagnetic mineral with a fair accuracy, even for the magnetization of a single grain of the ferromagnetic mineral. On the other hand, an ordinary magnetic balance was used for measuring the fairly accurate intensity of magnetization of specimens at the atmospheric temperature by means of the null method. S4. Change in Magnetization with Temperature--Thermo-. Magnetic Curves. The intensity of magnetization of all specimens described in+2 was measured at continual temperatures from the atmospheric temperature up to the Curie point by means of the above-mentioned apparatus in the atmosphere during both the heating and the cooling processes. Since the thermo-magnetic curves of the specimens of the titanomagnetite series and those of the ilmenite-hematite series are appreciably

7 Trzermo-Magnetic Study of Ferromagnetic Minerals Contained in Igneous Rocks 7 different from each other, it will be convenient to discuss about each of them separately. (A) Titanomagnetite series. The measurement was carried out for several grains which were sampled randomly from each specimen. The thermo-magnetic curve thus obtained in magnetic field of e. will be regarded approximately as the change of the No. 1 H=17000e. No. 2 H=24000e. No. 3 H=26000e. No. 4 H=21000e. No. 5 H=33000e. No. 6 H=24000e. No. 7 H=30000e. No. 8 H=24000e. No. 9 H=17000e. No. 10 H 30000e. No. 11 II=21000e. No. 12 II=24000e.

8 8 S. AKIMOTO No. 13 H=28000e. No. 14 H=30000e. No. 15 H=17000e. Fig. 3. Thernc-magnetic curves cf ferromagnetic minerals of titanomagnetite series. spontaneous magnetization with temperature. The results of the experiments are shown in Fig. 3. As will be seen in these figures, the type of these thermo-magnetic curves is very similar to that of the ferrite of magnetite type. However, the thermo-magnetic curve of some specimens changes irreversibly with temperature in heating and cooling processes. This is the same phenomenon that has been found previously in the measurement with temperature of the change of the specific susceptibility of these ferromagnetic minerals in a weak magnetic field in vacuum.) The specimens of the phenocryst minerals in the original rocks were classified into "reversible type" and "irreversible type" with respect to the change of their spontaneous magnetization with temperature, just as in the case of their magnetic susceptibility in a weak magnetic field. The reversible and irreversible types in the former correspond to the reversible and irreversible types in the latter, except No. 7 specimen. It is noticeable for all specimens that the degree of the irreversibility is more remarkable in the magnetization in ra weak magnetic field than that in such a strong field as the present study. The thermomagnetic curve of the irreversible specimen in re-heating and re-cooling processes coinsides with that of the initial cooling curve, as shown in Fig. 3 for No. 3 and No. 4 specimens. These facts may show that the initial cooling curve is more 'stable under the present experimental condition. This has been noticed in the previous experiments on the thermal change of the magnetic susceptibility in a weak field in vacuum. In the above-mentioned irreversible type specimens, the Curie point decreases apparently by heat-treatment. Such specimens as No. 12, No. 13 and No. 15 have somewhat different characteristics of the thermo-magnetic curves; the increase of the Curie point was found in the thermo-magnetic curve in the cooling process in comparison with that in the initial heating process. This seems to be due to some chemical reaction such as the oxidation owing to the smallness of the grains and the experimental procedure carried out in the atmosphere. It has been reported by Pouillard(3) that the Curie point of titanomagnetite

9 Thermo-Magnetic Study of Ferrcmagnetic Minerals Ccnfained in Igneous Rccks 9 series specimens decreases according as the content of TiFe2O4 increases in the solid solution between TiFe2O4 and Fe3O4. Hence, it may be inferred that the decrease of the Curie point of the irreversible type specimens is caused by the increase of the content of TiFe2O4 in the Fe3O4--TiFe2O4 specimen owing to the heat treatment up to about 600C. It will be found in Table I that the crystal parameter of the heat treated specimen is a little larger than that of the fresh specimen of the same mineral. As will be discussed later, the amounts of the increase of the lattice parameter is nearly in accord with the theoretical value derived from the amount of decrease of the Curie point, where the rate of increase in the lattice parameter corresponding to the decrease in the Curie point is estimated to be about A/ 100C. (B) Ilmenite-hematite series. The thermo-magnetic curves of the specimens belonging to this series are summarized in Fig. 4. As will be seen in these curves, some of the specimens changes stepwise in the heating process. This stepwise changes of such specimens as No. 1', No. 2', No. 3' and No. 4' will suggest that these specimens contain a certain amount of titanomagnetite. An important characteristic of all specimens of this series will be the peculiar type of the thermo-magnetic curve, that is their magnetization changes with temperature almost linearly over a fairly wide temperature range. It was ascertained especially for No. 6' specimen that the magnetization changes linearly from the Curie point down to the liquid air temperature. It may be noticed in Fig. 4 that all specimens of this series have a ferromagnetic character though the intensity of some of their magnetization is not so strong as in the case of the ordinary ferromagnetic substances. It will be a remarkable fact that there exists a ferromagnetic phase in this ilmenite-hematite series which has been considered hitherto, even in Pouillard's recent paper, to be nonmagnetic. On account of the heat treatment in the atmosphere, the thermo-magnetic curves of No. 1', No. 3' and No. 4' specimens showed the irreversible changes. This change may be due to development of titanomagnetite phase in these specimens, which may result in an increase in the intensity of magnetization. S5. Saturation Magnetization and. Curie Point. The intensity of saturation magnetization and the Curie point of all specimens will be discussed here, since they are the intrinsic magnetic properties of the specimen which chiefly depend upon the general chemical composition and the crystal structure, being =independent of the grain size and the internal stress. (A) Titanomagnetite series. The magnetization curve of the specimens of the titanomagnetite series was measured at the room temperature by means of an ordinary magnetic balance with null method. The intensity of saturation magnetization was determined by the following empirical formula by extrapolating to H=S D=6s(1-a/H), A (3)

10 10 S. AAIMOTo No. 1' H=30000e. No. 2' H=30000e. No. 3' H=30000e. No. 4' H=36000e. No. 5' H= e. No. 6' H= 36000e. Fig. 4. Thermo-n agretic curves of ferromagnetic minerals of ilmenite-hematite series.

11 Thermo-Magnetic Study of Ferromagnetic Minerals Contained in igneous Rocks 11 where A is the constant determined experimentally for every specimen. The intensity of saturation magnetization tabulated in Table I varies from about 30 e. m. u. /gr. to about 90 e. m. u. /gr., being always less than 93 e. m. u. /gr. which is expected to be the intensity of saturation magnetization of pure magnetite at the room temperature. (6) On the other hand, the Curie point in each specimen was determined from the thermo-magnetic curve described in+4. The Curie point values, tabulated also in Table I, spread over the temperature range from 300C to 500C. The Curie point, 0; thus determined is in good agreement with the Curie point of the same specimen, e', which was determined from the susceptibility in a weak magnetic field. Since the titanomagnetite is generally considered to be a solid solution between TiFe2O; and Fe3O4, it may be expected that there exists a close relation among the chemical composition, the lattice parameter, the Curie point and the intensity of saturation magnetization. The relation between the chemical composition and the lattice parameter is shown in Fig. 5, where the content of TiO2 in Mol percent was expressed as the measure of the substitution of Fe+3 ion by Fe+2 ion and Ti+4 ion in titanomagnetite. The general tendency that the lattice parameter becomes larger as the content of TiO2 increases is found in this figure. However, the relation between TbO2 and the lattice parameter for the natural titanomagnetite grain is a little less definite in comparison with that for the artificially synthesized specimen as obtained by Pouillard. Since the chemical composition of these natural titanomagnetites are deviated from the exact Fe3O4-TiFe2O4 series, as illustrated in Fig. 1, it seems likely that the discrepancy mentiond above is due to the condition that some amounts of TiO2 and FeO in the natural minerals make the phase of ilmenite which intergrows into the phase of titanomagnetite. Since both TiFe2O4 and FesO4 have the inverse spinel structures, the structure of any titanomagnetite may be generally expressed by xfe+2(ti+4fe+2)04(1-x) =Fe13Fe2(Fei Fe+3(Fe+2Fe+3)O4 2+Fei3Tix4)O4 where the cations inside the bracket respresent the 16c position and that outside the 8f position of the spinel structure. Hence, the number of Bohr magneton per molecule of any titanomagnetite, n, will be expressed by n=5(1-x)+4-5(1-x)-4x-4(1-x) (4) where the numbers of Bohr magneton of Fe+2, Fe+3 and Ti+4 are 4, 5 and 0 respectively. When the content of TiFe2O4i x, increases in the solid solution between TiFe2O4 and Fe3O4, the linear decrease of saturation magnetization from that of Fe3O4 is theoretically expected from (4). Fig. 5. Lattice constant of ferromagnetic minerals of titanomagnetite series as dependent on the content of Ti02. Q: Pouillard's artificial titanomagnetite.

12 12 S. AKMOTO Fig. 6. Specific intensity of saturation magnetization cf ferromagnetic minerals of titanomagnetite series as dependent on the content cf Ti02. Fig. 7. Specific intensity of saturation magnetization of, ferromagnetic minerals of titanomagnetite series as dependent on lattice constant. Fig. 8. Curie point of ferromagnetic minerals of titanomagnetite series as dependent on lattice constant. Q: Pouillard's artificial titanomagnetite. Fig. 9. Specific intensity of saturation magnetization of ferromagnetic minerals of titanomagnetite series as dependent on Curie, point. The Curie point may be expected to be proportional to the number of Bohr magneton, i. e. the intensity of saturation magnetization provided that the relation established in the case of the magnetism of binary alloy between ferromagnetic and non-ferromagnetic elements holds true in the present case of the solid solution between the ferromagnetic and non-ferromagnetic metallic oxides. As will be seen in Fig. 6, Fig. 7, Fig. 8 and Fig. 9, the linear relations between each two among the lattice parameter, the Curie point, the chemical composion and the intensity of saturation magnetization could be actually obtained as the first approximation for the natural titanomagnetite specimens. If one of the above four properties of the specimens of titanomagnetite series is given, we may be able to predict the, other three properties with a fair accuracy. (B) Ilmenite-hematite series. The intensity of the saturation magnetization of the specimens of ilmeni-te-

13 Tnermo-Magnetic Study of Ferromagnetic Minerals Contained in Igneous Rocks 13 hematite series at the room temperature is given in Table II. As will be seen in this table, the petrologically so-called ilmenites, No, 1'-No. 4' specimens, have the small value of the saturation magnetizatation such as 0. 2 e. m. u /gr, being com parable in order with that of hematite, while No. 5. and No. 6' specimens, which may be considered to be situated near the middle of the line DC in Fig. 1, have the fairly large value of the intensity of saturation magnetization, i. e. about 20 e. m. u. /gr., being comparable in order with that of the specimens of titanomagnetite series. Since the source of the feeble ferromagnetic property of hematite may be attributed to the parasitic ferromagnetism arised from the local imperfect compensation of the two sublattices which will be coupled antif erromagnetically, (8) the feeble ferromagnetic property of the so-called ilmenites may also be attributed to the same mechanism for that of hematite. However, it seems that the strong ferromagnetic property of No. 5' and No. 6' specimens are too strong to be explained by the same parasitic ferromagnetism as hematite. The Curie point of this series was also determined from the thermo-magnetic curves illustrated in Fig. 4. As will be seen in Table II, these values vary from about 100C to 250C. No. 1'-. -No. 4' specimens have the Curie point around 100C, while No. 5' and No. 6' specimens around 250C. Since the number of the specimens treated in the present study is too small, the problem whether or not the Curie point of this series changes continuously from about 100C of ilmenite to 6x75C of hematite will remain for future studies. S6. Conclusions. From the experimental results obtained in the present study, the intrinsic magnetic properties of ferromagnetic minerals contained in igneous rocks were clarified to some extent. Especially for the titanomagnetites which were commonly found in the igneous rocks, their Curie point, intensity of saturation magnetization could be interpreted reasonably as the magnetic properties of the solid solution between TiFe2O4 and Fe3O4, a linear relations between each two of the chemical composition, lattice parameter, the Curie point and the intensity of saturation magnetization being found as the first approximation. Another important point obtained in the present study will be that there exists a ferromagnetic phase in ilmenite-hematite series having the peculiar type of their thermo-magnetic curve. Thus, if the thermomagnetic curve of the ferromagnetic minerals contained in the igneous rocks is given, their chemical and crystallographic structure may be predicted with a fair accuracy. In concluding this paper, the writer wishes to thank Prof. T. Nagata who gave him the kind direction and encouragement throughout the study. The writer is indebted to Dr. H. Kuno for his useful discussions and suggestions in petrological side. His hearty thanks are also due to Messrs. T. Katsusa and J. Osaka who offered kindly him the data of the chemical composition of the specimens.

14 14 S. ABIMOTO References (1) T. Nagata, Rock-Magnetism, Tokyo (1953) (2) R. Chevallier et J. Pierre, Ann. de Physique, (10) 18, 383 (1932) (3) E. Pouillard, Ann. de Chimie, 5, 164 (1950) (4) S. Akimoto, Journ. Geomag. Geoele., 3, 47 (1951) (5) T. Nagata, S. Uyeda and S. Akimoto, Journ. Geomag. Geoele., 4, 22 (1952) T. Nagata, S. Uyeda, S. Akimoto and N. Kawai, Journ. Geomag. Geoele., 4, 102 (1952) T. Nagata, S. Akimoto and S. Uyeda, Journ. Geomag. Geoele., 5, 168, (1953) (6) P. weiss et R. Forrer, Ann de Physique, (10) 12, 279 (1929) (7) R. Chevallier et S. Mathieu, Ann. de Physique, 18, 258 (1943) (8) L. Neel, Rev. Mod. Phys., 25, 58 (1953)