**3. ESR spectra of ferrimagnetic minerals**

ESR has been geologically used for investigating the characteristics of paramagnetic lattice defect centers in natural minerals or ESR dating of rock-forming minerals such as quartz or feldspars. However, there are few studies on natural ferrimagnetic minerals using ESR because magnetometers are main equipments for investigating ferrimagnetic minerals in the field of geology or geophysics. ESR spectra give us important information on the magnetism of natural minerals as well as magnetometers. The g-value of the signal is an intrinsic physical parameter calculated from the microwave frequency and resonant magnetic field, which represent the energy level of unpaired electrons in minerals. The peak-to-peak linewidth and lineshape of the signal are also important physical parameters reflecting the spin-spin relaxation time. In this section, I will show ESR spectra obtained from main ferrimagnetic minerals and explain their characteristics.

#### **3.1 Magnetite: Fe3O4**

Magnetite is the most famous ferrimagnetic mineral. It is a cubic crystal of the spinel group and has the inverse spinel structure, which can be expressed byFe3+(Fe3+Fe2+)2O4, indicating that each formula unit of magnetite has one Fe3+ in the A sublattice and one Fe3+ plus one Fe2+ in the B sublattice. Magnetite can be produced by the oxidation of iron at high temperatures in air or steam, by heating maghemite (γ-Fe2O3) or hematite (α-Fe2O3) in a reducing atmosphere, by heating siderite (FeCO3) in steam or nitrogen at dull red heat, or by heating biotite (K(Mg,Fe2+)3(Al,Fe3+)Si3O10(OH)2) in nitrogen or vacuum (Deer et al., 1992). Fig.4 shows ESR spectra obtained from powder (1–10 μm) of various magnetite samples. The samples are respectively natural magnetite (≤10μm) distributed at Hanaidani Mine in Shimane, Japan (Nichika Corporation, MJ09) (a), synthetic magnetite (~1μm) with high purity (WAKO Pure Chemical Industries, JW090103) (b), magnetite produced from natural biotite (≤10μm) distributed in the Uchinoura granite in southern Kyushu, Japan by heating at 1000°C for 1 h under 0.5 Pa (c), and magnetite produced from natural siderite (≤10μm) distributed in Guelmin Es-Semara Region, Morocco (Hori Mineralogy Ltd.) by heating at 500°C for 5 min. under 0.6 Pa (d). The spectrum c) is magnified to 50×. Measurement conditions are as follows and the same conditions are used for the other spectra in this chapter: microwave frequency; 9.43 GHz, microwave power; 1 mW, modulation width; 100 kHz 0.05 mT, sweep speed; 8 min/sweep, accumulation; 3 times, measurement temperature; room temperature. The g-value calculated from the spectra is 2.70−7.15 and the peak-topeak linewidth of the FMR signals is 219−358 mT. The g-value tends to shift toward the lower magnetic field with increasing grain size due to the demagnetizing field. Siderite is a brownish trigonal mineral in the calcite group, while biotite is a black monoclinic mineral in the mica group. Since siderite and biotite are paramagnetic, both originally show paramagnetic signals, that is, EPR signals.

Fig. 4. ESR spectra obtained from various magnetite samples.

Fig. 5. Variation of the ESR spectrum of natural magnetite with step-by-step heating in air.

Shimane, Japan (Nichika Corporation, MJ09) (a), synthetic magnetite (~1μm) with high purity (WAKO Pure Chemical Industries, JW090103) (b), magnetite produced from natural biotite (≤10μm) distributed in the Uchinoura granite in southern Kyushu, Japan by heating at 1000°C for 1 h under 0.5 Pa (c), and magnetite produced from natural siderite (≤10μm) distributed in Guelmin Es-Semara Region, Morocco (Hori Mineralogy Ltd.) by heating at 500°C for 5 min. under 0.6 Pa (d). The spectrum c) is magnified to 50×. Measurement conditions are as follows and the same conditions are used for the other spectra in this chapter: microwave frequency; 9.43 GHz, microwave power; 1 mW, modulation width; 100 kHz 0.05 mT, sweep speed; 8 min/sweep, accumulation; 3 times, measurement temperature; room temperature. The g-value calculated from the spectra is 2.70−7.15 and the peak-topeak linewidth of the FMR signals is 219−358 mT. The g-value tends to shift toward the lower magnetic field with increasing grain size due to the demagnetizing field. Siderite is a brownish trigonal mineral in the calcite group, while biotite is a black monoclinic mineral in the mica group. Since siderite and biotite are paramagnetic, both originally show

paramagnetic signals, that is, EPR signals.

Fig. 4. ESR spectra obtained from various magnetite samples.

Fig. 5. Variation of the ESR spectrum of natural magnetite with step-by-step heating in air.

Fig. 6. Variation of the ESR spectrum obtained from natural siderite with step-by-step heating in air.

Fig. 5 shows the result from step-by-step heating experiments at 100–500°C in air. The heating duration is 5 minutes for each temperature. The natural magnetite same as the spectrum a) in Fig. 4 was used for the heating experiments. Magnetite is usually oxidized and changed into maghemite or hematite by heating in air, and the g-value of the FMR signal obtained from the natural magnetite shifts from 7.15 to 4.76 with heating. Although the signal peak becomes broader with the oxidation due to heating, its lineshape does not change so much; the peak-to-peak linewidth is in the range of 220−239 mT. This means that the oxidation may be limited to only the surface of the magnetite particles. On the other hand, Fig.6 shows ESR spectra obtained from natural siderite by step-by-step (5 min.) heating in air. The natural siderite same as the spectrum d) in Fig. 4 was used for the heating experiments. The FMR signal of magnetite produced from siderite by heating strikingly increases at 450°C, and then its lineshape is distorted at 500°C due to the production of maghemite or hematite. The FMR signal detected at 450°C in air has almost the same gvalue, peak-to-peak linewidth and lineshape as that of magnetite detected at 500°C in vacuum (Fig. 4d). However, the g-value somewhat shifts from 2.69 to 2.09 with heating, while the peak-to-peak llinewidth extremely changes from 170 to 272 mT. Since the magnetite produced by thermal decomposition of siderite may consist of extremely fine crystals or amorphous particles, it is easily oxidized under an oxidizing environment.

#### **3.2 Maghemite: γ-Fe2O3**

Maghemite is a popular ferrimagnetic mineral and widely used as a material for magnetic recorders. It is a cubic crystal of the spinel group and has the spinel structure expressed by Fe3+( Fe3+Fe3+2/3,V1/3)O4 where V is a vacancy, indicating that magnetite is oxidized to maghemite by changing the valence state of two thirds of the original Fe2+ to Fe3+ while simultaneously removing one third of the original Fe2+ from the B sublattice. This removal occurs by diffusion producing vacancies in the spinel structure where a Fe2+ cation had previously resided; these vacancies account for the name cation-deficient spinel. Since ferrimagnetism of magnetite results from Fe2+ in the B sublattice, the removal of one third of these cations decreases saturation magnetization from 480 G (4.8×105 A/m) for magnetite to 420 G (4.2×105 A/m) for maghemite (Butler, 1992). Pure maghemite is commonly metastable and irreversibly changes its crystal structure into a hexagonal one, that is, α-Fe2O3 (hematite) on heating to 300–500°C. However, the transformation temperature of natural maghemite into hematite is often over 500–700°C because maghemite is stabilized with impurities inside the crystal (e.g. Fukuchi et al., 2007). As for the formation of natural maghemite, besides the oxidation of magnetite, the following three processes are pointed out on the basis of the studies of ferrimagnetic minerals in soils (Butler, 1992):


Fig. 7 shows the ESR spectrum obtained from powder (≤1μm) of synthetic maghemite and its variation with step-by-step heating (5 min.) in air. The synthetic maghemite (≤1μm) with high purity (Kojundo Chemical Lab, FE006PB) was used for the heating experiments. Maghemite shows a quite different lineshape from magnetite. The signal peak around the lower magnetic field (~100 mT) obtained from maghemite is much sharper than that from magnetite (Fig.4). The g-value calculated from the spectra is in the range of 3.26−3.34 and the peak-to-peak linewidth of the FMR signals is 346−358 mT. The FMR signal of maghemite hardly changes its lineshape during heating until 500°C and thermally shows high stability although pure maghemite transforms into hematite by heating to 300–500°C.

On the other hand, Fig. 8 shows ESR spectra obtained from natural and synthetic hematites with those from synthetic lepidocrocite and goethite (α-FeOOH) and natural limonite (FeOOH(nH2O)). The samples are synthetic hematite (≤10 μm) with high purity (WAKO Pure Chemical Industries, JW090282) (a), natural hematite (specularite, ≤10 μm) distributed in Antananarivo, Madagascar (Hori Mineralogy Ltd.) (b), synthetic goethite (~1μm) with high purity (Kojundo Chemical Lab, FEI16PB) (c), synthetic lepidocrocite with high purity (≤10 μm, Kojundo Chemical Lab, FEI17PB) (d), and natural limonite (≤10 μm) distributed in Hwanghae-do, North Korea (Nichika Corporation, MU262) (e). Small signals like spines detected between 300 and 400 mT in the spectrum b) are Mn2+ markers. Hematite is a trigonal mineral of the corundum structure and shows parasitic ferromagnetism, which is a

Fig. 7. Variation of the ESR spectrum of synthetic maghemite with step-by-step heating in air.

and irreversibly changes its crystal structure into a hexagonal one, that is, α-Fe2O3 (hematite) on heating to 300–500°C. However, the transformation temperature of natural maghemite into hematite is often over 500–700°C because maghemite is stabilized with impurities inside the crystal (e.g. Fukuchi et al., 2007). As for the formation of natural maghemite, besides the oxidation of magnetite, the following three processes are pointed out on the basis of the

1. The formation of maghemite from iron oxides or oxyhydroxides by repeated oxidation-

2. The conversion of paramagnetic iron-bearing minerals by natural burning (above ~200

Fig. 7 shows the ESR spectrum obtained from powder (≤1μm) of synthetic maghemite and its variation with step-by-step heating (5 min.) in air. The synthetic maghemite (≤1μm) with high purity (Kojundo Chemical Lab, FE006PB) was used for the heating experiments. Maghemite shows a quite different lineshape from magnetite. The signal peak around the lower magnetic field (~100 mT) obtained from maghemite is much sharper than that from magnetite (Fig.4). The g-value calculated from the spectra is in the range of 3.26−3.34 and the peak-to-peak linewidth of the FMR signals is 346−358 mT. The FMR signal of maghemite hardly changes its lineshape during heating until 500°C and thermally shows high stability although pure maghemite transforms into hematite by heating to 300–500°C. On the other hand, Fig. 8 shows ESR spectra obtained from natural and synthetic hematites with those from synthetic lepidocrocite and goethite (α-FeOOH) and natural limonite (FeOOH(nH2O)). The samples are synthetic hematite (≤10 μm) with high purity (WAKO Pure Chemical Industries, JW090282) (a), natural hematite (specularite, ≤10 μm) distributed in Antananarivo, Madagascar (Hori Mineralogy Ltd.) (b), synthetic goethite (~1μm) with high purity (Kojundo Chemical Lab, FEI16PB) (c), synthetic lepidocrocite with high purity (≤10 μm, Kojundo Chemical Lab, FEI17PB) (d), and natural limonite (≤10 μm) distributed in Hwanghae-do, North Korea (Nichika Corporation, MU262) (e). Small signals like spines detected between 300 and 400 mT in the spectrum b) are Mn2+ markers. Hematite is a trigonal mineral of the corundum structure and shows parasitic ferromagnetism, which is a

Fig. 7. Variation of the ESR spectrum of synthetic maghemite with step-by-step heating in air.

studies of ferrimagnetic minerals in soils (Butler, 1992):

reduction cycles during soil formation.

°C) in the presence of organic matter. 3. The dehydration of lepidocrocite (γ-FeOOH). kind of antiferromagnetism. Hematite has an asymmetrical antiferromagnetic structure, so that it shows a weak spontaneous magnetization and ESR absorption (Fig. 8a). On the other hand, natural hematite (specularite) shows a quite different ESR signal from the synthetic hematite. It has the g-value of 10.8, which is much larger than that of 2.35 obtained from the synthetic hematite.

Fig. 8. ESR spectra obtained from powder of hematite, goethite, lepidocrocite and limonite.

Lepidocrocite is a monoclinic (or trigonal) paramagnetic crystal at room temperature, however shows antiferromagnetism at the Néel temperature of 77K. Therefore, lepidocrocite shows a paramagnetic signal at room temperature (Fig. 8d). A strong FMR signal of maghemite comes to be detected from heated lepidocrocite due to thermal dehydration; 2γ-FeOOH→γ-Fe2O3+H2O. Fig. 9 shows the variation of the ESR spectrum of lepidocrocite with step-by-step heating (5 min.) in air. The synthetic lepidocrocite same as the spectrum d) in Fig. 8 was used for the heating experiments. The FMR signal of maghemite produced from pure lepidocrocite by heating strikingly increases at 300°C. This suggests that the maghemite produced from lepidocrocite changes from a superparamagnet to a ferrimagnet between 250 and 300°C. The g-value is 2.26–2.31 and the peak-to-peak linewidth is 166–192 mT. The lineshapes of the FMR signals detected are different from those of maghemite with high crystallinity (Fig. 7). Moreover, Fig. 10 shows the result from isothermal annealing experiments at 250°C using the synthetic lepidocrocite. The lineshape and g-value of the signal obtained from the lepidocrocite characteristically change with heating time. The g-value shifts from 2.0 to 2.18 since the demagnetizing field causes the resonant magnetic field to shift toward the lower field (Fukuchi et al., 2007). The peak-to-peak linewidth also changes from 24 to 174 mT with heating time. In general, the maghemite produced from lepidocrocite has low crystallinity and shows an FMR signal with much lower g-value and peak-to-peak linewidth than maghemite with high crystallinity or one produced by the oxidation of magnetite (Figs. 5–7). Strictly speaking, such an FMR signal may be called superparamagnetic signal although it is difficult to distinguish between the superparamagnetic and ferrimagnetic signals. In this chapter, I use the term FMR signal for ESR signals derived from both superparamagnetic and ferrimagnetic maghemites. As mentioned later, the FMR signals detected from the Nojima fault rocks in Japan show almost the same characteristics as the superparamagnetic signal detected from the heated lepidocrocite.

Fig. 9. Variation of the ESR spectrum of lepidocrocite with step-by-step heating in air.

Fig. 10. Variation of the ESR spectrum of lepidocrocite with isothermal annealing at 250°C in air.

Fig. 11. Variation of the ESR spectrum of goethite with step-by-step heating in air.

Goethite is an orthorhombic mineral and a polymorph of lepidocrocite. It changes into hematite due to thermal dehydration by heating at 300–400°C; 2α-FeOOH→α-Fe2O3+H2O. Fig. 11 shows the variation of the ESR spectrum of goethite with step-by-step heating (5 min.) in air using the synthetic goethite (Fig. 8c).

Goethite is an antiferromagnet however exhibits a weak ferromagnetism as well as hematite. The signal detected once increases with producing hematite, and then decays above 400°C. The g-value is in the range of 2.13–2.22 and the peak-to-peak linewidth is 92–136 mT. The characteristics of the signal are very similar to those obtained from the heated lepidocrocite although the signal intensity is very weak. On the other hand, limonite consists of cryptocrystalline goethite and lepidocrocite along with absorbed water or some hematite, so that it has a complex ESR spectrum derived from these minerals (Fig. 8e).

#### **3.3 Pyrrhotite: Fe1-xS (0≤x<0.125)**

326 Earthquake Research and Analysis – Seismology, Seismotectonic and Earthquake Geology

Fig. 9. Variation of the ESR spectrum of lepidocrocite with step-by-step heating in air.

Fig. 10. Variation of the ESR spectrum of lepidocrocite with isothermal annealing at 250°C in air.

Fig. 11. Variation of the ESR spectrum of goethite with step-by-step heating in air.

Pyrrhotite has the approximate composition of FeS but always contains less iron than is indicated by this formula. Hence, it is expressed by the generic formula of Fe1-xS, where 0≤x<0.125. Pyrrhotite is a monoclinic or hexagonal crystal of the nickel arsenide (NiAs) structure and consists of several superstructures due to the presence and ordering of vacancies within the structure. In pyrrhotite, two antiparallel coupled sublattices containing iron cations exist and the number of iron cations in the opposing sublattices are unequal, so that it is responsible for the ferrimagnetism of pyrrhotite. Troillite (FeS) is antifferomagnetic and occurs mainly in meteories and lunar rocks. Pyrrhotite can be produced by the direct combination of iron and sulphur and by heating pyrite (FeS2) in an atmosphere of H2S at 550°C. In the Fe-S system, the pyrrhotite in equilibrium with pyrite above 400°C shows increasing iron deficit with increasing temperature (Deer et al., 1992). Fig. 12 shows ESR spectra obtained from natural pyrrhotite, synthetic troillite and synthetic pyrite. The samples used are natural pyrrhotite (≤10μm) distributed at Yanahara Mine in Okayama, Japan (Nichika Corporation, MU123) (a), synthetic troillite (≤10 μm) with high purity (Kojundo Chemical Lab, FEI06PB) (b), and synthetic pyrite (≤10 μm) with high purity (Kojundo Chemical Lab, FEI07PB) (c). The natural pyrrhotite shows very weak and broad ESR absorption, while the troillite has a similar ESR spectrum to natural hematite, that is,

Fig. 12. ESR spectra obtained from powder of natural pyrrhotite, and synthetic troillite and pyrite.

specularite (Fig. 8b). On the other hand, pyrite is a cubic mineral and shows paramagnetism. The synthetic pyrite shows an intermediate spectrum of specularite and troillite.
