American Mineralogist, Volume 81, pages 375-384, 1996 Magnetic properties of the magnetite-spinel solid solution: Curie temperatures, magnetic susceptibilities, and cation ordering RICHARD J. HARRISON ANDANDREW PurNIS * Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K. ABSTRACT Curie temperatures (Td of the (Fe304)x(MgA1204)1-xsolid solution have been deter- mined from measurements of magnetic susceptibility (x) vs. temperature. The trend in Te vs. composition extrapolates to 0 K at x = 0.27. This behavior is rationalized in terms of the trend in cation distribution vs. composition suggested by Nell and Wood (1989), with Fe occurring predominantly on tetrahedral sites for x < 0.27. High-temperature x-T curves are nonreversible because of the processes of cation or- dering and exsolution, which occur in the temperature range 400-650 0c. The Curie tem- perature of single-phase material is shown to be sensitive to the state of nonconvergent cation order, with a difference in Te of more than 70°C being observed between a sample quenched from 1400 °C and the same sample after heating to 650°C. This interaction between magnetic and chemical ordering leads to thermal hysteresis behavior such that Te measured during heating experiments is approximately 10°Chigherthan that measured during cooling. The hysteresis is due to a reversible difference in the state of cation order during heating and cooling caused by a kinetic lag in the cation-ordering behavior. Samples with compositions in the range 0.55 < x < 0.7 undergo exsolution to a mixture of ferrimagnetic and paramagnetic phases after heating to 650°C. Room-temperature hysteresis loops of the starting material and the high-temperature experiment products are compared. All starting materials are multidomain with coercivities He < 1.26 mT and Mr/ M, < 0.051. Samples that exsolved during the experiments have coercivities up to 20 mT and Mr/M, up to 0.36. This change in hysteresis properties is caused by grain sub- division during exsolution and implies a transition in the magnetic domain state from multi- to single-domain. INTRODUCTION dus exsolution and nonconvergent cation ordering. Exso- The magnetic properties of solid solutions are sensitive lution microstructures are often responsible for high to the processes of oxidation, cation ordering, and sub- coercivities in naturally occurring magnetic minerals such solidus exsolution. Both in nature and in laboratory ex- as the titanomagnetite solid solution (Tucker and O'Reil- periments these processes can occur concurrently, result- ly 1980; Shive and Butler 1969; Price 1980; Evans and ing in complex microstructures and a magnetic signature Wayman 1974). Although titanomagnetite is the most that is characteristic of the mineral processes that oc- important cause of remanence in rocks, it is not possible curred. In rock magnetism it is the magnetic properties to study the development of subsolidus exsolution mi- of minerals responsible for natural remanent magnetiza- crostructures in synthetic titanomagnetite because of the tions that are of interest. Studying the interaction be- low temperatures required to produce spinodal decom- tween cation ordering, exsolution, and magnetism helps position. However, the general principles of the relation- us to understand the acquisition, alteration, and stability ship between magnetism, chemical composition, micro- of natural magnetizations over geological time. In min- structure, and cation ordering can be studied in related eral physics magnetic properties of complex solid solu- systems. The magnetite-spinel solid solution is a more tions provide a sensitive probe for the study of intrinsic suitable system in that the chemical solvus has a conso- mineral properties and kinetic processes. lute point near 1000 °C (Mattioli and Wood 1988) and This paper is the second in a series detailing the mag- there are extensive changes in the degree of nonconver- netic properties of the magnetite-spinel solid solution gent cation order as a function of temperature (Nell and (Fe304)AMgA1204)I-x (Harrison and Putnis 1995). The Wood 1989; Nell et al. 1989). general aims of the project are to investigate the inter- In a previous paper (Harrison and Putnis 1995) we action between magnetism and the processes of subsoli- detailed low-temperature hysteresis properties of the ho- mogeneous solid solution. The trend in saturation mag- Permanent address: Universitat Muenster, Institut ftir Min- netization vs. composition can be understood in terms of eralogie,* Correnstrasse 24, 48149 Munster, Germany. the distribution of Fe atoms between tetrahedral (A) and 0003-004 X/96/0304-03 75$05 .00 375 376 HARRISON AND PUTNIS: MAGNETITE-SPINEL MAGNETIC PROPERTIES octahedral (B) sites, the net magnetization being the dif- literature (Carpenter et al. 1994; O'Neill and Navrotsky ference between the A and B sublattice magnetizations 1984; Nell and Wood 1989; Sack and Ghiorso 1991; Ste- (Brown et al. 1993). The results showed that for x > 0.3 phenson 1969). The term nonconvergent means that there the solid solution is ferrimagnetic with the B sublattice is no symmetry change involved in ordering cations be- magnetization greater than the A sublattice magnetiza- tween A and B sites. tion. For x < 0.3 the solid solution is ferrimagnetic with The distribution of cations between octahedral and tet- the A sublattice magnetization greater than the B sublat- rahedral sites for the (Fe30.)AMgAI20')I_x solid solution tice magnetization. The x = 0.3 composition is a com- has been experimentally determined at 1000 °C (Nell et pensation point corresponding to an antiferromagnetic al. 1989), and a thermodynamic model describing the spin structure with A and B sublattice magnetizations temperature dependence of cation distribution has been equal. The compensation point is associated with a pro- proposed (Nell and Wood 1989). The solid solution has nounced peak in the coercivity of 700 mT, which is re- a solvus at temperatures below 1000 °C (Mattioli and lated to the presence of fine-scale heterogeneities in com- Wood 1988). Exsolution occurs within the quaternary position and degree of cation order. system Fe30.-FeAI20.-MgFe20.-MgAI20., but tie-lines There is a discrepancy between the position of the com- joining Fe-rich and Fe-poor phases remain close to the pensation point reported by Harrison and Putnis (1995) binary join Fe30.-MgAI20. (Lehmann and Roux 1986). for samples quenched from 1400 °C and the position pre- The spin-only magnetic moments associated with FeH dicted by the Nell and Wood (1989) model for an equil- and FeH ions are 4 and 5 !La,respectively (1 !La= 9.27 X ibration temperature of 1400 0c. This discrepancy is due 10-2. Am2). Below the Curie temperature (Td of a mag- to cation reordering, which occurs when quenching from netic spinel Fe atoms on B sites have their moments high temperatures. The position of the compensation point aligned parallel to each other but antiparallel to those on in quenched samples is a function of the kinetics of cation A sites. The net saturation magnetization is given by the reordering and cannot be predicted directly from equilib- difference between the octahedral and tetrahedral sublat- rium models. The observed compensation point at x = tice magnetizations (Stephenson 1972). If the sublattice 0.3 is consistent with Nell and Wood (1989) if a kinetic magnetizations are unequal then the net magnetization is closure temperature of 1200 °C is assumed. nonzero and the material is ferrimagnetic. For example, Other experimental work on this solid solution in- magnetite with the inverse cation distribution has a net cludes measurements oflattice parameters, activities, Cu- moment of 4 !La. If the sublattice magnetizations are equal, rie temperatures, room-temperature low-field magneti- then the net magnetization is zero and the material is zation, and cation distribution vs. composition (Mattioli antiferromagnetic. et al. 1987; Mattioli and Wood 1988; Nishitani 1981; Nell et al. 1989). This paper presents the results of alter- Temperature dependence of magnetic susceptibility (x) nating-field susceptibility measurements on synthetic Here we present a brief outline of the theory relevant samples of the magnetite-spinel solid solution, initially to the interpretation of susceptibility-temperature curves. quenched from 1400 0c. We demonstrate that suscepti- For a detailed description of rock and mineral magnetism bility measurements can be used to monitor changes in the reader is referred to O'Reilly (1984). Magnetic sus- the Curie temperature associated with both composition- ceptibility (x) describes the response of a material to an al changes and changes in the degree of cation order. applied magnetic field. The magnetization per unit mass M (Am2/kg) acquired by a material in a field H (Aim) THEORETICAL BACKGROUND defines susceptibility using the equation X = M/H (m3/ Crystal structure and cation distributions of kg). If H is an alternating field, then the response of the magnetic spinels material may not be in phase with the field. In this case Magnetite (FeHFe~+O.) and spinel (MgAI20.) are both X has an imaginary component and hence X = x' + ix". cubic spinels with space-group Fd'Jm and eight formula In the majority of cases only the real component of sus- units per unit cell. The 32 0 atoms per unit cell are in an ceptibility is considered and this is denoted X in what approximately close-packed arrangement with cations follows. occupying eight tetrahedral (A) sites and 16 edge-sharing Below Tc the temperature dependence of Xmay be un- octahedral (B) sites. At room temperature MgA120. has derstood in terms of the relationship x(T) <X M.(T)/ K(T), the normal spinel cation distribution with 8 Mg atoms where M. is the spontaneous magnetization and K is the occupying A sites and 16 AI atoms on B sites. Fe30. has magnetic anisotropy. Both M. and K decrease with in- the inverse spinel cation distribution at room tempera- creasing temperature.