Curie temperatures of synthetic titanomagnetites in the Fe‐Ti‐O system: Effects of composition, crystal chemistry, and thermomagnetic methods

Abstract

[1] The present study is aimed at improving the calibration of the compositional dependence of the Curie temperature (T C ) of titanomagnetite (Tmt) on the basis of temperature-dependent magnetic susceptibility (c-T) curves measured on synthetic Tmts in the Fe-Ti-O system. In order to assess the possible influence of high-temperature cation vacancies onto the T C values, we have synthesized two types of assemblages in subsolidus conditions at 1 bar, 1100°C and 1300°C, under controlled oxygen fugacity conditions. Tmts synthesized in equilibrium with ilmenite-hematite ss (Ilm ss ) are expected to have the highest vacancy concentrations, those in equilibrium with wüstite (Wus) the lowest. The composition and homogeneity of the synthetic Tmts were carefully checked with a scanning electron microscope (SEM) and an electron microprobe (EMP). T C was determined from c-T curves using a kappabridge and, for comparison, from M s -T curves measured with a variable field translation balance. Our data set shows systematically higher T C values for Tmt coexisting with Ilm ss than for Tmt coexisting with Wus. Most c-T curves are nonreversible, whereby the largest DT C (40 K) concern Tmt(+Ilm ss ) of intermediate compositions synthesized at 1300°C. Nonreversibility is interpreted as reflecting cation reordering in Tmt during the high-temperature c-T measurements. T C values obtained from M s -T curves are higher than those obtained from the c-T curves, whereby the difference regularly increases (up to 40 K) with increasing Ti content, up to X Usp = 0.6. Our new calibration curves are suitable to retrieve Tmt compositions in basalts that were rapidly cooled and not oxidized by deuteric or hydrothermal fluids.Citation: Lattard, D., R. Engelmann, A. Kontny, and U. Sauerzapf (2006), Curie temperatures of synthetic titanomagnetites in the Fe-Ti-O system: Effects of composition, crystal chemistry, and thermomagnetic methods,

Inferred time- and temperature-dependent cation ordering in natural titanomagnetites

“…It is surprising to observe such large effects in these natural samples (DT C up to B150°C) given that previous studies have found either little or no evidence of temperature-dependent cation ordering in titanomagnetites 3,8,12 or relatively small changes in T C (DT C o40°C) arising from cation reordering 15 . The small amount of Mg and Al substitution in these titanomagnetites may have a role in the effect, but we note that a study of similarly impure synthetic titanomagnetites (Fe 2.6 Ti 0.2 Mg 0.2 O 4 ) also found no evidence for temperature-dependent cation ordering 8 .…”

“…This calculated range is significantly smaller than the range that we have found experimentally. Further experimental evidence from synthetic titanomagnetites 15 and from synthetic magnesioferrites 14,18 suggests that the Stephenson 33 model substantially underestimates the effects of cation ordering on T C , probably due to (a) A rapidly cooled sample will follow the grey curve, departing from equilibrium at relatively high temperature (at the rate-dependent T close ) and quenching in a low degree of order. A slowly cooled sample will follow the black curve, departing from equilibrium at a lower temperature and quenching in a higher degree of order.…”

“…associated effects including cation vacancies 15 and the interaction of magnetic ordering and cation ordering 14,18 . According to the same generalized structural formula, with atomic moments of 4 and 5 Bohr magnetons (m B ) for Fe 2 þ and Fe 3 þ , respectively, the net spontaneous magnetization at 0 K is 6(1 À x) À 2b m B per formula unit (pfu), assuming collinear A-B coupling.…”

“…Ferrimagnetism in these materials arises from an antiferromagnetic coupling of the A and B sublattices of the inverse spinel structure, with a net spontaneous magnetization that is determined by the proportions and the valence states of the magnetic cations in the A (tetrahedral) and B (octahedral) sites [1][2][3][4][5][6][7][8][9][10][11][12] . The Curie temperature (T C ) is directly related to the strength of the exchange interactions within and especially between the sublattices, which in turn depend on the site occupancy of the magnetic cations [13][14][15] .…”

“…This is thought to arise from a combination of a strong octahedral site preference for Ti 4 þ at all temperatures 12,23 and rapid reordering of the remaining Fe 2 þ and Fe 3 þ via electron hopping 4 . A notable exception is the work of Lattard et al 15 , in which cation reordering is invoked to explain irreversible thermomagnetic behaviour in synthetic cation-deficient titanomagnetites of intermediate composition (T C measured on warming oT C measured on cooling by r40°C). We show here for the first time experimental results documenting very large and systematic changes in T C in natural titanomagnetites, resulting from laboratory thermomagnetic measurements and isothermal annealing at moderate (350-400°C) temperatures, and we interpret the results in terms of time-and temperature-dependent cation distributions.…”