Mineral equilibria calculations in the system K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (KFMASHTO) using thermocalc and its internally consistent thermodynamic dataset constrain the effect of TiO2 and Fe2O3 on greenschist and amphibolite facies mineral equilibria in metapelites. The end-member data and activity–composition relationships for biotite and chloritoid, calibrated with natural rock data, and activity–composition data for garnet, calibrated using experimental data, provide new constraints on the effects of TiO2 and Fe2O3 on the stability of these minerals. Thermodynamic models for ilmenite–hematite and magnetite–ulvospinel solid solutions accounting for order–disorder in these phases allow the distribution of TiO2 and Fe2O3 between oxide minerals and silicate minerals to be calculated. The calculations indicate that small to moderate amounts of TiO2 and Fe2O3 in typical metapelitic bulk compositions have little effect on silicate mineral equilibria in metapelites at greenschist to amphibolite facies, compared with those calculated in KFMASH. The addition of large amounts of TiO2 to typical pelitic bulk compositions has little effect on the stability of silicate assemblages; in contrast, rocks rich in Fe2O3 develop a markedly different metamorphic succession from that of common Barrovian sequences. In particular, Fe2O3-rich metapelites show a marked reduction in the stability fields of staurolite and garnet to higher pressures, in comparison to those predicted by KFMASH grids.

The effect of TiO2 and Fe2O3 on metapelitic assemblages at greenschist and amphibolite facies conditions: mineral equilibria calculations in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3

This paper reviews the crystal structure of compounds with the general formula AB 2 X 4 , which crystallize with the same atomic structure as the mineral spinel, MgAl 2 O 4 . Three degrees of freedom associated with the detailed atomic arrangements of spinels are considered here: (i) the lattice parameter, a; (ii) the anion parameter, u; and (iii) the cation inversion parameter, i. Oxide spinels are used as examples to explore the interrelationships between these parameters.

Structure of Spinel

Preparation of layered double hydroxides and their applications as additives in polymers, as precursors to magnetic materials and in biology and medicine

In this paper a novel synthetic route of pure MFe2O4 (M = Mg, Co, and Ni) spinel ferrites is reported. The key feature of this method is using a single molecular precursor. The pure spinel ferrites can be obtained by calcination of tailored hydrotalcite-like layered double hydroxides (LDHs) of the type [M1-X-YFe2+YFe3+X(OH)2]X+(SO42-)X/2·mH2O (M = Mg, Co, and Ni) precursors at 900 °C for 2 h, in which the molar ratio of M2+/(Fe2+ + Fe3+) is adjusted to the same value as that in single spinel ferrite itself. The structural characteristics of the as-synthesized LDHs and their resulting calcined products are obtained by powder X-ray diffraction (XRD), chemical analysis, Fourier transform infrared spectroscopy (FT-IR), simultaneous thermogravimetric and differential thermal analysis (TG-DTA), and Mossbauer spectroscopy. The saturation magnetizations of the materials produced by calcinations of LDHs are higher than those of the spinel ferrites produced by the conventional ceramic and wet chemical routes, altho...

Stoichiometric Synthesis of Pure MFe2O4 (M = Mg, Co, and Ni) Spinel Ferrites from Tailored Layered Double Hydroxide (Hydrotalcite-Like) Precursors

The crystal structure and cation distributions in the spinels MgCr2O4, ZnCr2O4, Fe3O4 and a suite of ZnAl2O4 samples annealed at 900 to 1400° C and then rapidly quenched, have been determined by powder X-ray diffraction, using several different X-ray procedures and both conventional structure-factor refinement and whole-pattern (or Rietveld) refinement methods. The chromite spinels are expected from crystal chemical considerations to have an almost completely normal cation distribution (inversion parameter, x, equal to zero). In agreement with this expectation, three samples of MgCr2O4 annealed at 900, 1100 and 1300° C, and ZnCr2O4 were all found to have x=0 within two estimated standard deviations (esd), suggesting that the accuracy with which cation distributions in spinels may be determined by powder XRD is close to the estimated precision. Slightly better results are obtained assuming neutral-atom scattering curves rather than half-ionized or fully ionized, but the differences are small (within the esd). The results from the Rietveld refinements are similarly in good agreement with those using the conventional structure factor refinement approach (agreement within the combined esd's), although in detail the Rietveld procedure sometimes produces small systematic differences in refined parameters. The suite of ZnAl2O4 spinels show a smooth increase in x from 0.01 at 900° C to 0.05 at 1300° C, and this behaviour is well described by the simple thermodynamic model for disordering in spinels with αZn-Al=89 kJ/mol, assuming β=−20 kJ/mol. The oxygen positional parameters for Fe3O4 are similar to those from published single crystal studies, indicating that the powder method also yields accurate interatomic distances in spinels.

Crystal structures and cation distributions in simple spinels from powder XRD structural refinements: MgCr2O4, ZnCr2O4, Fe3O4 and the temperature dependence of the cation distribution in ZnAl2O4

Magnesioferrite (MgFe204 spinel) was synthesized in equilibrium with excess MgO using a flux method. Chemical analysis shows that the MgFe204 is stoichiometric, within analytical uncertainty. Samples were annealed at intervals of 50 °C between 450 and 1250 °C and quenched in H20. The time needed to reach equilibrium at low temperatures (450650 0C) was assessed by monitoring the change of lattice parameter (ao) with time. In addition, the changes in ao at 550-650 °C were reversed, using material annealed at 400 °C for 50 d. Some samples were further characterized by measuring their Neel temperatures, using DSC. The cation distributions in the quenched samples were determined using both powder X-ray diffraction and M6ssbauer spectroscopy. To increase the resolution in the latter, the samples were studied in an applied magnetic field of 4.5 T at 12-171 K. The cation distribution changes smoothly with temperature, with the cation inversion parameter (x) decreasing from 0.90 at 450 °C to 0.72 at 1100 0c. The precision with which x can be determined is ::to.004 from XRD and -::to.O! from the M6ssbauer spectroscopy. The two sets of measurements agree well with each other, the mean difference in x being 0.0056 ::t 0.0102. Thermodynamic modeling shows that the cation distribution can be described with a nonlinear enthalpy of disordering model, with aMg-Fe}+ = 26.6 ::t 0.4 and {1= - 21. 7 ::t 0.3 kJ/g-atom. Incorporation of a small excess (nonconfigurational) entropy term into the model gives a slightly better fit, however. The disordering in MgFe204 as a function of temperature is virtually identical to that found in Fe,04 from thermopower measurements. The present results differ to a greater or lesser extent from most of the previous work on MgFe204. This is most likely attributable to differences in stoichiometry. We also present some results on nonstoichiometric "MgFe204," i.e., solutions of MgFe204 and 1'-Fe20" which tend to confirm this hypothesis, at least for some of the earlier studies.

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H. St. C. O'Neill

Papers

The temperature dependence of the cation distribution in magnesioferrite (MgFe2O4) from powder XRD structural refinements and Mössbauer spectroscopy