Sillimanite and Ilmenite from High-grade Metamorphic Rocks of Antarctica and Other Areas |
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| Authors: | GREW EDWARD S |
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| Institution: | Department of Earth and Space Sciences, University of California Los Angeles, CA 90024 |
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| Abstract: | Sillimanite from a variety of high-grade metamorphic rocks containsfrom 0.13 to 1.82 weight per cent Fe2O3 and less than 0.1 weightper cent TiO2. The iron is trivalent and substitutes for Alonly. Ilmenite associated with the sillimanite contains no morethan 0.4 weight per cent Al2O3, SiO2, CaO, and MnO; and MgOdoes not exceed 1.6 weight per cent. It ranges in compositionfrom Ilm99Hem1 to Ilm85Hem15. A least squares fit of precision unit cell data on 10 analyzedsillimanites gives the following cell dimensions for iron-freesillimanite: a = 7.4830 Á, b = 7.6708 Á, c = 5.7694Á and V = 331.15 Á3. The projected increase incell volume with substitution of 10 mole per cent Fe2SiO3 is1.66 per cent. A regular increase in the Fe2O3 content of sillimanite withincreasing Fe2O3 content of associated ilmenite in 15 of 21samples analyzed suggests that sillimanite and ilmenite crystallizedin equilibrium in the 15 samples. The compositions of the tensillimanite-ilmenite pairs analyzed by the author fit the followingempirical curve (sol;(XFe2O3)Il = 1.110 x 10–3. This regularincrease in Fe2O3 contents fits a model of Fe3+ substitutionfor Al on two independent sites in sillimanite and a coupledsubstitution of for Fe2+ Ti on two sites in ilmenite. Sillimaniteand ilmenite are behaving as ideal solutions over the compositionalrange 0 < XFe2SIO3 < 0.013 in sillimanite and 0 < XFe2O3< 0.15 in ilmenite. Equations have been derived for expressing the variation inFe2O3 content of sillimanite associated with quartz and ilmeniteor hematite as a function of pressure, temperature, and Fe2O3content of the oxide minerals. For example, the Fe2O3 contentof a sillimanite with 1.5 mole per cent Fe2SiO3 coexisting withTi-free hematite is calculated to decrease 11 per cent witha 5 kb increase in pressure. The rate of increase with temperatureof the Fe2O3 content of sillimanite is greater in hematite-bearingassemblages than in ilmenite-bearing assemblages. |
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