Sillimanite and Ilmenite from High-grade Metamorphic Rocks of Antarctica and Other Areas

• Published in: Journal of Petrology
• Main Author: GREW, EDWARD S.
• Format: Text
• Language: English
• Published: Oxford University Press 1980
• Subjects: Articles; Antarc*; Antarctica
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/21/1/39; https://doi.org/10.1093/petrology/21.1.39

Sillimanite from a variety of high-grade metamorphic rocks contains from 0.13 to 1.82 weight per cent Fe 2 O 3 and less than 0.1 weight per cent TiO 2 . The iron is trivalent and substitutes for Al only. Ilmenite associated with the sillimanite contains no more than 0.4 weight per cent Al 2 O 3 , SiO 2 , CaO, and MnO; and MgO does not exceed 1.6 weight per cent. It ranges in composition from Ilm 99 Hem 1 to Ilm 85 Hem 15 . A least squares fit of precision unit cell data on 10 analyzed sillimanites gives the following cell dimensions for iron-free sillimanite: a = 7.4830 Á, b = 7.6708 Á, c = 5.7694 Á and V = 331.15 Á3. The projected increase in cell volume with substitution of 10 mole per cent Fe 2 SiO 3 is 1.66 per cent. A regular increase in the Fe 2 O 3 content of sillimanite with increasing Fe 2 O 3 content of associated ilmenite in 15 of 21 samples analyzed suggests that sillimanite and ilmenite crystallized in equilibrium in the 15 samples. The compositions of the ten sillimanite-ilmenite pairs analyzed by the author fit the following empirical curve (sol;(X Fe 2 O 3 )Il = 1.110 x 10−3. This regular increase in Fe 2 O 3 contents fits a model of Fe3+ substitution for Al on two independent sites in sillimanite and a coupled substitution of for Fe2+ Ti on two sites in ilmenite. Sillimanite and ilmenite are behaving as ideal solutions over the compositional range 0 < X Fe 2 SIO 3 < 0.013 in sillimanite and 0 < X Fe 2 O 3 < 0.15 in ilmenite. Equations have been derived for expressing the variation in Fe 2 O 3 content of sillimanite associated with quartz and ilmenite or hematite as a function of pressure, temperature, and Fe 2 O 3 content of the oxide minerals. For example, the Fe 2 O 3 content of a sillimanite with 1.5 mole per cent Fe 2 SiO 3 coexisting with Ti-free hematite is calculated to decrease 11 per cent with a 5 kb increase in pressure. The rate of increase with temperature of the Fe 2 O 3 content of sillimanite is greater in hematite-bearing assemblages than in ilmenite-bearing ...