Richmond, Nicola Claire; (2000) Computer simulation study of iron, aluminium and manganese in mantle silicates. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

A computer simulation study has been carried out on the incorporation of iron, manganese and aluminium into mantle silicates. Calculations have been carried out to determine the role aluminium has on the increased incorporation of ferric iron into perovskite which has been observed in experiment. It has been found that A1 substitutions into the Mg site in perovskite are energetically unfavourable. Consequently, it is more energetically favourable for Fe3+ to substitute for Mg and A1 for Si, directly charge balancing, than for independent Fe3+ or A1 substitutions charged balanced by vacancies or by a similar cation in another site. The possible effects of iron and aluminium on the elastic properties of perovskite have been considered. Ferric iron incorporation into perovskite, wadsleyite and forsterite has been considered to determine why some silicates (such as forsterite) do not contain significant quantities of ferric iron, while other silicates (such as wadsleyite and perovskite) do. The data obtained has indicated that substitutions of Fe3+ for Si are most important in determining ferric iron incorporation, with the differing size of the Si site particularly important. The site partitioning of Fe2+ and Mn2+ between the two cation sites in forsterite has been determined to assess the proposed use of olivine as a geospeedometer. This has been done by identifying the most energetically favourable Fe2+ and Mn2+ substitutions and by considering the other factors which could be important, including crystal field stabilisation energies and entropy. The results have indicated that both Fe2+ and Mn2+ preferentially occupy the M2 site in forsterite at low temperatures, with entropy causing disorder at high temperatures. This is in agreement with some recent experimental work on Mn2+ in olivine, but in contrast to experimental observations on Fe2+ incorporation. Possible reasons for the differences between calculation and experimental results are discussed.

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