Trace ferric ion in lunar and meteoritic titanaugites

Abstract

Evidence for presence of Fe3+ in lunar rocks is furnished by heating them in air to 200-225°C for two hours. This causes a large decrease in the same charge transfer bands attributed to Fe3+ that can be enhanced by heating the same rocks in air at 500°C. This data is interpreted as evidence that the Fe3+ was not in equilibrium in the melt but was produced by cosmic radiation subsequent to the rock formation. The decrease of the Fe3+ charge-transfer bands is accompanied by decrease in intensity of spin-allowed Fe2+ bands attributed toM 1 sites in the pyroxene in rock 12018. This decrease in the Fe2+ bands is attributed to decrease in the Fe2+ → Fe3+ charge-transfer intensification of these Fe2+ spin-allowed transitions when radiation-produced Fe3+ is partially-reduced by the low-temperature heating. The reaction of Fe3+ on heating to 200-225°C is probably Fe3+ + Ti3+ → Fe2+ + Ti4+. This is the reverse of the reaction caused by cosmic ray bombardment of the rock on the lunar surface. Possible tetrahedrally coordinated Fe3+ is present in the meteoritic and lunar augites as suggested by comparison of their spectra to that of terrestrial augite high in Fe3+. This would have been present in the original melt and is distinct from radiation produced Fe3+ in theM-sites. The polarized absorption spectra of single crystal pigeonite and augite from rock 12021 before heating, and augite from 12018 after heating are compared to that of meteoritic titanaugite in the Angra dos Reis meteorite and terrestrial titanaugite from Maui, Hawaii. The absorption spectrum of meteoritic hypersthene (Tatahouine) is also included for comparison to the pigeonite.