Abstract
The nuclear hyperfine doublets of 57Fe at the two nonequivalent octahedrally coordinated positions M1 and M2 in an olivine Mg 0.34Fe 0.66 SiO 4 from lunar basalt 12018 have been analyzed in the temperature range between 250 and 450°C. The Mg 2+, Fe 2+ is somewhat ordered, the Fe 2+ occupancy being greater at one of the two M positions by approximately 20 percent. After heating at 1155°C for seven days, the distribution was almost completely disordered. Studies of natural and heated olivines from terrestrial rocks show that the Mg 2+, Fe 2+ exchange reaction in olivines is considerably more sluggish than in chain silicates. Generally no change in site occupancy can be observed after heating at temperatures of approximately 900°C over several days. The observed degree of ordering in the lunar olivine is indicative of slow cooling during crystallization.
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