Chemistry and structure of lunar and synthetic armalcolite

A study of the chemical trends displayed by lunar armalcolites has been made in conjunction with single-crystal X-ray structure refinements of lunar and synthetic armalcolite in order to assess the possible importance of Ti 3+ in lunar armalcolite and to characterize the effects of cation substitutions on the structure. The apparent cation deficiencies found in lunar armalcolites analyzed with the electron microprobe most likely reflect the presence of Ti 3+, although the existence of vacancies cannot be ruled out. Structure refinements of an Apollo 17 armalcolite are consistent with either interpretation. These results support experimental evidence suggesting the presence of Ti 3+ in armalcolite and indicate that virtually all lunar armalcolites probably contain ∼4–11 mol.% Ti 2 3+Ti 4+O 5 component in solid solution. The cation distribution in lunar armalcolite is essentially completely ordered. However, synthetic crystals quenched from near 1200°C have been found to retain significant cation disorder.