Apollo 12 clinopyroxenes: Chemical trends

Abstract

Clinopyroxenes from Apollo 12 rocks 12021 and 12052 have been studied by single-crystal X-ray diffraction and electron microprobe techniques. Both rocks contain phenocrysts composed of pigeonite cores rimmed with sub-calcic augite in a groundmass including iron-rich pyroxenes. The phenocrysts have higher Al and Cr concentrations and higher Al/Ti ratios than the groundmass pyroxenes and pyroxenes of similar composition from Apollo 11 rocks. The chemical data suggest that two different coupled substitutions exist: Ti 4+(VI)+2Al(IV) for R 2+(VI)+2Si(IV) and Al(VI)−Al(IV) for R 2+(VI)+Si(IV). The latter couple is primarily responsible for the aluminous nature of the phenocrysts and may result from (1) crystallization at high pressures, (2) crystallization from a melt of low silica activity, or a combination of (1) and (2). Crystallization of the groundmass pyroxenes together with anorthite in the low pressure environment of the lunar surface results in a less aluminous clinopyroxene with the Ti+2Al couple predominating over the Al+Al couple. The Al accommodated in the pyroxene structure at low pressures in the presence of anorthite is essentially only that required to charge-compensate the octahedral Ti, i.e., Ti+2Al. Textural and chemical relationships indicate that clinopyroxene crystallization proceeded as follows: precipitation of skeleral crystals of low calcium, low iron pigeonite which became progressively enriched in CaO and FeO followed by the simultaneous crystallization of pigeonite and subcalcic augite and finally by the crystallization of augite trending towards pyroxferroite in composition.