Luna 20 soil: abundance and composition of phases in the 45–125 micron fraction

Abstract

Glass compositions in the Luna 20 soil indicate a minor contribution of mare rocks and a major contribution of highly feldspathic highland material. Glasses with the composition of Highland basalt (anorthositic gabbro or norite) predominate in a range of highly aluminous glasses. The analyses of minerals in the soil show that the highland rocks have a unique assemblage of minerals that can readily be distinguished from the mineral assemblages of either mare or KREEP basalts. The soils are characterized by abundant anorthitic (An 92–99), low-Fe plagioclase. Highly magnesian orthopyroxenes, pigeonites and augites are the most prominent pyroxenes. Unlike mare basalt pyroxenes, clinopyroxenes with intermediate Ca values are not abundant, but extreme iron enrichment towards pyroxferroite does occur. Olivines are more abundant than at other sites and are Mg-rich, low in Ca and Cr. Spinels with compositions approaching MgAl 2O 4 predominate over pleonastes and chromites. Ilmenite and metal are present but not abundant. These data establish the unique nature of the minerals in the highland soils. The mineral compositions are consistent with derivation from a suite of highly feldspathic rocks in which Highland basalt compositions predominate. Some of the mineral data, particularly from the pyroxenes, are suggestive of surface or near-surface processes, rather than plutonic crystallization.