The mineralogy and petrology of the Luna 20 soil sample

Fragments of igneous rocks, glasses and minerals comprise 25 per cent of the studied sample of the Luna 20 soil. Basalt fragments in the Luna 20 soil are similar to basalts from the mare regions of the Moon—in that they are characterized by the presence of iron-rich olivines and pyroxenes. On the basis of the FeO contents of plagioclases, it appears possible to distinguish between the plagioclase of the mare and highland regions of the Moon. Other igneous rock fragments are anorthosite, gabbroic anorthosite and anorthositic gabbro. The most abundant rock type (75 per cent of the sample) is microbreceia. One third of the fragments of microbreccia have undergone thermal metamorphism resulting in the homogenization of phases and the development of poikioblastic and hornfelsic textures. Excluding the basalt fragments, the dominant minerals in the Luna 20 soil are anorthite (An_93–98), magnesium-rich orthopyroxenes, intermediate clinopyroxenes and olivine (< Fa₅₀). Chemically, the Luna 20 and Apollo 16 soil samples are similar, but the Luna 20 soil is slightly depleted in aluminum and calcium and enriched in iron and magnesium relative to the Apollo 16 soils. The slight difference in bulk chemistry of the two soils may be a result of the presence of a minor amount of mare material in the Luna 20 soil and its apparent absence in the Apollo 16 soils.