Mössbauer studies of Luna 16 and 20 lunar soils
Samples of lunar soil returned by Luna 16 from Mare Fecunditatis and by Luna 20 from the highlands near the crater Appolonius G have been studied by 57 Fe Mössbauer spectroscopy. This enables the distribution of the total iron content between the various mineral phases to be determined. No troilite, magnetite or Fe 3+ was detected and the suggestion that ulvospinel (Fe 2 TiO 4 ) is present in substantial amounts in the fine-particle separates is discounted. The spectra were shown to arise from a metallic iron phase, and from Fe 2+ in olivine, in the M l and M2 sites of pyroxene, in ilmenite and in glasses. The slight asymmetry of the spectra is ascribed to superpara-magnetic iron in the finest particles and it is considered that this is formed by solar-wind reduction. The ilmenite content of the Luna 16 soil is higher than for Apollo 12, 14 and 15 soils but less than for Apollo 11 soil; the ilmenite content of the Luna 20 soil is exceptionally low. The soils from both Luna missions show an increased amount of iron in olivine and M1 pyroxene sites comparable with that found for Apollo 15 soils from the edge of Hadley Rille. The narrower linewidths in the Mössbauer spectra of the Luna 20 soil suggest that the proportion of iron-containing glassy phases in this sample is lower than for the Luna 16 soil.