Modal analyses of lunar soils by quantitative X-ray diffraction analysis

Abstract

We used X-Ray Diffraction (XRD) and Rietveld refinement to determine the modal mineralogy of 118 lunar regolith samples (<150 µm size fraction) from all landed Apollo missions. Data were calibrated with mineral mixtures and compared to results based on an X-ray digital imaging procedure for six soils obtained by the Lunar Soil Characterization Consortium. Agreement between XRD and digital imaging for all minerals detectable in the six soils is excellent (R2 = 0.953). XRD-based ternary plots (plagioclase-total pyroxene-olivine) vary from plagioclase-dominated (highlands as represented by Apollo 16 samples) to substantial mafic abundances at the mare sites. Olivine varies in relative abundance, with the Apollo 17 mare sites having the largest abundances. Olivine reaches 20 wt% at Apollo 17, but is a minor component at Apollo 14. The results agree with trends in mineral abundances obtained from reflectance spectroscopy for the Apollo sites. In a global context, however, the spectral data display a trend of increasing olivine at roughly constant pyroxene/plagioclase, reaching values of 40% olivine in the plagioclase-pyroxene-olivine ternary plot (e.g., Eratosthenian flows in Procellarum), indicating the presence of significant volumes of olivine-rich rock types on unsampled regions of the lunar surface.