Interpretation of lunar alpha-scattering data

Many ambiguities are inherent in the interpretation of chemical analyses if such analyses are used alone to interpret a rock genetically. Furthermore, rocks at the surface of the moon may differ from rocks below the surface because of contamination by meteoritic infall, bombardment by charged particles, and extensive mixing by impact. Nevertheless, the broad categories of igneous rocks and meteorites which are familiar on the earth are the best compositional models for rocks on the moon. Anhydrous igneous rocks and meteorites may be categorized on the basis of the atomic percentage of oxygen: (1) below 50%, sputtered rocks and those with free metal in excess of that in chondrites; (2) 50 to 56%, chondritic meteorites; (3) 57 to 58½%, dunites and olivine-rich peridotites; (4) 58½ to 60%, peridotites and pyroxenites; (5) 60 to 61%, basalts, some alkalic derivatives; (6) 61 to 64%, siliceous derivatives, some alkalic rocks. The Si/Na ratio is another reliable chemical parameter on which to base further subdivisions of genetic importance; terrestrial anhydrous igneous rocks and meteorites occupy well-defined fields in a plot of oxygen percentage versus Si/Na. The best parameter for resolving ambiguities in such a plot is the Mg/Si ratio. The concurrence of high Mg/Si and Mg/Na ratios with oxygen contents of over 59 to 60% will indicate water in the sample. The amount of water can be estimated by balancing the cations and anions in the actual analysis.