MINERALOGY,PETROLOGY AND CHEMISTRY OF LUNAR ROCK 12039

Rock 12039 belongs to the olivine-depleted group of magmatic rocks characterized by normative and modal SiO₂, absence or very low abundance of olivine, and high FeO/(FeO + MgO), Ti/Cr, and CaO/MgO ratios. Clinopyroxenes in this rock show a complex, essentially continuous, compositional zonation from augite cores through ferroaugite to ferrohedenbergite with an abrupt discontinuity at the pyroxferroite contact and, thus, are different from pyroxene in most other Apollo 12 rocks. Two grains contain thin subcalcic pigeonite zones. Texture, presence of very fine (< 1 μm) exsolution lamallae, and pyroxene zoning indicate a relatively rapid cooling history and pronounced in situ chemical fractionation. Rock 12039, on the basis of mineralogy and bulk composition, is the most highly differentiated member of the olivine-depleted basalt group