Compositional Zoning and Its Significance in Pyroxenes from Three Coarse-Grained Lunar Samples

Abstract

The calcium-rich pyroxenes in lunar samples 10047, 10058, and 10062 show pronounced sectoral and radial compositional variations which correlate with sharp to gradual variations in color and optical properties. The pyroxenes apparently grew as nearly euhedral crystals from melts of approximately the same composition as that of the samples. The coupled substitutions determined across sector boundaries suggest that Al is predominantly in the tetrahedral site and that Ti is predominantly quadrivalent. The pyroxene differentiation trend (unknown in terrestrial pyroxenes) is toward extreme enrichment in the ferrosilite molecule. The iron-enriched portions of the pyroxene grains may have grown with a triclinic pyroxenoid structure.