ZINCIAN ILMENITE ECANDREWSITE FROM A PELITIC SCHIST, DEATH-VALLEY, CALIFORNIA, AND THE PARAGENESIS OF (ZN,FE)TIO3 SOLID-SOLUTION IN METAMORPHIC ROCKS

Two compositionally and petrographically distinct populations of ilmen ite-ecandrewsite solid solution, FeTiO3 - ZnTiO3, coexist in a kyanite -bearing schist from the Black Mountains, Death Valley, California. Th e first population is extremely zinc-rich and has the stoichiometric f ormula (Zn0.17-0.81Fe0.18-0.64Mn0.01)TiO3. Single grains span the enti re compositional range. The mineral is orange-brown in transmitted lig ht, and occurs both as inclusions in garnet and as an abundant phase ( approximately 3-5 modal %) in the matrix. The second population is opa que in transmitted light, occurs exclusively as a matrix phase, and ha s the stoichiometric formula (Zn0-0.12Fe0.85-0.98Mn0.01-0.02)TiO3. Sin gle grains are generally homogeneous, but there is compositional varia tion among grains. The paragenesis of zinc-bearing ilmenite solid-solu tion in metapelitic rocks is problematic. Thermodynamic calculations a nd comparison with other reported occurrences indicate that ilmenite w ith greater than a few mol% ZnTiO3 component in metapelitic rocks shou ld be metastable relative to gahnite + quartz + rutile over the range of geologically relevant conditions of regional metamorphism.