On lithium and sodium trivalent-metal pyroxenes and crystal-field effects

Abstract

SummaryOf the metals in the series Sc, V, Cr, Mn, Fe, and Ga pyroxenes of the type LiM3+Si2O6 were obtained only from Sc, V, Cr, and Fe in dry synthesis experiments near 1000 °C. Ga and A1 gave keatite derivatives (so-called ‘β-spodumenes’) by dry heating; α-Ga-eucryptite was obtained from Ga hydrothermally at pressures between 4·5 and 6·5 kb. Mn gave Mn2O3. The lattice parameters of the pyroxenes vary irregularly with atomic number. This variation and the failure to produce a Mn3+-pyroxene can be explained by the crystal-field theory. At high temperatures A1 and Ga are too small to stabilize a silicate pyroxene phase even at moderately high pressures and form less dense tectosilicates. The octahedral stabilization energy for Cr3+ ensures that Cr, which has the same size as Ga, forms a pyroxene.