Colomeraite, NaTi3+Si2O6, a new clinopyroxene mineral from the Colomera iron meteorite

Abstract

Colomeraite (IMA 2021-061), with an end-member formula NaTi3+Si2O6, is a new Na pyroxene identified in the Colomera IIE iron meteorite. Colomeraite occurs with albite and K-feldspar in a silicate inclusion. The mean chemical composition of type colomeraite by electron probe microanalysis is (wt%) SiO2 54.75, Ti2O3 17.15, TiO2 5.58, NaO2 12.31, MgO 3.89, FeO 3.60, CaO 1.99, MnO 0.28, Al2O3 0.24, Cr2O3 0.14, K2O 0.02, total 99.95, giving rise to an empirical formula of (Na0.88Ca0.08Mg0.04)(Ti3+0.53Ti4+0.15Mg0.17Fe0.11Mn0.01Al0.01)Si2.01O6, with Ti3+ and Ti4+ partitioned, based on stoichiometry. Colomeraite has the C2/c diopside-type structure with a = 9.70(1) Å, b = 8.88(1) Å, c = 5.30(1) Å, β = 106.8(1)°, V = 437(2) Å3, and Z = 4, as revealed by electron back-scatter diffraction. The calculated density using the measured composition is 3.36 g/cm3. Colomeraite is a high-pressure and high-temperature Na-Ti3+-pyroxene, probably formed from an alkali plagioclase–Ti-rich phase melt via impact mixing of metal and silicates under extremely reducing conditions. The mineral is named after the host meteorite “Colomera”.