Crystal structure of a new (21)-clinopyribole synthesized at high temperature and pressure

Abstract

A (21)-clinopyribole with the composition K 1.10 Na 2.32 Ca 1.52 Mg 5.85 Al 1.23 Si 12.04 O 34 (OH) 2 has been syn­thesized at 10 GPa and 1250 °C in a multi-anvil apparatus. The unit-cell parameters are a = 9.8390(9), b = 26.6471(6), c = 5.2665(5) Å, β = 106.25(5)°, and V = 1325.6(4) Å 3 . The structure (space group A2/m) consists of an alternating arrangement of single- and double-chain silicate slabs along the b axis, with a + + + + configuration. This phase possesses all the features predicted by Veblen and Burnham (1978b) for a mixed-chain silicate intermediate between pyroxenes and amphiboles. The single-chain portion of the structure corresponds to a clinopyroxene with the omphacite composition Di 55 Jd 45 , whereas the double chain portion is essentially a potassic richterite. The MS2 cation in the single-chain portion occupies a coordination environment that is similar to the M4 site in richterite, whereas the MD4 cation coordination in the double-chain portion is comparable to the M2 site in C2/c omphacite. The observed unit-cell volume is 1.5% smaller than the equivalent mixture of Di 55 Jd 45 + richterite, accounting, in part, for its high-pressure stability relative to its pyroxene and amphibole components.