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

A (21)-clinopyribole with the composition K1.10Na2.32Ca1.52Mg5.85Al1.23Si12 .04O34(OH)(2) has been synthesized at 10 GPa and 1250 degreesC in a multi-a nvil apparatus. The unit-cell parameters are a = 9.8390(9), b = 26.6471(6), c = 5.2665(5) Angstrom, beta = 106.25(5)degrees, and V = 1325.6(4) Angstro m (3). The structure (space group A2/m) consists of an alternating arrangem ent of single- and double-chain silicate slabs along the b axis, with a + + + configuration. This phase possesses all the features predicted by Vebl en and Burnham (1978b) for a mixed-chain silicate intermediate between pyro xenes and amphiboles. The single-chain portion of the structure corresponds to a clinopyroxene with the omphacite composition Di(55)Jd(45), whereas th e double chain portion is essentially a potassic richterite. The MS2 cation in the single-chain portion occupies a coordination environment that is si milar 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. Th e observed unit-cell volume is 1.5% smaller than the equivalent mixture of Di(55)Jd(45) + richterite, accounting, in part, for its hi-h-pressure stabi lity relative to its pyroxene and amphibole components.