THE CRYSTAL STRUCTURE OF CALCIUM CATAPLEIITE

Abstract

Calcium catapleiite, CaZrSi3O9•2H2O, the calcium-dominant analogue of catapleiite, Na2ZrSi3O9•2H2O, firstly described as hexagonal, space group P63/mmc, with a 7.32, c 10.15 A, is actually orthorhombic, space group Pbnn, a 7.378(1), b 12.779(1), c 10.096(1) A, V 951.89 A 3 and Z = 4. The crystals display three-fold twinning (“Drilling”) about the c axis, with the three individuals rotated 120° one from the other. The crystal structure of calcium catapleiite from the type locality, the Burpala massif, northern Baikal Region, Siberia, Russia, has been determined by direct methods from single-crystal X-ray-diffraction data and refined to a final R = 0.0528 (wR 2 = 0.1604) for 2167 reflections with Fo > 6� (Fo). It is characterized by a mixed framework of regular [ZrO6] octahedra and three-membered silicate rings [Si3O9], very similar to the framework of catapleiite. The H2O molecules also are similarly placed in the two minerals. The Ca 2+ cations in calcium catapleiite are distributed over two distinct sites, with 0.8 and 0.2 occupancies, which contrasts with the fully disordered distribution of the Na + cations in catapleiite.