NATURAL KALSILITE, KALSIO4, WITH P31C SYMMETRY - CRYSTAL-STRUCTURE AND TWINNING

A new KAlSiO4 polymorph was found in a granulite facies gneiss from th e Punalur district, southern India. The structure was solved and refin ed on a twinned crystal to an R index value of 1.98% for 265 independe nt reflections. Metamorphic kalsilite is trigonal, space group P31c wi th a=5.157 (1) Angstrom c=8.706 (3) Angstrom, V=200.52 (9) Angstrom, Z =2. The overall diffraction symmetry 6/mmm exhibited from all the crys tals examined arises from a {000} twinning, related to a mistake in th e ordered Al-Si-Al-Si sequence along the c axis. The crystal structure is a stuffed derivate of tridymite, and is characterized by six-membe red tetrahedral rings with ditrigonal shape. Individual layers of this structure are the same as those of P6(3) kalsilite, but an stacked al ong the c axis in an eclipsed manner rather than in the staggered mann er of the P6(3) structure.