KIRCHHOFFITE, CsBSi2O6, A NEW MINERAL SPECIES FROM THE DARAI-PIOZ ALKALINE MASSIF, TAJIKISTAN: DESCRIPTION AND CRYSTAL STRUCTURE

Abstract

Kirchhoffite, CsBSi 2 O 6 , is a new species of silicate mineral from the Darai-Pioz alkaline massif in the upper reaches of the Darai-Pioz River, in the area of the joint Turkestansky, Zeravshansky and Alaisky ridges, Tajikistan. It occurs as equant grains 10 to 80 μm in diameter, associated with quartz, pectolite, baratovite, polylithionite, aegirine, fluorite, leucosphenite, pyrochlore, neptunite and reedmergnerite. Kirchhoffite is colorless, transparent with a white streak, and a vitreous luster; it does not fluoresce under ultraviolet light. No cleavage or parting was observed. The Mohs hardness is 6 to 6½, and it is brittle with a conchoidal fracture. The measured and calculated densities are 3.62(2) and 3.639 g/cm 3 , respectively. Kirchhoffite is uniaxial (+) with indices of refraction ω = 1.592(2), ɛ = 1.600(2). The mineral is tetragonal, space group I 4 1 / acd , a 13.019(2), c 12.900(3) A, V 2186.3(1) A 3 , Z = 16, a : c = 1 : 0.9909. The six strongest lines in the X-ray powder diffraction pattern are [ d in A( I )( hkl )] are 3.26(100) (040), 3.48(82)(132), 2.770(67)(332, 233), 2.294(41)(044), 2.109(34)(352, 253), 5.32(32)(121). Chemical analysis by electron and ion microprobes gave SiO 2 40.47, B 2 O 3 11.27, K 2 O 0.11, Cs 2 O 48.16, Rb 2 O 0.09, for a sum of 100.10 wt.%. The resulting empirical formula on the basis of six atoms of oxygen is (Cs 1.02 K 0.01 ) ∑1.03 B 0.96 Si 2.02 O 6 , ideally CsBSi 2 O 6 . The crystal structure of kirchhoffite was refined to an R 1 index of 3.1% based on 487 observed reflections collected on a four-circle diffractometer with Mo K α X-radiation. In the structure, there are two tetrahedrally coordinated sites: Si is occupied by silicon, Si –O> = 1.610 A, B is occupied by boron, B –O> = 1.465 A, and Cs is [12]-coordinated, = 3.301 A. Tetrahedra form a [BSi 2 O 6 ] framework, the topology of which is identical to that of tetragonal pollucite, CsAlSi 2 O 6 . The [12]-coordinated Cs atoms occupy the channels along [111] formed by six-membered rings of tetrahedra. Kirchhoffite, CsBSi 2 O 6 , is a B analogue of the tetragonal modification of pollucite.