CRYSTAL-STRUCTURE OF A NEW MINERAL TURKESTANITE - A CALCIUM ANALOG OFSTEACYITE

The crystal structure of a new mineral turkestanite Th(Ca,Na)(2)(K(1-z )square(z))[Si8O20], z = 0.47 (Dzhelisu massif, Alayskiy range, Tien S han)--a calcium analogue of steacyite-determined by the Rietveld metho d (ADP-2 diffractometer; lambda CuKalpha; Ni filter; 8.00 degrees < 2 theta < 140.00 degrees; 2 theta step, 0.02 degrees; count time per ste p, 10 s; 850 Bragg reflections). All calculations are performed with t he WYRIET program (version 3,3) in space group P4/mcc. The unit-cell p arameters are a = 7.5792(1) and c = 14.7042(2) Angstrom, and V= 844.68 (1) Angstrom(3). The structure of turkestanite is refined in the aniso tropic-isotropic approximation (the Pearson function) to R-p = 3.16, R -wp = 4.04, R-B = 2.18, R-F = 1.90, s = 1.41, DWD = 1.10, and sigma(x) = 1.839. The turkestanite sample contains thorite ThSiO4 as a second p hase; quantitative analyses show that the thorite content is 1.34 wt % . Turkestanite is the third mineral (along with steacyite and iraqite) that contains a discrete [Si8O20] radical, which consists of a pair o f four-membered rings formed by Si-O-tetrahedra. The predominance of C a2+ in the B position in distinction to Na+ in steacyite Th(Na,Ca)(2)( K(1-z)square(z))[Si8O20] (Z = 0.40) makes possible distinguishing turk estanite as an original mineral species.