Crystal structure of a new mineral turkestanite: A calcium analogoue of steacyite

The crystal structure of a new mineral turkestanite Th(Ca,Na)2(K1-z□z)[Si8O20], z=0.47 (Dzhelisu massif, Alayskiy range, Tien Shan)--a calcium analogoue of steacyite--determined by the Rietveld method (ADP-2 diffractometer: λCuKα: Ni filter; 8.00 deg. <2θ<140.00 deg.; 2θ step, 0.02 deg.; count time per step, 10 s; 850 Bragg reflections). All calculations are performed with the WYRIET program (version 3.3) in space group P4/mcc. The unit-cell parameters are a=7.5792(1) and c=14.7042(2) A, and V=844.68(1) A3. The structure of turkestanite is refined in the anisotropic-isotropic approximation (the Pearson function) to Rp=3.16, Rwp=4.04, RB=2.18, RF=1.90, s=1.41, DWD=1.10, and σx=1.839. The turkestanite sample contains thorite ThSiO4 as a second phase; 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 of four-membered rings formed by Si-O-tetrahedra. The predominance of Ca+ in the B position in distinction to Na+ in steacyite Th(Na,Ca)2(K1-z□z)[Si8O20] (z=0.40) makes possible distinguishing turkestanite as an original mineral species