Odigitriaite, CsNa (sub 5) Ca (sub 5) [Si (sub 14) B (sub 2) O (sub 38) ]F (sub 2) ; a new caesium borosilicate mineral from the Darai-Pioz alkaline massif, Tajikistan; description and crystal structure

Odigitriaite, a new Cs,Na,Ca borosilicate mineral, was discovered in moraine adjacent to the Darai-Pioz alkaline massif in the upper reaches of the Darai-Pioz river at the intersection of the Turkestansky, Zeravshansky and Alaisky mountain ridges, Tajikistan. It occurs as irregular thin flakes associated with quartz, pectolite, baratovite, fluorite, pekovite, polylithionite, aegirine, leucosphenite, pyrochlore, neptunite, reedmergnerite, mendeleevite-(Se), zeravshanite and sokolovaite. It is colourless with a white streak, is translucent and has a vitreous lustre; it does not fluoresce under ultraviolet light. Odigitriaite is brittle with an uneven fracture and a Mohs hardness of 5. The calculated density is 2.80(2) g/cm (super 3) The indices of refraction are alpha = 1.502, beta = 1.564, gamma = 1.576; 2Vobs = 46(2) degrees , dispersion is weak r > v, and there is no pleochroism. The chemical composition is as follows (electron microprobe, H (sub 2) O calculated from structure): SiO (sub 2) 55.30, Al (sub 2) O (sub 3) 0.09, Y (sub 2) O (sub 3) 0.44, MnO 0.94, FeO 0.10, PbO 0.21, K (sub 2) O 0.01 Cs (sub 2) O 8.36, B (sub 2) O (sub 3) 4.75, H (sub 2) O 0.37, F 1.74, O = F (sub 2) -0.74, total 99.43 wt.%. The empirical formula of odigitriaite is Cs (sub 0.90) Na (sub 5.12) Ca (sub 4.68) Mn (sub 0.20) Y (sub 0.06) Fe (sub 0.02) Pb (sub 0.01) [Si (sub 13.9 2) Al (sub 0.03) B (sub 2.06) O (sub 38) ]F (sub 1.39) (OH) (sub 0.62) The end-member formula is CsNa (sub 5) Ca (sub 5) [Si (sub 14) B (sub 2) O (sub 38) ]F (sub 2) The strong reflections in the powder X-ray diffraction pattern are: [(d, A), (I, %), (hkl)]: 5.45 (25) (1 1 3), 4.66 (33) (3 1 1), 4.40 (26) (0 2 2), 4.10 (36) (3 1 3), 3.95 (25) (3 1 3), 2.85 (31) (2 2 2), 2.68 (40) (0 0 6), 3.62 (45) (0 2 4), 3.35 (100) (2 2 4), 3.31 (30) (3 1 5), 3.25 (35) (4 0 4), 3.04 (60) (4 2 2), 2.925 (22) (4 2 3), 1.813 (23) (9 1 0). Odigitriaite is monoclinic, space group C2/c, a = 16.652(5), b = 9.598(3), c = 22.120(7) A, beta = 92.875(14) degrees , V = 3530.9(1.9) A (super 3) , Z = 4. The crystal structure of odigitriaite was solved by direct methods and refined to an R (sub 1) value of 2.75% based on single-crystal X-ray data. It is a double-layer sheet-borosilicate mineral; Cs and Na are intercalated within the double-layer sheet, and the double layers are linked by interstitial Ca and Na atoms.