Odigitriaite, CsNa₅Ca₅[Si₁₄B₂O₃₈]F₂, 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-(Ce), 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/cm³. The indices of refraction are α = 1.502, β = 1.564, γ = 1.576; 2Vobs = 46(2)°, dispersion is weak r > v, and there is no pleochroism. The chemical composition is as follows (electron microprobe, H₂O calculated from structure): SiO₂ 55.30, Al₂O₃ 0.09, Y₂O₃ 0.44, MnO 0.94, FeO 0.10, PbO 0.21, K₂O 0.01 Cs₂O 8.36, B₂O₃ 4.75, H₂O 0.37, F 1.74, O = F₂ –0.74, total 99.43 wt.%. The empirical formula of odigitriaite is Cs_0.90Na_5.12Ca_4.68Mn_0.20Y_0.06Fe_0.02Pb_0.01[Si_13.92Al_0.03B_2.06O₃₈]F_1.39(OH)_0.62. The end-member formula is CsNa₅Ca₅[Si₁₄B₂O₃₈]F₂. The strong reflections in the powder X-ray diffraction pattern are: [(d, Å), (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) Å, β= 92.875(14)°, V = 3530.9(1.9) ų, Z = 4. The crystal structure of odigitriaite was solved by direct methods and refined to an R 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.