Itsiite, ideally Ba2Ca(BSi2O7)2, is a new mineral from the Gun claim, just south of the Itsi Range, Yukon Territory, Canada. The new mineral occurs in low temperature, late-stage veins in direct association with cerchiaraite-(Fe), diopside, pyrite, quartz, sphalerite, and witherite. Itsiite occurs as colorless and light blue to medium greenish-blue tetragonal plates up to 1 mm across. The plates are flattened on {001} and exhibit the forms {001}, {101}, and {112}. The mineral is transparent, has a vitreous luster, and is non-fluorescent. It has a white streak and Mohs hardness of approximately 5½. It is brittle, with splintery fracture, and one perfect cleavage on {001}. The calculated density based upon the empirical formula and single-crystal unit cell is 3.644 g/cm3. The mineral is optically uniaxial (−), with ω = 1.623(1) and ε = 1.619(1) (white light). The mineral is nonpleochroic. Electron-microprobe compositions (average of 3) provided: Na2O 0.06, BaO 46.35, CaO 7.35, FeO 0.15, Al2O3 0.17, TiO2 0.06, SiO2 34.91, B2O3 10.41 (from structure), total 99.46 wt.%. The empirical formula (based on 14 O apfu) is Ba2.06(Ca0.89Al0.02Na0.01Fe0.01Ti0.01)∑0.94(Si3.96B2.04)∑6.00O14. Itsiite is tetragonal, I4¯2m, a 10.9515(5), c 10.3038(7) Å, V 1235.79(11) Å3, and Z = 4. The nine most intense lines in the X-ray powder diffraction pattern are [dobs in Å(I)(hkl)]: 5.50(42)(200); 3.746(100)(202); 3.446(60)(301); 3.100(51)(222); 2.899(96)(321,312); 2.279(44)(323); 2.145(69)(224,501); 1.8257(41)(503,334,305); 1.7584(43)(532,523). The crystal structure (R1 = 1.8% for 992 Fo > 4σF) is based upon a zeolite-like tetrahedral framework of corner-sharing tetrahedra consisting of four-membered silicate rings alternating with four-membered borate rings. The framework contains channels along each axis that host Ba2+ in ninefold coordination and Ca2+ in sixfold coordination. The structure is very similar to those of hyalotekite and kapitsaite-(Y).

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