Filatovite, ideally K[(Al,Zn)2(As,Si)2O8], has been found in products of fumarolic activity on the second cinder cone of the North breach of the great fissure Tolbachik eruption (1975–76, Kamchatka peninsula, Russia). The mineral occurs as prismatic crystals up to 0.3 mm and as intergrowths of crystals, in association with alumoklyuchevskite, lammerite, johillerite, sylvite, As-bearing orthoclase, hematite and tenorite. Filatovite is colourless, with vitreous luster and white streak. The mineral is brittle and transparent. It has a good {100} cleavage. Mohs' hardness is 5–6. The calculated density is 2.92 g/cm3. Biaxial, optically negative, α = 1.532(1), β = 1.535(1), γ = 1.537(1), 2Vmeas. = 60(10)°, 2Vcalc. = 78°. Optical orientation: X ∼ [100], Y and Z are normal to the {001} and {010} faces, respectively. No pleochroism has been observed. The mineral is monoclinic, space group I2/c, a = 8.772(1), b = 13.370(2), c = 14.690(2) Å, β = 115.944(6)°, V = 1549.1(4) Å3, Z = 8 (from single-crystal structure study). The diagnostic lines of the X-ray powder diffraction pattern are (I-d-hkl): 70-4.329-2̄02; 70-3.897-130; 100-3.364-2̄20, 204, 040; 50-3.300-004, 40-3.066-132, 60-2.981-042, 40-2.646-2̄42. Chemical composition (wt. %): P2O5 1.63, As2O5 40.60, SiO2 12.35, Al2O3 27.33, CuO 0.83, ZnO 3.85, FeO 0.28, Na2O 0.63, K2O 12.85, total 100.35. The empirical formula of filatovite, calculated on the basis of O = 8, is (K0.92Na0.07)∑0.99 [(Al1.81Zn0.16Cu0.04Fe0.01)∑=2.02(As1.20Si0.70P0.08)∑=1.98O8]. The simplified formula is K[(Al,Zn)2(As,Si)2O8]. The detailed chemical formula can be written as K[(Al2-xZnx)(As1+xSi1-x)O8], with × ∼ 0.20. Filatovite is a new mineral of the feldspar group and is structurally related to celsian. The name honors Prof. Stanislav K. Filatov (b. 1940), Department of Crystallography, St. Petersburg State University, St. Petersburg, Russia, for his important contributions to high temperature crystal chemistry and crystal chemistry of exhalation minerals.

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