Urusovite, Cu[AlAsO (sub 5) ], a new mineral from the Tolbachik volcano, Kamchatka, Russia

Urusovite, ideally Cu[AlAsO (sub 5) ], has been found in a fumarole in the North Breach of the great fissure Tolbachik eruption (1975-76, Kamchatka peninsula, Russia). It occurs as light-green plates up to 0.4 mm in maximal dimension, tabular on {100}, elongated along [001]; well-developed forms are {100}, {010}, {011}, {111}. Associated minerals are: ponomarevite, piypite, sylvite, dolerophanite, euchlorine, tenorite, hematite and two unknown As-bearing minerals. Urusovite is brittle and has a perfect cleavage on (100), a vitreous luster and a white streak. H = 378 kg/mm (super 2) . Biaxial, optically negative. alpha = 1.672(2), beta = 1.718(2), gamma = 1.722(2), 2V (sub meas) approximately 30(2) degrees , 2V (sub calc) = 32.2 degrees . Optical orientation is b = beta , c approximately alpha . Pleochroism is in the light-green tones: alpha - colourless, beta - light-green, gamma - light-green. The mineral is monoclinic, space group P2 (sub 1) /c, a = 7.314(2), b = 10.223(3), c = 5.576(2) Aa, beta = 99.79(3) degrees , V = 410.9(2) Aa (super 3) . The diagnostic lines of the X-ray powder diffraction pattern are (I-d-hkl): 100-7.20-100; 9-4.844-011; 23-4.327-111; 10-3.604-200; 10-3.174-121; 20-3.125-211; 6-2.656-012; 8-2.458-221. Urusovite is isostructural with Fe (super 2+) [AlPO (sub 5) ]. Microprobe analysis gave the following chemical composition (wt. %): CuO 32.23 (30.97-32.82), Al (sub 2) O (sub 3) 20.89 (20.44-21.44), Fe (sub 2) O (sub 3) 0.32 (0.10-0.72), ZnO 0.25 (0.10-0.43), As (sub 2) O (sub 5) 46.02 (44.97-47.17), V (sub 2) O (sub 5) 0.12 (0.00-0.40), Sigma 99.83 (98.71-100.64). The empirical formula of urusovite, (Cu (sub 0.99) Zn (sub 0.01) ) (sub Sigma 1.00) Al (sub 1.00) As (sub 0.98) O (sub 5.00) , calculated on the basis of 5 O atoms, is close to the ideal one, Cu[AlAsO (sub 5) ], confirmed by crystal-structure analysis. The mineral is named urusovite in honour of Vadim Sergeevich Urusov (1936-), crystal chemist, Corresponding Member of the Russian Academy of Sciences, chair of the Department of Crystallography and Crystal Chemistry of Moscow State University.