Nicksobolevite, Cu (sub 7) (SeO (sub 3) ) (sub 2) O (sub 2) Cl (sub 6) , a new complex copper oxoselenite chloride from Tolbachik fumaroles, Kamchatka Peninsula, Russia

Nicksobolevite, ideally Cu (sub 7) (SeO (sub 3) ) (sub 2) O (sub 2) Cl (sub 6) , was found in a fumarole of the second cinder cone of the North Breach of the Great Fissure Tolbachik volcano eruption (1975-1976), Kamchatka Peninsula, Russia. The mineral occurs as aggregates of red needle-like crystals up to 0.4 mm in maximal dimension, elongated along [001]. Associated minerals are chloromenite, prewittite, melanothallite, sophiite, ralstonite, ponomarevite, and native gold. Nicksobolevite is monoclinic, P2 (sub 1) /c, a = 10.958(9), b = 14.483(5), c = 10.494(14) Aa, beta = 113.61(7) degrees , V = 1526(3) Aa (super 3) , Z = 4 (from powder diffraction data). The eight strongest lines of the X-ray powder diffraction pattern are (I-d-hkl): 77-8.25-(110); 100-5.877-(120); 26-4.239-(112); 37-3.619-(040); 95-3.257-(310), (321); 50-2.715-(402); 26-2.668-(033), (411); and 40-2.278-(242), 134). Nicksobolevite is dark red, with vitreous luster and orange-red streak. The mineral is very brittle and transparent. Cleavage is perfect on {010} and {-101} and good on {120}. The Mohs hardness measured by microindentation is 2-2 1/2. The calculated density is 4.18 g/cm (super 3) (based on the empirical chemical formula). Nicksobolevite is optically positive, with alpha = 2.00(1), beta = 2.01(1), gamma = 2.08(1), 2V (sub calc.) = 43 degrees , and the orientation is X = b, Z^c = 36 degrees (in obtuse beta ). Pleochroism: X, Y = red, Z = brownish red. The chemical composition determined by the electron-microprobe analysis is (wt. %): CuO 56.17, ZnO 2.34, SeO (sub 2) 23.29, Cl 22.69, O = Cl -5.13, total 99.36. The empirical formula of nicksobolevite, calculated on the basis of 14 anions per formula unit, is (Cu (sub 6.71) Zn (sub 0.27) ) (sub Sigma 6.98) Se (sub 1.99) O (sub 7.92) Cl (sub 6.08) . The simplified formula is Cu (sub 7) (SeO (sub 3) ) (sub 2) O (sub 2) Cl (sub 6) , which requires CuO 59.02, SeO (sub 2) 23.52, Cl 22.54, O = Cl -5.09, total 100.00 wt%. The crystal structure was solved by direct methods and refined to an agreement index R (sub 1) = 0.075 on the basis of 3590 independent observed reflections. The structure contains eight symmetrically independent Cu (super 2+) positions with coordination numbers varying from 5 to 6. The crystal structure is based upon [O (sub 4) Cu (sub 13) ] tetramers consisting of four corner-sharing OCu (sub 4) tetrahedra. The tetrahedra are surrounded by the Se1O (sub 3) and Se2O (sub 3) selenite triangular pyramids to form linear {[O (sub 4) Cu (sub 13) ](SeO (sub 3) ) (sub 4) } complexes oriented parallel to [210] and [-210] and linked to each other into layers with a ladder-like configuration. The layers are perpendicular to the a axis and are surrounded by Cl (super -) anions, which provide their linkage in the [100] direction. The mineral is named in honor of Academician Nikolay (Nick) Vladimirovich Sobolev (b. 1935), in recognition of his important contributions to mineralogy and petrology. Nicksobolevite is the most structurally complex and Cl-rich copper selenite chloride known so far.