Abstract:
Allochalcoselite, Cu+Cu52+РbО2(SeО3)2Сl5, is a new mineral found in fumaroles of the second cinder cone at northern breach of the great fissure Tolbachik eruption (GFTE) (1975-1976) on Kamchatka peninsula, Russia. Associated minerals: cotunnite, sophiite, ilinskite, georgbokiite and burnsite. Allochalcoselite occurs as a small amount of prismatic crystals, up to 0.1 mm in length. Color dark-brown, streak brown; adamantine luster; transparent; non-fluorescent. Hardness on the Mohs scale: 3-4. Brittle, with perfect {100} cleavage. Density (calc.)= 4.653 g/cm3. Biaxial (+), α= 2.01(1), β= 2.01(1), γ= 2.08(1) (589 nm). 2V (meas.) = 50(15)°, 2V (cal.) = 68(10)°. Optical orientation: X= c, Y = b. Pleochroism: X- light-brown, Y- color not determined, Z- reddish-brown. Empirical formula of the mineral: Cu1+1.00Cu2+4.92Zn0.02 Pb1.07Se1.99O7.99Cl5.01. The simplified formula is Cu+Cu52+РbО2(SeО3)2Сl5. Single-crystal X-ray studies were carried out with a 3-circlc CCD diffrac-tometer and gave the following data: monoclinic, space group C2/m, a= 18.468(2), b= 6.1475(8), c = 15.314(2)Å, β = 119. 284(2)°, V= 1516.4(3) Å3, Z= 4. The crystal structure belongs to a new, previously unknown, structural type of inorganic compounds. There arc five Cu2+ sites in the structure that are coordinated by various combinations of O2- and Cl- anions. The structure contains also two Cu+ sites having linear coordination by two Cl- anions each, thus forming linear CuC-2 anions. There are one Pb and two Se sites; the Se4+ cations are coordinated by three O2- anions each and form the SeO2-3 selenite groups. The structure is based upon chains of apexsbound OCu6+4 and ОCu3РЬ6+ oxocentered tetrahedra running along the b axis. These chains arc linked via SeO2-3 groups into layers parallel to (100). Cl- and CuCl-2 anions arc located between the layers. The mineral is named after its chemical composition reflecting different oxidation states of copper and presence of Se: αλ- λος - «different» (Greek) + χαλχος; - «copper» (Greek) + selenium.