Abstract:
Burnsite, ideally KCdCu7O2(SeO3)(2)Cl-9, occurs in a fumarole in the North Breach of the great fissure Tolbachik volcano eruption (1975-1976), Kamchatka Peninsula, Russia. It occurs as dark red, anhedral, equidimensional grains. Associated minerals are cotunnite, sophiite, chloromenite, georgbokiite, ilinskite and an undefined Cu-Pb selenite. Burnsite is very rare and has only been found as several dozen grains that do not exceed 0.1 mm in maximum dimension. It is very similar megascopically to georgbokiite and the undefined Cu-Pb selenite, but differs from them by its red color. Burnsite has a strongly vitreous (metalloid) luster and a red streak. The mineral is brittle, opaque to translucent, with an uneven fracture. Cleavage is good on the (001) plane. Hardness VHN=12 kg/mm(2). The calculated density is 3.85 g/cm(2). It is nonfluorescent. It is uniaxial negative, E 1.912(5), O 1.920(5), with weak bireflectance and no pleochroism. Burnsite is hexagonal, space group P6(3)/mmc, a 8.7805(8), b 15.521(2) Angstrom, V 1036.3(2) Angstrom(3), Z=2. The strongest eight lines of the X-ray powder-diffraction pattern [d in Angstrom (I)(hkl)] are: 7.779(100)(002), 6.823(50)(101),4.391(60)(100), 3.814(80)(200), 3.066(70)(203), 2.582(50)(006), 2.501(60)(213) and 2.190(50)(220). Electron-microprobe analyses yield K2O 4.30, CuO 46.74,CdO 10.45, SeO2 19.91, Cl 25.46, less O=Cl-2 -5.75, total 101.11 wt. %. The empirical formula, derived from the crystal-structure analysis and electron-microprobe analyses, is K1.08Cd0.97Cu6.98O2.05 (Se1.07O3.21)(2)Cl-8.53, on the basis of O+Cl=17. The mineral name honors Professor Peter C. Burns, University of Notre Dame, Indiana, in recognition of his important contributions to structural mineralogy.
