Kurilite – Ag₈Te₃Se – a new mineral from the Prasolovskoe deposit, Kuril islands, Russian Federation

Abstract

Kurilite, with the simplified formula, Ag8Te3Se, is a new mineral from the Prasolovskoe epithermal Au-Ag deposit, Kunashir Island, Kuril arc, Russian Federation. It occurs as aggregates up to 2 mm in size, composed of brittle xenomorphic grains, up to several μm in size, in quartz, associated with tetrahedrite, hessite, sylvanite and petzite. Kurilite is opaque, grey, with a metallic lustre and a black streak. Under plane-polarized light, kurilite is white with no observed bireflectance, cleavage, or parting observed. Under crossed polars it appears isotropic without internal reflections. Reflectance values in air and in oil, are tabulated. It has a mean VHN (25 g load) of 99.9 kg/mm2 which equates roughly to a Mohs hardness of 3. Electron microprobe analyses yield a mean composition of Ag 63.71, Au 0.29, Te 29.48, Se 5.04, S 0.07, total 98.71 wt.%. The empirical formula (based on 12 atoms) is (Ag7.97Au0.02)Σ7.99Te3.00(Se0.86Te0.12S0.03)Σ1.01. The calculated density is 7.799 g/cm3 (based on the empirical formula and unit-cell parameters refined from single-crystal data). Kurilite is rhombohedral, R3 or , a 15.80(1), c 19.57(6) Å, V 4231(12)Å3, c:a 1.2386, Z = 15. Its crystal structure remains unsolved. The seven strongest lines of the X-ray powder-diffraction pattern [d in Å (I)(hkl)] are: 3.727(20)(131), 2.996(50)(232), 2.510(30)(226,422), 2.201(100)(128,416,342), 2.152(20)(603), 2.079(30)(253), 2.046(20)(336,434). The mineral is named after the locality.

TWINNING AND DISORDER IN THE CRYSTAL STRUCTURE OF KURILITE, Ag8Te3Se: TOWARD A NEW FAST IONIC CONDUCTOR?

“…The crystal structure is composed of three Te sites, three Se sites (Se3 with 50% occupancy; Table 2) and 10 Ag sites (Ag7 with 50% occupancy; Table 2). Results indicate that Z = 18, in contrast to Kovalenker et al (2010) who reported Z = 15. As such, the computed density should be 9.318 g/cm 3 rather than the value of 7.799 g/cm 3 previously reported.…”

“…The mineral occurs as aggregates up to 2 mm in size, composed of micron-sized brittle xenomorphic grains within quartz, occasionally as overgrowths on tennantitetetrahedrite-goldfieldite group minerals, and rarely as intergrowths with native gold, hessite, sylvanite and petzite. Additional details are given by Kovalenker et al (2010).…”

From Chalcogenides to Polychalcogenidehalides – First Identification in Mineral Samples

“…Quite recently this mineral was finally described as kurilite with a rhombohedral lattice but the crystal structure was not solved. [4] Kurilite is similar to hessite in structure and in chemical composition and to other silver chalcogenides, but not isostructural and seems to be the only trigonal mineral in this family.…”