VASILYEVITE, (Hg2)102+O6I3Br2Cl(CO3), A NEW MINERAL SPECIES FROM THE CLEAR CREEK CLAIM, SAN BENITO COUNTY, CALIFORNIA

Abstract

Vasilyevite, a new mineral species of ideal composition (involving complete anion order) (Hg 2 ) 2 + 10O 6 I 3 Br 2 Cl(CO 3 ), is is triclinic, P1, with unit-cell parameters measured on a single-crystal X-ray diffractometer: a 9.344(2), b 10.653(2), c 18.265(4) A, a 93.262(5), β 90.548(4), γ 115.422(4)°, V 1638.3(9) A 3 , a:b:c 0.8771:1:1.7145, Z = 2. The strongest seven lines of the X-ray powder-diffraction pattern [d in A(I)(hkl)] are: 7.645(60)(111), 4.205(80)(014), 3.296(50)(115,105), 3.132(90)(123,133), 2.894(100)(312,322), 2.722(80)(124) and 2.629(50)(130, 140). The mineral has been identified on five micromount specimens collected from a small prospect pit within the dumps surrounding the long-abandoned Clear Creek mercury mine, New Idria district, San Benito County, California. It is most closely associated with native mercury, eglestonite, montroydite, cinnabar and an undefined Hg oxyhalide in a host rock principally composed of quartz and ferroan magnesite. Vasilyevite occurs in small shallow quartz-lined vugs as anhedral cryptocrystalline masses, less than 0.5 mm in size, and as a somewhat elongate spheroidal anhedral mass, 0.3 mm in diameter, which is partly hollow and has a shell thickness of approximately 30 μm. The mineral is silvery grey to black to dark red-black with a red-brown streak. Physical properties include: adamantine to metallic luster, opaque to translucent (on very thin edges), nonfluorescent, no cleavage, very brittle, uneven fracture, estimated hardness approximately 3, density 9.57 g/cm 3 (calculated from chemical formula and unit-cell parameters derived from crystal structure). In polished section, vasilyevite is weakly bireflectant, nonpleochroic and moderately to strongly anisotropic in green, blue and grey tints. In reflected plane-polarized light, it is grey to white with abundant orange-red to blood-red internal reflections. Measured values of reflectance obtained in air and in oil for a single fragment are tabulated. Averaged and corrected results of electron-microprobe analyses yield Hg 2 O 89.1, I 7.0, Br 2.5, Cl 0.6, CO 2 [0.8] (from crystal structure), S 0.1, sum 100.1, less O = I + Br + Cl + S 0.88, total 99.22 wt.%, corresponding to Hg 1 + 20.82O 6 . 8 5 I 2 . 6 9 (Br 1 . 5 2 Cl 0 . 8 2 ) Σ 2 . 3 4 [(CO 3 ) 0 . 8 9 S 2 - 0.15] Σ 1 . 0 4 based on O + I + Br + Cl + S = 14.7 apfu (atoms per formula unit), as determined from the crystal structure. The original value for Hg, 85.7 wt.%, was converted to Hg 2 O after the crystal structure was determined. The infrared-absorption spectrum confirms the presence of CO 3 . The mineral name honors V.I. Vasilyev of the Institute of Geology of the Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia, for his contributions to the study of new and rare Hg-bearing minerals.