Gachingite, Au(Te(1-x)Se(x)) 0.2 = x <= 0.5, a new mineral from Maletoyvayam deposit, Kamchatka peninsula, Russia
Abstract
Gachingite, Au(Te(1-x)Se(x)), 0.2 = x <= 0.5, is a new mineral discovered in the Gaching ore occurrence of the Maletoyvayam epithermal deposit, Kamchatka, Russia. Gachingite forms individual droplet-like grains of sizes from 2 to 10 μm included in native gold (Au-Ag), associated with calaverite, maletoyvayamite, watanabeite and Au-Sb oxides.
The aggregates do not exceed 100 μm in diameter. In plane-polarised light, gachingite is grey with a bluish tint, has bireflectance (bluish-grey to deep grey), and strong anisotropy with rotation tints blue to dark blue to brown.
Reflectance values for gachingite in air (R-min, R-max in %) are: 39.9, 40.3 at 470 nm; 41.6, 43.3 at 546 nm; 42.0, 43.7 at 589 nm; and 43.0, 44.0 at 650 nm. Eighteen electron-microprobe analyses of gachingite gave an average composition: Au 62.40, Ag 0.57, Se 9.78, Te 27.33 and S 0.01, total 100.09 wt.%, corresponding to the formula (Au0.96Ag0.02)(Σ0.98)(Te(0.6)5Se(0.37))(Σ1.02) based on 2 apfu, the simplified formula is Au(Te0.65Se0.35); the average analyses of its synthetic analogue is Au 65.7, Se 13.1 and Te 21.1, total 99.9 wt.%, corresponding to Au(1.00)(Te0.50Se0.50).
The calculated density is 10.47 g/cm(3). The mineral is orthorhombic, space group Cmce (#64) with a = 7.5379 Å, b = 5.7415 Å, c = 8.8985 Å, V = 385.12 Å(3) and Z = 8.
The crystal structure was solved and refined from the single-crystal X-ray-diffraction data of synthetic Au(1.00)(Te(0.50)Se(0.50)). The crystal structure of gachingite represents a unique structure type, containing linear [Au-Au-Au] chains running along the b-axis indicating strong metallic interaction in one direction.
The structural identity of gachingite and its synthetic analogue Au(1.00)(Te(0.50)Se(0.50)) was confirmed by electron back-scatter diffraction and Raman spectroscopy. The formation of gachingite requires an abundant source of Au and Se and a high oxidising environment.
Gachingite is related to the gold-bearing productive stage of ore mineralisation, which is stable at 250 °C in logf(Se2) range of -12.4 and -5.7. The mineral is named after its type locality.