IMA 2021-008 = gachingite

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Marco E. Ciriotti
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IMA 2021-008 = gachingite

Messaggio da Marco E. Ciriotti » lun 24 gen, 2022 11:39

Prossima pubblicazione.

▪ Tolstykh, N.D., Tuhý, M., Vymazalová, A., Laufek, F., Plášil, J., Košek, F. (2022): Gachingite, Au(Te1-xSex) 0.2≈x≤0.5, a new mineral from Maletoyvayam deposit, Kamchatka peninsula, Russia. Mineralogical Magazine, 86, (in press).

Gachingite, Au(Te1-xSex), 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-polarized light, gachingite is grey with a bluish tint, has bireflectance (bluish-grey to deep grey), and strong anisotropy with rotations tints blue to dark blue to brown. Reflectance values for gachingite in air (Rmin, Rmax 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(Te0.65Se0.37)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 Au1.00(Te0.50Se0.50). The calculated density is 10.47 g/cm3. The mineral is orthorhombic, space group Cmce (#64) with a=7.54 Å, b=5.74 Å, c=8.90 Å, V=385.1 Å3 and Z=8. The crystal structure was solved and refined from the single-crystal X-ray-diffraction data of synthetic Au1.00(Te0.50Se0.50). 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 Au1.00(Te0.50Se0.50) was confirmed by EBSD 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 logfSe2 range of −12.4 and −5.7. The mineral is named after its type locality.
Marco E. Ciriotti

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