Pampaloite, AuSbTe, a new mineral from Pampalo gold mine, Finland

Abstract

Pampaloite, AuSbTe, is a new mineral discovered in the Pampalo gold mine, 65 km east of Joensuu, Finland. It forms anhedral grains (up to ~20 μm) intergrown with gold, frohbergite and altaite. Pampaloite is brittle and has a metallic lustre. Values of VHN25 lie between 245 and 295 kg/mm2, with a mean value of 276 kg/mm2, corresponding to a Mohs hardness of ~4–5 (measured on synthetic material). In plane-polarised light, pampaloite is white with medium to strong bireflectance, weak reflectance pleochroism from slightly pinkish brown to slightly bluish white (only visible in grains of synthetic material containing multiple orientations), and strong anisotropy, with blue to light brown rotation tints; it exhibits no internal reflections. Reflectance values of pampaloite in air (R1, R2 in %) are: 60.0, 62.5 at 470 nm, 62.5, 64.8 at 546 nm, 63.2, 65.6 at 589 nm and 63.7, 66.0 at 650 nm. Ten electron-microprobe analyses of natural pampaloite give an average composition: Au 44.13, Sb 27.44 and Te 28.74, total 100.31 wt.%, corresponding to the empirical formula Au1.00Sb1.00Te1.00 based on 3 atoms; the average of eleven analyses on synthetic pampaloite is: Au 44.03, Sb 27.26, and Te 29.08, total 100.38 wt.%, corresponding to Au0.99Sb1.00Te1.01. The density, calculated on the basis of the empirical formula, is 9.33 g/cm3.The mineral is monoclinic, space group C2/c, with a = 11.947(3), b = 4.481(1) Å, c = 12.335(3) Å, β = 105.83(2)°, V = 635.3(3) Å3 and Z = 8. The crystal structure was solved and refined from the single-crystal X-ray-diffraction data of synthetic AuSbTe. The pampaloite crystal structure can be considered as a monoclinic derivative of the CdI2 structure composed of [AuTe3Sb3] octahedra. The strongest lines in the powder X-ray diffraction pattern of synthetic pampaloite [d in Å (I) (hkl)] are: 4.846(24)($\bar{2}$02), 3.825(18)(111), 2.978(100)($\bar{3}$11), 2.968(50)(004), 2.242(25)(020), 2.144(55)(313), 2.063(33)($\bar{3}$15) and 1.789(18)(024).