Chukotkaite, AgPb7Sb5S15, a new sulfosalt mineral from Eastern Chukotka, Russia

The new sulfosalt chukotkaite, ideally AgPb7Sb5S15, was discovered in the valley of the Levyi Vulvyveem river, Amguema river basin, Iultin District, Eastern Chukotka, Chukotka Autonomous Okrug, North-Eastern region, Russia. The new mineral forms anhedral grains up to 0.4 × 0.5 mm intergrown with pyrrhotite, sphalerite, galena, stannite, quartz, and Mn-Fe-bearing clinochlore. Other associated minerals include arsenopyrite, benavidesite, diaphorite, jamesonite, owyheeite, uchucchacuaite, cassiterite, and fluorapatite. Chukotkaite is lead-grey and has metallic luster and a grey streak. It is brittle and has an uneven fracture. Neither cleavage nor parting were observed. Mohs hardness is 2–2½. Dcalc. = 6.255 g/cm3. In reflected light, chukotkaite is white, moderately anisotropic with rotation tints varying from bluish-grey to brownish-grey. No pleochroism or internal reflections are observed. The chemical composition of chukotkaite is (wt.%; electron microprobe) Ag 3.83, Pb 53.67, Sb 24.30, S 18.46, total 100.26. The empirical formula based on the sum of all atoms = 28 pfu is Ag0.93Pb6.78Sb5.22S15.07. Chukotkaite is monoclinic, space group P21/c, a = 4.0575(3), b = 35.9502(11), c = 19.2215(19) Å, β = 90.525(8)°, V = 2803.7(4) Å3, and Z = 4. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 3.52 (100) (045), 3.38 (50) (055), 3.13 (50) (065), , 2.82 (25) (066), 1.91 (50) (0 1 10). The crystal structure of chukotkaite was refined from single-crystal X-ray diffraction data to R = 0.0712 for 3307 observed reflections with Iobs > 3σ(I). Chukotkaite belongs to the group of rod-based sulfosalts. The new mineral is named after the region of its type locality: Chukotka Autonomous Okrug, North-Eastern Region, Russia.