CUPRONEYITE, Cu7Pb27Bi25S68, A NEW MINERAL SPECIES FROM BAITA BIHOR, ROMANIA

Abstract

Cuproneyite, with an ideal formula Cu 7 Pb 27 Bi 25 S 68 , is a new mineral species found in the skarn deposit of Baiţa Bihor, Romania. The mineral occurs in irregular aggregates, nests and veinlets up to 5 cm across, with predominant cosalite, hammarite–friedrichite and calcite, within the diopside skarn. Cuproneyite is opaque, has a silver-grey color and a metallic luster, and it displays a perfect cleavage along crystal elongation. The average Mohs hardness is about 3, approximated from the average micro-identation hardness, 222.5 kg/mm 2 . Cuproneyite forms irregular or short prismatic grains up to 300 μm in length, intergrown with lamellae and irregular patches of cosalite and hammarite–friedrichite. Bireflectance is barely visible in air and moderate in oil immersion, with a greyish white color and faint yellowish tints in the lightest position, to greyish white with faint bluish tints in the darkest position. Between crossed polars, the rotation tints of the most anisotropic grains are dark brownish grey to light brownish grey. Internal reflections are absent. The reflectance data (measured in air; R min , R max ) are: 40.4, 47.1% at 470 nm, 39.3, 45.7% at 546 nm, 38.3, 44.3% at 589 nm, and 38.1, 43.9% at 650 nm. Twenty-four electron-microprobe analyses gave, as an average, Cu 3.34(12), Ag 0.30(3), Pb 40.10(27), Bi 39.59(18), Se 0.14(3), Te 0.12(4), S 16.12(7) wt%, corresponding to Cu 7.1 Ag 0.38 Pb 26.04 Bi 25.49 (Te 0.12 Se 0.23 S 67.65 ) ∑68 , calculated on the basis of (S + Se + Te) = 68 atoms per formula unit. The simplified formula [after reconversion of x (Ag + Bi) into 2 x Pb] is Cu 7 Pb 27 Bi 25 S 68 , in agreement with the empirical formula and the one deduced through crystal-structure analysis. Cuproneyite is monoclinic, space group C 2 /m , with a 37.432(8), b 4.0529(9), c 43.545(9) A, β 108.800(5)°, V = 6254(2) A 3 , Z = 2, with a calculated density of 7.11 g/cm 3 . The strongest lines in the calculated X-ray powder pattern [ d in A(I) hkl ] are: 3.735(96)1003, 3.507(50)316, 3.464(53)8011, 3.347(84)4010, 2.956(77)713, 2.925(46)8014, 2.867(100)714, and 2.027(81)020. Cuproneyite is structurally related to neyite, the main difference residing in the fact that the independent Ag position present in the neyite structure is completely replaced by Cu in cuproneyite.