CUPROMAKOPAVONITE, Cu8Ag3Pb4Bi19S38, A NEW MINERAL SPECIES, ITS CRYSTAL STRUCTURE AND THE CUPROPAVONITE HOMOLOGOUS SERIES

Abstract

Cupromakopavonite, ideally Cu8Ag3Pb4Bi19S38, is a new mineral species found in quartz veins at the scheelite deposit Felbertal, Salzburg Province, Austria. It is associated with makovickyite, oversubstituted krupkaite ( bd 55), hodrusite, and kupcikite. The mineral is opaque and grey with a metallic luster; it is brittle without any discernible cleavage. In reflected light, it is greyish white, the bireflectance is weak, and the anisotropy is moderate in air and strong in oil. Internal reflections are absent. The reflectance data (%, air) are: 42.9, 46.6 at 470 nm, 42.1, 46.5 at 546 nm, 41.3, 45.8 at 589 nm and 40.2, 45.1 at 650 nm. The average results of 15 electron-microprobe analyses are: Cu 7.29(6), Ag 5.48(4), Pb 8.84(14), Cd 0.39(6), Bi 59.9(2), S 17.90(6), total 99.80(23) wt.%, corresponding to Cu7.82Ag3.46Pb2.91Cd0.24Bi19.53S38.05 (on the basis of Me + S = 72 apfu ). The simplified formula, Cu8Ag3Pb4Bi19S38, is in accordance with the results of our crystal-structure analysis. The density, 6.85 g/cm3, was calculated using the ideal (structural) formula. Cupromakopavonite has a monoclinic cell with a 13.380(2), b 4.0007(6), c 31.083(4) A, β 93.064(2)°, V 1661.5(4) A3, space group C 2/ m , and Z = 1. The strongest eight lines in the (calculated) powder-diffraction pattern [ d in A(I) hkl ] are: 3.607(57)(207), 3.457(99)(114), 3.436(37)(208), 3.340(34)(401), 2.953(28)(311), 2.874(33) (2010), 2.834(100)(313) and 2.256(29)(1111). The crystal structure was refined to R = 4.53% and wR = 6.0% for 1834 reflections [ F o > 4σ( F o )]. Our crystal-structure determination reveals that the unusually high value of the c parameter results from: (1) regular alternation of pavonite-like N = 4 and N = 5 layers in the structure, (2) regularly oriented distribution of the pairs of Bi pyramids and pairs of Pb–Bi prisms in the thin slabs of the structure, and (3) the appearance of a new three-coordinated Cu site instead of the octahedrally coordinated Ag site in the thin slabs. Cupromakopavonite is the N = 4.5 member of the newly defined cupropavonite homologous series, and its name underscores its close structural relationships to its two neighboring members, cupromakovickyite (N = 4) and cupropavonite (N = 5). We define the structural and chemical properties of this series, with the general, ideal formula, Cu8Ag2N−6Pb4Bi2N+10S4N+20 for Z = 1, where N is the order of the homologue. It includes N = 7 (“cuproplumbian benjaminite”) and N = 8 (“cuproplumbian mummeite”).