CUPROBISMUTITE HOMOLOGUES IN GRANITIC PEGMATITES FROM SZKLARSKA POREBA, KARKONOSZE MASSIF, SOUTHWESTERN POLAND

Abstract

We describe a complete sequence of cuprobismutite homologues (kupcikite, hodrusite, cuprobismutite and an unnamed N = 2.5 homologue) coexisting with one another, generally in the form of coherent, lamellar intergrowths, found in a granitic pegmatite cutting the Karkonosze granite at Szklarska Poreba, southwestern Poland. The minerals are associated with krupkaite, gladite, pekoite, bismuthinite varieties varying from bd 5 to bd 0 , native bismuth and chalcopyrite. The composition of kupcikite ranges from (Cu 6.67 Fe 1.22 Zn 0.10 ) ∑7.99 Bi 9.99 S 20.03 with N chem = 1.03(3) to (Cu 6.44 Fe 1.14 Zn 0.06 ) ∑7.64 (Bi 10.14 Ag 0.04 Cd 0.02 ) ∑10.20 S 20.16 with N chem = 1.19(3), whereas the composition of hodrusite ranges from (Cu 7.52 Fe 0.45 ) ∑7.97 (Bi 11.47 Ag 0.40 Cd 0.05 Pb 0.01 ) ∑11.93 S 22.10 with N chem = 1.49(4) to (Cu 7.26 Fe 0.44 ) ∑7.70 (Bi 11.51 Ag 0.63 Cd 0.09 Pb 0.01 ) ∑12.24 S 22.06 with N chem = 1.68(7). A phase with composition (Cu 7.28 Fe 0.53 ) ∑7.81 (Bi 13.84 Ag 1.88 Cd 0.16 ) ∑15.89 S 26.30 may represent an unnamed N = 2.5 homologue in the series. The slightly excessive order number of the homologues, N chem , connected with a deficiency of Cu-site cations and a surplus of Bi-site cations, has been interpreted as a result of intimate, two-phase submicrometric lamellar intergrowths of kupcikite with hodrusite and of hodrusite with cuprobismutite, existing at analytical spots that are apparently homogeneous under the normal magnification of the microprobe. Such intergrowths can result from structural changes at the lattice scale, related to modulations existing in the kupcikite, hodrusite and cuprobismutite structures, all similar to one another. Ideal compositions for N = 1 (kupcikite), 1.5 (hodrusite) and 2 (cuprobismutite) have been evaluated on the basis of linear dependencies between the contents of the major and minor components and the values of N chem in successive analytical spots. These compositions are close to the ideal stoichiometry of the minerals and, in light of the accuracy of the results, show neither vacancy at the Cu sites nor a surplus of cations at the Bi sites. Kupcikite, the mineral in the cuprobismutite series richest in Fe, does not contain Ag, Cd and Pb, which perfectly satisfies the structural requirements for the mineral; however, it contains traces of Zn. All the remaining homologues can contain Ag, Cd and Pb, with a distinct tendency to increase from hodrusite through cuprobismutite to the unnamed N = 2.5 homologue, whereas the Fe contents decrease and even attain zero, and Zn is virtually absent. The composition of cuprobismutite, which occurs in the Szklarska Poreba material only as nanoscale intergrowths with hodrusite, invisible at the μm scale in back-scattered electron images, ranges from Cu 8.00 (Bi 12.88 Ag 0.90 Cd 0.12 ) ∑13.89 S 24.11 to (Cu 7.46 Fe 0.37 ) ∑7.83 (Bi 12.76 Ag 0.84 Cd 0.18 Pb 0.02 ) ∑13.79 S 24.38 .