Discreditation of borishanskiite as a mineral species (= polarite, PdPb)

A re-examination by electron probe microanalysis of the holotype sample of the platinum-group mineral borishanskiite, previously defined as Pd (sub 1+x) (As,Pb) (sub 2) with x = 0-0.2, reveals that the mineral contains no detectable As. The new data also show the mineral to be stoichiometric, i.e., vacancy-free, with the average chemical composition (based on 2 apfu) of Pd (sub 0.99) (Pb (sub 0.72) Bi (sub 0.28) ) (sub Sigma 1.00) (n = 3) or ideally, PdPb. A comparison of the powder X-ray diffraction patterns for borishanskiite (PDF 00-029-0960) and polarite (PDF 00-023; ideally PdPb) show that, within error, the two are same. Further, the refined unit-cell parameters of borishanskiite, a = 7.18(2), b = 8.62(2), c = 10.66(2) Aa, agree well with those of polarite, a = 7.140, b = 8.563, c =10.501 Aa. Given the new chemical data for borishanskiite, which suggest the ideal chemical formula PdPb, along with its crystallographic similarity to polarite, it is concluded that borishanskiite is equivalent to polarite. Discussion of the extent of the PdBi-PdPb solid-solution series and confusion concerning the ideal formula for polarite is also presented.