The philipsbornite–segnitite solid-solution series from rędziny, eastern metamorphic cover of the Karkonosze granite (SW Poland)

Abstract

Supergene minerals of the philipsbornite–segnitite series, PbAl3(AsO4)(AsO3OH)(OH)6–PbFe3+3(AsO4) (AsO3OH)(OH)6, accompanied by carminite, PbFe3+2(AsO4)2(OH)2, were found in relics of hydrothermal quartz–chlorite–arsenopyrite veins, associated with subordinate polymetallic ores disseminated in contact zones of a dolomitic marble deposit at Rędziny, Western Sudetes, Poland, and recognized by means of electron microprobe and X-ray and electron-back-scattered diffraction (XRD and EBSD). Philipsbornite and segnitite, as the two minerals of the series, exhibit highly variable compositions, especially in terms of the range of Fe3+ ↔ Al3+ substitution at the G site, with a distinct gap between the values of 0.52 and 0.89 for the Fe/(Al+Fe) ratio; substitutions at the D and T sites are less important. In this respect, the minerals are almost identical with philipsbornite and segnitite, known from other localities. The gap might be a consequence of the limited miscibility of the end-members, but also might be attributed to crystallization under the changing and distinctly differing activities of Al3+ and Fe3+. The unit-cell parameters of philipsbornite, a = 7.1245(13) Å, c = 17.0967(45) Å, make the mineral comparable with philipsbornites from other occurrences. The EBSD analysis confirmed the rhombohedral structure of both minerals and the space group symmetry R-3m. The minerals crystallized in the sequence: philipsbornite → segnitite → carminite, which reflects (i) decreasing acidity in the oxidation zone, due to the leaching of sulphate ions and interaction of the solutions with a nearby dolomite lens, and (ii) varying activities of Al3+, Fe3+ and Pb2+cations, mobilized by the solutions through interaction with the silicate host containing disseminated arsenopyrite and subordinate sulphides, up to complete Pb2+ depletion.