Minerals with brackebuschite-like structures: A novel solid solution system involving Cr6+ and V5+

[Marco E. Ciriotti]

Referenza:
▪ Khanin, D.A. & Pekov, I.V. (2018): Minerals with Brackebuschite-Like Structures: A Novel Solid Solution System Involving Cr6+ and V5+. Geology of Ore Deposits, 59, 642-651.

Abstract:
A novel complex continuous system of solid solutions involving vauquelinite Pb2Cu(CrO4)(PO4)(OH), bushmakinite Pb2Al(VO4)(PO4)(OH), ferribushmakinite Pb2Fe3+(VO4)(PO4)(OH), and a phase with the endmember formula Pb2Cu(VO4)(PO4)(H2O) or Pb2Cu(VO4)(РО3ОН)(ОН) is studied based on samples from the oxidation zone of the Berezovskoe, Trebiat, and Pervomaisko-Zverevsky deposits in the Urals, Russia. This is the first natural system in which chromate and vanadate anions show a wide range of substitutions and the most extensive solid solution system involving (CrO4)2– found in nature. The major couple substitution is Cr6+ + Cu2+ ↔ V5+ + M3+, where M = Fe, Al. The correlation coefficients calculated from 125 point analyses are: 0.96 between V and (Fe + Al), 0.96 between Cr and (Cu + Zn), –0.96 between V and (Cu + Zn), –0.97 between Cr and (Fe + Al), and –0.97 between (Fe + Al) and (Cu + Zn). The substitutions V5+ ↔ Cr6+ (correlation coefficient –0.98) and to a lesser extent P5+ ↔ As5+ (correlation coefficient –0.86) occur at two types of tetrahedral sites, whereas the metal–nonmetal/metalloid substitutions, i.e., V or Cr for P or As, are minor. The substitution Fe3+ ↔ Al3+ is also negligible in this solid solution system.