Minerals with brackebuschite-like structures: a novel solid-solution system involving Cr6+ and V5+
Khanin D. A., Pekov I. V.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2016. V. 145. N 2. P. 96-112
A novel complex, continuous solid-solution system involving vauquelinite Pb2Cu(CrO4)(PO4)(OH), bushmakinite Pb2Al(VO4)(PO4)(OH), ferribushmakinite Pb2Fe3+(VO4)(PO4)(OH) and a phase with the end-member formula Pb2Cu(VO4)(PO4)(H2O) [or Pb2Cu(VO4)(PO3OH)(OH)] is studied based on samples from the Berezovskoe, Trebiatskoe and Pervomaisko-Zverevskoe deposits (Urals, Russia). This is (1) the first natural system in which the chromate and vanadate anions show the substitution in wide range and (2) the most extensive solid-solution system involving (CrO4)2– found in nature. The major substitution scheme is: Cr6+ + Cu2+ ↔ V5+ + M3+, with M = Fe, Al [correlation coefficients for 125 electron microprobe analyses, are: V: (Fe+Al) = 0.96; Cr: (Cu+Zn) = 0.96; V: (Cu+Zn) = –0.96; Cr: (Fe+Al) = –0.97; (Fe+Al):(Cu+Zn) = –0.97]. In the tetrahedral sites of two types, the substitutions V5+ ↔ Cr6+ (correlation coefficient for the pair Cr: V is –0.98) and, less, P5+ ↔ As5+ (correlation coefficient for the pair P: As is –0.86) happen, while the substitutions «metal for nonmetal/metalloid», i. e. of V or Cr for P or As, are minor. The substitution Fe3+ ↔ Al3+ is also insignificant in this solid-solution system.
Key words: vauquelinite, bushmakinite, ferribushmakinite, brackebuschite group, chromate, vanadate, oxidation zone, Berezovskoe deposit, Trebiatskoe deposit, the Urals.