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

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Language: Russian

Abstract

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.