Minerals with Brackebuschite-Like Structures: A Novel Solid Solution System Involving Cr6+ and V5+

摘要

A novel complex continuous system of solid solutions involving vauquelinite Pb2Cu(CrO4)(PO4)(OH), bushmakinite Pb2Al(VO4)(PO4)(OH), ferribushmakinite Pb2Fe(3+)(VO4)(PO4)(OH), and a phase with the endmember formula Pb2Cu(VO4)(PO4)(H2O) or Pb2Cu(VO4)(D D(3)DDe)(DDe) 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+ a dagger" V5+ + M (3+), 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+ a dagger" Cr6+ (correlation coefficient-0.98) and to a lesser extent P5+ a dagger" 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+ a dagger" Al3+ is also negligible in this solid solution system.