The Cr impurity in supergene oxosalts in chromate mineralization occurrences of the Urals.

Based on the results of more than 600 electron microprobe analyses of 25 minerals the distribution pattern of the Cr impurity in vanadates, phosphates, and arsenates collected in oxidation zones of six ore deposits of the Urals was studied. Among them are Pb minerals of the brackebuschite, apatite, adelite, and tsumcorite groups and alunite supergroup, as well as carminite, cornwallite, and bayidonite. Vanadates and arsenates with brackebuschite-type structures show a high affinity to Cr. The maximum content of the Cr impurity is characteristic of minerals with specified Fe trivalent cations (ferribushmakinite, arsenbrackebuschite, and gartrellite) or Al (plumbogummite and bushmakinite). The prevailing scheme of isomorphous substitution, according to which chromium enters into the compositions of these minerals, is heterovalent: Cr + M → Т + M (where Т = V, As, P; M = Fe, Al; M = Сu, Zn), whereas the role of isovalent substitutions Cr → S and Cr → Mo in oxosalts that formed in mineral occurrences of the Urals is insignificant. [ABSTRACT FROM AUTHOR]