Crystal chemistry and origin of REE-bearing mukhinite from carbonate veins of the Svetlinsky gold deposit, South Urals, Russia

A rare earth element (REE)-, Cr- and Mg-bearing variety of the vanadium epidote-group mineral mukhinite occurs in a calcite-dolomite carbonatite dyke cutting metamorphosed volcano-sedimentary rocks exposed in the walls of the quarry of the Svetlinsky gold deposit, South Urals. This mineral was found in a paragene assemblage including native sulphur, phlogopite and fluorophlogopite, together with accessory pyrite, other sulfides and sulfosalts, gold, Cr- and V-bearing muscovite, margarite, Cr- and V-bearing dravite, fluoro-tremolite, actinolite, fluoro-pargasite, anhydrite, apatite, uranium hydroxides, V-rich titanite, V- and Nb-rich rutile, spinel and corundum. The contents of sigma REE2O3 and V2O3 in mukhinite vary in the ranges of 4.01-9.69 and 5.34-7.46 wt.%, respectively. A Raman spectrum of REE-rich mukhinite is provided. The main schemes of isomorphic substitutions in mukhinite are sigma REE + Mg <-> Ca + Al and V+Cr <-> Al. The crystal structure of REE-rich mukhinite has been studied by single-crystal X-ray diffraction analysis. The mineral is monoclinic, with the space group P2(1)/m, and unit-cell parameters are: a = 8.8972(11) angstrom, b = 5.6221(6) angstrom, c = 10.1519(12) angstrom, beta = 115.169 degrees and V = 459.60(11) angstrom(3). The crystal structure of REE-rich mukhinite is similar to that of its synthetic analogue; the refined crystal-chemical formula of the sample studied is (Z = 2): {Ca-A(1)A(2)(Ca0.8REE0.2)}{(M)(1)(Al0.95Cr0.05)Al-M(2)M(3)[(V,Cr)3+0.40Al0.35Mg0.25]}(Si2O7)(SiO4)O(OH).