FERRIAN WINCHITE FROM THE ILMEN MOUNTAINS, SOUTHERN URALS, RUSSIA, AND SOME PROBLEMS WITH THE CURRENT SCHEME FOR AMPHIBOLE NOMENCLATURE

Abstract

The crystal structure of ferrian winchite (□0.87K0.13)∑1.00 (Ca0.65Na1.34)∑1.99 (Mg2.78Fe2+0.12 Mn0.07Ti0.01)∑2.98 (Fe2+0.88Fe3+0.92 Al0.20)∑2.00 (Si7.88Al0.12)∑8.00 O22 ((OH)1.77F0.23)∑2.00, a 9.8339(2), b 18.0357(2), c 5.2974(1) Å, β 104.195(1)°, V 910.9(2) Å3, C2/ m, Z = 2, D(calc) = 3.12 g/cm3, has been refined using the Rietveld method to the following values: Rp = 4.1 and RF = 3.6%, s = 2.74. The Fe3+/(Fe2+ + Fe3+) value, 0.48, was determined by Mössbauer spectroscopy. Site populations were assigned on the basis of the refined site-scattering values at the A-, B- and C-group sites. TAl was assigned to the T(1) site of the tetrahedral double chain on the basis of the observed distances. In the structure of ferrian winchite, all Fe 3+ occurs at the M(2) site, in accord with the observed distance of 2.05 Å. Fe2+ is partitioned between the M(2) and M(3) sites. The high amount of Fe 3+ (0.92 apfu) at the M(2) site focuses attention on some defects of the current system of amphibole nomenclature.