Crystal structure and crystal-chemistry of vanadio-pargasite: a new amphibole from southern Lake Baikal, Siberia, Russia

摘要

The crystal structure of a new member of the calcium amphibole subgroup, vanadio-pargasite ideally NaCa2(Mg4V3+) (SiAlO22)-Al-6-O-2(OH)(2) with empirical formula (K0.07Na0.90)(Na0.05Ca1.91Mg0.04)(Sigma 2.00)(Mg4.02Cr0.05V0.68 Al0.23Ti0.02)(Sigma 5.00)(Si6.09Al1.91)(Sigma 8.00)O-22(OH1.67F0.33)(Sigma 2.00,)was studied by single-crystal X-ray diffraction and refined to R1 of 0.0181. It is monoclinic, space group C2/m, unit-cell parameters alpha=9.8956(1), b = 17.9970(2), c = 5.2970(1) angstrom, beta = 105.391(1)degrees, and V = 909.52 angstrom(3)The mineral is isostructural with the amphiboles pargasite, magnesio-hastingsite, chromio-pargasite and Mn3+-rich pargasite. Site populations were derived from the structure refinement and electron-microbe analysis, and validated on the basis of OH-stretching FTIR spectroscopy. Accordingly, V is ordered at M(2) together with minor Al-[6], while a low amount of Al-[6] is present at M(3)..