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

The crystal structure of a new member of the calcium amphibole subgroup, vanadio-pargasite ideally NaCa2(Mg4V3+)Si6Al2O22(OH)2 with empirical formula (K0.07Na0.90)(Na0.05Ca1.91Mg0.04)Σ2.00(Mg4.02Cr0.05V0.68Al0.23Ti0.02)Σ5.00(Si6.09Al1.91)Σ8.00O22(OH1.67F0.33)Σ2.00, 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 a = 9.89560(10), b = 17.9970(2), c = 5.29700 Å, β = 105.391(1)°, and V = 909.52 Å3. 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 EMP analysis, and validated on the basis of OH-stretching FTIR spectroscopy. Accordingly, V is ordered at M(2) together with minor [6]Al, while a low amount of [6]Al is present at M(3).