Si-deficient, OH-substituted, boron-bearing vesuvianite from Sakha-Yakutia, Russia: a combined single-crystal, 1H MAS-NMR and IR spectroscopic study

Abstract

Single crystals of Si-deficient vesuvianite with significant degree of hydrogarnet-type (SiO 4 ) 4− –(O 4 H 4 ) 4− substitution occur as epitactic overgrowth on the surface of large wiluite crystals from the Wiluy River, Sakha–Yakutia, Russia. Electron-microprobe analysis revealed considerable Si-deficiency, Si ranging from 16.30 to 17.50 apfu . The crystal structure of the mineral has been refined in the P 4/ nnc space group, a = 15.5876(4), b = 11.8021(5) A to R 1 = 0.028 for 1533 unique observed reflections. The refinement of the site-occupancy factors confirmed significant vacancy at the Z (1) and Z (2) sites (27 % and 10 %, respectively), with associated increase of the Z (1)–O and Z (2)–O bond lengths to 1.697 and 1.655 A, respectively. The increased size of the Z (1) tetrahedra results in the compression of the X (1) polyhedra, while the expansion of the Z (2) tetrahedra is compensated by the compression of the X (4) polyhedra. As a result, the significant degree of the hydrogarnet-type substitutions does not have an essential influence upon the unit-cell parameters compared to the usual defect-free vesuvianite. Thermogravimetric (TGA) and differential scanning calorimetry (DSC) curves indicated two steps of the weight loss in the temperature ranges of 600–900°C (2.08 wt%) and 1014°C (2.22 wt%). The total weight loss is estimated as 4.30 %, which is in good agreement with the total content of OH estimated as 13.72 apfu . The 1 H solid state MAS-NMR demonstrates the presence of a strong additional line at 3.38 ppm compared to the usual vesuvianite, which is explained by the existence of the additional H(3) site. The infrared spectrum in the OH-stretching vibration region shows the additional (B′) absorption band at 3618 cm −1 typical for a hydrogarnet-type substitution.