Si-bearing, OH-dominant pertsevite Mg2(BO3)1-x(SiO4)x(OH,F)1-x (x = 0.2–0.3) from contact-metasomatic kotoitites and kotoite marbles at East Verkhoyan’ye region (northern Siberia) and at Gonochan, Dzhugdzhur Ridge (Russian Far East) was studied by electron microprobe (EMP) techniques, including boron analyses, and Raman spectroscopy. At both localities borate mineral-assemblages are very similar. Minerals associated with OH-dominant pertsevite (East Verkhoyan’ye region) are ludwigite, kotoite, szaibelyite, REE-bearing sakhaite, olivine and minerals of the humite group. Humite-group minerals and olivine were found to be slightly B2O3-bearing (ca. 1 wt.%). OH-dominant pertsevite, described here for the first time, formed at the expense of kotoite and hydroxylclinohumite and has a rather heterogeneous composition with respect to Si, B, F and OH between different grains and even within individual grains.
The crystal structure of one selected grain of OH-dominant pertsevite (Mg1.95Fe0.04Mn0.01)(BO3)0.75(SiO4)0.25[(OH)0.45F0.30], space group Pnma, a = 20.494(3), b = 11.890(2), c = 4.5880(6) Å, from the East Verkhoyan’ye region, has been determined by single-crystal X-ray methods yielding a strongly disordered distribution of SiO4 tetrahedra and triangular BO3 units within a hexagonal close-packed anion (O, OH, F) arrangement with Mg occupying half of the octahedral voids.