Rhabdoborite-(V), rhabdoborite-(Mo) and rhabdoborite-(W): a new group of borate minerals with the general formula ${{\text{Mg}}_{12}} {M_{1\nicefrac{1}{3}}} {{\text{O}}_{6}}$[(BO₃)_6–x(PO₄)_xF_2–x] (M = V⁵⁺, Mo⁶⁺ or W⁶⁺ and x < 1)

Abstract

Three new borate minerals rhabdoborite-(V), rhabdoborite-(Mo) and rhabdoborite-(W), forming the rhabdoborite group, are found in the Arsenatnaya fumarole, Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. They are closely associated with each other and anhydrite, diopside, hematite, schäferite, berzeliite, svabite, calciojohillerite, ludwigite, forsterite, magnesioferrite, baryte, fluorapatite, udinaite, arsenudinaite, and powellite. Rhabdoborites form long-prismatic to acicular crystals up to 0.04 × 0.04 × 7 mm³ typically combined in aggregates up to 1 cm. They are transparent, light yellow, with vitreous lustre. The empirical formulae of holotypes, calculated based on 26 O + F apfu, are: rhabdoborite-(V): (Mg_11.85Fe³⁺_0.11Mn_0.06Ca_0.02)_Σ12.04(V⁵⁺_0.70Mo⁶⁺_0.36W⁶⁺_0.21Te⁶⁺_0.01)_Σ1.28[(P_0.35As⁵⁺_0.16)_Σ0.51B_5.50]_Σ6.01O_24.35F_1.65; rhabdoborite-(Mo): (Mg_11.78Mn_0.07Fe³⁺_0.06Ca_0.06)_Σ11.97(Mo⁶⁺_0.65V⁵⁺_0.49W⁶⁺_0.19)_Σ1.33[(P_0.31As⁵⁺_0.14)_Σ0.45B_5.54]_Σ5.99O_24.33F_1.67; rhabdoborite-(W): (Mg_11.74Fe³⁺_0.09Ca_0.03Mn_0.02)_Σ11.88(W⁶⁺_0.75V⁵⁺_0.44Mo⁶⁺_0.13)_Σ1.32[(P_0.25As⁵⁺_0.13)_Σ0.38B_5.65]_Σ6.03O_24.42F_1.58. The simplified formulae are: rhabdoborite-(V): ${{\text{Mg}}_{12}}({\text{V}}^{5+}, M^{6+})_{1\nicefrac{1}{3}}{{\text{O}}_{6}}${(BO₃)_6–x(PO₄)_xF_2–x} (M⁶⁺  = Mo, W and x < 1); rhabdoborite-(Mo): ${{\text{Mg}}_{12}}{{\text{Mo}}_{\;\;\; 1\nicefrac{1}{3}}^{6+}}{{\text{O}}_{6}}$(BO₃)₆F₂; rhabdoborite-(W): ${{\text{Mg}}_{12}}{\text{W}}_{\;\;\; 1\nicefrac{1}{3}}^{6+}{{\text{O}}_{6}}$(BO₃)₆F₂. Rhabdoborites are hexagonal, P6₃, and Z = 1. The unit-cell parameters of rhabdoborite-(V)/rhabdoborite-(Mo)/rhabdoborite-(W) are: a 10.6314(4)/10.6304(3)/10.6366(5), c 4.5661(2)/4.56374(16)/4.5701(3) Å, and V 446.95(3)/446.63(3)/447.78(4) ų. The crystal structures were solved from single-crystal XRD data, R₁ = 0.0413, 0.0289 and 0.0418 for rhabdoborite-(V), rhabdoborite-(Mo) and rhabdoborite-(W), respectively. These isostructural minerals represent a novel structure type. The structure is based on a three-dimensional network (pseudo-framework) built by the (0001) layers formed by clusters Mg₆O₂₁F composed by Mg-centred octahedra and linked via BO₃ triangles. The distorted octahedra MO₆ (M = V⁵⁺, Mo⁶⁺, W⁶⁺) are located in gaps in the layers. The pair of anions, (BO₃)³⁻ triangle and adjacent F⁻, is partially substituted by the (P,As⁵⁺)O₄³⁻ tetrahedral anion. Rhabdoborites form a continuous solid-solution system in which contents of V, Mo and W are the main varying values. The rootname rhabdoborite reflects morphological (rhabdos is rod, in Greek) and chemical (borate) features of the minerals; the suffix-(V), -(Mo) or -(W) indicates predominant, species-defining M component.