Heterovalent isomorphism in the magnesium-iron borates

Orthorhombic magnesium-iron ludwigite-vonsenite forms a continuous isomorphic series Mg₂Fe³⁺BO₃]O₂-Fe ₂ ²⁺ FeBO₃]O₂; its composition at the magnesioskarn and other deposits varies from magnesian to ferriferous members. In addition, they demonstrate isovalent substitution of Mn for Mg (in pinakiolite, blatterite, and others) and practically complete substitution of Ni for Mg (in bonaccordite). Ferric iron in the borates is substituted by isovalent Al and Cr. The incorporation of Ti, Sn, Sb, and V via heterovalent substitution has been studied in less detail. Our research revealed new manifestations of Ti-and Sn-bearing borates. They are magnesioludwigite and azoproite with variable Ti content, as well as by Sn-bearing aluminian borates formed via the 2Fe³⁺ → (Ti⁴⁺ + Mg)⁶⁺ and/or (Sn⁴⁺ + Mg)⁶⁺ substitution. The incorporation of pentavalent elements according to the scheme 3Fe³⁺ → (Sb⁵⁺ + 2Mg)⁹⁺ or (V⁵⁺ + 2Mg)⁹⁺ is not excluded. The highest Ti borates were found in the marbles and calciphyres of the Tazheran deposit in the Baikal region and Nalednoe, Dokuchan, and Titovskoe deposits in Yakutia, where azoproites contain more than 50 and even higher 75 mol % of the Mg₂(TiMg)_0.5BO₃]O₂ end member. Aluminum magnesioludwigites from Yakutia and Chukotka simultaneously contain tin and titanium. Mount Brooks, Alaska, contains tin-bearing azoproite or its tin-bearing varieties. New data are reported on Sb-and V-bearing orthoborates. Calciphyres of Alaska contain monoclinic magnesiohulsite (Mg,Fe)₂(SnMg) _0.5 ⁶⁺ BO₃]O₂, which is replaced by schoenfliesite MgSn(OH)₆. The studied borate occurrences belong to hypabyssal magnesian skarns of the periclase and monticellite metasomatic PT facies at contacts of dolomites with granitoid intrusions of increasing alkalinity or leucocratic granites. Their formation was related to interaction between disequilibrium kotoite and early oxides and spinellides of various compositions, on the one hand, and, on the other hand, to the influx of Ti-and Sn-bearing hydrothermal solutions.