Alumino-magnesiohulsite, a new member of the hulsite group, in kotoite marble from east of Verkhoyansk, Sakha-Yakutia, Russia

Alumino-magnesiohulsite, the Al- and Mg-dominant analogue of hulsite, (Fe²⁺,Mg)₂(Fe³⁺,Sn, Mg)O₂BO₃, is a new, optically spectacular mineral with pleochroism from brown to blue-green discovered in a magnesian skarn from northeastern Siberia. It forms prismatic, twinned crystals in a spinel-bearing kotoite marble at the contrct of a Mesozoic granosyenite against Palaeozoic dolomite marbles. EMP analyses give: MgO 33.94; FeO 15.97; Al₂O₃ 15.86; SnO₂ 11.88; TiO₂ 0.75; MnO 0.42; CaO 0.11; B₂O₃(calc.) 17.07; total 96.00 wt.%, which can be recalculated to the formula (Mg1.55Fe²⁺_0.45)Σ2.00(Al_0.63Mg_0.17Mn_0.01Ti_0.02Sn_0.16)Σ0.99 O₂(BO₃). Optically, it is biaxial (+), α'about 1.78, γ'about 1.805, 2V_z (measured from extinction data) = 33(5)°, α is parallel to the prism axis b. Alumino-magnesiohulsite is monoclinic, space group P2/m, with a = 5.3344(7), b = 3.0300(5), c = 10.506(1) Å, β = 94,46°, V = 169.29(4) ų, Z = 2 and D_calc = 3.84 g/cm³. Its cell parameters are significantly smaller than those of previously studied members of the hulsite group. A single-crystal X-ray study provided a pattern of cation distribution over the five distinct octahedral sites (M1-M5), which is not consistent with the formula given above. Despite this discrepany, the IMA-Commission on New Minerals and Mineral Names agreed for nomenclature and classification purposes with using the conventional stoichiometric formula in which the amount of R²⁺ cations attributed to the tin-bearing unit equals that of the tetravalent ions, so that this attains an overall charge of 3.0⁺. Taking into account variable tin contents as usual in hulsites, an ideal general formula of alumino-magnesiohulsite is Mg₂(Al_1–2xMg_xSn_x)O₂(BO₃) with x expected to be in the range 0.15–0.20.

Alumino-magnesiohulsite occurs within bimineralic aggregates in the rock together with aluminous, tin-bearing ludwigite of the formula (Mg_1.62Fe²⁺_0.38)(Fe³⁺_0.50Al_0.31Fe²⁺_0.07Mn_0.01Ca_0.01Sn_0.05Ti_0.05) O₂(BO₃). These aggregates may be pseudomorphs after a preexisting high-temperature mineral intermediate in composition between ludwigite and alumino-magnesiohulsite. The reason for the exceptionally high Al-content of the new hulsite-group mineral can only partly be due to its Al-saturation by coexisting spinel, because hulsites from other localities also coexisting with spinel contain much less Al.