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1 Institute of Geology of Ore Deposits (IGEM), 35 Staromonetny, Moscow 119017, Russia
2 Institut für Geologic, Mineralogie und Geophysik, Ruhr-Universität, D-44780 Bochum, Germany
3 Laboratorium für chemische und mineralogische Kristallographie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland
* Corresponding author, e-mail: werner.schreyer{at}ruhr-uni-bochum.de
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)
2.00(Al0.63Mg0.17Mn0.01Ti0.02Sn0.16)0.99 O2(BO3). Optically, it is biaxial (+), 
'about 1.78, 
'about 1.805, 2Vz (measured from extinction data) = 33(5)°, 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) Å3, Z = 2 and Dcalc = 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(Al1–2xMgxSnx)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 (Mg1.62Fe2+0.38)(Fe3+0.50Al0.31Fe2+0.07Mn0.01Ca0.01Sn0.05Ti0.05) 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.
Key-words: new mineral: alumino-magnesiohulsite, berates, chemical analysis (mineral), crystal structure, ludwigite, Siberia.
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