Mengxianminite, ideally Ca2Sn2Mg3Al8[(BO3)(BeO4)O6]2, is a new borate mineral from Xianghualing skarn, Hunan Province, southern China. It occurs in the hsianghualite vein of Xianghualing skarn, associated with fluorite, phlogopite, hsianghualite, magnetite, dravite, magnesiotaaffeite-2N′2S, and calcite. Mengxianminite forms subhedral to euhedral green crystals from 20 to 200 μm long, translucent to transparent, with a vitreous luster. The crystals show perfect cleavage on {100} and good cleavage on {010}, and do not fluoresce in long- or short-wave ultraviolet light. The estimated Mohs hardness is 8, and the tenacity is brittle with irregular fracture. The calculated density is 4.170 g/cm3. Optically, mengxianminite is biaxial (–), with α = 1.80(2), β = 1.83(2), γ = 1.84(2) (589 nm). The mean chemical composition of mengxianminite is Al2O3 40.00, SnO2 25.96, MgO 6.57, CaO 8.56, FeO 4.83, B2O3 6.53, BeO 4.37, ZnO 1.81, MnO 1.23, Na2O 1.13, TiO2 0.10, SiO2 0.04, sum 101.12 with a corresponding empirical formula calculated on the basis of 26 O atoms of (Ca1.64,Na0.39)∑2.03(Sn1.85,Zn0.24)∑2.09(Mg1.75, Fe0.72,Al0.42,Mn0.19,Ti0.01)∑3.09Al8[(B1.01O3)(Be0.94O4)O6]2. (Be and B were measured by secondary ion mass spectrometry, average of six electron microprobe analyze points and in wt%.) The strongest eight lines of the powder XRD pattern [d in Å (I)(hkl)] are: 3.000 (35)(16.2.0); 2.931 (100)(17.1.1); 2.475 (29)(022); 2.430 (30)(13.3.1); 2.375 (100)(14.0.2/640); 2.028 (52)(21.3.1); 1.807 (35)(913); 1.530 (98)(14.6.0/15.3.3). Mengxianminite is orthorhombic, space group Fdd2; unit-cell parameters refined from single-crystal X-ray diffraction data are: a = 60.699(4), b = 9.914(1), c = 5.745(1) Å, V = 3457.4(4) Å3, and Z = 8. The structure of mengxianminite is characterized by the alternating O-T1-O-T2-O′-T2 layers stacked along the a axis, which are equal to two alternating modules: the module A (O-T1-O) corresponding to the spinel module with an additional O layer (AlO6 octahedra layer), and the module B (T2-O′-T2) showing the simplified formula CaSnAl(BeO4)(BO3), where SnO6 octahedra are isolated in the T2 layers, connected via BeO4 and CaO11 groups, and AlO6 edge-sharing octahedra in the O′ layer form chains running along the {011} or {011} direction, connected in the c direction by the BO3 triangular groups. Mengxianminite is the first borate mineral with both Sn and Be, likely crystallized under F-rich conditions at late stages of the Xianghualing skarn.

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