Mengxianminite (Ca2Sn2Mg3Al8[(BO3)(BeO4)O6]2) a new borate mineral from Xianghualing skarn, Hunan Province, China, with a highly unusual chemical combination (B + Be + Sn)

Mengxianminite, ideally Ca 2 Sn 2 Mg 3 Al 8 [(BO 3 )(BeO 4 )O 6 ] 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-2 N ′2 S , 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/cm 3 . 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 Al 2 O 3 40.00, SnO 2 25.96, MgO 6.57, CaO 8.56, FeO 4.83, B 2 O 3 6.53, BeO 4.37, ZnO 1.81, MnO 1.23, Na 2 O 1.13, TiO 2 0.10, SiO 2 0.04, sum 101.12 with a corresponding empirical formula calculated on the basis of 26 O atoms of (Ca 1.64 ,Na 0.39 ) ∑2.03 (Sn 1.85 ,Zn 0.24 ) ∑2.09 (Mg 1.75 , Fe 0.72 ,Al 0.42 ,Mn 0.19 ,Ti 0.01 ) ∑3.09 Al 8 [(B 1.01 O 3 )(Be 0.94 O 4 )O 6 ] 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 A ( 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 Fdd 2; unit-cell parameters refined from single-crystal X-ray diffraction data are: a = 60.699(4), b = 9.914(1), c = 5.745(1) A, V = 3457.4(4) A 3 , 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 (AlO 6 octahedra layer), and the module B (T2-O′-T2) showing the simplified formula CaSnAl(BeO 4 )(BO 3 ), where SnO 6 octahedra are isolated in the T2 layers, connected via BeO 4 and CaO 11 groups, and AlO 6 edge-sharing octahedra in the O′ layer form chains running along the {011} or {011} direction, connected in the c direction by the BO 3 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.