Geology

Biography

Biography: Can Rao

Abstract

Mengxianminite, Ca2Sn2Mg3Al8[(BO3)(BeO4)O6]2, is a new borate mineral from Xianghualing skarn, Hunan Province, southern China. It occurs in the Hsianghualite vein from this skarn, and is associated with fluorite, phlogopite, hsianghualite, magnetite, tourmaline, magnesiotaaffeite-2N2S 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 4.17 g/cm3. Optically, mengxianminite is biaxial (–), with α = 1.80(2), β = 1.83(2), γ = 1.84(2) (589 nm). Chemical analysis by electron microprobe (average of 6) gave Al2O3 40.00, SnO2 25.96, MgO 6.57, CaO 8.56, FeO 4.83, B2O3 6.52, BeO 4.68, ZnO 1.81, MnO 1.23, Na2O 1.13, TiO2 0.10, SiO2 0.04, sum 101.42 wt%. The empirical formula, calculated on the basis of 26O, 2Be and 2B atoms per formula unit, is (Ca1.63,Na0.39)Æ©2.02(Sn1.84,Zn0.24)Æ©2.08(Mg1.74,Fe0.72,Al0.38,Mn0.19,Ti0.01)Æ©3.04Al8 [(BO3)(BeO4)O6]2. The stronger eight lines of the powder XRD pattern [d in Å (I)(hkl)] are: 3.000(35)(16 20); 2.931(100)(17 11); 2.475(29)(022); 2.430(30)(13 31); 2.375(100)(14 02/640); 2.028(52)(21 31); 1.807(35)(913); 1.530(98)(14 60/15 33). Mengxianminite is orthorhombic, space group Fdd2; unit-cell parameters refined from single-crystal X-ray diffraction data are: a = 60.689 (3), b = 9.907 (1), c = 5.740 (1) Å, V = 3451.0 (3) Å3, Z = 8. The structure of mengxianminite is composed of alternating O-T1-O-T2-O’-T2 layers stacked along the a axis, equal to two alternating modules: A module (O-T1-O) consists of the spinel modular and another O layer (AlO6 octahedra layer); B modular (T2-O’-T2) shows the simplified formula CaSnAl(BeO4)(BO3), SnO6 octahedra are isolated in the T2 layers, connected via BeO4 and CaO11 groups; AlO6 edge-sharing octahedra in the O’ layer form chains running along the b axis; these chains are connected in the c direction by the BO3 triangular groups. Mengxianminite is of hydrothermal origin, crystallized during the late stage of the xianghualing skarn.