Journal of Earth Science & Climatic Change
Open Access

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 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 = 1.80(2), = 1.83(2), = 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: (Ca_1.63,Na_0.39)_2.02(Sn_1.84,Zn_0.24)_2.08(Mg_1.74,Fe_0.72,Al_0.38,Mn_0.19,Ti_0.01)_3.04Al₈ [(BO₃)(BeO₄)O₆]₂. The stronger eight lines of the powder XRD pattern [d in (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) , V = 3451.0 (3) 3, 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.

Biography

Can Rao has completed his PhD and Post-doctoral studies from Nanjing University, China.. He is an Associate Professor at Zhejiang University, China. He has found 3 new minerals (strontiohurlbutite, minjiangite and mengxianminite), which have been approved by IMA, and published 16 papers in reputed journals.

Email: canrao@zju.edu.cn