Minjiangite, BaBe2(PO4)(2), a new mineral from Nanping No. 31 pegmatite, Fujian Province, southeastern China

摘要

Minjiangite, ideally BaBe2(PO4)(2), is a new mineral species which has been found in the Nanping No. 31 pegmatite, Fujian Province, southeastern China. It occurs in the fractures of montebrasite from pegmatite zone IV, and is associated with quartz, muscovite, hydroxylapatite and palermoite. Minjiangite forms subhedral to euhedral white crystals from 5 to 200 mu m long, transparent to translucent, with a vitreous lustre. The estimated Mohs hardness is similar to 6, the tenacity is brittle and no cleavage was observed. The calculated density is 3.49 g/cm(3). Optically, minjiangite is uniaxial (+), with omega = 1.587(3), epsilon = 1.602(2) (lambda = 589 nm). Electron-microprobe analyses (average of 8) give P2O5 40.16, BaO 43.01, BeO 14.06 (measured by Secondary Ion Mass Spectrometry), SiO2 0.17, CaO 0.17, SrO 0.08, FeO 0.03, MgO 0.01, TiO2 0.07, K2O 0.05, Na2O 0.11, total 97.92 wt.%. The empirical formula, calculated on the basis of 8 O a.p.f.u., is (Ba0.99Ca0.01Na0.01)(Sigma 1.01)Be-1.98(P1.99Si0.01)(Sigma 2.00)O-8. The powder X-ray diffraction (XRD) pattern of minjiangite perfectly fits that of synthetic BaBe2(PO4)(2); the strongest eight lines of the powder XRD pattern of the natural phosphate [d in angstrom (I)(hkl)] are: 3.763(100)(101); 2.836(81.3)(102); 2.515(32.3)(110); 2.178(25.6)(200); 2.1620(19)(103); 2.090(63.9)(201); 1.770(16.2)(113); 1.507(25.4)(212). Unit-cell parameters, refined from the powder XRD pattern of natural minjiangite, are a = 5.030(8), c = 7.467 (2) angstrom, V = 163.96(3) angstrom(3). These unit-cell parameters confirm that minjiangite is the natural analogue of synthetic BaBe2(PO4)(2) (P6/mmm, a = 5.029(1), c = 7.466 (1) angstrom, V = 163.52(1) angstrom(3), Z = 1); its crystal structure is topologically similar to that of dmisteinbergite, CaAl2Si2O8, a hexagonal polymorph of anorthite. The formation of minjiangite is related to the hydrothermal alteration of montebrasite by late Ba-and Be-rich fluids.