Geochemical and morphological characteristics of coarse-grained tabular beryl from the Xuebaoding W–Sn–Be deposit, Sichuan Province, western China

Abstract

The dominant crystal faces on beryl are usually hexagonal prisms, a direct reflection of the lattice structure. However, coarse-grained beryl from the Xuebaoding W–Sn–Be deposit, China, has a tabular habit. To evaluate the genesis of this well-crystallized tabular beryl, we analysed the crystals using inductively coupled plasma mass spectrometry (ICP–MS), X-ray fluorescence (XRF) spectrometry, X-ray powder diffraction (XRD), and differential interference contrast microscopy (DICM). The beryl unit cell dimensions are 9.223–9.232 Å for a 0 and 9.183–9.244 Å for c 0. The ratio c 0 /a 0 = 0.995–1.002. The beryl has a high alkali content, including Li2O (2.60–3.10 wt.%), Rb (65–120 ppm), Cs2O (0.70–0.92 wt.%), and Na2O (1.29–1.66 wt.%); other components include TiO2 (0.00–0.01 wt.%), TFe2O3 (1.03–1.29 wt.%), MnO (0.01 wt.%), MgO (0.09–0.12 wt.%), and CaO (0.03–0.04 wt.%). These analyses indicate that the Xuebaoding material is a Na–Li–Cs beryl with a dominance of tetrahedral substitution in the crystal structure. Based on a systematic statistical study of the relationship between chemical composition and habit, a high content of alkalis leads to a lower growth rate of c {0001} compared with m , producing the short prismatic/columnar habit that typifies the Xuebaoding beryl. The difference in the number of pyramidal growth hillocks on s faces that face in opposing directions strongly suggests the influence of unidirectional hydrothermal fluid flow on growth rates. The oriented flow also led to variable growth rates for other crystal faces, including m and c {0001}, and caused the growth of many thick, distorted crystals of scheelite, cassiterite, tabular apatite, etc. Apparently, the m faces were outgrown by other, more rapidly growing faces and consequently tended to disappear. Accordingly, we discuss two mechanisms that underlie the formation of tabular beryl crystals: (1) the geochemical compositions and especially the high amount of alkalis at the Be site and in the channel structures of the crystals; and (2) the directional flow and supply of hydrothermal fluids in narrow fractures during crystal growth.