Mineralogical classification and crystal water characterisation of beryl from the W–Sn–Be occurrence of Xuebaoding, Sichuan province, western China

Abstract

Beryl from Xuebaoding, Sichuan Province, western China is known for its unusual tabular habit and W–Sn–Be paragenesis in a greisen-type deposit. The crystals are typically colourless transparent to pale blue, often with screw dislocations of hexagonal symmetry on the (0001) crystal faces. Combining electron microprobe analyses and laser ablation inductively coupled plasma mass spectrometry with single-crystal X-ray diffraction (XRD), correlated with Raman and micro-infrared (IR) spectroscopy and imaging, the crystal chemical characteristics are determined. The contents of Na + (0.24–0.38 atoms per formula unit (apfu)) and Li + up to 0.38 apfu are at the high end compared to beryl from other localities worldwide. Li + substitution for Be 2+ on the tetrahedral (T2) site is predominantly charge balanced by Na + on the smaller channel (C2) site, with Na + ranging from 91.5% to 99.7% (apfu) of the sum of all other alkali elements. Cs + and minor Rb + and K + primarily charge balance the minor M 2+ substitution for Al 3+ at the A site; all iron at the A site is suggested to be trivalent. The a axis ranges from 9.2161(2) to 9.2171(4) Å, with unit-cell volume from 678.03(3) to 678.48(7) Å 3 . The c/a ratio of 1.0002–1.0005 is characteristic for T2-type beryl with unit-cell parameters controlled primarily by Be 2+ substitution. Transmission micro-IR vibrational spectroscopy and imaging identifies coordination of one or two water molecules to Na + (type IIs and type IId, respectively) as well as alkali free water (type I). Based on IR absorption cross section and XRD a C1 site water content of 0.4–0.5 apfu is derived, i.e. close to 50% site occupancy. Secondary crystal phases with a decrease in Fe and Mg, yet increase in Na, suggest early crystallisation of aquamarine, with goshenite being late. With similar crystal chemistry to beryl of columnar habit from other localities worldwide, the tabular habit of Xuebaoding beryl seems to be unrelated to chemical composition and alkali content.