Abstract
AbstractBeryl 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 Be2+ 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 M2+ substitution for Al3+ 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) Å3. The c/a ratio of 1.0002–1.0005 is characteristic for T2-type beryl with unit-cell parameters controlled primarily by Be2+ 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.
Highlights
IntroductionBackgroundBeryl as the most common beryllium mineral is of important geochemical significance in different geological contexts
Beryl as the most common beryllium mineral is of important geochemical significance in different geological contexts
Be3Al2Si6O18, beryl is a cyclosilicate mineral of hexagonal crystal structure
Summary
BackgroundBeryl as the most common beryllium mineral is of important geochemical significance in different geological contexts. Al3+ (A site) in planes parallel to (0001) (Aurisicchio et al, 1988, 1994; Černý, 2002; Groat et al, 2008). The rings form channels parallel to the crystallographic c axis, which in natural beryl typically contain variable amounts of primarily alkali cations and neutral molecules of water, CO2 and CH4 (Ginzburg, 1955; Wood and Nassau, 1968; Hawthorne and Černý, 1977; Aines and Rossman, 1984; Artioli et al, 1993; Charoy et al, 1996; Pankrath and Langer, 2002; Gatta et al, 2006; Groat et al., 2008; Fridrichová et al, 2016). The smaller alkali Na+ occupies the narrower channel site (C2/2b), while the larger K+, Cs+ or Rb+ as well as H2O and CO2 prefer the wider channel site
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