Barton, Mark D. Department of Earth and Space Sciences, University of California, Los Angeles, California.
- Additional Readings
The most common beryllium mineral. Beryl, Al2[Be3Si6O18], crystallizes in the hexagonal system with space group P6/mcc. The crystal structure consists of six-membered rings of corner-sharing silicon-oxygen (SiO4) tetrahedra cross-linked by corner-sharing beryllium-oxygen (BeO4) tetrahedra to make a three-dimensional honeycomb structure; aluminum-oxygen (AlO6) octahedra lie between the Si6O18 rings (see illustration). Cordierite, (Mg,Fe)2[Al4Si5O18], has a similar structure. Open channels parallel to the sixfold axis contain variable amounts of alkalies (up to 5–8 wt %), such as lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs), and water molecules (up to ∼2.5 wt %). Minor amounts of iron (Fe), chromium (Cr), manganese (Mn), and other metals substitute for aluminum, contributing to the various colors of natural beryl. Beryl has a vitreous luster and is typically white to bluish- or yellowish-green, but it can also be shades of yellow, blue, and pink. Its hardness is 7.5–8 on Mohs scale; it has an imperfect basal cleavage and a specific gravity of 2.7–2.9 (increasing with alkali content). Weakly colored varieties can be confused with quartz or apatite. See also: Crystal structure; Hardness scales
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