Moore, Paul B. Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois.
- Additional Reading
An orthorhombic magnesium aluminosilicate mineral of composition Mg2[Al4Si5O18], a = 1.710 nanometers, b = 0.973 nm, c = 0.936 nm, Z = 4, space group Cccm. The crystal structure is related to beryl; magnesium is octahedrally coordinated by oxygen atoms, and the tetrahedrally coordinated aluminum and silicon atoms form a pseudohexagonal honeycomb, admitting limited amounts of K+ and Na+ ions and water molecules in the open channels. Limited amounts of Fe2+ may substitute for Mg2+, and Fe3+ for Al3+. The hardness is 7 (Mohs scale), specific gravity 2.6; luster vitreous; cleavage poor; and color greenish-blue, lilac blue, or dark blue, often strongly pleochroic colorless to deep blue (see illustration). Transparent pleochroic crystals are used as gem material. The disordered cordierite structure, Mg2[(Al,Si)9O18], is called indialite and is hexagonal, space group P6/mcc, isotypic with beryl. Osumilite, KMg2Al3· [(Al,Si)12O30]·H2O, is also related but has a structure built of double six-membered rings of tetrahedrons. Cordierite, indialite, and osumilite are difficult to distinguish. Pale colored varieties are often misidentified as quartz, since these minerals have many physical properties in common. See also: Beryl; Silicate minerals
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