Burnham, Charles W. Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts.
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A group of silicate minerals whose physical properties resemble those of pyroxenes. In contrast with the two-tetrahedra periodicity of pyroxene single silicate chains, the pyroxenoid crystal structures contain single chains of (SiO4)4- silicate tetrahedra having repeat periodicities ranging from three to nine (see illustration). The tetrahedron is a widely used geometric representation for the basic building block of most silicate minerals, in which all silicon cations (Si4+) are bonded to four oxygen anions arranged as if they were at the corners of a tetrahedron. In pyroxenoids, as in other single-chain silicates, two of the four oxygen anions in each tetrahedron are shared between two Si4+ cations to form the single chains, and the other two oxygen anions of each tetrahedron are bonded to divalent cations, such as calcium (Ca2+), iron (Fe2+), or manganese (Mn2+). These divalent cations bond to six (or sometimes seven or eight) oxygen anions, forming octahedral (or irregular seven- or eight-cornered) coordination polyhedra. See also: Pyroxene; Silicate minerals
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