Pyroxenoid

Article By:

Burnham, Charles W. Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts.

Last reviewed:December 2019

DOI:https://doi.org/10.1036/1097-8542.757357

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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