Anorthite

Article By:

Ribbe, Paul H. Formerly, Department of Geological Sciences, Virginia Polytechnic Institute, Blacksburg, Virginia.

Last reviewed:January 2021

DOI:https://doi.org/10.1036/1097-8542.037000

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• Anorthite, published October 2019:Download PDF Get Adobe Acrobat Reader

The calcium-rich plagioclase feldspar with composition Ab₀An₁₀₀ to Ab₁₀An₉₀ (Ab = NaAlSi₃O₈; An = CaAl₂Si₂O₈). Anorthite (see illustration) occurs in olivine-rich igneous rocks and rare volcanic ejecta (for example, at Mount Vesuvius, Italy, and Miyakejima, Japan). Hardness is 6 on Mohs scale, specific gravity 2.76, melting point 1550°C (2822°F). The crystal structure of Ab₀An₁₀₀ (triclinic space group Pī) consists of an infinite three-dimensional array of corner-sharing [AlO₄] and [SiO₄] tetrahedra, alternately linked together in a framework of [Al₂Si₂O₈]_∞²⁻ composition in which charge-balancing calcium (Ca²⁺) cations occupy four distinct, irregular cavities. The crystal structure of anorthite inverts to higher symmetry (Iī) with each of the following: (1) heating above 242°C (468°F), (2) adding ∼10 mole % Ab component (substituting Na⁺ + Si⁴⁺ ↔ Ca²⁺ + Al³⁺), (3) increasing hydrostatic pressure above ∼2.6 gigapascals (26,000 atmospheres), and (4) effectively disordering the aluminum (Al) and silicon (Si) atoms at very high temperatures. Each inversion is evidenced by certain types of structurally out-of-step (antiphase) domains that can be imaged by transmission electron microscopy. Natural anorthite has no commercial uses, but the synthetic material [CaO · Al₂O₃ · 2SiO₂ (known as CAS2)] is important in the ceramic industry and in certain composite materials with high-temperature applications. See also: Crystal structure; Feldspar; Igneous rocks

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