Ribbe, Paul H. Formerly, Department of Geological Sciences, Virginia Polytechnic Institute, Blacksburg, Virginia.
Last reviewed:January 2021
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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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