anorthite

n.
A rare plagioclase feldspar with high calcium oxide content occurring in igneous rocks.

[From Greek an-, not; see a-¹ + orthos, straight (from its oblique crystals).]

anorthitic an'or·thit'ic (-thĭt'ĭk) adj.

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The calcium-rich plagioclase feldspar with composition Ab₀An₁₀₀ to Ab₁₀An₉₀ (Ab = NaAlSi₂O₈; An = CaAl₂Si₂O₈), occurring 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 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. 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 Feldspar.

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The noun has one meaning:

Meaning #1: rare plagioclastic feldspar occurring in many igneous rocks

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