All Minerals

Anorthite

Formula: CaAl2Si2O8

A plagioclase feldspar (vs. typical alkali feldspars used in ceramics). More precisely, it is a calcium end-member of the plagioclase feldspar series.

Anorthite is also a crystal phase that makes bone china possible (it forms and grows during firing). Bone china is termed "anorthite porcelain" (vs typical "mullite porcelains"), more precisely it is a CaO - P₂O₅ - Al₂O₃ - SiO₂ glass-ceramic system. During the firing, feldspar melts, kaolin decomposes to source free SiO₂, bone ash provides CaO. The melt becomes CaO -Al₂O₃ - SiO₂ rich. Anorthite crystallizes out of that melt. It is a recrystallization process driven by CaO reacting with an aluminosilicate melt.

Anorthite is also formed in high-calcium glazes, especially cone 9–10 lime mattes (or calcia mattes). The characteristic microcrystalline matte surfaces are typically CaO - Al₂O₃ - SiO₂ rich. In glaze textbooks, references to “lime matte crystallization” or “calcium aluminosilicate crystals” is frequently referring to anorthite.

In Portland cement and kiln clinkers: It appears in phase diagrams and ceramic engineers studying kiln reactions encounter it frequently.

Calcium porcelains (explored historically as alternatives to feldspathic porcelain): Where CaO replaces part of the alkali flux, anorthite becomes a major crystalline phase.

In devitrification in high-CaO glazes (especially slow-cooled): Anorthite, wollastonite and other Ca silicate crystals can form.

In glaze defect analysis: Anorthite may appear in XRD results when investigating matte surfaces, devitrification, stiff calcium glazes, high calcium feldspars and calcareous clays.

Phase diagram literature: In the CaO - Al₂O₃ - SiO₂ phase triangle (a classic in ceramic texts), anorthite is a primary phase field.

Other things get more attention in pottery literature or studio-level discussions: E.g. Mullite, quartz, cristobalite.

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