Mayenite

Chlormayenite
Crystal structure of chlormayenite (Cl− ions are omitted for clarity)
General
Category	Oxide mineral Mayenite supergroup
Formula (repeating unit)	Ca12Al14O32[☐4Cl2]
Strunz classification	4.CC.20
Crystal system	Cubic
Crystal class	Hextetrahedral (43m) H-M symbol: (4 3m)
Space group	I43d
Unit cell	a = 11.98 Å; Z = 2
Identification
Color	Colorless
Crystal habit	Microscopic anhedral grains
Streak	White
Diaphaneity	Transparent
Specific gravity	2.85
Optical properties	Isotropic
Refractive index	1.614–1.643
Alters to	Absorbs water on exposure
References

Chlormayenite (after Mayen, Germany), Ca₁₂Al₁₄O₃₂[☐₄Cl₂], is a rare calcium aluminium oxide mineral of cubic symmetry.

It was originally reported from Eifel volcanic complex (Germany) in 1964. It is also found at pyrometamorphic sites such as in the Hatrurim Formation of Israel and in some burned coal dumps.

It occurs in thermally altered limestone xenoliths within basalts in Mayen, Germany and Klöch, Styria, Austria. In the Hatrurim of Israel it occurs in thermally altered limestones. It occurs with calcite, ettringite, wollastonite, larnite, brownmillerite, gehlenite, diopside, pyrrhotite, grossular, spinel, afwillite, jennite, portlandite, jasmundite, melilite, kalsilite and corundum in the limestone xenoliths. In the Hatrurim it occurs with spurrite, larnite, grossite and brownmillerite.

Synthetic Ca₁₂Al₁₄O₃₃ and Ca₁₂Al₁₄O₃₂(OH)₂ are known, they are stabilized by moisture instead of chlorine. The formula can be written as [Ca₁₂Al₁₄O₃₂]O, which refers to the unique feature: anion diffusion process.