Chlormayenite
Chlormayenite (after Mayen, Germany), CaAlO[☐Cl], 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 CaAlO and CaAlO(OH) are known, they are stabilized by moisture instead of chlorine. The formula can be written as [CaAlO]O, which refers to the unique feature: anion diffusion process. Chlormayenite is also found as calcium aluminate in cement where its formula is also written as 11CaO·7 AlO·CaCl, or CACaCl in the cement chemist notation.