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. 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.