Abstract
Aenigmatite is common in many trachytes, phonolites and agpaitic nepheline syenites. Petrographic evidence suggests that the aenigmatite in these rocks arises by the reaction of Ti-magnetite with a peralkaline silica-undersaturated liquid, and it is postulated that a no-oxide field, where aenigmatite is stable, exists in alkaline undersaturated magmas. This field is similar to that found in silicic liquids but lies below the FMQ buffer curve in \(T - f_{{\text{O}}_{\text{2}} } \) space and is probably confined within narrow limits of temperature and oxygen fugacity. The hydrated equivalent of aenigmatite is possibly astrophyllite and the latter mineral is frequently associated with Na-amphiboles in natural rocks. This suggests that the stability field of astrophyllite is similar to that of Na-amphiboles with respect to temperature and \(f_{{\text{O}}_{\text{2}} } \).
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