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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 \(f_{{\text{O}}_{\text{2}} } \).

Alkaline igneous complexes form distinct lineaments in Angola, South West Africa, Brazil, and Uruguay. Each of these lineaments lie along small circles centered on the Cretaceous and present poles of rotation for the South Atlantic and can be correlated with a unique fracture zone/transform fault that offsets the Mid-Atlantic ridge. It is suggested that the lineaments are continental expressions of these fracture zones. Isotopic age determinations from complexes in the lineaments lie in the range 120–135 my and 50–80 my and are consistent with the current model for the evolution of the South Atlantic and the origin of the lineaments as suggested here.