Describes the compositions and modes of occurrence of amphiboles of the riebeckite-glaucophane group, with special reference to stability relationships in terms of their significance for interpreting the range of temperature-pressure conditions under which glaucophane schists are formed, as distinct from those governing the formation of green schists and amphibolites. The presence of riebeckite, magnesioriebeckite, and aegirine molecules in solid solution is considered to extend the stability field of glaucophanic amphiboles considerably. Glaucophane schists of the Omi district and the Sambagawa-Mikabu zone, Japan, are described as examples.