Alkali Amphiboles from the Blueschists of Cazadero, California

Alkali amphiboles from Type III and Type IV metamorphic zonesin blueschist facies rocks of Cazadero, California, and fromcomparable New Caledonian rocks have been characterized by X-raycrystallographic, optical, and chemical methods. The compositionof any particular alkali amphibole is strongly controlled bythe bulk composition of the host rock. Within the blueschistfacies, metamorphic zones are not characterized by changes inamphibole composition. All the alkali amphiboles studied hereinbelong to the C2/m space group and complete miscibility betweenglaucophane and riebeckite has been demonstrated for the conditionsprevailing during metamorphism in the Cazadero and New Caledonianblueschists. Linear relationships are found between unit-celldimensions and variations in composition between glaucophaneand riebeckite. The alkali amphiboles of glaucophane compositionsbelong to the high pressure-low temperature series, glaucophaneII-riebeckite. Limited miscibility of actinolite in glaucophanemay be characteristic of blueschist facies metamorphism.