Zoisite-clinozoisite relations in low- to medium-grade high-pressure metamorphic rocks and their implications

Summary Coexisting zoisite and clinozoisite in seventeen specimens from six localities in Japan have been studied with the electron-probe microanalyser. Zoisite and clinozoisite are commonly zoned, but compositional gaps between them are systematic. Referring to the metamorphic grade of the host rocks, a temporary and schematic phase-diagram for the system Ca 2 Al 3 Si 3 O 12 -(OH)-Ca 2 Al 2 Fe 3+ Si 3 O 12 (OH) is presented. With increasing temperature, in the range of low- to medium-grade metamorphism, the compositional gap between the two epidote-group minerals shifts towards higher Fe 3+ compositions.