PHASE RELATIONS ON THE ACTINOLITE-PARGASITE JOIN

Abstract

Phase relations along the join Ca 2 Mg 4 Fe2+Si 8 O 22 (OH) 2 (Actinolite)-NaCa 2 Mg 3�2 Fe 0�8 2+AlSi 6 Al 2 O 22 (OH) 2 (Pargasite) have been studied at P H 2 O = 1 kb and the oxygen fugacities defined by the iron-wustite(IW) buffer. Actinolite and bornblende are separated by a solvus and the field of actinolite+hornblende+vapor is present in the region between Ac 85 Pa 15 and Ac 55 Pa 45 at 680 �C. Complete miscibility is achieved at 720 �C. At temperatures higher than the solvus there is a continuous solid solution series between the two end members. The stability field of amphibole solid solution gradually increases with increasing pargasite content in actinolite. The phase assemblages at temperatures higher than those of a solid solution series between the two end members change with increasing pargasite content in the bulk composition as follows; Act+Cpx+Qz+V, ActHbl+Cpx+Opx+Qz+V, Hbl+Cpx+Opx+Pl+V and Hbl+Cpx+Pl+Ol+V. In comparison with the Fe-free system, the extent of the miscibility gap between actinolite and hornblende is reduced by an increase in the Fe2+ content. The present study should provide an adequate basis for the interpretation of actinolite-hornblende pairs in metamorphic rocks.