Symmetry of actinolite in low grade metamorphic rock and its implication for the problem of the compositional gap between actinolite and hornblende.

Abstract

Actinolite and hornblende commonly coexist in metabasite of upper greenschist-lower amphibolite facies. Two contradictory interpretations for the coexistence of two calcic amphiboles are suggested, that is, miscibility gap and transitional loop between actinolite and hornblende. We determined the space group of actinolite obtained from a greenschist of the Sanbagawa belt in central Shikoku. The greenschist was taken from lower metamorphic temperature area than that of the hornblende first appearance. Weissenberg and precession photographs indicate that the actinolite have C-lattice, and the existence of the center of symmetry is confirmed by treating the reflected X-ray intensities obtained by 4-circle diffractometer with statistical method (Nz method). The space group of the actinolite is C2/m. Since the actinolite has the same space group as that of hornblende, no crystallographic transformation can be considered in calcic amphibole. Hence, the coexistence of actinolite and hornblende should be explained by the miscibility gap between them.