Clinopyroxene exsolution in wollastonite from Namaqualand granulite, South Africa

摘要

Chemical and crystallographic properties of clinopyroxene exsolution in wollastonite are described from metamorphosed calc-silicate granulite, Namaqualand, South Africa. The wollastonite is Ca1.9Fe0.01Al0.01Si2.01O6 belonging to space group P2(1)/a (2M polytype) and the clinopyroxene is Ca0.99Mg0.75-0.80Fe0.17-0.2tNa0.02Al0.03Si1.99-2.00O6 belonging to C2/c. An electron backscattered diffraction investigation suggests that the clinopyroxene lamellae elongated along [111] lie on (120) and (100) of the wollastonite-2M, and [110] of both lamellae are parallel to [00 1] of the wollastonite-2M. The formation of the exsolution probably results from the relatively high peak metamorphic temperature (800-860 degrees C) of the Namaqualand granulite and its slow cooling rate.