Polymorphism of wollastonite

Abstract

Different superstructures of wollastonite from the regional metamorphic Lepontine zone (Southern Alps) and other localities (e.g., Sierra Nevada, California) have been found. Besides well ordered normal 1T, 2M (2T ?), 4T-wollastonite, all disordered intermediate states exist. The various polymorphs were studied with the precession method. The superstructure is observed in reflections hkl k=odd. 4T-wollastonite is described with a ₀=31.659 Å and is shown to be a common polymorph. The superstructure seems to be independent of metamorphic grade (in the well-zoned Lepontine area it does not follow any mineralogical isograds) and occurs only in strained rocks with well developed lineation and strong preferred orientation. Thus the formation of the superstructure is explained as a deformation effect. Contrary to temperature and pressure, both stress and strain show large local variations and inhomogeneities. Experimental deformation of a 2T-wollastonite at 4 kb and 700° C produced complete disorder. It is suggested that consecutive annealing of strained disordered wollastonite causes periodic stacking sequences of [SiO₃]-chains along the a^*-axis. It appears from these studies that superstructures in chainsilicates are more common than presently known and that they might be useful petrogenetic indicators for the deformation and cooling history of a rock.