An experimental study of the plastic deformation of quartzite has produced microstructures and preferred orientations similar to those found in many natural rocks, and has identified the operative orienting mechanisms in most cases. The microstructures vary widely with conditions and presumably are related to the deformation mechanisms. Below 850°C at 10−5/sec (or 650°C at 10−7/sec), no recrystallization occurs; the deformation of the original grains is very inhomogeneous and deformation lamellae of many orientations are observed. At higher temperatures or slower strain rates, grain boundary recrystallization is present; the original grains are continuously flattened with increasing strain and only basal and prismatic deformation lamellae are observed. Above 800°C at 10−7/sec, recrystallization is complete after low strain.

Below 800°C at 10−5/sec (or 600°C at 10−7/sec), a maximum of c axes develops parallel to the compression direction (σ1), while at higher temperatures and slower strain rates, a small-circle girdle of c axes develops about σ1. The opening half-angle of this girdle ranges from 20° to 45° and increases with increasing temperature and decreasing strain rate. Super-imposed on both of these c axis patterns is a tendency for the poles to positive trigonal forms, and the pole to the second order prism to be aligned parallel to σ1. The preferred orientations of the c axes and the prisms are consistent with external rotations produced by the observed intragranular glide. The difference in the preferred orientations of the positive and negative forms is due to mechanical Dauphiné twinning. Strong evidence exists that these same orienting mechanisms have operated in many naturally deformed rocks.

First Page Preview

First page PDF preview
You do not currently have access to this article.