Optical and electron microscope investigation of temperature-dependent microstructures in anorthoclase

Abstract

Optical and transmission electron microscopy has been used to study the cross-hatched microstructure (associated with the intersection of albite and pericline twin lamellae) that is characteristic of b*c* sections of anorthoclase. With the aid of a specimen-heating stage fitted to each microscope, the changes that occur in the microstructure on heating and subsequent cooling have been observed over the temperature range from ~20 to 950 °C. Observations of unheated crystals indicate that the cross-hatched microstructure consists of a chessboard pattern of triclinic areas (diagonal pairs of which are essentially albite-twin related) and of optically monoclinic areas that consist of domains of fine-scale albite-twin lamellae and domains of pericline-twin lamellae. The four-spot diffraction pattern characteristic of M twinning is observed only from volumes of specimen containing both types of domain. The complex twin interactions at the domain boundaries are described in detail. The changes in optical contrast of the twins when the crystals are heated are shown to be due to changes in the orientation of the refractive index ellipsoid; the disappearance of twins does not necessarily indicate a change to monoclinic symmetry. The observations also indicate that the relative stability of albite and pericline twins is a function of temperature, the pericline twin being more stable than the albite twin at high temperatures near the monoclinic–triclinic transformation and the opposite at room temperature. Evidence is also given that the twin microstructure that develops at the transformation is strongly influenced by external stress.