Identification of twins in muscovite: an electron backscattered diffraction study

Abstract

Abstract Twins in micas are difficultly identified due to mica’s hexagonal pseudosymmetry. Many theoretic studies on mica twins have been reported but experimental observations are very limited. In this paper, we present an electron backscattered diffraction analysis to identify twins in the muscovite in a quartz schist occurring in the UHP-HP metamorphic rock belt in Dabie Mountain, China. A trilling twin with twin law <310>/{110} is common in the muscovite. A six-couplet twin consisting of two trilling twins related by twin laws <110>/{130} and <001>/{001}(or <100>/{100}) has been discovered. This six-couplet twin contains many cross-twin relationships among the most common mica twin laws <310>/{110}, <110>/{130} and <001>/{001}. The composition plane for twin laws <110>/{130} and <001>/{001} is {001} which is reasonable in mica structure to form a twin by rotation around twin axes, and that for twin law <310>/{110} is irregular based on EBSD resolution. A possible misindexation of a trilling twin or a 3T polytype during EBSD test is discussed, which is helpful to distinguish a twin from a polytype in micas. The occurring frequency of twin law <310>/{110} is higher than that of twin laws <110>/{130} and <001>/{001}, which is consistent to the deducing result from mica structure analysis. This research provides a convenient and effective EBSD method to identify mica twins and an experimental method to distinguish a twin from a polytype in micas, which is a problem confusing researchers for many years.