Electron Back-Scatter Diffraction Analysis for Twins of Plagioclase in Some Rocks

Twins in plagioclase are very common, and the twin laws are of more than 10 types. However, these twin laws are difficultly distinguished under optic microscope. In this paper, it provides a method based on electron back-scatter diffraction (EBSD) technique to determine accurately twin laws of plagioclase in three kinds of rocks (granite, diorite and leptite), occurring in Dabie Mountain, China. It is found that albite-carlsbad compound twin (including Albite law, Carlsbad law and Albite-Carlsbad law) is the most common twin type in the plagioclase in these three rocks. However, this Albite-Carlsbad compound twin (including 3-4 individuals) under EBSD test is just a polysynthetic twin (only 2 individuals) under optic microscope, because there are 1-2 individuals appearing as small domains inside other individuals, which cannot be resolved under optic microscope. A small amount of Pericline law, Prism {110}, {130} and {130} laws are also found. A new compound twin among Carlsbad law, Prism{110} law and Prism{130} law is found. The Albite-Carlsbad law, Prism {130} law and Prism {130} law can co-exist together to form an intergrowth with three-six fold symmetry, but they cannot form a compound twin. Based on the limited amount of statistics in this paper, the probability of twin laws of plagioclase is unvaried among these three kinds of rocks, possibly reflecting that the twin laws of plagiocalse are not related closely to the formation condition of rocks. The method provided in this paper to analyze twin laws of plagioclase based on EBSD is effective and convenient, which will be helpful to statistic twin laws of plagioclase in rocks on a large scale, and find some new twin laws as well as new compound twins.