Plagioclase twins in a basalt: an electron backscatter diffraction study

Abstract

Twins in plagioclase, which are abundant in rocks, have important implications for the growth history and subsequent transformation. There are many twin laws in plagioclase and some of them are difficult to identify. This paper presents an electron backscatter diffraction (EBSD) analysis combined with electron microprobe analysis of plagioclase twinning in the Niutoushan alkali basalt. Theoretical pole figures of 12 different twin laws for plagioclase were generated. By comparing the pole figures obtained from EBSD with the theoretical ones, 11 twin laws have been identified in the alkali basalt, and their frequency has also been determined: Albite (28%), Carlsbad (25%), Albite–Carlsbad (34%), Pericline (3%), Ala (2%), Manebach (1%), Albite–Ala (1%), Prism {110} (2%), Prism {1\bar 10} (1%), Prism {130} (1%) and Prism {1\bar 30} (1%). All the plagioclase twins in the alkali basalt are growth twins. The anorthite content of the plagioclase has a negative correlation with the frequency of the Pericline and Albite twin laws but a positive correlation with the frequency of all the other twin laws, which is consistent with previous results. The theoretical pole figures of twin laws for plagioclase introduced in this paper can be applied to investigating plagioclase twin laws in all plagioclase-bearing rocks, and the relationship between twin pattern and crystallization conditions of the rocks can be established.