• twinning;
  • intergrowth;
  • quartz;
  • electron backscatter diffraction

Quartz has various types of twin law and twinned quartz is more common than untwinned quartz. However, it is difficult to determine the twin laws of quartz by optical or diffraction methods, so the distribution of the different twin laws of quartz in rocks is still unknown. This paper introduces a method to determine quartz twin laws using electron backscatter diffraction (EBSD). Based on this method, the relationships between quartz crystals in the granite of Tiantangzhai pluton, located at the centre of the Dabie Mountain area, China, are analysed. It is found that the Dauphiné twin is very abundant but the other twin laws (Japan, Esterel, Sardinian, Breithaupt and Cornish) are very rare in this granite. Besides twin laws, many kinds of intergrowths among the quartz crystals are also found. In the intergrowths the overlapping faces are {11inline image2}, {10inline image1}, {10inline image2}, {11inline image1} and {20inline image1}, which are simply the twin planes of the Japan, Esterel, Sardinian, Breithaupt and Cornish twin laws, respectively. It is interesting that quartz crystals like to form intergrowths based on these faces but do not like to form twins with these faces as a twin plane. This research shows that the relationship between quartz crystals in the granite is controlled by the crystallographic orientation, resulting in the twins and intergrowths occurring at frequencies far exceeding random contact. It is also suggested that EBSD is the best method to determine the twin laws and intergrowths of quartz in a rock.