Symmetric Intergrowth of Quartz in Igneous Rocks

Crystal intergrowth (or crystal clustering) is an important phenomenon during crystal growth and textural development of igneous rocks,which can provide much information about magmatic processes. We used electron backscattered diffraction (EBSD) technique and pole figures to analyze the quartz orientation relationships in Meichuan granodiorite and Tiantangzai granite. Dauphiné twins and intergrowths (by overlapping the same plane) are very abundant in these two rocks. Beside Dauphiné twins and intergrowths,we found a special kind of symmetric intergrowths among quartz crystals in these two rocks. Considering quartz's Laue Group 3 2/m (except the point group 32),Dauphiné twin plane (0001) or (1010) overlaps the symmetry plane (1120) of the third crystal in some intergrowths of quartz. The intergrowths composed by these three crystals retain some symmetry of Dauphiné twins because of the overlapping. The overlapping between twin plane and the symmetry plane of the third crystal has also been found in the aggregation of NH₄Cl crystallites and diopside dendritic crystals reported in our previous work. It is reflected that symmetry controls the intergrowth of crystals. Laue Group is related to lattice symmetry but not to crystal structure symmetry. The lattice,rather than the crystal structure,plays an important role in quartz intergrowth in rocks.