岩浆岩中石英晶体的对称性连生

矿物晶体连生是岩浆岩中晶体生长及岩石结构形成过程中的一个重要现象,可以提供岩浆演化过程信息.用电子背散射衍射技术(EBSD)分析了天堂寨花岗岩和梅川花岗闪长岩中石英晶体的结晶学取向关系.这2种岩石中石英的道芬双晶和交生(两晶体以某个相同的面重合连生)出现概率非常高.除了道芬双晶和交生外,还发现了一种特殊的石英晶体对称性连生现象.在这种特殊的对称性连生体中,考虑石英的劳埃群32/m(而不是石英的点群32),两晶体形成的道芬双晶的双晶面(0001)或(10-10)与第3个晶体的对称面(11-20)重合.这样的重合使得这3个晶体的连生体保留了道芬双晶的某些对称性.这种双晶面与第3个晶体的对称面重合的现象在我们以前报道的NH4Cl晶体聚集体和透辉石枝晶聚集体中也出现过.这种现象说明晶体的连生受控于对称性,即:晶体连生会选择对称性高的形式.由于劳埃群是与晶体空间格子的对称性相关而并不是与晶体结构的对称性相关的,所以,空间格子的对称性可能是控制晶体对称性连生的主要因素.

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.