Symmetric Intergrowth of Quartz in Igneous Rocks
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摘要: 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3 2/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(1010)与第3个晶体的对称面(1120)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.Abstract: 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.
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图 1 梅川花岗闪长岩中石英晶体的取向扫描图
扫描范围大约为1.5 cm × 3.5 cm；其中圈定了92个颗粒.颗粒中石英晶体之间的黑线是由EBSD系统软件自动生成的道芬双晶界，其定义为： < 0001 > /60°
Fig. 1. Quartz orientation map of the scanning area (about 1.5 cm × 3.5 cm) of the section of Meichuan granodiorite, with the delineated circles and numbers for each grain
图 2 G30号颗粒中石英晶体的极图分析(右上角是G30号颗粒的取向扫描图)
Fig. 2. Pole figures of Grain G30 (the upper-right-hand corner shows the orientation map of Grain G30)
图 3 D7号颗粒中石英晶体的极图分析(右上角是D7号颗粒的取向扫描图)
Fig. 3. Pole figures of Grain D7 (the upper-right-hand corner shows the orientation map of Grain D7)
图 4 D54号颗粒中石英晶体的极图分析(右上角是D54号颗粒的取向扫面图)
Fig. 4. Pole figures of Grain D54 (the upper-right-hand corner shows the orientation map of Grain D54)
图 5 G50号颗粒中石英晶体的极图分析(右上角是G50号颗粒的取向扫面图)
Fig. 5. Pole figures of Grain G50 (the upper-right-hand corner shows the orientation map of Grain G50)
图 6 D68号颗粒中石英晶体的极图分析(右上角是D68号颗粒的取向扫面图)
Fig. 6. Pole figures of Grain D68 (the upper-right-hand corner shows the orientation map of Grain D68)
图 7 NH4Cl晶体聚集体中晶体A, B, A’, B’之间的双晶关系示意(a)和透辉石枝晶聚集体中晶体1, 3, 5之间的双晶关系示意(b)
a.详细分析见Zhao et al.(2007); b.详细分析见Zhao et al.(2010)
Fig. 7. A sketch of the twin relationship among Crystals A, B, A' and B' in NH4Cl crystallite aggregation (a) and a sketch of the twin relationship among Crystals 1, 3 and 5 in diopside dendrites (b)
图 8 各种石英对称性连生体的理想立体图(Shape V7.1.2)
对于G30, D7，D68：第2个图是表示另外一种方位的立体图；对于D54和G50：第2个图是将晶体3或晶体3-4与晶体1-2分离后的立体图
Fig. 8. Ideal stereogram of the quartz symmetric intergrowths (by Shape V7.1.2)
表 1 石英各种双晶律、交生及无关取向连生的概率
Table 1. The frequency of quartz twin laws, intergrowths and no-relationships
岩石 道芬双晶律 其他双晶律 交生 无关取向连生 天堂寨花岗岩 71 3.2 19.3 6.5 梅川花岗闪长岩 70.7 5.2 21 3.1 注：(1)表中双晶律概率为：(某双晶数×2)/所有石英单晶数，因为双晶是由两个单体组成的，所以具有双晶关系的单体数为：双晶数×2；(2)表中无关取向连生概率为：与别的晶体没有双晶和交生关系的单体数/所有石英单晶数；(3)表中交生的概率为：100 -(各种双晶概率+无关取向连生概率)，因为交生概率如果按照某个交生面统计的话，会有重复计数，不太好统计, 但是，本文不报道这些道芬双晶与交生的具体情况，本文着重研究在这些石英双晶与交生中所发现的对称性连生现象. 
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