黑硼锡镁矿孪晶的电子衍射分析

Electron Diffraction Analysis of Magnesiohulsite Twin

黑硼锡镁矿是1985年在我国发现的一种新矿物,微孪晶特别发育,其电子衍射斑点往往具规律的分裂现像。用选区电子衍射获得[010]、[141]和[121]定向带轴电子衍射斑点分裂图,经矩阵分析,确定两套斑点分裂是由其呈挛晶结构关系所致,孪晶面为(101),属于面律孪晶,这种孪晶的微形貌由明场像(BF)得到证实。根据不同取向的电子衍射图,确定该矿物属单斜晶系,a_0=10.7,b_0=3.01,c_0=5.39,β=94.5°。上述挛晶结构特征及其晶体学数据与X射线单晶研究结果有很好的可比性。

Magnesiohulsite is a new mineral species found in China in 1985. It occurs in the magnesian skarn zone between biotitic granite (γ_5~2)and Carboniferous dolomitic limestone in Dayi Mountain, Hunan Province. Quite a lot of its crystals have microtwins. The electron diffraction spots split regularly. Using the selected area electron diffraction method (SAED), We have obtained the splitting electron diffraction spots patterns of [010],[41] and [21] zone axes. Through matrix analysis of the microtwins, it is determined that the two sets of splitting spots are of twin structure relationship, belonging to face-law twin with (101) twinning plane. Its microshapes are demonstrated by bright field image. Different kinds of electron diffraction patterns show that the mineral belongs to monoclinic system with a_0 10.7, b_0 3.01, c_0 5.39 and β 94.5°. Characteristics of this twin structure and its crystallographic data are quite comparable with the result of X-ray singlecrystal analysis. This paper provides a quite good mineralogical method for studying twins, especially microtwins of the mineral.