X-ray diffraction topographic study of twinning and growth of natural spinels

摘要

Two spinel twinned crystals coming from Pegu (Myanmar) have been studied by means of X-ray topographic methods, optical microscopy and electron microprobe analysis. Topographs, taken with conventional and synchrotron sources, showed the presence of a twin lamella in both samples as well as the presence of cross-twinning in one of them. In particular, white beam synchrotron radiation source topography allowed to detect the twin individuals, to identify and to locate the twin planes, and to determine spatial relationships and mutual orientations of the twin components. By combining optical and topographic observations, an unusual morphology, flattened crosswise the twin planes bounding the twin lamella, has been outlined in both samples and related to the growth mechanism. Microchemical analyses showed that the twin individuals in each sample have nearly identical compositions, whereas the samples differ mainly for the Mg and Zn contents. These observations suggest that they grew in similar and chemically closed microenvironments. The results, taken as a whole, indicate that twinning and cross-twinning occurred since nucleation of the samples, and they mark similar growth conditions.