Some X-ray topographic observations on natural fluorite

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Abstract
Clear, generally flawless specimens, coloured and uncoloured, were studied. X-ray topographs confirmed that cold concentrated H2SO4 is a reliable dislocation etchant. The occurrence of Burgers vectors parallel towas also confirmed. Bundles of very straight dislocations fanning out nearly perpendicular to the local growth face constituted the common grown-in dislocation configuration. In between dislocation bundles dislocation-free volumes of millimetre dimensions could be found. One colourless crystal possessed a dislocation configuration closely resembling that of many melt-grown crystals, and was interpreted as evidence of crystallization of this specimen at a high temperature. Twin boundaries of interpenetrating twins appear to trap impurities which cause long-range distortions. An occurrence of fault surfaces coinciding with growth sector boundaries was discovered, indicating a growth-direction-dependent departure from cubic symmetry. Absolute measurements of 111 and 220 structure factors from x-ray Pendellosung fringe spacings could not be made with less than several per cent error because of lattice distortions.