Transmission electron microscope study of crystal defects in natural fluorite

Large loops (>1000 Å) and an ordered arrangement of defect aggregates (∽100 Å) in the (111) plane of natural fluorite have been observed by transmission electron microscopy. The loop images are observed as double-arc contrast or strain-field contrast. Loops lie on {111} planes and have Burgers vectors of the type (a/2) 〈110〉. The defect aggregate array gives rise to superlattice reflections in (111) electron diffraction patterns which are indexed on a cubic unit cell with lattice parameter as = {article}{empty}{document}√ 3{document} a0, where a0 is the fluorite (CaF2) lattice parameter. The ordered defect array is observed to intensify in the electron microscope, and electron irradiated areas possess a deep blue coloration after electron microscopy. The associated coloration leads to the conclusion that the defect aggregates are F-aggregate (color) centers, and conversely that color centers or color-center aggregates are responsible for coloration in natural fluorites.