Compositional control of plane group symmetry in tourmalines; an experimental and computer simulated TEM, crystallographic image processing and Moessbauer spectroscopy study
Compositional control of plane group symmetry in tourmalines; an experimental and computer simulated TEM, crystallographic image processing and Moessbauer spectroscopy study
European Journal of Mineralogy (June 1993) 5 (3): 479-492
Tourmalines with different compositions show different plane group symmetries in experimental HRTEM when projected along the c axis. Fe-poor elbaite shows p31m symmetry, while Fe-bearing elbaite shows p3ml symmetry. Rubellite shows p6 symmetry, while Fe-Mg-Al- bearing tourmaline shows a 'pseudo' p6 type of symmetry. Symmetries p3ml and p6 are not compatible with the space group R3m assigned to tourmaline from XRD structure determinations. The perturbations of symmetry depend on the magnitude of the scattering amplitude of the atoms occupying the 3a and 9b sites. Computer simulations of elbaite, schorl and dravite show different projected potentials for images down the c axis. In Fe-poor elbaite regions with high mean concentration of light or heavy elements a few unit cells apart are common, giving a marked difference in the distribution contrast in the HRTEM image. Experimental HRTEM, computer simulation and Mossbauer spectroscopy all indicate that in tourmaline the composition controls symmetry.