XRDT AND TEM STUDY OF THE STRIATIONS OF THE TOURMALINE PRISM FACES

Natural crystals of tourmaline are characterized by striations parallel to the c axis on prismatic faces. Crystals without striations are really rare, so it is important to understand the origin and the meaning of striations. Different hypotheses were made about the origin of the striations and the most important of which are: 1) juxtaposition of prismatic facets, that make curved prism surfaces, 2) multiple parallel twinning. A different conclusion was made in a study of tourmaline crystals from pegmatite pockets of island of Elba (Italy) [1]: the origin of striations was attributed to a particular type of growth mechanism. Two growth stages were identified in the quoted study from both defect analysis and crystalchemical characterization of the samples. The former stage was the pegmatitic, corresponding to the inner greenish-yellow region dislocation-free, that was characterized by growth bands and growth sector boundaries. Therefore, 3D nucleation occurred and the bulk tourmaline crystals grew either by 2D mechanism or by direct adsorption of growth units, or both these mechanisms at once. The latter stage was hydrothermal and regarded the colorless overgrowth characterized by screw dislocations. During this last growth stage striations were formed. Small crystal grains, in fact, developed parallel to the c axis by spiral growth mechanism. The imperfect juxtaposition of prismatic facets of each crystal grain caused the striations. Nevertheless, it should be emphasized that the absence of striations on prismatic facets of core during the pegmatitic growth stage has not been found yet. In order to deeply investigate the origin of striations, in this study other samples of tourmaline from island of Elba with a greenish-yellow core and colorless overgrowth have been analyzed. In particular, sample fragments with missing external overgrowth have been investigated. Hence it has been possible to observe optically that the prismatic facets of inner core did not show striations. XRDT studies of basal slices confirm that the core is dislocation-free, while the colorless overgrowth is characterized by a high density of dislocations. Therefore these observations confirm the conclusions of Agrosì et al. [1]. In addition, to investigate the interface between core and rim, smaller fragments of tourmalines have been studied by TEM. Preliminary HRTEM results show a perfect structural continuity between inner core and external rim. Moreover, the interface does not correspond to a integer surface but correspond to a transition region characterized by a high concentration of impurities.