Striations and hollow channels in rounded beryl crystals

Abstract

Periodico di Mineralogia (2011), 79, 1 (Special Issue), 75-87 - DOI: 10.2451/2011PM0006 Special Issue in memory of Sergio Lucchesi Striations and hollow channels in rounded beryl crystals Gioacchino Tempesta * , Eugenio Scandale and Giovanna Agrosi Dipartimento Geomineralogico, Universita di Bari, Italy *Corresponding author: tempesta@geomin.uniba.it Abstract Structural defects in natural colourless beryl crystals from Minas Gerais (Brazil) were studied using X-ray Diffraction Topography (XRDT). The samples are characterised by a strongly rounded morphology and by the presence of hollow channels parallel to the c-axis, some of them visible to the naked eye and partially filled with kaolinite. The analysis of structural defects as dislocations, growth bands, solid inclusions and precipitates has been essential for the reconstruction of growth history in this study. The formation of hollow channels was attributed to the corrosion and post-genetic alteration of strongly deformed areas surrounding branches of dislocations parallel to the c-axis. The final rounded morphologies and the striations may have been attained either as the results of a parallel growth of some individuals or as the result of a variation of the growth rates in the final stages of growth. Due to the occurrence of kaolinite, post-genetic corrosion has been speculated. However, this process appears to have contributed solely to the hollow channel formation and not to the rounded morphology evident in these samples. Key words : X-ray topography; beryl; channels; morphology and striations.