Growth defects of a melanite crystal

A melanite crystal from the volcanic complex of Colli Albani (Italy) was st udied by means of Optical Microscope (OM), X-ray Diffraction Topography (XR DT) and Electron Microprobe Analysis (EMPA). Extended defects - dislocation s, inclusions, precipitates and growth sector boundaries - were observed, a llowing the crystal growth history to be reconstructed and three successive growth stages to be determined. The chemical zonings of the sample, invest igated by means of EMPA, were related to the morphological evolution vs. ti me, obtained by XRDT, and this allowed the discrimination between concentri c and sector zonings to be made. Concentric zoning concerned continuous smo oth concentration variations of SiO2, Al2O3 and TiO2. Sector zoning occurre d sharply in crossing the boundaries between n{211} and d{110} growth secto rs where TiO2 concentrations resulted higher in the {211} growth sectors wi th respect to {110} whereas Al2O3 resulted lower.