Chemical investigation of coloured minerals in natural stones of commercial interest.

The economic interest in natural stones provides an interesting input to the study of coloured rock-forming minerals. Generally, the colour of a mineral is attributed either to the presence of intrinsically-coloured constituents or of trace elements substituting major elements in the crystallographic sites, or to physical factors such as crystal lattice defects. We have analysed the blue and blue-green silicate minerals dumortierite and amazonite, coming from Mozambique quartzites, and from Malawi and Brazil granitoids, respectively. This study was performed by a multiple analytical approach including (a) a comparison of different signals (optical light-polarized images, backscattered electron images and X-ray multielemental maps); and (b) a collection of quantitative data obtained by WDS systems for major elements, and by WDS and/or micro-PIXE facility for trace elements. A good correlation between Ti concentration zoning and pleochroism variations was observed in dumortierite. In addition, the blue dumortierite of Mozambique, with its unusual significant amounts of antimony (about 5 wt.% Sb2O3), represents a mineral with an intermediate composition between dumortierite and holtite. In amazonite the aquamarine colour seems to be related to the Pb content on the order of some hundreds of ppm.