Combined micro-PIXE facility and monochromatic cathodoluminiscense spectroscopy applied to colored minerals of natural stones: an example from amazonite.

Quantitative micro-PIXE and cathodoluminescence spectra obtained for amazonite crystals from Malawi and Brazil granitoids, in order to check the trace elements presumably responsible for their color. These granitoids, used as Natural Stones for inside coverings and furnishings, are precious and rare, and owe their preciousness to the color of the rock-forming aquamarine amazonite. The trace elements investigated by means of micro-PIXE show the occurrence of Fe, Rb, Pb and Ga at concentrations above the minimum detection limit. In particular, the Malawi amazonite has a higher Pb content than the Brazil amazonite, but for Pb content both samples show a nearly flat pattern profile along the whole crystals. The cathodoluminescence spectra in both samples display three main peaks at 284, 558 and 680 nm which may be attributed to the presence of Pb2+, Mn2+ and Fe3+, respectively. As no Mn was detected by PIXE we suggest that its occurrence is below the PIXE detection limit. Micro-PIXE analysis and cathodoluminescence spectra of most common and uncolored K-feldspars do not reveal the presence of lead; therefore we support the hypothesis of Pb as the chromophore source.