Chemical determination of coloured zoned minerals in natural stones by EDS/WDS electron microprobe: An example from dumortierite quarzites.

Dumortierite is a rather uncommon borosilicate [(Al1-xx) Al2Al4Si3BO18-3x(OH)3x] occurring in rare and precious commercial natural stones which hold an economic interest related to their properties and colour. These stones are petrographically known as quartzites, whose blue colour is given by variable amounts of dumortierite. A petrographic and mineral chemistry study of blue dumortierite quartzites from Brazil, Mozambique and Madagascar mines, in order to elucidate the relationships between colour and chemical variations, was performed. Two different dumortierite generations were detected. The first occurs as millimetric porphyroblasts from the Mozambique sample and shows a marked zoning for Al, Si, Ti and Sb. The second is characterised by submillimetric prismatic crystals, which are lesser zoned. For both generations, a good correlation between Ti concentration zoning and pleochroism variations was observed. In particular, the comparison between polarised light images of single dumortierite crystals and the Ti distribution X-ray maps evidences the good correlation existing between Ti content and blue colour variations. Moreover, dumortierite from Mozambique quartzites displays an unusual occurrence of Sb, with concentrations ranging up to 5 wt.% (expressed as Sb2O3).