Major- and Trace-element Composition of Paraiba-type Tourmaline from Brazil, Mozambique and Nigeria

摘要

Copper-bearing tourmalines are highly prized for their vivid coloration. We analysed the major and trace elements of some gem-quality Cu-bearing tourmalines (e.g. blue, greenish blue, yellowish green, green, violet and pink) from Brazil, Mozambique and Nigeria. Most of them contained significant amounts of Cu, Mn or a combination of both elements. There was no clear-cut correlation of the Cu and Mn contents with coloration. Blue colour was in most cases due to Cu2+. Pink and violet coloration (due to Mn3+) was shown by Mn-bearing tourmalines that contained no significant Fe. Green colour in the Nigerian tourmaline was most probably due to a combination of Mn, Cu and Fe. Some of the green samples from Brazil contained up to 0.6 wt.% V2O3. Among the trace elements, remarkable contents of Pb (up to 4,000 ppm) and Bi (up to 2,900 ppm) were detected rarely in samples from all three countries. Based on a comparison of unheated pink and violet samples with data for blue Parafba-type tourmalines, CuO/MnO_(tot) is usually >0.5 for unheated blue samples. Hence, we suggest that blue Cu-bearing tourmalines with CuO/MnO_(tot) <0.5 may have been heat treated to reduce the contribution of the reddish component of Mn3+.