CONCLUSIONS

High valuable copper-bearing tourmalines occur in different color and large size and are produced in Brazil, Mozambique and Nigeria. The sizes and distribution of these gemstones in the gem market were studied and a statistic about the trade has been published in this report. These types of tourmalines include many different color varieties and are found as heated as well as unheated gems (Fig. Par07 and 12). Unusual large sizes have been found from the mines in Mozambique. The majority of the stones have been heat-treated in the greenish-blue to blue colors, whereas the purple and green colors are more frequently spared of thermal enhancement (Fig. Par07). These findings are in good agreement with the findings from heat-treatment experiments (Lit. Par33), which predicted that heat-treatment of green colors, is possible with limited success only. Copper-bearing tourmalines from Brazil such as from the state of “Paraiba” were more often found to be unheated than Mozambique tourmalines. Gem quality “Paraiba”-tourmalines does occur in much smaller sizes than the African counterparts, but they are generally more saturated in color. According to our opinion, large-sized unheated gem-quality tourmalines from Brazil (Paraiba or Rio Grande do Norte) with intense “neon”-blue colors can be described as extremely rare (See Fig. Par07, 08, 11 and 12).

We found it useful to investigate the origin based on chemical fingerprints using different methods, including EMPA, LIBS, LA-ICP-MS and ED-XRF analyses. Based on these analyses, it was found that the chemical composition of copper-bearing tourmalines from Mozambique, Nigeria and Brazil show distinct differences, which are sufficient selective to determine their origin. The findings on LA-ICP-MS-analyses are in agreement with earlier work in the literature (Lit. Par11 and Lit Par33). The usefulness of diagrams from these earlier works (Lit. Par11) were tested and found to be mostly applicable, such as the ternary diagram Mg-Zn-Pb (See Fig. Par 91a-b), the Cu+Mn versus Ga+Pb-diagram and the Cu+Mn versus Pb/Be-ratio-diagram (Fig. Par88 and Fig. Par89).

However, the data of Abduriyim et al. (Lit. Par11) could not be plotted in some of our diagrams as they did only publish concentration ranges and ratios and no individual quantitative results and the standard deviation, respectively. However, the trends between different colored copper-bearing Mozambique tourmalines including the variations in Fe and Ti as published in the literature (Lit. Par 33) were plotted in diagrams and confirmed (Fig. Par76c, 78f, 81, 82 and 88). Because we focused on chemical profile analyses (LA-ICP-MS) and chemical distribution mapping (EMPA-analyses) of chemical zoning within single tourmalines, however, it was possible to investigate the chemical evolution of the copper-bearing tourmalines of samples from Mozambique and Brazil (Fig. Par91) in more detail. We correlated different zones of different tourmalines with the chemical evolution within the tourmalines. Different evolution patterns were found in Brazilian and Mozambique tourmalines (Fig. Par91). This helped us to explain the spread of the data published elsewhere (Lit. Par 11), such as in the Mg-Zn-Pb ternary diagram that are proposed for use of origin determination. With chemical profile analyses it was possible to evaluate the limitations of the diagrams proposed for origin determination in the literature (Lit. Par11). It can be shown that sudden changes of ratios and sum of elements occurred within the same crystal depending on the different growth zones (e.g. see Box Par2 and 4). The diagrams based on Cu, Mn, Pb, Ga and Be should be used for each color group separately. In the light of strong variations within single tourmalines, the generalization about the chemical differences regarding Ga, Ge, Pb, Mg, Zn, Sb, Sc and Bi in tourmalines (See abstract Lit. Par 11) are not useful for origin determination purposes.

For example, due to the different approach of concentrating the research on studies within single crystals, it was possible to find different trends regarding the element Bi (See Fig. Par91d). High Bi-concentrations were found in tourmalines from both, Brazil and Mozambique, respectively. Different Bi-Cu-Mn-concentration trends were found in tourma-