ELECTRON MICROPROBE ELEMENT DISTRIBUTION MAPPING
Fig. Par31 Elemental distribution maps were produced by electron microprobe analysis of important elements. The analyzed sample is a “Paraiba” tourmaline from the Batalha mine. The tourmaline has been cut perpendicular to the c-axis (compare Fig. Par28-29). The trail of the craters are form LA-ICP-MS analysis (Compare Fig. Par32-39). The picture of the tourmaline is shown in Fig. Par31 for comparison of the visual and the electron microprobe electron concentration mappings. The colors of the pictures shown in Fig. Par31a-l are artificial and arbitrary chosen to achieve maximum contrast to show the differences in element concentrations. To calibrate the chemical concentrations, average chemical analyses by LA-ICP-MS (Tab. Par02) are projected into the picture (µg/g).

Fig. Par31a Back scattered electron image (BSE)

The BSE-image provides different grey tones that are produced by the different atoms present. Higher total number of heavy atoms result in brighter grey tones. Fig. Par31a shows a dark core and lighter rim. This can be explained by the increase of heavy elements from the core to the rim of the crystal. Lighter grey tones can be correlated with increase in Mn, Cu and Bi (Fig. Par31k). The dark-grey core can be correlated with low concentrations of Mn and Cu (Fig. Par31j). The outer core is dominated by elevated copper- (Cu) and low Mn-concentrations in the BSE image (Fig. Par31k) correlates well with the visual color zoning (Fig. Par32). “Neon”-blue colors are found in the areas of the crystal at these elevated Cu and depleted Mn-concentrations. Elemental distribution of other elements such Si, Al, Ca, Na Mg, Zn, F and Bi may not be directly correlated with the visible color zoning as they are not the cause of blue color in the tourmaline (e.g. Lit Par10, 11, 33).

Fig. Par31b-l Element distribution mapping of various elements

The element distribution mapping shows that Al-concentrations are slightly higher in the core than in the rim of the tourmaline (Fig. Par28a, 31b) whereas Si does not show any variation (Fig. Par31c). Ca- and Na-concentration revealed an “inner triangle” which was not evident in the microscope (Fig. Par31d-e). In this “inner triangle”, the Ca-concentrations are depleted and Na-concentrations are enriched (Fig. Par31d-e). Higher Ca- and lower Na-concentrations are found in the zone of highest Cu-concentrations (compare Fig. Par31d and Par31k).