Tetrahedrally coordinated boron in tourmalines from the liddicoatite-elbaite series from Madagascar: Structure, chemistry, and infrared spectroscopic studies

Four colorless tourmalines of the liddicoatite-elbaite series from pegmatites from Anjanabonoina, Madagascar, have been characterized by crystal-structure determination and by chemical analyses. Optimized formulae range from (x)(Ca0.57Na0.29 rectangle(0.14)) (Y)(Al1.41Li1.33Mn0.072+rectangle(0.19)) Al-z(6)T (Si5.86B0.14)O-18(BO3)(3) v(OH)(3.00) W[F-0.76(OH)(0.24)] [a = 15.8322(3), c = 7.1034(3) angstrom] to (x)(Na0.46Ca0.03 rectangle(0.24)) (Y)(Al1.82Li0.89Fe0.012+ Mn-0.01(2+)rectangle(0.27)) Al-Z(6) (T)(Si5.56B0.44)O-18 (BO3)(3) (V)(OH)(3.00) (w)[(OH)(0.50)F-0.50] [a = 15.8095(9), c = 7.0941(8) angstrom] (R=1.3-1.7%). There is a high negative correlation (r(2)=0.984) between the < T-O > bond-lengths (similar to 1.618-1.614 angstrom) and the amount of B-IV (from the optimized formulae). Similar to the olenites (from Koralpe, Austria) the liddicoatite-elbaite samples show a positive correlation between the Al occupancy at the Y site and B-IV (r(2)=0.988). Short-range order configurations show that the presence of B-IV is coupled with the occupancy of (Al2Li) and (Al-2 rectangle) at the Y site. The structural formulae of the Al-rich tourmalines from Anjanabonoina, Madagascar, show similar to rectangle(0.2) (vacancies) on the Y site. We believe that short-range order configurations with (Y)(Al-2 rectangle) are responsible for these vacancies. Hence, an oft-used calculation of the Li content by difference on the Y site may be problematic for Al-rich tourmalines (olenite, elbaite, rossmanite). Fourier transform infrared (FTIR) spectra were recorded from the most B-IV-rich tourmaline sample. The bands around 5195 and 5380 cm(-1) can be assigned to H2O. Because these bands still could be observed in FTIR spectra at temperatures from -150 to +600 degrees C, it seems unlikely that they result from H2O in fluid inclusions. Interestingly, another FTIR spectrum from a dravite in which the X site is filled completely with Na, does not show bands at similar to 5200 and similar to 5400 cm-1. Although not definitive, the resulting spectra are consistent with small amounts of H2O at the X site of the elbaite. The rare-earth element (REE) pattern of the B-rich elbaite (Sigma REE: similar to 150 ppm) demonstrates that this sample is strongly enriched in LREEs compared to HREEs and exhibits a negative Eu anomaly. This sample shows the strongest enrichment of LREEs and a high La-N/Yb-N ratio of similar to 351, which seems to confirm an important role of the fractional crystallization process.