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(Ca_0.57Na_0.29□_0.14) ^Y(Al_1.41Li_1.33Mn²⁺_0.07□_0.19)^Z Al₆^T(Si_5.86B_0.14)O₁₈ (BO₃)₃^V (OH)_3.00^W[F_0.76(OH)_0.24] [a = 15.8322(3), c = 7.1034(3) Å] to ^X(Na_0.46Ca_0.30□_0.24) ^Y(Al_1.82Li_0.89Fe²⁺_0.01 Mn²⁺_0.01□_0.27) ^ZAl₆^T(Si_5.56B_0.44)O₁₈ (BO₃)₃^V(OH)_3.00^W[(OH)_0.50F_0.50] [a = 15.8095(9), c = 7.0941(8) Å] (R = 1.3–1.7%). There is a high negative correlation (r² = 0.984) between the <T-O> bond-lengths (~1.618–1.614 Å) and the amount of ^IVB (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 ^IVB (r² = 0.988). Short-range order configurations show that the presence of ^IVB is coupled with the occupancy of (Al₂Li) and (Al₂□) at the Y site. The structural formulae of the Al-rich tourmalines from Anjanabonoina, Madagascar, show ~ □_0.2 (vacancies) on the Y site. We believe that short-range order configurations with ^Y(Al₂□) 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 ^IVB-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 °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 ~5200 and ~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 (∑REE: ~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 ~351, which seems to confirm an important role of the fractional crystallization process.