Tetrahedrally coordinated boron in tourmalines from the liddicoatite-elbaite series from Madagascar: Structure, chemistry, and infrared spectroscopic studies
- Author(s)
- Andreas Ertl, John M. Hughes, Stefan Prowatke, Thomas Ludwig, Pinnelli S.R. Prasad, Franz Brandstätter, Wilfried Körner, Ralf Schuster, Franz Pertlik, Horst Marschall
- Abstract
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 ?0.14)Y(Al1.141Li1.33 Mn0.072+? 0.19) ZAl6T(Si5.86 B0.14)O18(BO3)3V(OH)3.00W[F0.76 (OH)0.24] [a = 15.8322(3), c = 7.1034(3) Å] to X(Na0.46Ca0.30?0.24) Y(Al1.82Li0.89Fe0.012+Mn0.012+?0.27) ZAl6T(Si5.56 B0.44)O18(BO3)3V(OH)3.00W[(OH)0.50 F0.50] [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 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 LaN/YbN ratio of ?351, which seems to confirm an important role of the fractional crystallization process.
- Organisation(s)
- Department of Mineralogy and Crystallography
- External organisation(s)
- Miami University, Scientific Software Center, National Geophysical Research Institute (NGRI), Naturhistorisches Museum Wien (NHM), Geologische Bundesanstalt
- Journal
- American Mineralogist: an international journal of earth and planetary materials
- Volume
- 91
- Pages
- 1847-1856
- No. of pages
- 10
- ISSN
- 0003-004X
- Publication date
- 2006
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 105116 Mineralogy, 105904 Environmental research
- Portal url
- https://ucrisportal.univie.ac.at/en/publications/b2f1dd38-7757-4aef-b0b5-8297e7415ee2