Tetrahedral boron in natural and synthetic HP/UHP tourmaline: Evidence from Raman spectroscopy, EMPA, and single crystal XRD

Author(s)
Martin Kutzschbach, Bernd Wunder, Diether Rhede, Monika Koch-Müller, Andreas Ertl, Gerald Giester, Wilhelm Heinrich, Gerhard Franz
Abstract

Olenitic tourmaline with high amounts of tetrahedral B (up to 2.53

[4]B pfu) has been synthesized in a piston-cylinder press at 4.0 GPa, 700 °C, and a run duration of 9 days. Crystals are large enough (up to 30 × 150 μm) to allow for reliable and spatially resolved quantification of B by electron microprobe analysis (EMPA), single-crystal X-ray diffraction, and polarized single-crystal Raman spectroscopy. Tourmalines with radial acicular habit are zoned in

[4]B-concentration [core: 2.53(25)

[4]B pfu; rim: 1.43(15)

[4]B pfu], whereas columnar crystals are chemically homogeneous [1.18(15)

[4]B pfu]. An amount of 1.4(1)

[4]B pfu was found in the columnar tourmaline by single-crystal structure refinement (SREF) (R = 1.94%). The EMPA identify

[T]Si

-1

[V,W]O

-1

[T]B

1

[V,W](OH)

1 as the main and [X]

-1

[T]Si

-1

[X]Na

1

[T]B

1 as minor exchange vectors for

[4]B-incorporation, which is supported by the SREF. Due to the restricted and well-defined variations in chemistry, Raman bands in the OH-stretching region (3000-3800 cm

-1) are unambiguously assigned to a specific cation arrangement. We found the sum of the relative integrated intensity (I

rel) of two low-frequency bands at 3284-3301 cm

-1 (ν1) and 3367-3390 cm

-1 (ν2) to positively correlate with the

[4]B concentrations:

[4]B [pfu] = 0.03(1) × [I

rel (ν1) + I

rel (ν2)]. Hence, those bands correspond to configurations with mixed Si/B occupancy at the T site. Our semi-quantitative correlation also holds for well-characterized natural

[4]B-bearing tourmaline from the Koralpe, Austria. This work shows the potential for Raman spectroscopy as a non-destructive method for the chemical classification of (precious) natural tourmaline, and as a tool to rapidly characterize chemical zonation of tourmalines in thin section.

Organisation(s)
Department of Mineralogy and Crystallography
External organisation(s)
Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum, Naturhistorisches Museum Wien (NHM), Technische Universität Berlin
Journal
American Mineralogist: an international journal of earth and planetary materials
Volume
101
Pages
93-104
No. of pages
12
ISSN
0003-004X
DOI
https://doi.org/10.2138/am-2016-5341
Publication date
01-2016
Peer reviewed
Yes
Austrian Fields of Science 2012
104026 Spectroscopy, 105116 Mineralogy, 105113 Crystallography
Keywords
ASJC Scopus subject areas
Geochemistry and Petrology, Geophysics
Portal url
https://ucrisportal.univie.ac.at/en/publications/tetrahedral-boron-in-natural-and-synthetic-hpuhp-tourmaline-evidence-from-raman-spectroscopy-empa-and-single-crystal-xrd(c9e6a734-0e27-484a-a204-9a9e825caa23).html