Special collection; advances in ultrahigh-pressure metamorphism; tetrahedral boron in natural and synthetic HP/UHP tourmaline; evidence from Raman spectroscopy, EMPA, and single-crystal XRD

Olenitic tourmaline with high amounts of tetrahedral B (up to 2.53 (super [4]) B pfu) has been synthesized in a piston-cylinder press at 4.0 GPa, 700 degrees C, and a run duration of 9 days. Crystals are large enough (up to 30X150 mu m) 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 (super [4]) B-concentration [core: 2.53(25) (super [4]) B pfu; rim: 1.43(15) (super [4]) B pfu], whereas columnar crystals are chemically homogeneous [1.18(15) (super [4]) B pfu]. An amount of 1.4(1) (super [4]) B pfu was found in the columnar tourmaline by single-crystal structure refinement (SREF) (R=1.94%). The EMPA identify (super [T]) Si (sub -1) (super [V,W]) O (sub -1) (super [T]) B (sub 1) (super [V,W]) (OH) (sub 1) as the main and (super [X]) [] (super -1) (super [T]) Si (sub -1) (super [X]) Na (sub 1) (super [T]) B (sub 1) as minor exchange vectors for (super [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 (super -1) ) are unambiguously assigned to a specific cation arrangement. We found the sum of the relative integrated intensity (Irel) of two low-frequency bands at 3284-3301 cm (super -1) (v1) and 3367-3390 cm (super -1) (v2) to positively correlate with the (super [4]) B concentrations: (super [4]) B [pfu]=0.03(1)X[I (sub rel) (v1)+I (sub rel) (v2)]. 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.