Original paper
Chemical and structural evidence for [4]B [4]Si substitution in natural tourmalines
Hughes, Karrie-Ann; Hughes, John M.; Dyar, M. Darby
European Journal of Mineralogy Volume 13 Number 4 (2001), p. 743 - 747
published: Jul 18, 2001
Abstract
Because tourmaline is the most common boron-bearing mineral, the crystal chemisty of B in the phase is of fundamental importance to understanding the boron budget in the Earth. Until recently, the presence of [4]B as a substituent in the tourmaline tetrahedral (T) ring was unrecognized. However, a study of an Austrian olenite has shown that the substitution is not only possible but can occur in amounts up to B1.00 per T6.00. Through Magic-Angle-Spinning Nuclear Magnetic Resonance (MAS-NMR) spectroscopy, it was recently shown that [4]B in tourmaline can be identified by its spectroscopic signature. This study characterizes the structural response to small (< 0.50 apfu) amounts of substituent [4]B. Two tourmaline samples ([4]B0.28, [4]B0.37 by chemical analysis) that have been demonstrated by MAS-NMR to contain [4]B were analyzed by single-crystal X-ray techniques. It was found that reasonable agreement is obtained between [4]B content as determined by Secondary Ion Mass Spectrometry and X-ray site refinement, although the X-ray refinement significantly overestimates [4]B (by 3σ) in one sample. The response of the tourmaline atomic arrangement to the incorporation of substituent [4]B is subtle, perhaps explaining the lack of previous recognition of the substitution. The P-T-X conditions required for the substitution are not known, but are being studied by mineral researchers; it has been observed that all of the [4]B-bearing tourmalines discovered to date contain little or no Mg on the octahedral sites.