Chemical and structural evidence for B-[4] <-> Si-[4] substitution in natural tourmalines

Because tourmaline is the most common boron-bearing mineral, the crystal ch emistry of B in the phase is of fundamental importance to understanding the boron budget in the Earth. Until recently, the presence of B-[4] as a subs tituent in the tourmaline tetrahedral (T) ring was unrecognized. However, a study of an Austrian olenite has shown that the substitution is not only p ossible but can occur in amounts up to B-1.00 per T-6.00. Through Magic-Angle-Spinning Nuclear Magnetic Resonance (MAS-NMR) spectrosc opy, it was recently shown that B-[4] in tourmaline can be identified by it s spectroscopic signature. This study characterizes the structural response to small (< 0.50 apfu) amounts of substituent B-[4]. Two tourmaline sample s (B-[4](0.28), B-[4](0.37) by chemical analysis) that have been demonstrat ed by MAS-NMR to contain B-[4] were analyzed by single-crystal X-ray techni ques. It was found that reasonable agreement is obtained between B-[4] cont ent as determined by Secondary Ion Mass Spectrometry and X-ray site refinem ent, although the X-ray refinement significantly overestimates B-[4] (by 3 sigma) in one sample. The response of the tourmaline atomic arrangement to the incorporation of s ubstituent B-[4] is subtle, perhaps explaining the lack of previous recogni tion 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 o bserved that all of the B-[4]-bearing tourmalines discovered to date contai n little or no Mg on the octahedral sites.