Chemical and structural evidence for (super ) <--> (super ) Si substitution in natural tourmalines
Because tourmaline is the most common boron-bearing mineral, the crystal chemistry of B in the phase is of fundamental importance to understanding the boron budget in the Earth. Until recently, the presence of (super ) 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 B (sub 1.00) per T (sub 6.00) . Through Magic-Angle-Spinning Nuclear Magnetic Resonance (MAS-NMR) spectroscopy, it was recently shown that (super ) B in tourmaline can be identified by its spectroscopic signature. This study characterizes the structural response to small (<0.50 apfu) amounts of substituent (super ) B. Two tourmaline samples ( (super ) B0.28, (super ) B0.37 by chemical analysis) that have been demonstrated by MAS-NMR to contain (super ) B were analyzed by single-crystal X-ray techniques. It was found that reasonable agreement is obtained between (super ) B content as determined by Secondary Ion Mass Spectrometry and X-ray site refinement, although the X-ray refinement significantly overestimates (super ) B (by 3sigma ) in one sample. The response of the tourmaline atomic arrangement to the incorporation of substituent (super ) 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 (super ) B-bearing tourmalines discovered to date contain little or no Mg on the octahedral sites.