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Chemical and structural evidence for ^[4]B ^[4]Si substitution in natural tourmalines

Karrie-Ann HUGHES^1,*, John M. HUGHES^1, and M. Darby DYAR²

¹ Department of Geology, Miami University, Oxford, Ohio 45056, U.S.A.
² Department of Geography and Geology, Mount Holyoke College, South Hadley, Massachusetts 01075, U.S.A.

E-mail: hughesjm{at}muohio.edu

^* Present address: Civil Engineering and Geological Sciences, Notre Dame, Notre Dame, Indiana 46556, U.S.A.

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 ^[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 B_1.00 per T_6.00.

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]B._0.028, ^[4]B_0.37 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 30) 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.

Key-words: tourmaline, boron, crystal-structure refinement.

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