Abstract

Significant amounts of boron in both trigonal and tetrahedral coordination have been found through 11B magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy of natural olenite (aluminum-rich tourmaline) from Stoffhütte, Koralpe, Styria, Austria. The 11B MAS NMR spectrum consists of a superposition of two quadrupolar MAS peaks. A broad peak with δiso = 18.3 ppm, νQ = 1410 KHz, and η = 0.11 demonstrates the trigonal-planar environment of the BO3 group (relative area ratio = 80%). The narrow symmetrical peak (δiso = 0.0 ppm, νQ = 300 KHz, η = 0.00) represents tetrahedral BO4 groups (relative area ratio = 20%). An infrared spectrum shows hydroxyl stretching bands at low wavenumbers indicating that underbonded O atoms of the hexagonal ring (due to a partial replacement of [4]Si4+ by [4]B3+) form relatively strong hydrogen bonds with the protons of the hydroxyl groups. A 29Si MAS NMR spectrum shows a peak consisting of a main signal at −90 ppm and a shoulder at about −85 ppm. The main signal originates from Si atoms connected (via oxygen bridges) to two other Si atoms in the hexagonal ring, and the minor signal is from Si atoms connected to one Si and one B atom. No signal corresponding to [6]Si was detected in this natural olenite sample.

You do not currently have access to this article.