Tetrahedral substitutions in tourmaline: a review
The Si4+ cation, typically the overwhelming occupant of the tetrahedral site of the tourmaline structure, can be partially replaced by Al3+, B3+, and Be2+. Tetrahedrally coordinated Al (up to ~0.9 Al atom per formula unit, apfu) has been found in different Mg- and/or Fe2+-rich tourmalines from high-temperature aluminous lithologies as well as in Li-rich pegmatites. It is relatively common for Li- and Al-bearing tourmalines to also contain some B. While the highest boron content (~0.8 B apfu) in a natural tourmaline was found in an Al-rich tourmaline (olenite) from the Eastern Alps, Austria, much higher contents of up to ~2.8 apfu have been reported for synthetic Al-rich tourmalines. Natural tourmalines with a higher total Al content (>7.2 Al apfu) can also contain significant B. The highest verified beryllium content in a tourmaline is still below 100 ppm, but current data show that in pegmatitic tourmaline crystals the Be content is positively correlated with the overall Al content. Therefore Li-rich tourmalines can contain higher Be contents in pegmatites that originated from differentiated granitic magma, than in pegmatites that have evolved as granitic pegmatitic melts from the surrounding country-rocks during decompression.
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Document Type: Research Article
Publication date: June 1, 2018
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