Tetrahedral substitutions in tourmaline: a review

Abstract

The Si 4þ cation, typically the overwhelming occupant of the tetrahedral site of the tourmaline structure, can be partially replaced by Al 3þ , B 3þ , and Be 2þ. Tetrahedrally coordinated Al (up to ∼0.9 [4] Al atom per formula unit, apfu) has been found in different Mg-and/or Fe 2þ-rich tourmalines from high-temperature aluminous lithologies as well as in Li-rich pegmatites. It is relatively common for Li-and [4] Al-bearing tourmalines to also contain some [4] B. While the highest boron content (∼0.8 [4] 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 [4] 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.

Magnesio-lucchesiite, CaMg3Al6(Si6O18)(BO3)3(OH)3O, a new species of the tourmaline supergroup

“…http://www.minsocam.org/ fractionated Al-rich tourmalines (e.g. Ertl et al 2018), and hence unlikely in magnesiolucchesiite. The amount of H or OH was calculated in agreement with structural results and in order to achieve the electrostatic neutrality, under the assumption of (Y+Z+T) = 15.000 apfu and 31 anions.…”

Cation partitioning among crystallographic sites based on bond-length constraints in tourmaline-supergroup minerals

“…DOI: https://doi.org/10.2138/am-2021-7804. http://www.minsocam.org/ liddicoatite-elbaite series -the excess of Al at the Y site in Y (Al 2 Li) compositions results in the presence of tetrahedral B (Ertl et al 2006(Ertl et al , 2018.…”

“…In contrast, extensive "tschermak-type" Al Si -1 Mg -1 substitution indicates high-grade metamorphism in the Al-rich and, to a lesser extent, silica-poor environment (Henry and Dutrow 1996;Ertl et al 2018). Higher T Al is quite often connected with oxy-tourmalines (Cempírek et al 2006;Bosi et al 2010Bosi et al , 2017Bačík et al 2013Bačík et al , 2015b.…”

“…In short, relatively high amounts of T Al may be expected in Mg-and/or Fe 2+ -rich tourmalines in aluminous bulk compositions and high-temperature metamorphic conditions. Significant amounts of T Al are also found in pegmatitic Al-rich and Li-bearing tourmaline species including Li-rich schorl, elbaite, fluor-liddicoatite, olenite, "oxy-rossmanite" (Cámara et al 2002;Bosi et al 2005;Lussier et al 2008;Ertl et al 2018). Moreover, adachiite, which was found in a hydrothermal vein (with margarite and diaspore) crosscutting a lateritic metamorphic rock in Japan, is the first end-member of the tourmaline supergroup formed via Tschermak-like substitution , this value can be used for discrimination of cation site-occupancy preference.…”

Mn-bearing purplish-red tourmaline from the Anjanabonoina pegmatite, Madagascar

“…Tourmaline minerals are complex borosilicates that have been studied extensively in terms of their crystal structure and crystal chemistry (e.g. Foit, 1989; Grice and Ercit, 1993; Ertl et al , 2002; Novák et al , 2004; Bosi and Lucchesi, 2007; Bosi 2013, 2018; Novák et al , 2011; Henry and Dutrow, 2011; Henry et al , 2011; Cempírek et al , 2013; Ertl et al , 2018; Andreozzi et al , 2020).…”