Tetrahedral substitutions in tourmaline: a review

Tourmaline is an extremely important mineral because of its relevance in the geosciences and also for its technical applications (piezo- and pyroelectric material) and its use as gemstone. The tourmaline composition gives information on the thermal and fluid history of rocks in which it develops, is intimately associated with some of the world's premier metallic ore deposits and yields isotopic evidence for the environmental sources of the boron that is an essential constituent of all tourmalines. Thus tourmaline is an extremely critical link in the boron cycle on the Earth and has many other useful petrogenetic features (e.g., Dutrow & Henry, 2000). Varying chemical compositions of tourmaline reflect the chemical conditions of rocks where tourmaline forms and have been used as an important petrologic indicator of different geologic environments in previous investigations (e.g., Henry & Guidotti, 1985). Tourmaline can be stable in a wide range of pressure-temperature conditions (e.g., Ertl et al., 2010).

The tourmaline group consists of more than 30 species, which occur in many different geological environments (Henry & Dutrow, 1996; Henry et al., 2011). Tourmalines are complex aluminium-borosilicate minerals with strongly varying compositions related to isomorphous substitutions. The tourmaline mineral group is chemically one of the most complicated groups of silicate minerals, with the general formula XY₃Z₆[T₆O₁₈](BO₃)₃V₃W (Henry et al., 2011), with the most common site occupants:

X = Ca²⁺, Na⁺, K⁺, □ (vacancy)

Y = Li⁺, Mg²⁺, Fe²⁺, Mn²⁺, Al³⁺, Cr³⁺, V³⁺, Fe³⁺, Ti⁴⁺

Z = Al³⁺, Mg²⁺, Fe³⁺, Mn³⁺, V³⁺, Cr³⁺, Ti⁴⁺, (Fe²⁺)

T = Si⁴⁺, Al³⁺, B³⁺, Be²⁺

V = OH⁻, O²⁻

W = OH⁻, F⁻, O²⁻, (Cl⁻).

The structure is characterized by six-membered tetrahedral rings (T sites), whose apical oxygens point toward the (–) c-pole, producing the acentric nature of the structure. These planar rings of tetrahedra are linked by two types of octahedra, Z and Y, which share edges to form brucite-like fragments. Triangular BO₃groups lie parallel to the (0001) plane. The Z octahedra are relatively small, somewhat distorted, and are usually occupied predominantly by trivalent Al.

r_A, ...  = effective ionic radius of atom ^[4]A, …, in (Å) (Si, 0.26; Al, 0.39; B, 0.11; Shannon, 1976);

x_A, ...  = concentration (apfu) of elements A, … which occupy the T site of tourmaline (note that the sum of all element concentrations at the T site is 6.00 apfu);

〈T–O〉 = average T–O bond length (Å) of tourmaline.

In most tourmalines, especially in Mg-bearing samples from non-pegmatitic environments, the substitution of Si at the T site is limited. In the following sections we discuss documented T-site cation occupants in addition to Si and consider the implications. To date, there is no conclusive evidence for the presence of either Ti⁴⁺ (Grice & Ercit, 1993) or Fe³⁺ (Dyar et al., 1998) at the T site in significant amounts (Hawthorne, 1996; Andreozzi et al., 2008; George Rossman, pers. comm., 2016).

An effective way to incorporate Al in the tetrahedral sites of minerals is through a Tschermak-type substitution, i.e.^[6]R²⁺ + ^[4]Si = ^[6]Al + ^[4]Al, where R²⁺ represents divalent cations. In general, operation of this type of substitution to enhance ^[4]Al will be favoured by increasing temperature and by bulk compositions that are aluminous and, to a lesser extent, silica poor (e.g.Pattison, 1987; Padrón-Navarta et al., 2013). Using a limited set of data, Henry & Dutrow (1996) described 0.25 apfu ^[4]Al from tourmalines which have been crystallized above 750 °C. Such tourmalines from high-grade aluminous metapelites have relatively low vacancies (0.05 ± 0.05) at the X site when they crystallized above 750 °C (Henry & Dutrow, 1996). However, it is not uncommon for silica-undersaturated upper-amphibolite to granulite-facies rocks with corundum, kornerupine and/or grandidierite to contain tourmaline with 0.3–0.4 apfu ^[4]Al (e.g.Grew et al., 1990, 1997).

The T sites can be occupied by significant amounts of Al (Povondra, 1981), especially in aluminous, high-grade metamorphic rocks (e.g.Cempírek et al., 2006). The relationship between 〈T–O〉 distances and Al occupancy in the ring was first defined by Foit & Rosenberg (1979) and Foit (1989). Subsequently, MacDonald & Hawthorne (1995) re-examined this relationship and found a linear relation between 〈T–O〉 and the aggregate cation radius approximating hard-sphere behaviour.

Small, but significant amounts of tetrahedrally coordinated Al (0.15 apfu ^[4]Al, 〈T–O〉 = 1.624(1) Å) were described from a V-, Cr-, and Mg-bearing oxy-tourmaline from the Sludyanka crystalline complex, Lake Baikal, Russia, by Bosi et al. (2017). Even higher amounts of ^[4]Al were found in Li-bearing schorl (0.17 apfu ^[4]Al, 〈T–O〉 = 1.623(1) Å) from the border zone of the Cruzeiro pegmatite, Minas Gerais, Brazil (Bosi et al., 2005) and from a Mn- and Li-bearing schorl (0.24 apfu ^[4]Al, 〈T–O〉 = 1.624(1) Å) from a black nodule from the Alto Lighona pegmatite field, Zambezia, Mozambique (Cámara et al., 2002). Similar amounts of ^[4]Al are found in an Mg-rich tourmaline (0.19 apfu ^[4]Al, 〈T–O〉 = 1.624(1) Å; oxy-dravite) from a metapyroxenite from Jabal Zalm, Saudi Arabia, embedded in serpentinite, which derived from a dunite-peridotite protolith (Bosi et al., 2010). Another tourmaline sample, a Mg-rich schorl (#108749) from a syenite pegmatite from the Raglan Township, Renfrew Co., Ontario, Canada, exhibits even higher amounts of ^[4]Al (0.34 apfu ^[4]Al, 〈T–O〉 = 1.625(1) Å; Bloodaxe et al., 1999). Manganese- and Al-rich, Li-bearing tourmaline (fluor-elbaite) from a pegmatite near village Eibenstein, Austria, can also contain significant amounts of ^[4]Al (0.33 apfu ^[4]Al, 〈T–O〉 = 1.626(1) Å) as was shown by chemical analyses (including light elements) in combination with crystal-structure determination (Prowatke et al., 2003). Similar amounts of ^[4]Al were found in an Mn-bearing elbaite from Mogok, Myanmar (0.34 apfu ^[4]Al; Lussier et al., 2008) and in Fe²⁺-bearing olenite from an abyssal pegmatite at the locality Kuklík, near Kutná Hora, Czech Republic (0.40 apfu ^[4]Al, 〈T–O〉 = 1.626(1) Å; Cempírek et al., 2006). Foit & Rosenberg (1979) and MacDonald & Hawthorne (1995) showed using crystal-structure analyses in combination with chemical analyses that Mg-rich and V-bearing tourmaline samples can also contain relatively high amounts of ^[4]Al (〈T–O〉 = 1.625(1) Å for a tourmaline from a quartz–graphite schist from Silverknob, Mariposa County, California, USA, with ∼0.37 apfu ^[4]Al; 〈T–O〉 = 1.627(2) Å for green fluor-uvite samples from East Africa (Landanai, Arusha Region, Tanzania) with 0.45 ± 0.04 apfu ^[4]Al; MacDonald & Hawthorne, 1995; see also Lussier et al., 2009). An Mg-bearing oxy-schorl from fracture fillings cutting altered metarhyolite pyroclasts near Zlatá Idka village, Slovak Ore Mountains, Slovak Republic, was described with a relatively high amount of ^[4]Al (∼0.50 apfu ^[4]Al; 〈T–O〉 = 1.632(2) Å) by Bačík et al. (2013). In the new Ca- and Fe²⁺-rich tourmaline adachiite, which was found in a hydrothermal vein (with margarite and diaspore), crosscutting a lateritic metamorphic rock (mainly consisting of corundum and hercynite) near Saiki City, Oita Prefecture, Japan, Nishio-Hamane et al. (2014) described the highest amount of ^[4]Al (0.85 apfu ^[4]Al; 〈T–O〉 = 1.634(2) Å). Adachiite is the first end-member of the tourmaline group formed via Tschermak–like substitution. The measured 〈T–O〉 distance of this tourmaline is consistent with the calculated 〈T–O〉 distance for such a high amount of ^[4]Al (Ertl et al., 2012a). Hence, a maximum amount of ∼0.9 apfu ^[4]Al, which seems to be verified by single-crystal structure data, might be near the upper limit.

In sum, relatively high amounts of ^[4]Al may be expected in Mg- and/or Fe²⁺-rich tourmalines, especially when they are in aluminous bulk compositions and experienced high-temperature metamorphic conditions. Significant amounts of ^[4]Al are also found in pegmatitic Al-rich and Li-bearing tourmaline species (elbaite, liddicoatite, olenite, “oxy-rossmanite”). Quite commonly these ^[4]Al- and Li-bearing tourmalines also contain significant amounts of ^[4]B (Ertl et al., 2005, 2009, 2010, 2012a, 2012b; Hughes et al., 2000; Lussier et al., 2009; Schreyer et al., 2002).

Over the last 20 years many examples of substitution of Si by B have been found in natural and synthetic tourmalines (Ertl et al., 1997, 2005, 2006, 2007, 2008, 2012a, 2012b, 2015; Hughes et al., 2004; Kutzschbach et al., 2016, 2017; Lussier et al., 2008, 2009; Schreyer et al., 2000, 2002). Using the crystallographic and analytical techniques that are most commonly employed today it is difficult to verify the presence of B at the T site. If analyses are done only with the electron microprobe, as is typical, tetrahedrally coordinated B goes unnoticed due to uncertainty of analyses and common site-assignment assumptions. However, it is important to determine the amount of B incorporation at the T site in tourmaline because it appears to be best expressed at lower tem­peratures of crystallization and likely reflects local petrologic environmental conditions (Schreyer et al., 2000; Ertl et al., 2008).

The initial report of natural Al-rich tourmaline with ^[4]B was substantiated by Ertl et al. (1997) using single-crystal structure refinement and independently by chemical analysis (including the light elements B, Li, H). These authors reported the unit-cell parameters as a = 15.7395(2), c = 7.0656(2) Å (cell volume: 1515.86(5) ų) and a 〈T–O〉 bond length of 1.610(1) Å. This colourless tourmaline, identified as olenite, from the Koralpe, Eastern Alps, Styria, Austria, exhibits relatively high amounts of ^[4]B. An updated formula of ^X(Na_0.40Ca_0.29□_0.31) ^Y(Al_2.40Li_0.35□_0.25) ^ZAl₆ (BO₃)₃^T[Si_4.89B_0.83Al_0.27Be_0.01]O₁₈^V(OH)₃^W[O_0.58(OH)_0.36F_0.06] was given by Ertl et al. (2007). This tourmaline was found in a pegmatite that intruded discordantly into mylonitic garnet–biotite schists (Hughes et al., 2000; Kalt et al., 2001). The mylonitization of the gneisses in the shear zone of the Koralpe Complex occurred during the eo-Alpine metamorphism. The peak conditions of the pressure-temperature (PT) path of the shear zone as reported by Stüwe & Powell (1995) involved decompression from >1800 MPa to 1400–1600 MPa at temperatures above 620 °C and reaching the peak conditions of 1400–1600 MPa and 700 °C. The PT conditions of the tourmaline-bearing pegmatite were described with 500–700 MPa/∼630 °C (Ertl et al., 2008). Even 20 years after the first publication, the ^[4]B content in this olenite is still the highest known in natural tourmalines.

Another tourmaline with a relatively high amount of ^[4]B was described by Ertl et al. (2007). This sample is an almost colourless olenite (a = 15.7561(6), c = 7.0703(5) Å) from a “mushroom” tourmaline from Momeik Township, northeast of Mogok, Mandalay Division, Shan State, Myanmar. This tourmaline has the second highest amount of ^[4]B (∼0.7 apfu ^[4]B) with a 〈T–O〉 distance of 1.604 Å, the shortest observed in natural tourmaline to date. This tourmaline was found in a pegmatitic pocket emplaced in a peridotite body. The olenite from Koralpe contains a greater amount of ^[4]B than the olenite from Mogok, but the Koralpe olenite also contains a significant amount of ^[4]Al, which enlarges the 〈T–O〉 distance. “Mushroom” tourmalines from Mogok with larger unit-cell parameters (〈T–O〉 distances: 1.609–1.614 Å) and lower amounts of ^[4]B were described by Lussier et al. (2008, 2009).

An Al-rich tourmaline with unit-cell parameters a = 15.777(1), c = 7.086(1) Å from the Sahatany Pegmatite Field at Manjaka, Sahatany Valley, Madagascar, contains also a significant amount of ^[4]B (∼0.4 apfu, no ^[4]Al; 〈T–O〉 = 1.611(1) Å; Ertl et al., 2008). A microthermometric study of fluid inclusions indicate PT conditions of 200–300 MPa/350–500 °C (Ranorosoa, 1986).

Elbaite samples from Li-pegmatites with small amounts of ^[4]B were also described (∼0.1–0.2 apfu ^[4]B; e.g., Ertl et al., 2009, 2010; Lussier et al., 2009). Most of these samples also contain small amounts of ^[4]Al. Therefore, it is not very easy to prove the presence of ^[4]B by crystal-structure refinements alone, but it requires allied evidence.

The first description of synthetic Al-rich tourmaline with verified ^[4]B was by Schreyer et al. (2000). They successfully synthesized tourmaline in the range 550–900 °C/1–5 GPa. Similar tourmalines were synthesized later by different groups (e.g.Wodara & Schreyer, 2001; Schlager, 2003; Kempl, 2008; London, 2011; Berryman et al., 2016; Kutzschbach et al., 2016, 2017). A pronounced negative correlation (r² = 1.0) between crystallization tem­perature (at constant pressure) and ^[4]B (from refinement) in synthetic Al-rich tourmalines (synthesis by London, 2011) was found by Ertl et al. (2012a). Wodara & Schreyer (2001) and Marler et al. (2002) found an inverse correlation between the ^[4]B content and the unit-cell volume. When plotting this relationship including recent data from structure refinements from Al- and ^[4]B-rich tourmaline, a pronounced negative correlation (r² = 0.95) is observed (Fig. 1). An extrapolation to an Al-rich tourmaline with 3 apfu ^[4]B implies a hypothetical cell volume of ∼1454 ų (corresponding to a = 15.48, c = 7.00 Å). A theoretical end-member with the highest possible amount of ^[4]B and having the formula CaAl₃Al₆(BO₃)₃[B₄Si₂O₁₈](OH)₄ has a calculated cell volume of ∼1425 ų and would have cell parameters of a = 15.36, c = 6.97 Å, i.e. smaller than observed in any tourmaline characterised to date. The relationship between temperature during crystal growth and the maximal possible amount of ^[4]B in Al-rich tourmaline is plotted in Fig. 2. The amount of ^[4]B was calculated (by using the correlation of Fig. 1) from synthetic tourmalines with the lowest cell volumes. Above a pressure of ∼1000–1500 MPa (depending on the temperature) the highest observed ^[4]B content does not change significantly at a given temperature (Ertl et al., 2008). Hence, the pressure conditions during the syntheses have only a minor influence on the amount of ^[4]B, but at higher pressures larger crystals can be grown. The temperature (over a range of ∼400–700 °C; see Fig. 3 in Ertl et al., 2008) is the most important factor.

Very high ^[4]B tourmalines appear to be most likely to occur in low-temperature environments. To generate tourmalines with a maximum content of 3 apfu ^[4]B a temperature of ∼350 °C was extrapolated (Fig. 2). Such tourmalines could theoretically occur in distal contact metamorphic, hydrothermal and greenschist-facies rocks, and possibly in high-pressure metamorphic rocks associated with subduction zones (blueschist and eclogite facies). It only can be speculated that tourmalines with more than 50% B at the tetrahedral site occur. For a maximum content of 3.5 apfu ^[4]B a temperature of ∼220 °C was calculated. Tourmalines with even higher amounts of ^[4]B might hypothetically occur as tiny crystals in diagenetic sediments.

Beryllium, which also occupies the T site, was found in a strongly zoned tourmaline from a Li-pegmatite from the Himalaya Mine, Mesa Grande, California. Beryllium is very low in the Fe²⁺-rich core (oxy-schorl; 7 ppm Be), but increases concomitantly with the Al content towards the fluor-elbaite rim zone (Ertl et al., 2010). The zone with highest Al content also has the highest Be content (76 ppm Be). Liddicoatite samples from Li-pegmatites from Anjanabonoina, Madagascar, exhibit Be contents of 26–28 ppm (Ertl et al., 2006). A ^[4]B-bearing elbaite from the same locality has 19 ppm Be. An elbaite from Nuristan, Afghanistan, contains 20 ppm Be (Ertl et al., 2006). Fluor-elbaites from pegmatites, which may have evolved from granitic pegmatitic melts during decompression from the surrounding country rocks during exhumation of the Moldanubian nappes (Lower Austria) after the peak of the Variscan metamorphism, exhibit lower Be contents of 5–13 ppm (Ertl et al., 2012b). The ^[4]B-rich olenite samples (with varying Fe²⁺contents) from Koralpe contain up to 37 ppm Be and, similar to the zoned tourmaline from the Himalaya Mine, California, the Be content increases simultaneously with the Al content (Ertl et al., 2006; Hughes et al., 2004).

In sum, ^[4]B may be expected in Al-rich tourmalines with >7.2 apfu Al (>1.2 apfu Al the Y site and 6 apfu at the Z site), because short-range order configurations with ^[4]B always include Al at the sixfold coordinated positions (Ertl et al., 2007, 2010). Based on the correlation in Fig. 1 the unit-cell volume of ^[4]B-bearing tourmalines should be smaller than ∼1540 ų. Especially colourless or pale coloured samples are candidates for such tourmalines. Petrologic environments with four-coordinated B in associated minerals, such as some of the borates in evaporites, may also be potential carriers of ^[4]B-bearing tourmalines.

To date, the highest verified Be contents of tourmaline are still below 100 ppm. It might be possible to synthesize Al-rich tourmalines with higher Be contents. It can be concluded that in pegmatites that originated from differentiated granitic magma, Li-rich tourmalines can contain higher Be contents (∼19–76 ppm) than in pegmatites that have evolved as granitic pegmatitic melts from surrounding country-rocks during decompression (∼5–13 ppm Be). In all investigated zoned tourmaline crystals, the Be content is positively correlated with the Al content.

Based on this review, future research might include (1) search for and evaluation of tourmalines that develop in low-temperature Ca-Al-rich environments and may have maximal levels of ^[4]B and ^[4]Al, (2) determination of the influence of temperature and pressure on ^[4]B and ^[4]Al in natural settings and (3) continued chemical investigation of Be-enriched tourmaline found in Be-rich environments such as beryl-bearing lithologies. Because of the difficulties in analyzing B and Be and in determining their crystallographic locations in tourmaline, improved determination techniques such as SIMS and NMR Magic Angle Spinning will be critical for proper characterization of these cations on tourmaline (e.g.Lussier et al., 2008).

This work was funded by the Austrian Science Fund (FWF) project no. P 26903-N19 and P 31049-N29 granted to AE and National Science Foundation grant no. 1551434 to DH. We thank Dmitry Yu. Pushcharovsky, Aaron J. Lussier and an anonymous reviewer for their constructive reviews.

We dedicate this paper to two great scientists and friends, Giovanni Ferraris and Stefano Merlino. The warmest congratulations to their 80th birthdays and a lot of more wonderful years, including some interesting work on scientific questions.