Alumino-oxy-rossmanite from pegmatites in Variscan metamorphic rocks from Eibenstein an der Thaya, Lower Austria, Austria: A new tourmaline that represents the most Al-rich end-member composition

Chemical analyses

The two crystal fragments selected for crystal-structure determination (PINK1, from the 1994 pegmatite, and PINK2, from the 2017 pegmatite) were embedded in epoxy on a single 2.5 cm diameter round glass slide and polished. All elements reported here except B, Li, Be, and H were determined with a Cameca SX51 electron microprobe (EMP) equipped with five wavelength-dispersive spectrometers (Universität Heidelberg). Operating conditions: 15 kV accelerating voltage, 20 nA beam current, and beam diameter 5 μm. Peaks for all elements were measured for 10 s, except for Mg (20 s), Ti (20 s), Zn (30 s), and F (40 s). We used the following (natural and synthetic) reference materals and X‑ray lines for calibration: topaz (FKα), albite (NaKα), wollastonite (SiKα) and (CaKα), corundum (AlKα), periclase (MgKα), orthoclase (KKα), rutile (TiKα), rhodonite (MnKα), hematite (FeKα), and gahnite (ZnKα). The analytical data were reduced and corrected using the PAP routine (Pouchou and Pichoir 1991). A modified matrix correction was applied assuming stoichiometric O atoms and all non-measured components as B₂O₃. The accuracy of the electron-microprobe analyses and the correction procedure was checked by measuring three samples of reference tourmalines (98114: elbaite, 108796: dravite, 112566: schorl). Compositions of these tourmaline samples were determined as part of an interlaboratory comparative study (Dyar et al. 1998, 2001). Under the described conditions, the accuracy of all analyses is ±1% relative for major elements and ±5% relative for minor elements.

H, Li, and B were determined in PINK2 by SIMS with a CAMECA ims 3f ion microprobe (Universität Heidelberg). Primary O^– ions were accelerated to 10 keV. The mass spectrometer’s energy window width was 40 eV. An offset of 75 V was applied to the secondary accelerating voltage of 4.5 kV so that secondary ions with an initial energy of 75 ± 20 eV were analyzed (energy filtering), which minimizes potential matrix effects (Ottolini et al. 1993). The primary current was 10 nA, resulting in a spot diameter of ~20 μm. The spectrometer’s mass resolving power (MRP) M/ΔM for B, Li, and Si was set to ~1000 (10%) to suppress interferences (⁶LiH⁺, ¹⁰BH⁺, Al³⁺). Secondary ⁷Li, ¹¹B, and ³⁰Si ions were collected under an imaged field of 150 μm diameter. For H (and Si) the MRP M/ΔM was set to ~400 (10%), and the imaged field was limited to a diameter of ~12 μm. In situ water contamination was reduced by using a liquid nitrogen cold-trap attached to the sample chamber (Ludwig and Stalder 2007; and references therein). The count rates of the analyzed isotopes (¹H, ⁷Li, and ¹¹B) were normalized to the count rate of ³⁰Si and relative ion yields (RIY) were used for quantification of the results (e.g., Hinton 1990, 1995; Ottolini et al. 1993). The analytical procedures for the SIMS analyses (which included Be) of sample PINK1 were almost identical (details see Ertl et al. 2005).

The relative ion yields for H and B were determined using three tourmalines as reference material: elbaite, dravite and schorl (Dyar et al. 1998, 2001). The reference material for Li and Be was the NIST SRM610 standard glass with concentrations for Li (464.2 μg g⁻¹) and Be (469.0 μg g⁻¹) average taken from Pearce et al. (1997). The relative reproducibility (1σ) for the RIY of H, Li, Be, and B was <1%. Matrix effects and the uncertainty of the element concentrations in the reference material limit the accuracy of the analysis. The relative accuracy is estimated to be <20% for H and <10% for Li, Be, and B. Table 1 contains complete chemical analyses of the two studied crystal fragments of pink Al-rich tourmaline.

Powder X‑ray diffraction

The eight strongest X‑ray diffraction lines in the (calculated) powder pattern [d in Å (I) hkl] are: 2.5534 (100) 051, 3.9508 (85) 220, 2.9236 (78) 122, 4.1783 (61) 211, 2.4307 (55) 012, 2.0198 (39) 152, 1.8995 (30) 342, 6.294 (28) 101 (Online Materials^[1] Table OM1). The X‑ray powder diffraction pattern had to be calculated because only a very small amount of material was available for scientific studies of the single small pink tourmaline crystal, which was originally found in 1994. Although pink tourmaline was found more frequently in the 2017 pegmatite, the chemistry of the studied crystals was not as close to the ideal end-member as that of the 1994 material.

Crystal-structure refinement

A fragment of the holotype crystal (PINK1) was mounted on a Bruker Apex CCD X‑ray diffractometer equipped with graphite-monochromatized MoKα radiation (Department of Geology, Miami University, Oxford, Ohio, U.S.A.). Redundant data were collected for an approximate sphere of reciprocal space, and were integrated and corrected for Lorentz and polarization factors, and for absorption, using the Bruker program SaintPlus (Bruker AXS 2001). The structure was refined with the Bruker SHELXTL v. 6.10 package of programs, with neutral-atom scattering factors and terms for anomalous dispersion.

A fragment of PINK2 was structurally investigated at the Institut für Mineralogie und Kristallographie, Geozentrum, Universität Wien, Austria. As a first step, the quality of different crystal fragments was checked with a Bruker APEXII diffractometer equipped with a CCD area detector and an Incoatec Microfocus Source IμS (30 W, multilayer mirror, MoKα). The crystal with the best diffracting quality was subsequently measured on this diffractometer. Single-crystal X‑ray diffraction intensity data, up to 82.84°2θ, were collected at room temperature, integrated, and corrected for Lorentz and polarization factors, and with a multi-scan absorption correction (Sheldrick 2002). The structure was refined with SHELXL-97 (Sheldrick 2008) using scattering factors for neutral atoms.

The refinements were performed with anisotropic displacement parameters for all non-hydrogen atoms. The various site occupancies were refined according to well-known characteristics of the tourmaline structure (Henry et al. 2011) and considering the results of the EMP, SIMS, and spectroscopic analyses. Hence, the X-site occupancy was refined using a Na scattering factor, the Y site with Al and Li scattering factors, the T site with Si and B, and the Al occupancy at the Z site was fixed at 1.00, typical for Al-rich tourmalines. The W site was preliminarily refined with O vs. F scattering factors, but because the resulting value for F was lower than the standard deviation, in the final cycles the W site was only refined with O scattering factors.

Refined unit-cell parameters and the most important average bond lengths for PINK1 and PINK2 are listed in Table 2. The CIF¹ provides crystal data and details of both structure refinements (atomic coordinates, individual bond lengths, and angles).

Optical spectroscopy

A 380 × 185 μm² crystal fragment of the holotype PINK1 was prepared as a 92.8 μm thick parallel plate polished on both sides to study its color properties. Polarized optical absorption spectra in the 390–1100 nm range were obtained at about 1 nm resolution with a locally built microspectrometer system consisting of a 1024-element Si diode-array detector coupled to a grating spectrometer system via fiber optics to a highly modified NicPlan infrared microscope containing a calcite polarizer. A pair of conventional 10× objectives was used as an objective and a condenser. Spectra were obtained through the central area of the sample, which included a veil of two-phase (liquid/gas) inclusions.

Infrared spectra

Infrared spectra of holotype material to examine the OH content were measured in the main compartment of a Nicolet Magna 860 FTIR spectrometer at 2 cm⁻¹ resolution using a 200 μm aperture. Near-IR spectra were obtained using a CaF₂ beam splitter, tungsten-halogen source, MCT-A detector, and LiIO₃ polarizer and were averaged over 256 to 4000 scans. For the smallest crystal fragments, a silica beam splitter and InSb detector were used to collect the spectra. Additional spectra were obtained with a Nicolet Continuum infrared microscope using a MCT-A/CaF₂/ tungsten-halogen combination.

The total integrated band area (area_tot = area_E||c + 2 × area_E┴c) was determined for the mid-IR OH overtone bands using Omnic E.S.P. 5.2 software. Integrated absorbances were determined by establishing a suitable baseline and measuring the area between the curve and the baseline over the region of the OH bands. The baseline connects with the spectral trace on both the high- and low-energy side of the OH region. This was done on a spectrum normalized for a 1 cm thick crystal. Before integration, a minor, visually estimated background correction was needed to compensate for a sloping background.

Physical properties

Holotype alumino-oxy-rossmanite is brittle and has a Mohs hardness of 7; it is non-fluorescent, has no observable parting and cleavage, and has a vitreous luster. The megascopic color of alumino-oxy-rossmanite is pink, and the streak is white. It has a measured density of 3.07(3) g/cm³ (pycnometer method; Syromyatnikov 1935; Ksanda and Merwin 1939) and a calculated density of 3.092(1) g/cm³ based on the empirical formula and the unit-cell volume refined from the single-crystal X‑ray diffraction data. The fracture is conchoidal. The mineral has a prismatic habit, but only crude prismatic forms are developed. Twinning was not observed.

Optical spectroscopy and optical properties

The optical absorption spectrum of alumino-oxy-rossmanite (Fig. 3) is most intense in the E┴c direction. It consists of a band at about 555 nm and weaker features near 450 and 415 nm. The E||c spectrum also contains the 555 nm band that overlaps with another broad band near 475 nm. There is also an indication of a weak band near 780 nm. The broad rise in absorption from high- to low-energy could be due in part to scattering from imperfections in the crystal, and partly to an ultraviolet absorption band. The spectrum is broadly similar to the spectrum of rossmanite (Ertl et al. 2005), but the major bands are shifted to longer wavelengths. For example, in rossmanite, the main band is observed at 517 nm.

Figure 3

Optical absorption spectrum of a 0.0928 mm thick crystal of alumino-oxy-rossmanite (sample PINK1).

The pink color of alumino-oxy-rossmanite is due to the band at 555 nm that is associated with Mn³⁺ probably produced by natural irradiation of Mn²⁺ as has been previously described for elbaite (Reinitz and Rossman 1988). In plane-polarized light, alumino-oxy-rossmanite is pleochroic, O = pink, E = near-colorless (polarized parallel to the c-axis; orientation: E||c). Alumino-oxy-rossmanite is uniaxial negative, ω = 1.648(5), ε = 1.628(5) (590 nm).

Infrared spectroscopy

The infrared spectrum obtained in the mid-infrared region (Fig. 4) of sample PINK1 shows OH bands in the E||c spectrum at 7175, 7144, 7022, ~6964 cm⁻¹, and the most prominent band at 6708 cm⁻¹. These bands are shifted to lower wavenumbers as compared to the corresponding bands in fluor-elbaite of similar divalent-cation composition (Ertl et al. 2005). Mattson (1985) concluded that the 7001 cm⁻¹ band in an elbaitic tourmaline (sample GRR 598-Afgh-1) is associated with H bound to the O3 site with a local configuration involving Li⁺. This band is of low intensity in alumino-oxy-rossmanite compared to the elbaitic tourmaline of Mattson (1985). The low intensity of these OH bands suggests that the absolute OH concentration is lower than that in sample GRR 598-Afgh-1. Because no absolute calibration of the intensity of the OH overtone bands in the tourmaline spectrum has yet been done, it is not possible at this time to establish an absolute concentration. By comparing the OH intensity of alumino-oxy-rossmanite to other relevant tourmalines (25 elbaites, fluor-elbaites, fluor-liddicoatites, and rossmanites), alumino-oxy-rossmanite remains the tourmaline with integrated intensity that is signifiantly lower than all the other tourmaline samples (Ertl et al. 2015). All of these observations are highly suggestive that alumino-oxy-rossmanite contains a lower amount of OH than most other tourmalines. The infrared spectrum confirms that hydroxyl groups in holotype alumino-oxy-rossmanite (sample PINK1) are present at a lower concentration than commonly found in other Li-bearing tourmalines.

Figure 4

Near-infrared spectrum of alumino-oxy-rossmanite (sample PINK1) from Eibenstein in the OH overtone region. A spectrum of a 0.0928 mm thick crystal was plotted normalized to 1.0 mm thickness.

By applying the Beer’s Law ε value (estimated to be 7.5 for Mn³⁺; Reinitz and Rossman 1988) to the optical absorption spectrum (Fig. 2) and by determination of the Mn³⁺ band heights, considering the chemistry and the density (3.09) of aluminooxy-rossmanite, we were able to estimate the percentage of Mn, which was oxidized to Mn³⁺ (~74%). Because iron is oxidized under less oxidizing conditions than manganese, which is shown in Figure 3 of Huebner (1969), we consider the percentage of the oxidized Fe to be similar, if not higher than it is for Mn. It is not uncommon that the small amount of total Fe in pink Al-rich and Li-bearing tourmalines is oxidized to a relatively high percentage (~60–90% as determined by Mössbauer spectroscopy; Ertl et al. 2010). The oxidation of relatively high amounts of Mn and Fe in alumino-oxy-rossmanite is in agreement with the observation that the OH groups are present at a lower concentration than commonly found in other Al-rich and Li-bearing tourmalines.

Crystal structure

The crystal structure of alumino-oxy-rossmanite [holotype material, sample PINK1; space group R3m; a = 15.803(1), c = 7.088(1) Å; V = 1532.8(3) ų] was refined to an R1(F) value of 1.68%. The crystal structure of tourmaline sample PINK2 [a = 15.809(1), c = 7.082(1) Å; V = 1532.9(3) ų] was refined to an R1(F) value of 1.37%. The empirical formulas of these samples were determined on the basis of EMPA, SIMS, spectroscopical methods (optical absorption and infrared spectroscopy), and SREF as follows:

No H could be found at the W site (O1 site) by refinement, and also the difference-Fourier map around the W site indicates this site to be mainly occupied by O. The map shows a spherical electron density similar to that described by Cámara et al. (2002) [Fig. 1a of that paper, crystal 1, dravite, with 0.18 (OH) at the W site]. Because of a small, but still visible OH1 band in the IR spectrum, only 0.15 (OH) pfu have therefore been assigned to the W site. Ertl et al. (2002) showed that the bond-angle distortion (σ₂^oct) of the ZO₆ octahedron in a tourmaline is largely a function of the <Y-O> distance of that tourmaline, although the occupant of the O3 site (V site) also affects that distortion. For all investigated tourmalines in which the V site is completely occupied by 3 (OH) groups, including the samples from Hughes et al. (2004), the covariance, r, of <Y-O> and the σ²_oct of the ZO₆ octahedron is –0.991 (Fig. 5). In Figure 5, the data of the holotype material (PINK1) of alumino-oxy-rossmanite lies between the tourmalines that contain 3 (OH) at the V site and natural fluor-buergerite, which contains 0.3 (OH) and 2.7 O at the V site. In addition, the tourmaline sample PINK2 is slightly shifted in the direction of OH-poor fluor-buergerite. The calculated wt% for H₂O is ~14% lower than the measured value (by SIMS) for sample PINK2. One possible explanation for the higher value as obtained by SIMS is that alumino-oxy-rossmanite contains H₂O-bearing fluid inclusions (Ertl et al. 2005). However, these errors are still lower than the relative uncertainty of the SIMS analyses.

Figure 5

Relationship between bond-angle distortion σ²_oct of the ZO₆ octahedron and the average Y-O distance. Modified from Figure 3 from Ertl et al. (2002), including the structural data from Hughes et al. (2004). Alumino-oxy-rossmanite (type material, PINK1), Al-rich oxy-tourmaline (with a major alumino-oxy-rossmanite component, PINK2).

The structural refinement of the X site of the type material (PINK1) confirms an occupancy of Na <50% (only very small amounts of Ca were found by chemical analyses; Table 1). Only Al was found to occupy the Z site, and also the refined Y-site occupancies are in good agreement with the chemistry (Table 1). Although the structure refinements show significant amounts of ^[4]B [0.28(1)–0.33(2) apfu], the <T-O> bond lengths are in the range 1.617(1)–1.619(1) Å (Table 2), which is, within the standard deviation, close to a T site fully occupied with Si (MacDonald and Hawthorne 1995; Bloodaxe et al. 1999; Bosi et al. 2005). Because of a relatively low-Si content in both samples (~5.4 apfu Si; Table 1), these bond-lengths can only be explained by significant amounts of ^[4]Al in addition to ^[4]B. A substitution of Si by Al in tourmaline was described for the first time by Foit and Rosenberg (1979). The type material of alumino-oxy-rossmanite (sample PINK1) contains 0.41 apfu ^[4]Al³⁺. The simplified T-site occupation can also be written as (R₅⁴⁺R³⁺) or as [Si₅⁴⁺(Al,B)³⁺]. Hence, alumino-oxy-rossmanite is an Al-rich tourmaline that contains not only Si but also significant amounts of B and Al at the T site. Elbaitic and olenitic tourmaline samples from pegmatites often contain ^[4]B as well as ^[4]Al (Ertl et al. 2007, 2009, 2010; Lussier et al. 2009).

The lattice parameters of both investigated samples are among the lowest observed among natural tourmalines, also lower than other samples from this locality (Kolitsch et al. 2020). Alumino-oxy-rossmanite has the highest known Al content of all natural tourmalines (up to 47.08 wt% Al₂O₃; up to 8.78 apfu Al). Very high-Al contents were also found in colorless olenite from Koralpe, Styria, Austria [up to 46.53–46.71 wt% Al₂O₃ and 8.46 apfu Al (Ertl et al. 1997; Hughes et al. 2000)], and in olenite from the type locality Olenii Range, Voron’í Tundry, Kola Peninsula, Russia [up to 45.79–46.43 wt% Al₂O₃ and 8.52 apfu Al (Sokolov et al. 1986; Schreyer et al. 2002)]. Every tourmaline can be described very well by the mol% of different end-members, like every garnet. Considering the end-member formula of alumino-oxy-rossmanite (see next section), the major components of sample PINK1 (holotype material) are 41 mol% alumino-oxy-rossmanite, 33 mol% olenite, and 9 mol% elbaitic tourmaline (fluor-elbaite, elbaite). The sum of these components is 83 mol%. The major components of sample PINK2 are 34 mol% alumino-oxy-rossmanite, 28 mol% olenite, and 19 mol% elbaitic tourmaline, and hence this sample is a complex intergrowth of different end-members, mainly of the alumino-oxy-rossmanite–olenite solid solution. The sum of these components is 81 mol%. Minor components in both samples are fluor-liddicoatite and tsilaisite with ≤3 mol%. The rest of the cations cannot yet be assigned to a special tourmaline because there is no end-member established, which includes Mn³⁺ or ^TB. We conclude that in both samples, alumino-oxy-rossmanite is the dominant component. In contrast to PINK1, sample PINK2 is characterized by a lower alumino-oxy-rossmanite component and a higher elbaitic component.

End-member formula and relationship to other tourmalines

The empirical formula of the holotype material, calculated on the basis of 31(O,OH,F), is x ◻ 0.53 N a 0.46 C a 0.01 Y A l 2.37 Mn 0.21 3 + Li0.16□0.14Mn0.072+Fe0.033+Fe0.012+Ti0.014+zAl6(BO3)3(Si5.37Al0.41B0.22O18) ^V[(OH)_2.77O_0.23] ^W[O_0.80(OH)_0.15F_0.05].

Details of the occupation of both (OH)/O sites are given in the Crystal structure section above. The simplified formula is (□,Na) (Al,Mn,Li,□)₃Al₆(BO₃)₃[(Si,Al,B)₆O₁₈][(OH),O]₃[O,(OH)]. When applying the IMA-CNMNC recommended use of the dominant-valency rule for tourmaline-supergroup minerals, the Y-site cations are ordered as follows: (Al,Mn,Fe)2.63+>(Li)0.2+> (Mn,Fe)0.12+, that is R2.63+>R0.2+>R0.12+. Hence, our new tourmaline corresponds to the “X-site vacant tourmaline group” (Henry et al. 2011). The most abundant cation at the Y site with the charge 3+ is aluminum: Al2.43+>Mn0.23+>Fe<0.13+.

If we use the empirical formula and apply the site total charge approach (Bosi et al. 2019), which is useful for identifying new tourmaline end-members, two configurations are possible:

In accord with the chemical composition of the Y and T sites of the studied sample, the possible atomic arrangements are quantitatively evaluated in terms of apfu as follows:

Configuration 1.

Configuration 2. Only the following end-member charge arrangement is in agreement with configuration 2:

In the studied sample, the arrangement compatible with the Y-site and T-site constituents are: ^Y(R₃³⁺)^{Σ9+ T}(R₅⁴⁺R³⁺)^Σ23+ = Y(R33+)0.87T(R54+R3+)0.87=Y(Al2.613+)T(Si4.34+R0.873+)=7.83 apfu, limited by ^TR³⁺ contents. Actually, this arrangement is limited by the □ content at the X site (0.53 vacancies pfu), and consequently the actual number of atoms decreases to 4.77 apfu. We assume that the measured contents of B and Li are reasonably accurate, because the excess B (B > 3.00 apfu) as well as the Li content are relatively small. Hence, we conclude that ^TAl > ^TB. When we use the empirical formula, the amount of the end-member charge arrangement of configuration 2 is the largest one, and since ^TAl > ^TB, the end-member formula of the studied sample can be written as □Al₃Al₆(BO₃)₃(Si₅AlO₁₈)(OH)₃O. This ideal formula requires SiO₂ 31.90, Al₂O₃ 54.14, B₂O₃ 11.09, H₂O 2.87, total 100 wt%.

Hawthorne and Henry (1999) were the first to propose “oxy-rossmanite,” with the ideal formula □(Li_0.5Al_2.5)Al₆(Si₆O₁₈) (BO₃)₃(OH)₃O, as a hypothetical tourmaline end-member, because of its relationship to rossmanite (Selway et al. 1998). While rossmanite contains an (OH) at the W site, “oxy-rossmanite” contains an oxygen (O²⁻) at this site. Because the new tourmaline contains not only more Al, which fills the Y and Z sites entirely and the Si site partially, than “oxy-rossmanite,” but also O²⁻ at the W site, we suggest the name alumino-oxy-rossmanite, with the end-member formula □Al₃Al₆(BO₃)₃(Si₅AlO₁₈)(OH)₃O. Alumino-oxy-rossmanite is named for its chemical relationship to rossmanite, □(LiAl₂)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃(OH), which in turn was named after George R. Rossman, Professor of Mineralogy at the California Institute of Technology (Pasadena, California). With the proposed name, it is evident how this end-member is related to the quite common tourmaline rossmanite and to “oxy-rossmanite.” We also want to keep the tourmaline nomenclature as simple as possible so that the name already gives some information about the composition.

How the theoretical chemical composition of aluminooxy-rossmanite is related to the tourmalines rossmanite, “oxy-rossmanite,” elbaite, fluor-elbaite, darrellhenryite, olenite, fluor-liddicoatite, and liddicoatite is shown in Online Materials^[1] Table OM2. For further comparison, chemical analyses and selected physical properties of the valid species of Li-bearing tourmaline end-members of the tourmaline supergroup (Henry et al. 2011) are given in Table 3. The comparison shows that alumino-oxy-rossmanite and rossmanite have the smallest unit-cell parameters of all these species. The derivation of alumino-oxy-rossmanite from rossmanite appears to be a two-step process, as shown in plots in Figures 6 and 7. The second step involves the T sites, which is unique to alumino-oxy-rossmanite among tourmalines. For example, ^YLi_0.5 + ^VOH in rossmanite = ^YAl_0.5 + ^VO in “oxy-rossmanite” while Si remains equal to 6. In contrast, ^YAl_0.5 + ^TAl in alumino-oxy-rossmanite = ^YLi_0.5 + ^TSi in “oxy-rossmanite,” that is, the T site is involved. Ignoring the changes in Si occupancy results in the two analyzed tourmaline samples (PINK1 and PINK2) plotting between end-member “oxy-rossmanite” and end-member olenite and not between end-member alumino-oxy-rossmanite and end-member “oxy-rossmanite.” This omission is overcome by having two plots side-by-side (see Figs. 6 and 7).

Figure 6

Total Al vs. Si in different Al-rich tourmaline end-members. Note: a = Olenite; b = Darrellhenryite; c = Elbaite, fluorelbaite; d = Fluor-liddicoatite. Measured samples: PINK1 = Alumino-oxy-rossmanite (type material); PINK2 = Pink tourmaline with a major alumino-oxy-rossmanite component.

Figure 7

Total Al vs. Li in Al-rich tourmaline end-members. Note: a = Olenite; b = Darrellhenryite; c = Elbaite, fluor-elbaite; d = Fluorliddicoatite. Measured samples: PINK1 = Alumino-oxy-rossmanite (type material); PINK2 = Pink tourmaline with a major alumino-oxy-rossmanite component.