Lowering R3m Symmetry in Mg-Fe-Tourmalines: The Crystal Structures of Triclinic Schorl and Oxy-Dravite, and the Mineral luinaite-(OH) Discredited

Abstract

Discreditation of the monoclinic tourmaline mineral species luinaite-(OH), ideally (Na,▯)(Fe²⁺,Mg)₃Al₆(BO₃)₃Si₆O₁₈(OH)₄ was approved by the IMA-CNMNC (proposal 21-L) and is described. We analyzed two luinaite-(OH) samples: one from the type locality Cleveland tin mine, Luina, Waratah, Tasmania, Australia, and the other from Blue Mountain Saddle (Bald Hornet Claim), North Bend, King County, Washington, DC, USA. Biaxial (−) crystals representative of the studied samples were spectroscopically (Mössbauer, polarized Fourier transform infrared, optical absorption spectroscopy), chemically (nuclear microprobe analysis and electron microprobe analysis), and structurally characterized (single-crystal X-ray diffraction). Results show the occurrence of a triclinic structure for the studied luinaite-(OH) samples, which differs only in terms of a slight structural distortion from typical trigonal tourmaline structure (the topology of the structure is retained). As a result, following the IMA-CNMNC and tourmaline nomenclature rules, the triclinic luinaite-(OH) from the type locality (Australia) can be considered as the triclinic dimorph of schorl, as its chemical composition corresponds to schorl, and thus it should be referred as schorl-1A. Similarly, the triclinic sample from the USA can be considered as the triclinic dimorph of oxy-dravite, as its chemical composition corresponds to oxy-dravite, and then is referred to as oxy-dravite-1A.

1. Introduction

Tourmalines are complex borosilicates that usually crystallize in the trigonal crystal system, space-group type R3m, and whose general chemical formula can be written as: XY₃Z₆T₆O₁₈(BO₃)₃V₃W, where X = Na⁺, K⁺, Ca²⁺, ▯ (= vacancy); Y = Al³⁺, Fe³⁺, Cr³⁺, V³⁺, Mg²⁺, Fe²⁺, Mn²⁺, Li⁺; Z = Al³⁺, Fe³⁺, Cr³⁺, V³⁺, Mg²⁺, Fe²⁺; T = Si⁴⁺, Al³⁺, B³⁺; B = B³⁺; V = (OH)⁻, O²⁻; W = (OH)⁻, F⁻, O²⁻. Note that the letters X, Y, Z, and B represent groups of cations at the ^[IX]X, ^[VI]Y, ^[VI]Z, ^[IV]T, and ^[III]B crystallographic sites (designated by italicized letters). The letters V and W in the formula represent groups of anions accommodated at the [III]-coordinated O(3) and O(1) crystallographic sites, respectively.

This study concerns the discreditation of the tourmaline mineral species luinaite-(OH), ideally (Na,▯)(Fe²⁺,Mg)₃Al₆(BO₃)₃Si₆O₁₈(OH)₄, which was approved by the International Mineralogical Association’s Commission on New Minerals, Nomenclature and Classification (IMA-CNMNC) in 2009 (proposal 2009-046) and discredited by the Commission in 2022 (proposal 21-L; [1]). Luinaite-(OH) was identified as monoclinic, space group Cm, and the structure was determined with representative cell dimensions (type material): a = 10.408(3), b = 15.991(5), c = 7.189(2) Å, β = 117.44(2)° and V = 1061.88 ų [2]. The symmetry reduction of this monoclinic-pseudotrigonal member of the tourmaline supergroup is caused by cation order. The type locality where luinaite-(OH) was recognized is the Cleveland tin mine, Luina, Waratah, Tasmania, Australia (41°28′57′′ S, 145°23′7′′ E; type locality).

There are two main reasons to discredit luinaite-(OH):

(1) Luinaite-(OH) violates the procedures for mineral nomenclature: if the crystal structures of the polymorphs have essentially the same topology, differing only in terms of a structural distortion or in the order-disorder relationship of some of the atoms comprising the structure, such polymorphs are not regarded as separate species [3];

(2) Luinaite-(OH) violates the procedures for tourmaline nomenclature: any deviation from the reference trigonal space-group type R3m symmetry is accommodated by adding a suffix to the root-name that indicates any atypical symmetry and not by a new name [2]. In this regard, however, the luinaite-(OH) name was approved in 2009, that is, before the approval of tourmaline nomenclature in 2011 [2].

We will show that the crystal structure of luinaite-(OH) has the same topology as an existing trigonal Fe-Mg-rich tourmaline. Throughout the paper, the name luinaite-(OH) is written in italics to indicate that it is not an IMA-CNMNC approved mineral.

2. Materials and Methods

2.1. Samples

In this study, we analyzed two samples: one (labelled as LUI-AUS) from the type locality Cleveland tin mine, Luina, Waratah, Tasmania, Australia, and another (labelled as LUI-USA) from Blue Mountain Saddle (Bald Hornet Claim), North Bend, King County, Washington, DC, USA. The latter is also a locality where luinaite-(OH) was recognized [2]. Other localities are listed in Section 5.

2.2. Crystal Chemical Characterization

The sample material was ground under acetone and analyzed by ⁵⁷Fe Mössbauer spectroscopy (Swedish Museum of Natural History, Stockholm, Sweden) to determine the Fe³⁺/ΣFe ratio. Due to limited amounts of material, a Mössbauer point source was used. Data were collected over 1024 channels, and the raw spectra were folded and calibrated against an α-Fe foil.

Biaxial (−) crystals representative of samples LUI-AUS and LUI-USA were selected with a polarized light microscope for crystal chemical investigations.

Polarized Fourier transform infrared absorption spectra were measured on 41 μm (LUI-AUS) and 68 μm (LUI-USA) thick doubly polished single-crystal sections oriented parallel to the c-axis. A Bruker Vertex spectrometer attached to a Hyperion 2000 microscope (Swedish Museum of Natural History, Stockholm, Sweden) was used to collect spectra in the range 2000–13,000 cm⁻¹ at a resolution of 4 cm⁻¹.

Polarized optical absorption spectra were acquired at room temperature on the same polished crystals that were used for the collection of infrared spectra. An AVASPEC-ULS2048X16 spectrometer (Swedish Museum of Natural History, Stockholm, Sweden), connected via a 400 μm UV fiber cable to a Zeiss Axiotron UV-microscope, was used. A 75 W Xenon arc lamp was used as a light source, and Zeiss Ultrafluar 10× lenses served as both objective and condenser. A UV-quality Glan-Thompson prism, with a working range from 40,000 to 3704 cm⁻¹, was used as the polarizer.

Nuclear microprobe analysis (Lund University, Lund, Sweden) was used to quantify lithium (⁷Li) according to the procedure reported in [4].

Electron microprobe analysis was obtained using a wavelength-dispersive spectrometer (WDS mode) with a Cameca SX50 instrument (CNR-Istituto di Geologia Ambientale e Geoingegneria, Roma, Italy), operating at an accelerating potential of 15 kV a sample current of 15 nA and 10 μm beam diameter. Minerals and synthetic compounds were used as standards: wollastonite (Si, Ca), magnetite (Fe), rutile (Ti), corundum (Al), vanadinite (V) fluorophlogopite (F), periclase (Mg), jadeite (Na), orthoclase (K), sphalerite (Zn), rhodonite (Mn), metallic Cr and Cu. The PAP routine was applied [5].

Single-crystal X-ray studies were carried out with a Bruker KAPPA APEX-II single-crystal diffractometer (Dipartimento di Scienze della Terra, Sapienza Università di Roma, Roma, Italy) equipped with a CCD area detector (6.2 cm × 6.2 cm active detection area, 512 × 512 pixels) and a graphite crystal monochromator, using MoKα radiation from a fine-focus sealed X-ray tube. The sample-to-detector distance was 4 cm. A total of 4055 exposures (step = 0.2°, time/step = 20 s) covering a full reciprocal sphere were collected at room temperature.

3. Results

3.1. Mössbauer Spectroscopy (MS)

The recorded Mössbauer spectra (Figure 1) were fitted using the software MossA [6] with four doublets assigned to Fe²⁺ and one doublet assigned to Fe³⁺, resulting in an Fe³⁺/ΣFe ratio of 0.056 for sample LUI-AUS and 0.13 for sample LUI-USA (

Table 1. Mössbauer parameters for luinaite-(OH).

Sample	δ	ΔEQ	Γ	% Area	Assignment
LUI-AUS	1.10	2.56	0.23	22.4	Fe2+
	1.09	2.32	0.26	34.3	Fe2+
	1.11	2.00	0.29	22.0	Fe2+
	1.12	1.58	0.38	15.8	Fe2+
	0.41	0.86	0.39	5.6	Fe3+
LUI-USA	1.10	2.61	0.24	13.5	Fe2+
	1.10	2.38	0.28	30.7	Fe2+
	1.11	1.98	0.41	28.1	Fe2+
	1.14	1.61	0.43	14.5	Fe2+
	0.42	0.86	0.44	13.2	Fe3+

δ = centroid shift (mm/s), ΔE_Q = quadrupole splitting (mm/s), Γ = full width at half maximum (mm/s).

).

3.2. Fourier Transform Infrared (FTIR) Spectroscopy

The single-crystal FTIR spectra recorded in polarized mode parallel to the crystallographic c-axis show a very intense band around 3530 cm⁻¹ and two weaker but significant bands at 3629–3631 and 3720–3723 cm⁻¹ (Figure 2). As typically observed for tourmaline spectra in the (OH) range, the main band is off-scale for the E//c direction due to excessive absorption. Spectra obtained perpendicular to the c-axis show considerably weaker bands centered at 3491 and 3550 cm⁻¹ for sample LUI-AUS and 3529 cm⁻¹ for sample LUI-USA. The occurrence of bands above ~3600 cm⁻¹ is worth noting, which is the region where bands due to (OH) at the O(1) site (≡W) are expected [7,8].

3.3. Optical Absorption Spectroscopy (OAS)

The recorded optical absorption spectra (Figure 3) show broad and strongly E⊥c-polarized (i.e., O > E) absorption bands at ~22,700, ~14,000 and ~9100 cm⁻¹. In agreement with previous optical studies of tourmaline [9], the bands at ~14,000 and ~9100 cm⁻¹ are assigned to Fe^3+-enhanced spin-allowed d-d transitions in six-coordinated Fe²⁺. These two bands are distinctly non-Gaussian in shape, which suggests that ferrous iron is located at several non-equal octahedrally coordinated sites. The broad, intense, and strongly E⊥c-polarized band at ~22,700 cm⁻¹ is due to Fe²⁺-Ti⁴⁺ intervalence charge transfer processes [10,11]. Additional sharp absorption bands, observed in the E//c-spectrum in the range 6700–7000 cm⁻¹, mark overtones of the fundamental (OH)-stretching modes.

3.4. Chemical Data and Mineral Formula

The electron microprobe analysis (EMPA) results represent the mean values of 12 spot analyses for each sample. Vanadium, Cr, Ni, Cu, and Zn were below detection limits (<0.03 wt%). The number of atoms per formula unit (apfu) was determined as follows. In accord with the structural information, the B content was assumed to be stoichiometric (B = 3.00 apfu): the values of the B- and T-site occupancy factors are consistent with the B fully occupied by B and T sites with no or insignificant B. The iron oxidation state was determined by MS. In accordance with the MS results and Fe and Mn redox potential arguments, all Mn was considered as Mn²⁺. In accordance with nuclear microprobe analysis, the Li content is insignificant: Li₂O < 0.01 wt%. The (OH) content and the formulae were then calculated by charge balance with the assumption (T + Y + Z) = 15.00 apfu and 31 anions pfu. The excellent agreement between the number of electrons per formula unit (epfu) derived from EMPA and SREF (respectively, 246.07 and 245.73 epfu for sample LUI-AUS, and 236.81 and 237.76 epfu for sample LUI-USA) supports the stoichiometric assumptions. Chemical data are given in

Table 2. Chemical data (in wt%) for luinaite-(OH).

Constituent	LUI-AUS	LUI-USA
	12 Spots	12 Spots
SiO2	35.24 (67)	36.01 (51)
TiO2	0.09 (3)	0.23 (12)
B2O3(calc) a	10.24	10.45
Al2O3	33.66 (65)	33.69 (62)
FeOtot	12.51 (40)	7.12 (71)
MnO	0.08 (4)	-
MgO	1.94 (39)	5.62 (53)
CaO	0.18 (09)	0.44 (13)
Na2O	1.78 (27)	2.12 (16)
K2O	0.04 (1)	-
F	0.37 (10)	0.06 (5)
H2O(calc) a	2.94	3.03
O=F	−0.16	−0.03
Fe2O3 b	0.77	1.04
FeO b	11.82	6.18
Total	99.00	98.85

Note: Errors for oxides and fluorine are standard deviations (in brackets). ^a Calculated by stoichiometry. ^b From Mössbauer spectroscopy.

. It should be noted that the samples are rather homogeneous from a chemical viewpoint, as shown by relatively low standard deviation values (Table 2).

The ordered empirical chemical formulae, with appropriate grouping of sites, required for classification purposes [2] are:

• Sample LUI-AUS

^X(Na_0.59K_0.01▯_0.37Ca_0.03)_Σ1.00^Y(Fe²⁺_1.68Mg_0.49Mn_0.01Al_0.71Fe³⁺_0.10Ti_0.01)_Σ3.00^ZAl₆^T[(Si_5.98Al_0.02)_Σ6.00O₁₈] (BO₃)₃ (OH)₃ ^W[(OH)_0.33F_0.20O_0.47]_Σ1.00

• Sample LUI-USA

^X(Na_0.68▯_0.24Ca_0.08)_Σ1.00^Y(Mg_1.39Fe²⁺_0.86Al_0.59Fe³⁺_0.13Ti_0.03)_Σ3.00^ZAl₆^T[(Si_5.99Al_0.01)_Σ6.00O₁₈] (BO₃)₃ (OH)₃ ^W[(OH)_0.36F_0.03O_0.60]_Σ1.00

These chemical formulae lead to the ideal formulae:

• Sample LUI-AUS: NaFe²⁺₃Al₆(Si₆O₁₈)(BO₃)₃(OH)₃(OH).
• Sample LUI-USA: Na(Mg₂Al)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃O.

As a result, the compositions of LUI-AUS and LUI-USA are consistent with mineral species schorl and oxy-dravite, respectively.

3.5. Crystal Structure

The observed biaxial interference figures of samples LUI-AUS and LUI-USA are inconsistent with the typical R3m space-group type of tourmaline. This result is also supported by the significant difference in unconstrained unit-cell refinements from the expected hexagonal cell values (a = b, α = β = 90°, and γ = 120°). The possible biaxial subsymmetries of space group R3m are the subgroups Cm (monoclinic) and P1 (triclinic). Thus, in accord with the studies of [12,13,14], we selected the space-group type P1 for the refinement in line with the significant departure of α and β from 90° in unconstrained unit-cell refinements. To facilitate the comparison between triclinic and trigonal structures, the primitive triclinic cell used in this study [e.g., sample LUI-AUS, a = 7.20820(10), b = 9.5000(2), c = 9.4947(2) Å, α = 113.7519(8)°, β = 104.5539(8)°, γ = 104.6823(9)° and V = 527.835(18) ų] was recast in the non-standard space-group R1, defining a pseudohexagonal unit-cell similar to the typical hexagonal triple R cell but with unconstrained unit-cell parameters (

Table 3. Single crystal X-ray diffraction data details for luinaite-(OH).

-	LUI-AUS	LUI-USA
Crystal sizes (mm)	0.07 × 0.10 × 0.18	0.09 × 0.12 × 0.19
a (Å)	15.9513 (5)	15.9084 (3)
b	15.9421 (5)	15.9374 (3)
c	7.1921 (2)	7.20839 (15)
α (°)	90.0354 (17)	90.1073 (9)
β	89.9359 (16)	89.9118 (9)
γ	119.8527 (14)	119.9399 (7)
V (Å3)	1586.24 (9)	1583.71 (6)
Range for data collection, 2θ (°)	5.08–72.07	5.11–74.14
Reciprocal space range hkl	−26 ≤ h ≤ 26 −20 ≤ k ≤ 26 −11 ≤ l ≤ 11	−26 ≤ h ≤ 26 −26 ≤ k ≤ 26 −12 ≤ l ≤ 12
Set of measured reflections	17,779	18,250
Unique reflections, Rint (%)	8874, 3.51	9253, 2.83
Restraints, refined parameters	6, 471	6, 471
Flack parameter	0.05 (2)	0.07 (3)
wR2 (%)	8.57	6.49
R1 (%) all data	4.83	3.33
R1 (%) for I > 2σI	3.83	2.94
GooF	0.922	0.902
Diff. peaks (±e−/Å3)	0.74; –0.75	0.47; –1.01

Notes: Total number of frames = 4055. Data collection temperature = 293 K. Space-group type R1. Z = 3. Radiation: Mo-Kα = 0.71073 Å. Absorption correction method: multi-scan (SADABS). Refinement method: full-matrix last-squares on F². Structural refinement program: SHELXL-2013. R_int. = merging residual value; R₁ = discrepancy index, calculated from F-data; wR₂ = weighted discrepancy index, calculated from F²-data; GooF = goodness of fit; Diff. peaks = maximum and minimum residual electron density.

).

The starting coordinates were taken from [14]. Structure refinement was undertaken using the SHELXL-2013 program [15]. Variable parameters were scale factor, extinction coefficient, atom coordinates, site-scattering values (for X, Ya, Yb, Yc, Za, Zb, Zc, Zd, Ze, and Zf sites), and atomic-displacement factors. Attempts to refine the extinction coefficient yielded values very close to zero and within standard uncertainty; thus, it was not refined. Neutral atom scattering factors were used. In detail, the X site was modeled using the Na scattering factor. The occupancy of the three and six non-equivalent Y and Z sites, respectively, were obtained considering the presence of Al versus Fe. All the T, B, and anion sites were modeled, respectively, with Si, B, and O scattering factors and with a fixed occupancy of 1, because refinement with unconstrained occupancies showed no significant deviations from this value. The position of the H atom bonded to the oxygen at three non-equivalent O(3) sites in the structure was taken from the difference-Fourier map and incorporated into the refinement model; the O(3)–H(3) bond length was restrained (by DFIX command) to be 0.97 Å with isotropic displacement parameter constrained to be equal to 1.2 times that obtained for the respective O(3) sites. There were no correlations greater than 0.7 between the parameters at the end of the refinement. Details concerning data collection and refinement are reported in Table 3.

Final atom positions, equivalent isotropic displacement parameters, bond distances, and refined site occupancy factors are given in Tables S1 and S2. Crystallographic information files (CIF) are on deposit (see Supplementary Materials).

The population of the Ya, Yb, Yc, Za, Zb, Zc, Zd, Ze, and Zf sites was optimized by the method of Wright et al. [16], in which structural and chemical data were used along with the default setting of the program, but with the chemical variability constrained by electroneutrality. Results are reported in

Table 4. Optimized cation site populations ^a (in atoms per formula unit) and mean atomic number ^b for luinaite-(OH).

Site	Site Population	Mean Atomic Number
-	Sample LUI-AUS	Observed	Calculated
X	0.59 Na + 0.01 K + 0.03 Ca + 0.37 ▯	7.82 (8)	7.25
Ya	0.34 Al + 0.43 Fe2+ + 0.11 Mg + 0.10 Fe3+ + 0.11 Mn + 0.12 Ti4+	19.83 (14)	19.93
Yb	0.54 Al + 0.42 Fe2+ + 0.04 Mg	18.30 (13)	18.40
Yc	0.57 Al + 0.38 Fe2+ + 0.06 Mg	17.72 (12)	17.82
Za	0.95 Al + 0.03 Fe2+ + 0.02 Mg	13.22 (8)	13.32
Zb	0.88 Al + 0.09 Fe2+ + 0.03 Mg	13.98 (8)	14.08
Zc	0.97 Al + 0.02 Fe2+ + 0.01 Mg	13.16 (8)	13.26
Zd	0.78 Al + 0.14 Fe2+ + 0.08 Mg	14.61 (9)	14.71
Ze	0.92 Al + 0.05 Fe2+ + 0.03 Mg	13.57 (8)	13.67
Zf	0.77 Al + 0.13 Fe2+ + 0.10 Mg	14.52 (9)	14.62
Ta,b,c,d,e	6 Si	14 c	14
Tf	5.98 Si + 0.02 Al	14 c	14
B1,2,3	3 B	5 c	5
-	Sample LUI-USA	Observed	Calculated
X	0.68 Na + 0.08 Ca + 0.24 ▯	9.78 (6)	9.09
Ya	0.57 Al + 0.26 Fe2+ + 0.17 Mg	16.24 (6)	16.21
Yb	0.43 Al + 0.15 Fe2+ + 0.27 Mg + 0.13 Fe3+ + 0.03 Ti4+	16.63 (7)	16.60
Yc	0.66 Al + 0.18 Fe2+ + 0.16 Mg	15.18 (6)	15.17
Za	0.95 Al + 0.02 Fe2+ + 0.03 Mg	13.21 (5)	13.18
Zb	0.76 Al + 0.07 Fe2+ + 0.17 Mg	13.75 (5)	13.73
Zc	0.94 Al + 0.02 Fe2+ + 0.04 Mg	13.29 (5)	13.26
Zd	0.72 Al + 0.06 Fe2+ + 0.22 Mg	13.60 (5)	13.57
Ze	0.80 Al + 0.05 Fe2+ + 0.15 Mg	13.55 (5)	13.52
Zf	0.77 Al + 0.05 Fe2+ + 0.18 Mg	13.55 (5)	13.52
Ta	5.99 Si + 0.01 Al	14 c	14
Tb,c,d,e,f	6 Si	14 c	14
B1,2,3	3 B	5 c	5

^a Using the method of Wright et al. [16] ^b According to [17] ^c Fixed in the final stages of refinement.

.

4. Discussion

In accord with the current IMA-CNMNC rules, the polymorphic forms of a mineral are regarded as different species if their structures are topologically different [2]. Thus, if the crystal structures of LUI-AUS and LUI-USA show a different way of connecting the atoms (for example, showing at least an atom with a different coordination number) compared to that of the trigonal schorl and oxy-dravite (respectively) structure, they may be regarded as different species.

Figure 4 displays the crystal structure of sample LUI-AUS (a similar figure can be obtained for sample LUI-USA). From this figure, it is quite apparent that the triclinic structure is very similar to the typical trigonal structure: e.g., no change in the link between atoms or coordination number as the topology of the structure is retained. Both the triclinic structures of LUI-AUS and LUI-USA differ only in terms of a slight structural distortion from trigonal tourmaline structure. As a result, following the IMA-CNMNC and tourmaline nomenclature rules [2,3], the triclinic luinaite-(OH) from the type locality (LUI-AUS) can be considered as the triclinic dimorph of schorl, and then referred as schorl-1A (for Anorthic; [3]). Similarly, the triclinic sample LUI-USA can be considered as the triclinic dimorph of oxy-dravite, and thus, it can be referred to as oxy-dravite-1A.

5. World Locations Where non Trigonal Fe-Mg-Tourmalines Occur

Several lower symmetry tourmalines have also been discovered by S.J.M. and Uwe Kolitsch. These include fibrous tourmalines from: Mount Bendoc, Victoria (Australia), Mount Bischoff, Tasmania (Australia), Bald Hornet claim, North Bend, King Co., Washington (DC, USA), the Itatiaia mine, Conselheiro Pena, Doce Valley, Minas Gerais (Brazil), the Sn-bearing greisen deposit at Ehrenfriedersdorf, Saxony (Germany), and the following four Norwegian localities: Hundholmen, Midtfjellet quarry, A/S Granit quarry and E18 roadcut (these samples were preliminarily described in [19,20]. In all these cases, the crystal structure was solved in the monoclinic space group Cm with a ≈ 10.4, b ≈ 16.0, c ≈ 7.2 Å, and β ≈ 117°. Further details will be published in a separate paper.

Cámara et al. [14] reported a single crystal of schorl composition from Langesundsfjord, Telemark (Norway) showing a change in crystallographic symmetry, accompanied by a different optical behaviour, i.e., trigonal-uniaxial in the dark brownish core and triclinic-biaxial in the darker brownish rim.

6. Conclusions

Atom distributions in the R1 tourmaline structure are consistent with the ordering scheme of space groups Cm and R3m [12]. In general, the structure topology is preserved in both biaxial (triclinic and monoclinic) and uniaxial (trigonal) tourmalines, whereas the geometry of their structures differs only in terms of structural distortion. Thus, these polymorphic forms should not be regarded as separate mineral species. This conclusion can also be shown by the general structural formulae, along with the site coordination numbers, corresponding to R3m, Cm, and R1 (or P1) tourmaline crystal structures:

• R3m

^IXX ^VIY₃ ^VIZ₆ (^IVT₆O₁₈) (^IIIBO₃)₃ ^IIIO(3)₃ ^IIIO(1);

• Cm

^IXX (^VIYa ^VIYb₂) (^VIZa₂ ^VIZb₂ ^VIZc₂) [(^IVTa₂ ^IVTb₂ ^IVTc₂)O₁₈] [(^IIIB1 ^IIIB2₂)O₃]₃ [^IIIO(3A) ^IIIO(3B)₂] ^IIIO(1);

• R1 (or P1)

^IXX (^VIYa ^VIYb ^VIYc) (^VIZa ^VIZb ^VIZc ^VIZd ^VIZe ^VIZf) [(^IVTa ^IVTb ^IVTc ^IVTd ^IVTe ^IVTf)O₁₈] [(^IIIB1 ^IIIB2 ^IIIB3)O₃]₃ [^IIIO(3A) ^IIIO(3B) ^IIIO(3C)] ^IIIO(1).

In general, distortions from the trigonal tourmaline structure can be identified using high-resolution single-crystal X-ray diffraction data, better if accompanied by a polarizing microscope analysis to assess the biaxiality of the crystal.