Anortho-schorl from Langesundsfjord (Norway)

Some previous studies report of optically anomalous tourmaline with biaxial character, which is incompatible with its putative R3m symmetry. A crystal fragment of sample GEO-NRM19252409 (Swedish Museum of Natural History) coming from Langesundsfjord (Norway) showing zonation with biaxial optic behavior in the rims was studied by means of electron microprobe, singlecrystal X-ray diffraction, Mössbauer, infrared and optical absorption spectroscopy and optical measurements. The latter were performed with a spindle stage in a Leitz Dialux microscope equipped with a CCD camera using Excelibr spreadsheet (Steven & Gunter, 2018), using a 30 μm thin section of the crystal cut perpendicular to c axis. Conoscopic image showed a biaxial interference figure, with negative optic sign. Measure 2Vx was 15.6° (white light). The biaxial character of the sample is not due to internal stress since it cannot be removed by heating and cooling. Orientation of the optical indicatrix was obtained using the same crystal mounting at a Rigaku XtaLAB Synergy-S diffractometer (MoKα) and showed c^X = 2°, b^Z = 164.3° and a^Z = 75.8°. Complete data collection was obtained on a full sphere up to 0.50 Å. Diffraction data were refined with a standard R3m space group model [a 16.0013(2) Å, c 7.2263(1) Å] and with a nonconventional triclinic R1 space group model from Hughes et al. (2011), keeping the same hexagonal triple cell for comparison purposes but with unconstrained unit-cell parameters: a 16.0093(5) Å, b 16.0042(5) Å, c 7.2328(2) Å, α 90.008(3)°, β 89.856(3)°, γ 119.90(9)°. Trigonal Rint = 6.6%, Rall = 1.75% (3136 unique reflections) vs. triclinic Rint = 4.1%, Rall = 2.53% (17342 unique reflections). In both structure models, protons bonded to O3 sites were located and their coordinates refined. No proton was observed close to O1, in agreement with chemistry. Crystal- chemical analysis resulted in the chemical formula (Na0.98K0.010.01)Σ1.00 Y(Fe2+ 1.73Mg0.12Al0.90Ti0.21Mn0.02V0.0 1Zn0.01)Σ3.00 Z(Al4.88Fe3+ 0.30Fe2+ 0.19Mg0.63)Σ6.00(Si6O18)(BO3) (OH)3[(OH)0.29F0.22O0.49]Σ1.00, which agrees well in terms of calculated site scattering (X 10.9 epfu, Y 63.7 epfu, Z 83.7 epfu) and refined site scattering (X 11.4 epfu, Y 63.4 epfu, Z 83.6 epfu). The ordering of about 0.19 apfu of Fe2+ at the Z sites comes from the structure refinement of the R1 model that showed that one out of 6 independent Z sites (Zd site) shows higher refined site scattering (15.5 eps vs. mean 13.3 eps for the other 5 sites) as well as larger mean bond length (1.968 Å vs. 1.925, s.d. 0.006, Å for the other 5 sites) and larger octahedral angle variance (53° vs. 42°, s.d. 3). All these features support the local ordering of Fe2+ at the Zd site. A less pronounced ordering of Al is also observed at the Yc site (lower refined scattering and smaller mean bond length than the other two Y sites), which shares one edge with the Zd site. Optical absorption spectra also show evidence of Fe2+ at the Z sites. Interestingly, the elongation of the Zd-octahedron is along a direction which forms an angle of ca. 73° with a unit-cell parameter and is coincident with the direction for γ-refraction index. All these evidences support the triclinic character of the tourmaline structure from Langesundsfjord, and put on evidence that even in the presence of excellent statistical residual factors from excellent X-ray diffraction data, the lowering of symmetry due to cation ordering may have been overseen in many other tourmaline samples in absence of an opportune check of the optical behavior. Although the symmetry of the present sample is not trigonal, the basic structure type of tourmaline is retained. Thus, it can be classified as the triclinic dimorph of schorl.