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.

Blue growth zones caused by Fe2+ in tourmaline crystals from the San Piero in Campo gem-bearing pegmatites, Elba Island, Italy

“…A slightly higher energy of 23900 cm -1 for the Fe 2+ -Ti 4+ IVCT-band as compared to a previously reported energy range of 22000-22900 cm -1 (e.g. Smith, 1978;Taran et al, 1993;Bosi et al, 2022b) could be explained by differences in local cation ordering. This suggests that Ti 4+ acts as colourcausing agent in combination with Mn 2+ and Fe 2+ in the yellow-green zone of the San Silvestro sample, consistent with the Ti enrichment of this zone (TiO 2 up to 0.11 wt.%).…”

“…In the spectra of the San Silvestro sample, the broad and strongly polarized (E⊥c) absorption in the near UV-region extending from 19000 to 32000 cm -1 can be tentatively assigned to a Mn 2+ -Ti 4+ IVCT-band at ⁓30100 cm -1 (Rossman and Mattson, 1986) and to a Fe 2+ -Ti 4+ IVCT-band at ⁓23900 cm -1 (data not shown). A slightly higher energy of 23900 cm -1 for the Fe 2+ -Ti 4+ IVCT-band as compared to a previously reported energy range of 22000-22900 cm -1 https://doi.org/10.1180/mgm.2022.101 Published online by Cambridge University Press (e.g., Smith, 1978;Taran et al, 1993;Bosi et al, 2022b) may possibly be explained by differences in local cation ordering. This suggests that Ti 4+ acts as colour-causing agent in combination with Mn 2+ and Fe 2+ in the yellow-green zone of San Silvestro sample, consistent with the Ti enrichment of this zone (TiO2 up to 0.11 wt%).…”

Schorl-1A from Langesundsfjord (Norway)

“…Recently, however, luinatite-(OH) has been formally discredited (IMA proposal n. 21-L; Bosi et al 2022), and any deviation from the reference space group R3m symmetry should be accommodated in the nomenclature by adding a suffix to the root name that indicates any atypical symmetry (Henry et al 2011).…”