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

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.

Skogby, HenrikNaturhistoriska riksmuseet,Enheten för geovetenskap(Swepub:nrm)henrskog (author)
Hålenius, UlfNaturhistoriska riksmuseet,Enheten för geovetenskap(Swepub:nrm)ulfhalen (author)
Ciriotti, Marco E.Università di Torino, Italy (author)
Mills, Stuart J.Geosciences, Museums Victoria, Melbourne, Australia (author)
Naturhistoriska riksmuseetEnheten för geovetenskap (creator_code:org_t)

By the author/editor: Bosi, Ferdinando; Skogby, Henrik; Hålenius, Ulf; Ciriotti, Marco ...; Mills, Stuart J.

About the subject

NATURAL SCIENCES: NATURAL SCIENCES; and Earth and Relate ...; and Geochemistry

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