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Princivalleite, Na(Mn2Al)Al6(Si6O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup from Veddasca Valley, Varese, Italy

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BOSI Ferdinando PEZZOTTA Federico SKOGBY Henrik ALTIERI Alessandra HALENIUS Ulf TEMPESTA Gioacchino CEMPÍREK Jan

Rok publikování 2022
Druh Článek v odborném periodiku
Časopis / Zdroj Mineralogical Magazine
Fakulta / Pracoviště MU

Přírodovědecká fakulta

Klíčová slova princivalleite; new mineral species; crystal-structure refinement; electron microprobe; Mossbauer spectroscopy; infrared spectroscopy; optical absorption spectroscopy; micro-laser induced breakdown spectroscopy
Popis Princivalleite, Na(Mn2Al)Al6(Si6O18)(BO3)3(OH)3O, is a new mineral (IMA2020-056) of the tourmaline supergroup. It occurs in the Veddasca Valley, Luino area, Varese, Lombardy, Italy (46°03’30.74’’N, 8°48’24.47’’E) at the centre of a narrow (2–3 cm wide) vertical pegmatitic vein, a few metres long, crosscutting a lens of flaser gneiss. Crystals are subhedral (up to 10 mm in size), azure with a vitreous lustre, conchoidal fracture and white streak. Princivalleite has a Mohs hardness of ~7, a calculated density of 3.168 g/cm3 and is uniaxial (–). Princivalleite has trigonal symmetry, space group R3m, a = 15.9155(2) A, c = 7.11660(10) A, V = 1561.15(4) A3 and Z = 3. The crystal structure was refined to R1 = 1.36% using 1758 unique reflections collected with MoK? X-ray intensity data. Crystal-chemical analysis resulted in the empirical crystal-chemical formula X(Na0.54Ca0.11?0.35)?1.00Y(Al1.82Mn2+0.84Fe2+0.19Zn0.07Li0.08)?3.00Z(Al5.85Fe2+0.13Mg0.02)?6.00[T(Si5.60Al0.40)?6.00O18](BO3)3O(3)[(OH)2.71O0.29]?3.00O(1)[O0.66F0.22(OH)0.12]?1.00 which recast in its ordered form for classification purposes is: X(Na0.54Ca0.11?0.35)?1.00 Y(Al1.67Mn2+0.84Fe2+0.32Zn0.07Mg0.02Li0.08)?3.00 ZAl6.00[T(Si5.60Al0.40)?6.00O18](BO3)3V[(OH)2.71O0.29]?3.00W[O0.66F0.22(OH)0.12]?1.00. Princivalleite is an oxy-species belonging to the alkali group of the tourmaline supergroup. The closest end-member compositions of valid tourmaline species are those of oxy-schorl and darrellhenryite, to which princivalleite is related by the substitutions Mn2+ - Fe2+ and Mn2+ - 0.5Al3+ + 0.5Li+, respectively. Princivalleite from Veddasca Valley is a geochemical anomaly, originated in a B-rich and peraluminous anatectic pegmatitic melt formed in situ, poor in Fe and characterised by reducing conditions in the late-stage metamorphic fluids derived by the flaser gneiss. The Mn-enrichment in this new tourmaline is due to absence of other minerals competing for Mn such as garnet.
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