IMA 2020-056 = princivalleite

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Marco E. Ciriotti
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IMA 2020-056 = princivalleite

Messaggio da Marco E. Ciriotti » ven 14 gen, 2022 20:22

Prossima pubblicazione.

▪ Bosi, F., Pezzotta, F., Skogby, H., Altieri, A., Hålenius, U., Tempesta, G., Cempírek, J. (2022): Princivalleite, Na(Mn2Al)Al6(Si6O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup from Veddasca Valley (Varese, Italy). Mineralogical Magazine, 86, (in press).

Princivalleite, Na(Mn2Al)Al6(Si6O18)(BO3)3(OH)3O, is a new mineral 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 center of a narrow (2-3 cm wide) vertical pegmatitic vein, a few meters long, crosscutting a lens of flaser gneiss. Crystals are subhedral (up to 10 mm in size), azure with a vitreous luster, conchoidal fracture and white streak. Princivalleite has a Mohs hardness of approximately 7, a calculated density of 3.168 g/cm3 and is uniaxial negative. Princivalleite has trigonal symmetry, space group R3m, a = 15.9155(2) Å, c = 7.11660(10) Å, V = 1561.15(4) Å3, Z = 3. The crystal structure was refined to R1 = 1.36 % using 1758 unique reflections collected with MoKα X-ray intensity data. Crystal-chemical analysis resulted in the empirical crystal-chemical formula X(Na0.54Ca0.110.35)Σ1.00Y(Al1.82Mn2+0.84Fe2+0.19Zn0.07Li0.08)Σ3.00Z(Al5.85Fe2+0.13Mg0.02)Σ6.00 [T(Si5.60Al0.40)Σ6.00O18](BO3)3 O(3)[(OH)2.71O0.29]Σ3.00 O(1)[O0.66F0.22(OH)0.12]Σ1.00 which recast in its ordered form for classification purposes is: X(Na0.54Ca0.110.35)Σ1.00Y(Al1.67Mn2+0.84Fe2+0.32Zn0.07Mg0.02Li0.08)Σ3.00ZAl6.00T[(Si5.60Al0.40)Σ6.00O18(BO3)3[(OH)2.71O0.29]Σ3.00W[O0.66F0.22(OH)0.12]Σ1.00. 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). The new mineral was approved by the International Mineralogical Association’s Commission on New Minerals, Nomenclature and Classification (IMA 2020-056). 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 characterized 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.
Marco E. Ciriotti

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