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

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/cm3and 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 MoKa X-ray intensity data. Crystal-chemical analysis resulted in the empirical crystal-chemical formula {equation presented} which recast in its ordered form for classification purposes is:{equation presented} 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.