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)Al-6(Si6O18)(BO3)(3)(OH)(3)O, is a new mineral (IMA2020-056) of the tourmaline supergroup. It occurs in the Veddasca Valley, Luino area, Varese, Lombardy, Italy (46 degrees 03'30.74 '' N, 8 degrees 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 similar to 7, a calculated density of 3.168 g/cm(3) and is uniaxial (-). Princivalleite has trigonal symmetry, space group R3m, a = 15.9155(2) angstrom, c= 7.11660(10) angstrom, V= 1561.15(4) angstrom(3) and Z = 3. The crystal structure was refined to R-1 = 1.36% using 1758 unique reflections collected with MoKa. X-ray intensity data. Crystal-chemical analysis resulted in the empirical crystal-chemical formula (X)(Na0.54Ca0.11 square(0.35))Sigma(Y)(1.00)(Al1.82Mn0.84Fe0.192+Zn0.07Li0.08)(Sigma 3.00)(Z)(Al5.85Fe0.132+Mg0.02)(Sigma 6.00)[ ((T)(Si5.60Al0.40)(Sigma 6.00)O-18](BO3)(3)(O(3)) [(OH)(2.71)O-0.291](Sigma 3.00)(O(1))[O0.66F0.22(OH)(0.12)](Sigma 1.00 )which recast in its ordered form for classification purposes is: (X)(Na0.54Ca0.11 square(0.35))(Sigma 1.00)(Y)(Al1.67Mn0.842+Fe0.322+Zn0.07Mg0.02Li0.08)(Sigma 3.00)(Z)(Al-6.00[(T)(Si5.60Al0.4040)(Sigma 6.00)O-18](BO3)(3)(V(3)) [(OH)(2.71)O-0.291](Sigma 3.00)(W)[O0.66F0.22(OH)(0.12)](Sigma 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+ <-> 0.5Al(3+) + 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.

Automated summary

Princivalleite is a new mineral belonging to the tourmaline supergroup, with a unique crystal structure and chemical composition, found in Italy.