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

Ferdinando Bosi,Federico Pezzotta,Jan Cempírek,Ulf Hålenius,Gioacchino Tempesta,Alessandra Altieri,Henrik Skogby

doi:10.1180/mgm.2022.3

Abstract

AbstractPrincivalleite, 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) Å, c = 7.11660(10) Å, V = 1561.15(4) Å3 and 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 formulaX(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)3O(3)[(OH)2.71O0.29]Σ3.00O(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)Σ3.00ZAl6.00[T(Si5.60Al0.40)Σ6.00O18](BO3)3V[(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. 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.

Highlights

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
Tourmalines are complex borosilicates that have been extensively studied in terms of their crystal structure and crystal chemistry (e.g., Foit, 1989; Hawthorne, 1996; Hawthorne and Henry, 1999; Ertl et al, 2002; Novák et al, 2004; Bosi and Lucchesi, 2007; Bosi, 2013, 2018; Henry and Dutrow, 2011; Cempírek et al, 2013; a k and Fridrichová, 2020)
The mineral is named after Francesco Princivalle (1956), professor of mineralogy at Department of Mathematics and Geosciences, University of Trieste, Italy, for his contributions to the understanding of the crystal chemistry and geothermometery of several group minerals such as spinels, olivines and pyroxenes

Summary

Introduction

Tourmalines are complex borosilicates that have been extensively studied in terms of their crystal structure and crystal chemistry (e.g., Foit, 1989; Hawthorne, 1996; Hawthorne and Henry, 1999; Ertl et al, 2002; Novák et al, 2004; Bosi and Lucchesi, 2007; Bosi, 2013, 2018; Henry and Dutrow, 2011; Cempírek et al, 2013; a k and Fridrichová, 2020). Determination of number of atoms per formula unit (a.p.f.u.) In agreement with the structure-refinement results, the boron content was assumed to be stoichiometric (B3+ = 3.00 a.p.f.u.) Both the site-scattering results and the bond lengths of B and T are consistent with the B site fully occupied by boron and no amount of B3+ at the T site (e.g., Bosi and Lucchesi, 2007). Princivalleite originated in a B-rich and peraluminous anatectic pegmatitic melt formed in situ, poor in Fe and characterized by reducing conditions, as evidenced by the occurrence of rare pyrite and only Fe2+ in tourmaline Such reducing conditions are related to a low fugacity of oxygen in the late-stage metamorphic fluids derived by the flaser gneiss, which promoted the formation of very low levels of melting. Another explanation might be an unexposed magmatic source of melt as in the case of (apparently anatectic) kyanite-bearing Li-rich pegmatites at Virorco, Argentina (Galliski et al 2012)

Flack parameter

Σcation MFCa