Princivalleite, Na(Mn2Al)Al6(Si6O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup from Veddasca Valley, Varese, Italy. (14th February 2022)

• Record Type: Journal Article
• Authors: Bosi, Ferdinando; Pezzotta, Federico; Skogby, Henrik; Altieri, Alessandra; Hålenius, Ulf; Tempesta, Gioacchino; Cempírek, Jan
• Is Part Of: Mineralogical magazine. Volume 86:Number 1(2022)
• Page Start: 78
• Page End: 86
• Publication Date: 2022-02-14
• Subjects: princivalleite -- new mineral species -- crystal-structure refinement -- electron microprobe -- Mössbauer spectroscopy -- infrared spectroscopy -- optical absorption spectroscopy -- micro-laser induced breakdown spectroscopy; Mineralogy -- Periodicals; Mineralogy -- Great Britain -- Periodicals; 549.05
• DOI: 10.1180/mgm.2022.3
• Languages: English
• ISSNs: 0026-461X

Abstract

Abstract: Princivalleite, Na(Mn2 Al)Al6 (Si6 O18 )(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°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/cm 3 and is uniaxial (–). Princivalleite has trigonal symmetry, space group R 3 m, a = 15.9155(2) Å, c = 7.11660(10) Å, V = 1561.15(4) Å 3 and Z = 3. The crystal structure was refined to R 1 = 1.36% using 1758 unique reflections collected with Mo K α X-ray intensity data. Crystal-chemical analysis resulted in the empirical crystal-chemical formula X (Na0.54 Ca0.11 □0.35 )Σ1.00 Y (Al1.82 Mn 2+ 0.84 Fe 2+ 0.19 Zn0.07 Li0.08 )Σ3.00 Z (Al5.85 Fe 2+ 0.13 Mg0.02 )Σ6.00 [ T (Si5.60 Al0.40 )Σ6.00 O18 ](BO3 )3 O(3) [(OH)2.71 O0.29 ]Σ3.00 O(1) [O0.66 F0.22 (OH)0.12 ]Σ1.00 which recast in its ordered form for classification purposes is: X (Na0.54 Ca0.11 □0.35 )Σ1.00 Y (Al1.67 Mn 2+ 0.84 Fe 2+ 0.32 Zn0.07 Mg0.02 Li0.08 )Σ3.00 Z Al6.00 [ T (Si5.60 Al0.40 )Σ6.00 O18 ](BO3 )3 V [(OH)2.71 O0.29 ]Σ3.00 W [O0.66 F0.22 (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 Mn 2+ ↔ Fe 2+ and Mn 2+ ↔ 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.