Uvite, CaMg₃(Al₅Mg)(Si₆O₁₈)(BO₃)₃(OH)₃(OH), a new, but long-anticipated mineral species of the tourmaline supergroup from San Piero in Campo, Elba Island, Italy

Abstract

Uvite, CaMg3(Al5Mg)(Si6O18)(BO3)3(OH)3(OH), is a new mineral of the tourmaline supergroup. It occurs in the Facciatoia quarry, San Piero in Campo, Elba Island, Italy (42°45′04.55″N, 10°12′50.89″E) at the centre of a narrow (2–3 cm wide) vein composed of aggregates of dark brown to black tourmaline, penetrating (magnesite + dolomite)-rich hydrothermally altered metaserpentinite. Crystals are euhedral and up to 1 cm in size, brown with a vitreous lustre, conchoidal fracture and grey streak. Uvite has a Mohs hardness of ~7½, a calculated density of 3.115 g/cm3 and is uniaxial (–). Uvite has trigonal symmetry, space group R3m, a = 15.9519(10) Å, c = 7.2222(5) Å, V = 1597.3(1) Å3 and Z = 3. The crystal structure was refined to R1 = 1.77% using 1666 unique reflections collected with MoKα X-rays. Crystal-chemical analysis resulted in the empirical crystal-chemical formula $^X ({\rm Ca}_{0.61}{\rm Na}_{{0.35}} \square_{{0.04}})_{\Sigma 1.00}{}^{Y} \left( {{\rm Mg}_{1.50}{\rm Fe}^{2 + }_{0.47} {\rm Al}_{0.71}{\rm Fe}^{3 + }_{0.14} {\rm Ti}_{0.18}} \right)_{\Sigma 3.00}$ ${}^{Z} \left( {{\rm Al}_{4.54}{\rm Fe}^{3 + }_{0.18} {\rm V}^{3 + }_{0.02} {\rm Mg}_{1.27}} \right)_{\Sigma 6.00}{}^{T}\left[ {{\left( {{\rm Si}_{5.90}{\rm Al}_{0.10}} \right)}_{\Sigma 6.00}{\rm O}_{18}} \right]{\rm } \left( {\rm BO_3} \right)_3^{} {^{\rm O(3)}}\left( {\rm OH} \right)_3{}^{{\rm O}\left( 1 \right)} [\left( {\rm OH} \right)_{0.55}{\rm F}_{0.05}{\rm O}_{0.40}]_{\Sigma 1.00}$ which recast in its ordered form for classification purposes is: $$\eqalign{& ^{\rm X} ({\rm Ca}_{0.61}{\rm Na}_{0.35}\squ _{0.04})_{\Sigma 1.00}{}^{\rm Y} \left( {{\rm Mg}_{2.35}{\rm Fe}^{2 + }_{0.47} {\rm Ti}_{0.18}} \right)_{\Sigma 3.00} \cr & {}^{\rm Z} \left( {{\rm Al}_{5.25}{\rm Fe}^{3 + }_{0.32} {\rm V}^{3 + }_{0.02} {\rm Mg}_{0.42}} \right)_{\Sigma 6.00}{}^{\rm T} \left[ {{\left( {{\rm Si}_{5.90}{\rm Al}_{0.10}} \right)}_{\Sigma 6.00}{\rm O}_{18}} \right]{\rm }\left( {\rm BO_3} \right)_3{}^{\rm V} \left( {\rm OH} \right)_3{}^{\rm W} [\left( {\rm OH} \right)_{0.55}{\rm F}_{0.05}{\rm O}_{0.40}]_{\Sigma 1.00}}$$ Uvite is a hydroxy-species belonging to the calcic-group of the tourmaline supergroup. The closest end-member compositions of valid tourmaline species are fluor-uvite and feruvite, to which uvite is related by the substitutions W(OH)– ↔ WF– and YMg2+ ↔ YFe2+, respectively. The occurrence of a solid-solution between uvite and magnesio-lucchesiite, according to the substitution ZMg2+ + W(OH)– ↔ ZAl3+ + WO2–, is supported by experimental data. The new mineral was approved by the IMA–CNMNC (IMA 2019-113). Uvite from Facciatoia formed by the reaction between B-rich fluids, released during the crystallisation process of LCT pegmatites, and the surrounding metaserpentinites, altered by contact metamorphism in the aureole of the Miocene Mt. Capanne monzogranitic pluton.

Nickel- and Fe3+-rich oxy-dravite from the Artana Mn prospect, Apuan Alps (Tuscany, Italy)

“…Nickel-and Fe 3+ -rich oxy-dravite from the Artana Mn prospect (Apuan Alps) is the fourth well-characterized non-pegmatitic tourmaline supergroup minerals from Tuscany. Along with the recently approved new minerals dutrowite (Biagioni et al 2020), uvite (Bosi et al 2022b), and magnesio-lucchesiite (Scribner et al 2021), it suggests that further crystal-chemical investigations on the many tourmaline occurrences from this Italian sector may be able to improve the tourmaline systematics and our understanding of its crystal chemistry. Moreover, the studied sample is one of the Ni-richest so far described and it occurs in a different geological setting concerning the previously-known occurrences for Ni-bearing tourmalines.…”

“…Three additional species have been discovered during the crystal-chemical characterization of non-pegmatitic tourmalines from Tuscany. Uvite and magnesio-lucchesiite were identified in veins within metabasite of the thermal aureole of the Monte Capanne intrusion (Bosi et al 2022b;Scribner et al 2021), whereas dutrowite was taining sub-millimetric prismatic crystals of a potential tourmaline supergroup mineral was identified. The mineralogical study confirmed the first identification, pointing out a high Ni content and suggesting the necessity of its full crystal-chemical investigation.…”

In situ high-temperature behaviour and breakdown conditions of uvite at room pressure

“…Texture effects were corrected using spherical harmonics (8th-order, nine refinable parameters), selecting the number of appropriate terms by using the approach of Ballirano (2003). Starting structural data were taken from Bosi et al (2022) and each refined structure at a given non-ambient T was used as input for the subsequent T. Experimental conditions and miscellaneous information regarding the refinements are listed in Table 1, a magnified 3D-plot of the full data set is shown in Fig. 1, and a representative example of Rietveld plots is shown in Fig.…”

“…The occurrence of Ca-rich tourmalines in nature was experimentally demonstrated to be a function of pressure, temperature and metamorphic fluids composition (Berryman et al 2016), but calcic tourmalines remain so far the less investigated in terms of thermal behaviour, dehydration and breakdown products, also because uvite, ideally CaMg 3 (Al 5 Mg)(Si 6 O 18 ) (BO 3 ) 3 (OH) 3 (OH), was only recently officially approved by the International Mineralogical Association's Commission on Nomenclature and Classification as a new mineral species of tourmaline supergroup (Bosi et al 2022).…”

Physical properties and crystal structure of near end-member oxy-dravite from the Beluga occurrence, Nunavut territory, Canada

“…Occupancies of octahedral sites were refined using bond-valence optimisation (Table 3) which confirmed disorder of Mg and Al between the Y- and Z-sites, typical for Mg-rich tourmalines (e.g. Grice and Ercit, 1993; Hawthorne et al , 1993; Ertl et al , 2003; Bosi and Lucchesi, 2004; Bosi and Skogby, 2013; Bačík, 2015; Pieczka et al , 2018; Scribner et al , 2021; Bosi et al , 2022; Sȩk et al , 2022).…”