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- Altieri, Alessandra, et al.
(författare)
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Blue growth zones caused by Fe2+ in tourmaline crystals from the San Piero in Campo gem-bearing pegmatites, Elba Island, Italy
- 2022
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Ingår i: Mineralogical magazine. - : Mineralogical Society. - 0026-461X .- 1471-8022. ; 86:6, s. 910-919
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Tidskriftsartikel (refereegranskat)abstract
- Two tourmaline crystals with a blue growth zone at the analogous pole, respectively from the San Silvestro and the Fucili pegmatites, located in the San Piero in Campo village, Elba Island (Tyrrhenian Sea, Italy), have been described for the first time using compositional and spectroscopic data to define their crystal-chemical aspects and the causes of the colour. Compositional data obtained by electron microprobe analysis indicate that both tourmalines belong to the elbaite–fluor-elbaite series. The upper part of each crystal is characterised by an increased amount of Fe (FeO up to ~1 wt.%) and a Ti content below the detection limit. Optical absorption spectra recorded on the blue zone of both samples show absorption bands caused by spin-allowed d-d transitions in [6]-coordinated Fe2+, and no intervalence charge transfer Fe2+-Ti interactions, indicating that Fe2+ is the only chromophore. Mössbauer analysis of the blue zone of the Fucili sample confirmed the Fe2+ oxidation state, implying that the redox conditions in the crystallisation environment were relatively reducing. The presence of colour changes at the analogous termination during tourmaline crystal growth suggests a change in the composition of the crystallisation environment, probably associated with a partial opening of the system.
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| 2. |
- Bosi, Ferdinando, et al.
(författare)
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Princivalleite, Na(Mn2Al)Al6(Si6O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup from Veddasca Valley, Varese, Italy
- 2022
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Ingår i: Mineralogical magazine. - : Mineralogical Society. - 0026-461X .- 1471-8022. ; 86:1, s. 78-86
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Tidskriftsartikel (refereegranskat)abstract
- Princivalleite, 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.
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