History of ardennite and the new mineral ardennite-(V)
Ardennite-(V), ideally Mn4[Al4(AlMg)][Si5V]O22(OH)6, is the analogue of ardennite, ideally Mn4[Al4(AlMg)][Si5As]O22(OH)6, with V dominant at the T4 site. The history of the discovery of ardennite is outlined, with special accent on its chemical composition; revision of the old literature evidenced that even if vanadium-containing specimens were considered since the first description, the analytical data contained in the first papers are very questionable, and ardennite has been so far considered an arsenate-silicate. A new occurrence of ardennite-(V) is described from Sparone, Locana Valley (TO), Piedmont, Italy. Here ardennite-(V) occurs within a piemontite-bearing micaschist. Crystals are elongated along  and very thin (up to 1 mm in length and a few microns in diameter), are yellow with a white streak, transparent with a vitreous lustre, and brittle. No cleavage or parting were observed. The calculated density is 3.55 g/cm3. Optically, ardennite-(V) is biaxial, with α > 1.765(5), β < 1.775(5), and γ = 1.775(5), and is weakly pleochroic, with X, Y = pale yellow; Z = yellowish. Electron microprobe analyses (P2O5 0.38 wt.%, As2O5 0.37, V2O5 4.60, SiO2 31.21, TiO2 0.21, Al2O3 22.60, Fe2O3 1.64, Cr2O3 0.34, MgO 4.44, MnO tot 23.28, CaO 4.24, Na2O 0.02, F 0.17, H2O calc 5.70), gave the following empirical formula: [Mn2+ 3.221Ca0.774Na0.006]Σ = 4.001 [Al4.456Mg1.127Fe3+ 0.210Mn3+ 0.137Cr0.046Ti0.027]Σ = 6.003 [Si5.000] [V0.518Si0.315Al0.080P0.055As0.033]Σ =1.001 O21.433 (OH)6.475 F0.092. The five strongest peaks in the X-ray powder diffraction pattern [d(Å)(I)(hkl)] are: 2.948(90)(115), 2.609(100)(310/116), 2.033(55)(225), 1.585(75)(425), 1.477(45)(2.2.10). Ardennite-(V) is orthorhombic, space group Pnmm, with a 8.760(3), b 5.838(2), c 18.56(2) Å. Both the new mineral and its name were approved by the IMA CNMNC. Chemical analyses which point to the occurrence of ardennite-(V) were recorded also at Varenche mine, S. Barthelemy, Aosta Valley, Italy, but the crystal showed a marked zoning.
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Document Type: Research Article
Publication date: 2007-08-01
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