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History of ardennite and the new mineral ardennite-(V)

¹ Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
² Dipartimento di Scienze della Terra, Università di Pisa, via S. Maria 53, 56126 Pisa, Italy

^* Corresponding author, e-mail: pasero{at}dst.unipi.it

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 [010] 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 > 1.765(5), ß < 1.775(5), and = 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⁺_3.221Ca_0.774Na_0.006] _{= 4.001} [Al_4.456Mg_1.127Fe³⁺_0.210Mn³⁺_0.137Cr_0.046Ti_0.027] _{= 6.003} [Si_5.000] [V_0.518Si_0.315Al_0.080P_0.055As_0.033] _=1.001 O_21.433 (OH)_6.475 F_0.092. 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.

Key-words: ardennite-(V), new mineral species, Sparone, Piedmont, Italy, ardennite.