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

Ardennite-(V), ideally Mn (sub 4) [Al (sub 4) (AlMg)][Si (sub 5) V]O (sub 22) (OH) (sub 6) , is the analogue of ardennite, ideally Mn (sub 4) [Al (sub 4) (AlMg)][Si (sub 5) As]O (sub 22) (OH) (sub 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 luster, and brittle. No cleavage or parting were observed. The calculated density is 3.55 g/cm (super 3) . Optically, ardennite-(V) is biaxial, with alpha > 1.765(5), beta < 1.775(5), and gamma = 1.775(5), and is weakly pleochroic, with X, Y = pale yellow; Z = yellowish. Electron microprobe analyses (P (sub 2) O (sub 5) 0.38 wt.%, As (sub 2) O (sub 5) 0.37, V (sub 2) O (sub 5) 4.60, SiO (sub 2) 31.21, TiO (sub 2) 0.21, Al (sub 2) O (sub 3) 22.60, Fe (sub 2) O (sub 3) 1.64, Cr (sub 2) O (sub 3) 0.34, MgO 4.44, MnO (sub tot) 23.28, CaO 4.24, Na (sub 2) O 0.02, F 0.17, H (sub 2) O (sub calc) 5.70), gave the following empirical formula: [Mn (super 2+) (sub 3.221) Ca (sub 0.774) Na (sub 0.006) ] (sub Sigma = 4.001) [Al (sub 4.456) Mg (sub 1.127) Fe (super 3+) (sub 0.210) Mn (super 3+) (sub 0.137) Cr (sub 0.046) Ti (sub 0.027) ] (sub Sigma = 6.003) [Si (sub 5.000) ] [V (sub 0.518) Si (sub 0.315) Al (sub 0.080) P (sub 0.055) As (sub 0.033) ] (sub Sigma =1.001) O (sub 21.433) (OH) (sub 6.475) F (sub 0.092) . The five strongest peaks in the X-ray powder diffraction pattern [d(Aa)(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) Aa. 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.