Abstract :
[en] Ardennite is a Mn-rich silicate mineral which contains arsenate and/or vanadate groups. This mineral was first discovered in a quartz vein in Salmchâteau (Ardennes, Belgium), in association with apatite and albite. The two mineral species, ardennite-(As) and ardennite-(V), are occurring in highly oxidized and manganiferous metasediments that were affected by low- to high-grade metamorphism (1).
The general formula of ardennite may be written as A4M6T6O22(OH)6. The large A sites mainly contain Mn2+ and smaller amount of Ca2+ and Mg2+, the octahedral M sites can host Al3+, Fe3+, Mn3+, Mg2+ and other minor six-fold coordinated cations, and the tetrahedral T sites can host Si4+, As5+, V5+, and P5+ (1). The mineral is orthorhombic, with space group Pnnm, and a ≈ 8.8 Å, b ≈ 5.8 Å, c ≈ 18.6 Å (2).
Single-crystal X-ray diffraction measurements were performed on three new occurrences of Belgian ardennites, from Bihain, Thier del Preu, and Regné, respectively. Structure refinements were performed to R1 = 2.16-5.1%, in the Pmmn space group, with a = 5.7981(0)-5.8035 Å, b = 18.4670(7)-18.4792(2) Å, c = 8.6888(3)-8.6959(5) Å, V = 930.58(6)- 932.59(11) Å3, Z = 2. The crystal structure of ardennite is formed by chains of edge-sharing [MO6] octahedra running along the a axis and connected by [TO4] and [T3O10] groups. The large A cations occur in the cavities of the structure in six- and seven-fold coordination polyhedra. In the asymmetric unit of the ardennite structure, occur four independent T sites, three M sites and two larger A sites. T1, T2 and T3 are occupied by Si4+ while the T4 site hosts As5+. Samples from Bihain and Regné present lower refined occupancies in this site (0.81, 0.85), thus indicating a substitution towards ardennite-(V). The A sites are occupied by Mn2+, the charge balance is then compensated by a coupled substitution (T4+ + M3+ ↔ T5+ + M2+) in the M3 site, which mainly contains Mg2+. The knowledge of the cation distributions in these three new occurrences, combined with chemical data, will help us to better understand the crystal chemistry of the complex ardennite group.
