Crystal chemistry of Belgian ardennites

The mineral ardennite-(As), which belongs to the ardennite group, was originally described in the locality of Salmchâteau, Stavelot Massif, Belgium. In the past 10 years, several new samples of ardennites have been found at seven localities of this region, motivating us to reinvestigate the crystal chemistry of the ardennite group. Under the polarizing microscope, most ardennites form lamellae or needles included in quartz veins or constituting the matrix of red Ordovician schists. Electron-microprobe analyses, as well as single-crystal structure refinements, show a homovalent substitution of As 5+ by V 5+ at the T 4 tetrahedral site, leading to a complete solid solution between ardennite-(As) and ardennite-(V). Minor substitutions on that site allow the incorporation of a maximum of 0.28 P 5+ atoms per formula unit (apfu) and of less than 0.1 Si 4+ pfu, except in a sample from Arbrefontaine, where Si reaches 0.74 apfu. The main substitution mechanism, affecting both the T 4 and M3 sites, is T 4 Si 4+ + M3 (Al,Fe) 3+ ↔ T 4 (As,V,P) 5+ + M3 Mg 2+. Crystal-chemistry calculations indicate a positive correlation between the unit-cell parameters and the (Ca + Mg + Fe) contents of ardennites, as well as a negative correlation between the bond length distortion coefficients at the T 4 and M3 sites. The existence of possible Si-rich and P-rich end-members, as well as the nomenclature of the ardennite group in which dewalquite could be revalidated, is discussed.