Häggite from the Gambatesa mine, Liguria, Italy: a refinement of the crystal structure

The crystal structure and crystal-chemical features of häggite have been confirmed on the basis of new X-ray diffraction data collected on a small single crystal from Gambatesa mine (Italy). The crystal structure of häggite is monoclinic, C2/m space group, with a = 12.2203(21), b= 2.9991(5), c = 4.8334(9) Å; = 98.384(4), V = 175.25(5) ų, Z = 2. The crystal structure contains one independent V octahedron populated by V3+ and V4+ and the final bond valence analysis confirms the crystal-chemical formula of natural häggite : [V](V3+V4+)2[O1]O2-[O2](OH)2[O3](O2-OH)2. Only the O1 atom site, shared between two octahedra, shows an anomalously low incident-bond valence sums (1.72 valence units). The possible hydrogen bond motif in presence of 3 OH groups imposes the use of half populated H sites, which cannot be observed in the final ΔF map. The refinement of the complete structural model converges to a final Robs of 0.039 for 244 reflection with IO>3σ(IO).