Häggite from the Gambatesa mine, Liguria, Italy: a refinement of the crystal structure
Callegari, Athos Maria; Boiocchi, Massimo
published: Jan 1, 2015
ArtNo. ESP154019201001, Price: 29.00 €
The crystal structure and crystal-chemical features of häggite: V+3V4+O2(OH)3; 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; β = 98.384(4)°, V = 175.25(5) Å³, 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 R obs of 0.039 for 244 reflection with IO > 3 σ(IO). The crystallographic data obtained in our natural sample have been compared with those of häggite from the type locality and with a synthetic phase named “haggite” and described as V4+ 2O3(OH)2.