GRAVEGLIAITE, MNSO3 3H2O, A NEW MINERAL FROM VAL GRAVEGLIA (NORTHERN APENNINES, ITALY)

A crystallographic and crystal chemical study has been performed on a new mineral, an orthorhombic MnSO3 . 3H2O occurring in manganese ores at the Gambatesa Mine (Val Graveglia, Italy), associated with tephroite, bementite, braunite, hausmannite and hematite. Gravegliaite, appearing mainly as few radial aggregates of prismatic crystals, elongated parallel to [010] and up to 0.5 mm long, crystallizes in Pnma space group with a = 9.763(1) angstrom, b = 5.635(1) angstrom and c = 9.558(1) angstrom. It is colourless, transparent, optically biaxial and nonpleochroic with n-alpha = 1.590, n-beta = 1.596, n-gamma = 1.636 and 2 V-gamma = 41-degrees (Z = b). The strongest lines in the powder pattern are (d angstrom) 6.83, 4.33 and 3.43. Microprobe analyses reveal a good compositional homogeneity and a quasi-ideal chemical composition. The crystal structure refinement confirms this mineral to be the natural analogue of synthetic MnSO3 . 3H2O described by previous authors. The attempt to localize hydrogen atoms and to assign hydrogen bonds is discussed, since hydrogen bonds seem to play the major role in assuring cohesion to the structure.