Structure refinement and new crystal-chemical data for tiragalloite (Mn2+ 3.86Ca0.10)σ3.96(As5+ 0.85V5+ 0.02Si0.19)σ1.06 Si3O12(OH) from the Scerscen glacier, Val Malenco, Italy

A new sample of tiragalloite from the Scerscen Glacier of the Malenco valley (Lombardy, Italy) has been studied by single-crystal X-ray diffraction and microchemical analyses. Structure refinement in space group P21/n converge to R1=0.0373 for 6149 reflections with I ≥ 2 Σ(I) and 0.0583 for all 8242 data. The refined unit-cell parameters are: A=6.6702(2) Å, b=19.9336(7) Å, c=7.5759(2) Å, β=95.518(1)°, V=1002.63(5) Å; and the crystal-chemical formula is (Mn2+ 3.86Ca0.10)σ3.96(As5+ 0.85V5+ 0.02Si0.19)σ1.06 Si3O12(OH), with Z=4. The new data of tiragalloite from Scerscen confirm the general organization of the crystal structure, as previously reported in literature for two samples from Graveglia valley, Liguria, Italy, which contained a different amount of V5+ or As5+ cations. However, refinement of the site populations at the Mn(3) and Mn(4) sites suggests that the distribution of Ca among these atom sites might be different than that reported in literature. In particular, in tiragalloite from Scerscen, Ca seems to be preferentially located at the [6]Mn(4)(O5OH) octahedron rather than within the large [7]Mn(3)O7 polyhedron. The topology of the [AsSi3O12(OH)]8- unit of tiragalloite is compared with natural and synthetic phases containing similar groups of four tetrahedra.