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

Abstract

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 ≥ 2σ(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) ų; and the crystal-chemical formula is (Mn²⁺_3.86Ca_0.10)_Σ3.96 (As⁵⁺_0.85V⁵⁺_0.02Si_0.19)_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.