The crystal structure of the elusive huemulite

COLOMBO, FERNANDO; BAGGIO, RICARDO; KAMPF, ANTHONY R.

CANADIAN MINERALOGIST

MINERALOGICAL ASSOC CANADA

Año: 2011 vol. 49 p. 849 - 864

0008-4476

The crystal structures of natural (from the West Sunday Mine, Utah, USA) and synthetic huemulite, Na4Mg(V10O28)·24H2O, have been solved and refined to R1 = 0.0313 (for 3535 unique Fo > 4s F reflections) and 0.0246 (for 3672 unique Fo > 4s F reflections), respectively. Huemulite is triclinic, space group P, with Z = 1; unit-cell dimensions of the natural sample are  a 9.0453(2), b 11.3337(3), c 11.7372(8) Å, α 105.223(7), β 97.383(7), γ 100.790(7)°, V 1120.30(9) Å3, whereas those of the synthetic sample are a 9.0425(2), b 11.3303(2), c 11.7353 (8) Å, α 105.222 (7), β 97.377 (7), γ 100.791 (7)°, V 1119.47(8) Å3. The structure consists of decavanadate oxyanions (V10O28)6- linked via an interstitial complex composed of isolated [Mg(H2O)6]2+ octahedra and an [Na4(H2O)14]4+ cationic group (defining an infinite zig-zag chain). There are also four isolated H2O groups, two of them positionally disordered. All except 4 H atoms have been located, showing a network of H-bonds that further links the interstitial complex and the structural unit, stabilizing the atomic arrangement. The Lewis acidity of the interstitial complex (0.18) is almost coincident with the upper limit of basicity of the structural unit (0.17), thus showing that the valence-matching principle is maintained in this structure. It is probable that the X-ray pattern and the unit-cell dimensions informed in the original description of huemulite were measured using a mixture that included fully hydrated and partially dehydrated material. Huemulite is closely related to a synthetic family of general formula Na4M(V10O28)·23H2O (M=Ni,Mg). Unit cell and atomic positions of huemulite are related to those of the synthetic family by the transformation matrix M = [1 0 0 / 0 1 1 / 0 1 -1].   However, some of the symmetry restrictions (inversion centers, cell centering, etc.) present in huemulite are relaxed, with the consequence that fewer atoms are symmetry-related in the synthetic family.