The crystal structure of the elusive huemulite

Abstract

The crystal structures of natural huemulite from the West Sunday mine, Utah, USA, and synthetic huemulite, Na4Mg(V10O 28)·24H2O, have been solved and refined to R 1 = 0.0313 (for 3535 unique F0 > AσF reflections) and 0.0246 (for 3672 unique F0 > 4σF reflections), respectively. Huemulite is triclinic, space group P1̄, 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(H 2O)6]2+ octahedra and an [Na4(H 2O)14]4+ cationic group (defining an infinite zigzag chain). There are also four isolated H2O groups, two of them positionally disordered. All except four H atoms have been located, showing a network of hydrogen 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 valencematching principle is maintained in this structure. It is probable that the X-ray pattern and the unit-cell dimensions in the original description of huemulite were measured using a mixture that included fully hydrated and partially dehydrated material. Huemulite is a member of the pascoite group, which is closely related to a synthetic family of general formula Na4M(V10O 28)·23H2O (M: Ni, Mg).The unit cell and atomic positions of huemulite are related to those of the synthetic family by the transformation matrix M = [100/011/011̄]. However, some of the symmetry restrictions (e.g., inversion centers, cell centering) present in huemulite are relaxed, with the consequence that fewer atoms are symmetry-related in the synthetic materials.