The Crystal Structure of the Elusive Huemulite

Abstract

The crystal structures of natural huemulite from the West Sunday mine, Utah, USA, and synthetic huemulite, Na 4 Mg(V 10 O 28 )•24H 2 O, have been solved and refined to R 1 = 0.0313 (for 3535 unique F o > 4sF reflections) and 0.0246 (for 3672 unique F o > 4sF reflections), respectively. Huemulite is triclinic, space group P1, Z = 1; unit-cell dimensions of the natural sample are a 9.0453(2), b 11.3337(3), c 11.7372(8) Å, a 105.223(7), b 97.383(7), g 100.790(7)°, V 1120.30(9) Å 3 , whereas those of the synthetic sample are a 9.0425(2), b 11.3303(2), c 11.7353(8) Å, a 105.222(7), b 97.377(7), g 100.791(7)°, V 1119.47(8) Å 3 . The structure consists of decavanadate oxyanions (V 10 O 28 ) 6linked via an interstitial complex composed of isolated [Mg(H 2 O) 6 ] 2+ octahedra and an [Na 4 (H 2 O) 14 ] 4+ cationic group (defining an infinite zigzag chain). There are also four isolated H 2 O 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 Na 4 M(V 10 O 28 )•23H 2 O (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 / 011]. 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.soMMaire Nous avons résolu la structure cristalline de la huémulite, Na 4 Mg(V 10 O 28 )•24H 2 O, provenant de la mine West Sunday, au Utah, et de son équivalent synthétique, et nous l'avons affiné jusqu'à un résidu R 1 de 0.0313 (pour 3535 réflexions uniques, F o > 4sF) et de 0.0246 (pour 3672 réflexions uniques, F o > 4sF), respectivement. La huémulite est triclinique, groupe spatial P1, Z = 1; les paramètres réticulaires de l'échantillon naturel sont a 9.0453(2), b 11.3337(3), c 11.7372(8) Å, a 105.223(7), b 97.383(7), g 100.790(7)°, V 1120.30(9) Å 3 , tandis que ceux de l'échantillon synthétique sont a 9.0425(2), b 11.3303(2), c 11.7353(8) Å, a 105.222(7), b 97.377(7), g 100.791(7)°, V 1119.47(8) Å 3 . La structure contient des oxyanions de décavanadate (V 10 O 28 ) 6en liaison grâce à un complexe interstitiel contenant des octaèdres [Mg(H 2 O) 6 ] 2+ isolés et un groupe cationique [Na 4 (H 2 O) 14 ] 4+ dans un agencement en chaînes en zigzag infinies. Il y a de plus quatre groupes H 2 O isolés, dont deu...

A Versatile Polyoxovanadate in Diverse Cation Matrices: A Supramolecular Perspective

“…These sodium-water chains are laterally linked by the decavanadate anions to form a two-dimensional coordination polymeric structure; [Co(H 2 O) 6 ] 2+ remains as an isolated discrete coordination complex cation located in the void space (Figure 3) to counterbalance the negative charges of the decavanadate anion. In the language of decavanadate mineral crystallography, these cations (e.g., sodium and cobalt ions in compound 1 ) are called interstitial units (Hughes et al, 2002, 2005, 2008; Colombo et al, 2011; Kampf et al, 2011b, 2013b, 2014). For example, in the crystal structure of the kokinosite mineral (Kampf et al, 2014), the interstitial unit has a composition of (Na 2 Ca 2 •24H 2 O) 6+ and the decavanadate anion (V 10 O 28 ) 6− forms the structural unit of the mineral.…”