Crystal Chemistry of Hydrous Aluminium Arsenate Minerals

Abstract

The hydrous aluminium arsenate minerals form a small but diverse group of crystal structures. They range from mansfieldite, AlAsO4 ⋅ 2H2O, with isolated Al-centred octahedra, coordinated only to AsO4 tetrahedra, to the pharmacoalumite group minerals containing tetrahedral clusters of edge-shared octahedra. In between are representatives of the alunite and laueite supergroup mineral structure types, ceruleite, CuAl4(AsO4)2(OH)8 ⋅ 4H2O, with planar tetrameric clusters of octahedra and pharmacoalumite derivative structures with polyoxometalate-type clusters. An interesting feature of the latter minerals is their high structural flexibility to undergo heat-induced rotations of the octahedral clusters about corner-linkages with AsO4 tetrahedra. This is reflected in very large negative thermal expansion (NTE) coefficients for the minerals, which locates them in the class of colossal-NTE materials. The pharmacoalumite derivative minerals form open-framework or layer structures containing high levels of zeolitic water, and heating the minerals results in phase changes associated with dehydration, displacement of the layers and rotations of the heteropolyhedral components. The different structure types have been ordered based on their octahedral compositions AlOm/n(OH)p/q(H2O)r/s where m, p, and r are the numbers of anions in the octahedron and n, q, and s are the coordination numbers of the anions, averaged over the independent Al-centred octahedra. This gives a smooth progression in the anion-to-Al ratio from 4 for mansfieldite to 2.5 for pharmacoalumite, reflecting increasing condensation of the octahedra associated with increasing dehydration of the octahedra.