The crystal structure of kastningite (Mn, Fe, Mg) (H2O)4[Al2(OH)2(H2O)2(PO4)2] · 2 H2O – a new hydroxyl aquated orthophosphate hydrate mineral

Abstract

Abstract The crystal structure of kastningite, a new mineral from Waidhaus, Bavaria, Germany, has been determined. Ideal formula from X-ray structure analysis is Mn(H2O)4[Al2(OH)2(H2O)2(PO4)2] · 2 H2O. Lattice parameters are a = 10.205(1) Å, b = 10.504(1) Å, c = 7.010(1) Å, α = 90.38(1)°, β = 110.10(1)°, γ = 71.82(1)° (space group P[unk], Z = 2). Kastningite is isostructural to stewartite Mn(H2O)4 · [Fe2 3+(OH)2(H2O)2(PO4)2] · 2H2O. Chains of corner sharing octahedra run in the [102] direction. Within the chain, octahedra of composition [Al(OH)2(O P )2(H2O)2]3− alternate with others of composition [Al(OH)2(O P )4]7− (with O P designating oxygen atoms which form part of a phosphate group). The resulting chain composition is [Al(O P )3(OH)(H2O)]4−. Symmetry equivalent chains are bridged by [PO4]3− tetrahedra and sheets parallel to {010} of composition [Al2(OH)2(H2O)2(PO4)2]2− are formed. [Mn(H2O)4(O P )2]2− octahedra link these sheets in the direction of [010] thus resulting in a three dimensional structure. The structure is further held together by a complex system of hydrogen bonds. Chains of corner-sharing octahedra which are further connected to [PO4]3− tetrahedra are a common feature within other aquated orthophosphate hydrate minerals. Yet there are differences with respect to the number of symmetrically equivalent octahedra within the chain, their composition as well as to the way they are connected to the tetrahedra. A brief comparison of kastningite to these other minerals is given.