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The crystal structure of magnussonite, ideally Mn2+18[As3+6(Mn1+x)O18]2[(H2O, Clx, ☐) (H2O, ☐)]2, from Långban, Sweden, was refined to an R1-index of 1.19% and the structure proposed by Moore and Araki (1979) is confirmed. Magnussonite has a densely packed structure of (Mnφn) polyhedra, φ = (O2−, H2O, Cl−), and (As3+O3) triangular pyramids that is best envisaged as layers of polyhedra in the same way as many of the other manganese-arsenite-arsenate structures from Långban. There are two distinct layers in magnussonite; the two layers may be combined into a slab that stacks along the a-direction with rotations between adjacent slabs. A surprising feature of the dense-packed magnussonite atomic arrangement is an array of structural channels along [111] that contain much of the disorder that occurs in the magnussonite structure. The channels contain the partly occupied MX site on the central axis of the channel, and the CLW2 site (with extremely low occupancy), also on the central axis of the channel. The CLW2 site, previously unrecognized in the magnussonite structure, contains H2O, whereas the minor Cl in the structure resides in the CLW1 channel site, balancing the charge of the MX-site occupant. The MX site on the central axis of the channels displays a coordination known only in Långban minerals. In the local arrangement around the unoccupied MX site, the neighboring (As3+O3) groups project their associated stereoactive lone-pairs of electrons into the channel. Where the MX site is occupied by Mn, there are six lone-pairs of electrons pointing toward Mn; the 18-electron rule predicts/rationalizes formulae for this stable transition-metal cluster. The (As3+O3) groups and MX occupant form a [Mn+(As3+O3)6] arrangement in accord with the 18-electron rule where Mn+ contributes 6 3d electrons and the six lone-pairs of the [(As3+O3)6] arrangement contribute 12 electrons for a total of 18 electrons that form nine molecular orbitals that are metal-ligand bonds or non-bonding. Magnussonite and dixenite, another basic manganese-iron arsenate-arsenite-silicate mineral of the Långban-type deposits in Bergslagen, Sweden, are the only two minerals known with such local [M+(As3+O3)n] transition-metal clusters. The presence of these exotic clusters in structures containing densely packed Mn2+ octahedra is not understood at present. |
