Abstract
Single crystals of synthetic francoanellite K3Al5(HPO4)6(PO4)2-12H2O could be obtained for the first time by topochemical dehydration of taranakite crystals. An X-ray structure determination showed francoanellite to be the mineral with the second longest crystallographic axis described hitherto. Crystal data: space group R3c, a = 869.0(2), c = 8227(1)pm, Z = 6 , Rg = 0.042. Francoanellite is a layer structure mineral having six layers of composition [K3Al5(HPO4)6(PO4)2(H2O)12], connected by hydrogen bonds. The rigid layer is formed by columns of corner sharing hydrogen phosphate tetrahedra and AIO6-octahedra which are inter connected by additional six-coordinated Al ions. In trigonal holes of the layer orthophosphate ions are situated. The structure of francoanellite is very similar to the structure of taranakite K3H6Al5(PO4) 8 · 18H2O which has planar water interlayers between the Al-phosphate layers. A neutron scattering experiment with subsequent Rietveld refinement of the powder pattern gave the H-atom positions. Hydrogen bonds in francoanellite are formed within the rigid layers and between them.
During the reaction taranakite → francoanellite crystals in an intermediate stage of dehydration could be obtained. From the c-axis of 8858 pm and one-dimensional electron density projections it can be proposed that in these crystals every second water interlayer was lost and a first order staging product of the deintercalation of water from taranakite was formed.
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