Abstract

The crystal structure of the mineral graftonite has been determined by single crystal X-ray diffraction methods. The lattice parameters for thecrystal examined are a = 8.91(1) Å, b= 11.58(1) Å, c = 6.239(8) Å, β = 98.9(1)° and Z = 4. The space group is P21/c. The anions are slightly irregular PO43- tetrahedra with individually averaged PO bond lengths of 1.55 Å for both anion groups. Three cation polyhedra are found, and in two the cations have five-fold coordination while for the third the coordination number is seven. The Ca2+ is found to be preferentially dissolved in the latter site. The structure is composed of chains of trigonal bipyramidal cation polyhedra running parallel to the c axis formed through edge-sharing. The seven coordinated cations occur in edge-shaped pairs which are comer-shared with similar groups to form sheets in the b-c plane while the remaining cation polyhedra join these sheets with the chains. This site has four-fold coordination in CdZn2-(PO4)2 which otherwise has esssentially the same structure as graftonite. The major cationic species in graftonite are Fe2+ and Mn2+ whose orthophosphates can be crystallized with the graftonite structure.

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