Abstract

The crystal structure of meurigite, ideally [K(H2O)2.5][Fe83+ (PO4)6(OH)7(H2O)4], monoclinic, C2/c, a = 29.018(5), b = 5.1892(6), c = 19.695(3) Å, β = 106.987(1)°, Z = 4, from the Santa Rita mine, New Mexico, has been solved and refined to R1 = 4.69%, wR2 = 12.6% using 3325 unique [Fo > 4σ (Fo)] reflections collected using a Bruker 6000 SMART CCD diffractometer and synchrotron radiation of wavelength 0.41328 Å. The structure of meurigite is a framework consisting of face-sharing octahedral Fe23+ O9 dimers, which are linked by sharing corners with corner-sharing dimers and isolated Fe3+O6 octahedra to form thick slabs of octahedra parallel to the ac plane. PO4 tetrahedra further link octahedra within the slabs and also link slabs to one another perpendicular to the ac plane. Relatively large channels through the framework along the b axis contain disordered K atoms and H2O molecules, which take part in two overlapping arrays. Partial vacancies in the Fe and P sites may account for discrepancies between the empirical and ideal chemical formulas. Packing considerations suggest that the empirical formula should be based on the total number of large ions (K + Na + O = 38.5 per formula unit), which for the chemical analysis provided in the original description yields [(K0.91Na0.03)∑0.94(H2O)2.56]∑3.50[(Fe7.523+Al0.17Cu0.03)∑7.72(PO4)5.48(CO3)0.21(OH)7.20(H2O)5.23]. The meurigite structure is related to those of other fibrous ferric phosphates with 5 Å fiber axes and shows a particularly close relationship with the structure of dufrénite. Crystal chemical evidence suggests that, even if meurigite and phosphofibrite are isostructural, phosphofibrite may qualify as a distinct species based upon its low K content (<0.5 apfu based on a recalculation of the original chemical analysis).

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