THE CRYSTAL STRUCTURE OF MEURIGITE

Abstract

The crystal structure of meurigite, ideally [K(H2O)2.5][Fe3+8 (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 Fe3+2 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 a-c plane. PO4 tetrahedra further link octahedra within the slabs and also link slabs to one another perpendicular to the a-c 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 [(K0.91 Na0.03)Σ0.94 (H2O)2.56]Σ3.50 [(Fe3+7.52Al0.17 Cu0.03)Σ7.72(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).