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The crystal structure of meurigite

¹ Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Blvd., Los Angeles, California 90007, U.S.A.
² Department of Geophysical Sciences, Center for Advanced Radiation Sources and Materials Research Science and Engineering Center, University of Chicago, 5734 S. Ellis Avenue, Chicago, Illinois 60637-1434, U.S.A.
³ Center for Advanced Radiation Sources, University of Chicago, 5640 S. Ellis Avenue, Chicago, Illinois 60637-1434, U.S.A.

Correspondence: ^* E-mail: akampf{at}nhm.org

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, ß = 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 [F_o > 4 (F_o)] 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 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 [(K_0.91Na_0.03)_0.94(H₂O)_2.56]_3.50[(Fe_7.52³⁺Al_0.17Cu_0.03)_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).

Key Words: Meurigite • phosphofibrite • crystal structure • crystal chemistry • fibrous iron phosphates

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