Hydrogen bonding in coquimbite, nominally Fe-2(SO4)(3)center dot 9H(2)O, and the relationship between coquimbite and paracoquimbite

作者：Majzlan Juraj^*; Dordevic Tamara; Kolitsch Uwe; Schefer Juerg

来源：Mineralogy and Petrology, 2010, 100(3-4): 241-248.

DOI：10.1007/s00710-010-0128-4

Using single-crystal X-ray diffraction at 293, 200 and 100 K, and neutron diffraction at 50 K, we have refined the positions of all atoms, including hydrogen atoms (previously undetermined), in the structure of coquimbite (P (3) over bar 1c, a=10.924(2)/10.882(2) angstrom, c=17.086(3) / 17.154(3) angstrom, V=1765.8(3)/1759.2(5) angstrom(3), at 293 / 50 K, respectively). The use of neutron diffraction allowed us to determine precise and accurate hydrogen positions. The O-H distances in coquimbite at 50 K vary between 0.98 and 1.01 angstrom. In addition to H2O molecules coordinated to the Al3+ and Fe3+ ions, there are rings of six "free" H2O molecules in the coquimbite structure. These rings can be visualized as flattened octahedra with the distance between oxygen and the geometric center of the polyhedron of 2.46 angstrom. The hydrogen-bonding scheme undergoes no changes with decreasing temperature and the unit cell shrinks linearly from 293 to 100 K. A review of the available data on coquimbite and its "dimorph" paracoquimbite indicates that paracoquimbite may form in phases closer to the nominal composition of Fe-2(SO4)(3)center dot 9H(2)O. Coquimbite, on the other hand, has a composition approximating Fe1.5Al0.5(SO4)(3)center dot 9H(2)O. Hence, even a "simple" sulfate Fe2-xAlx(SO4)(3)center dot 9H(2)O may be structurally rather complex.

• 出版日期2010-11