Crystal chemistry of iowaite, Mg6FeIII 2 (OH)16Cl2 x 4H2O, a natural layered double hydroxide

The paper reports the crystal structure, chemistry, and vibrational spectroscopy of iowaite, a natural chloride member of the industrially important class of layered double hydroxides (LDH). The samples from the Talnakh and Korshunovskoe deposits, both Siberia, Russia were studied. The powder X-ray diffraction data show that iowaite in both samples belongs to the 3R polytype. The crystal structure of iowaite from Talnakh has been refined to R-1 = 0.053 based on 136 reflections with I > 3 sigma I using single-crystal X-ray diffraction data. The mineral crystallizes in space group R-3 m and has the unit-cell parameters a = 3.1077(9), c = 23.885(13) angstrom, V = 199.78(16) angstrom(3), Z = 3/8. The chlorine atoms have been localized in iowaite structure for the first time. The IR bands in the ranges 3430-3440, 3520-3530, and 3630-3640 cm(-1) are characteristic of LDH with Cl as interlayer anion with M-II:M-III = 3:1 and correspond to the hydrogen bonds HO-H center dot center dot center dot Cl, O-H center dot center dot center dot Cl and O-H center dot center dot center dot OH2, respectively. The aforementioned wavenumbers can be considered as characteristic only to species with M-II:M-III = 3:1 and Cl- as an interlayer anion. The bond valence calculations using the example of the local crystal structure of iowaite show that two (H2O)(0) molecules are required for coordination of one Cl- ion in order to satisfy the valence-matching principle from bond-valence theory. This explains the consistent stoichiometry of iowaite in terms of the ratio of interlayer components as Cl:H2O similar to 1:2, which cannot be explained from the standpoint of long-range order, since the ordering of neither interlayer components nor trivalent cations (according to any superstructure) is observed.