Crystal chemistry of iowaite, Mg6FeIII2(OH)16Cl2 × 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 3 R polytype. The crystal structure of iowaite from Talnakh has been refined to R ₁ = 0.053 based on 136 reflections with I > 3σ 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) Å, V = 199.78(16) Å 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···Cl, O–H···Cl and O–H···OH ₂, 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 (H ₂O) 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:H ₂O ∼ 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.