Composition and anion ordering in some Fe II–III hydroxysalt green rusts (carbonate, oxalate, methanoate): The fougerite mineral

Abstract

Main features of Fe II–III hydroxysalts (green rusts) are obtained from XRD. Moreover, Mössbauer spectroscopy revealed that several Fe II sites exist. The structure is classified in green rusts one and two (GR1 and GR2) according to the stacking sequence of Fe(OH) 2 brucite-like layers depending on the shape and type of anions inserted within interlayers. Long range order as determined for hydroxysulphate GR2(SO 4 2−) is extended to distributions of cations and anions within GR1s even though these are not observed by XRD. Abundances of Fe II and Fe III environments within GR1s that intercalate carbonate, oxalate and methanoate (formate) are found for compositions [Fe 6 IIFe 2 III(OH) 16] 2+·[CO 3 2−·5H 2O] 2−, [Fe 4 IIFe 2 III(OH) 12] 2+·[CO 3 2−·3H 2O] 2−, [Fe 6 IIFe 2 III(OH) 16] 2+·[C 2O 4 2−·4H 2O] 2− and [Fe 5 IIFe 2 III(OH) 14] 2+·[2HCOO −·3H 2O] 2−, which correspond to orders α, β and γ where the cation distances are (2 × a 0), (√3 × a 0) or a mixture of both, with a ferric molar ratio x = {[Fe III]/[Fe total]} = 1/4, 1/3 and 2/7, respectively. Anion distributions within interlayers are devised and long range orders in other layered double hydroxides could questionably be extended from these models. The formula [Fe 6 II (1− x) Fe 6 x IIIO 12H 2(7−3 x) ] 2+·[CO 3 2−·3H 2O] 2− for the fougerite mineral, which is the oxyhydroxycarbonate obtained by deprotonation of Fe II–III hydroxycarbonate [Fe 4 IIFe 2 III(OH) 12] 2+·[CO 3 2−·3H 2O] 2− where x ∈ [1/3,2/3] is confirmed.