Anion and cation distributions in Fe(II–III) hydroxysalt green rusts from XRD and Mössbauer analysis (carbonate, chloride, sulphate, …); the “fougerite” mineral

Abstract

Main crystallographic features of Fe(II–III) hydroxysalts usually named green rusts are known from XRD. However, Mössbauer spectroscopy often used to characterise them revealed that several Fe(II) sites exist that have not been elucidated yet. The structure is classified in green rust one, GR1, and green rust two, GR2, according to the sequence of stacking of Fe(OH) 2 brucite-like layers that depends on the shape and type of anions inserted within interlayers. A long-range order had been determined by Rietveld analysis for the hydroxysulphate GR2(SO 4 2−), that correspond to the distribution of cations and anions. Using similar interactions in the GR1 structure where long-range ordering is not observed, it is possible to devise models of correlated anion and cation distributions that link layers along the 3 ̄ axis. The predicted abundance of Fe(II) environments permits to attribute all hyperfine components. Anion distribution within interlayers for GR1(Cl −) and GR1(CO 3 2−) explains that the composition lies around [Fe II 3Fe III(OH) 8] +•[Cl, ∼2H 2O] − for the first one whereas a domain around [Fe II 4Fe III 2(OH) 12] 2+•[CO 3 2−, ∼3H 2O] 2− exists for the second one. “Fougerite” mineral (IMA 2003-057) is [(Fe II, Mg II) (1− x) (Fe III, Al III) x (OH) 2] x+ •[( x/2) CO 3 2−, yH 2O] x− , where (∼1/4)⩽ x⩽(1/3) and y⩽[1−(3/2) x]. Poles are pyroaurite [Mg II 6Fe III 2(OH) 16] 2+•[CO 3 2− • ∼ 4H 2O] 2− and hydrotalcite [Mg II 4Al III 2(OH) 12] 2+•[CO 3 2− • ∼ 3H 2O] 2−.