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

Main features of FeII-III hydroxysalts (green rusts) are obtained from XRD. Moreover, Mossbauer 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(SO42-) 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 [(Fe6Fe2III)-Fe-II(OH)(16)](2+)center dot[CO32-center dot 5H(2)O](2-), [(Fe4Fe2III)-Fe-II(OH)(12)](2+)center dot[CO32-center dot 3H(2)O](2-), [(Fe6Fe2III)-Fe-II(OH)(16)](2+)center dot[C2O42-center dot 4H(2)O](2-) and [(Fe5Fe2III)-Fe-II(OH)(14)](2+)center dot[2HCOO(-)center dot 3H(2)O](2-), which correspond to orders alpha, beta and gamma where the cation distances are (2 x a(0)), (root 3 x 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 [(Fe6(1-x)Fe6xO12H2(7-3x))-Fe-II-O-III](2+)center dot[CO32-center dot 3H(2)O](2-) for the fougerite mineral, which is the oxy-hydroxycarbonate obtained by deprotonation of FeII-III hydroxycarbonate [(Fe4Fe2III)-Fe-II(OH)(12)](2+)center dot[CO32-center dot 3H(2)O](2-) where x is an element of [1/3, 2/3] is confirmed. (c) 2008 Elsevier Masson SAS. All rights reserved.