A method has been worked out for the reproducible preparation of Green Rust substances involving SO42-, Cl-, Br-, and I- anions. The chemical composition of the substances prepared has been followed in dependence on the synthesis conditions. The powder X-ray and electron diffraction patterns and infrared and Moessbauer spectra have been measured and discussed.

The thermal decomposition of synthetic lepidocrocite, )~-FeOOH, was studied using DTA, infrared and M6ssbauer spectroscopy and X-ray diffraction, and the dependence of the phase composition of the dehydration products on the method of preparing the initial ~,-FeOOH was described. The results demonstrated that 7'-Fe203 formed by dehydration of ~-FeOOH consists of very small particles with a broad size distribution and that, depending on the method of preparation, the initial 7-FeOOH can contain a large amount of amorphous ferric hydroxide. A preparation method yielding 7-FeOOH which is not contaminated with other ferric oxide-hydroxides is recommended.From a thermodynamic point of view, lepidocrocite (~-FeOOH) is unstable ferric oxide-hydroxide; however, it can be quite stable under suitable conditions. It also occurs naturally as the mineral lepidocrocite in mesozoic formations [1]. A layered structure is typical for the compound, where the layers of close-packed FeO 6 octahedra are connected by hydrogen-bonds. Lepidocrocite is isotypical with boehmite (7-AIOOH) and has an orthorhombic structure [2, 3] with symmetry space group D~ (CmCm).On heating in the air, transformation to ;~-Fe,,O 3 (maghemite) first occurs, followed on further heating by transformation [4] to ~-Fe20 3 (haematite): 7-FeOOH TI____~ y_Fe203 r~ 9 e_F%O3"The temperatures T1 and T2 at which these reactions proceed are very dependent on the method of sample preparation.Synthetic lepidocrocite is usually prepared [5] by oxidation of an aqueous solution of FeC12 with sodium nitrite in a medium buffered with urotropine. This modification can be prepared in the pure form [6] by oxidation of an aqueous solution of FeSO4 with air at pH 7. This work deals with differences in behaviour of the two thus-prepared types of v-FeOOH on heating, and with some properties of the dehydration products, found by analysis of the M6ssbauer and infrared spectra.

The course of the transformation of synthetic γ‐FeOOH (lepidocrocite) samples, prepared by different methods was investigated and the effect of a preliminary heat treatment of the initial material on the product phase composition and morphology studied. Both the goethite‐tohematite ratio and the particle size of the products depend on the preparation method and on the preliminary heat treatment of the initial lepidocrocite. The results are discussed in terms of the dissolution of the less stable lepidocrocite followed by the crystallization of the more stable phases α‐FeOOH and α‐Fe2O3 from the solution. The final product phase composition and the particle size are affected considerably by a presence of goethite and/or hematite germs in the initial γ‐FeOOH.

No abstract

Oxidation of FeSO4 by air oxygen has been studied in aqueous solutions of constant acidity. The effect of various reaction conditions (of acidity, temperature, concentration of FeSO4, and of the air flow through the suspension) on the yield of various modifications of ferric oxide-hydroxides in the product mixture is described and the mechanism of the process is discussed. Reaction products prepared in the neutral and weakly alkaline solutions contain predominantly lepidocrocite (γ-FeOOH), amorphous Fe2O3.n H2O (ferrihydrite), at higher pH values also goethite (α-FeOOH) is formed. Their mutual ratios depend strongly on the solution acidity.