ISSN 10642293, Eurasian Soil Science, 2015, Vol. 48, No. 3, pp. 240–249. © Pleiades Publishing, Ltd., 2015. Original Russian Text © Yu.N. Vodyanitskii, S.A. Shoba, 2015, published in Pochvovedenie, 2015, No. 3, pp. 277–287. SOIL CHEMISTRY Ephemeral Fe(II)/Fe(III) Layered Double Hydroxides in Hydromorphic Soils: A Review Yu. N. Vodyanitskii and S. A. Shoba Faculty of Soil Science, Moscow State University, Moscow, 119991 Russia email: [email protected] Received July 11, 2014 Abstract—Ephemeral green rust is formed seasonally in some hydromorphic soils. It consists of Fe(II)/Fe(III) layered double hydroxides with different types of interlayer anions and different oxidation degrees of iron (x). In synthetized stoichiometric green rust, x = 0.25–0.33; in soil fougerite, it may reach 0.50–0.66. The mineral sta bility is provided by the partial substitution of Mg2+ for Fe2+. The ephemeral properties of the green rust are manifested in the high sensitivity to the varying redox regime in hydromorphic soils. Green rust disappears dur ing oxidation stages, which complicates its diagnostics in soils. For green rust formation, excessively moist min eral soil needs organic matter as a source of energy for the vital activity of ironreducing bacteria. In a gleyed Cambisol France, where fougerite is formed in the winter, the index of hydrogen partial pressure rH2 is 7.0–8.2, which corresponds to highly reducing conditions; upon the development of oxidation, fougerite is transformed into lepidocrocite. In the mineral siderite horizon of peatbogs in Belarus, where green rust is formed in the sum mer, rH2 is 11–14, which corresponds to the lower boundary of reducing conditions (rH2 = 10–18); magnetite is formed in these soils in the winter season upon dehydration of the soil mass. Keywords: green rust, fougerite, hydrogen partial pressure rH2, lepidocrocite, magnetite DOI: 10.1134/S106422931503014X INTRODUCTION anions in the interlayer space and with different Fe(II)/Fe(III) ratios, i.e., with different oxidation Minerals containing Fe(II) are formed in hydro degrees of iron. Thus, fougerite is the name of a min morphic soils providing their cold colors [4, 5]. Fe(II) eral that was earlier referred to as green rust according minerals are studied to very different extents. More to its color. stable minerals (siderite, magnetite, vivianite) were well studied long ago, whereas ephemeral compounds, At present, a considerable amount of naturally rapidly oxidized in the air, are poorly investigated. occurring LDHs are known with most of them con taining magnesium, aluminum, and iron with the Starting with the works of Bernal, the ephemeral – – 2− Fe(II) compounds have been called “green rust” [2, main interlayer anions being OH , Cl , and CO3 . 22]. Green rusts are found in paddy soils, as well as in Many of them were officially recognized long ago, 2− ⋅ other soils with an unstable moisture regime [7, 31]. In e.g., hydrotalcite [Mg6Al2(OH)16] [CO3 4H2O], the , green rust 2− World Reference Base for Soil Resources pyroaurite [Mg Fe (OH) ][ ) ⋅ 4H O], iowaite is referred to in the description of gleyic properties 6 2 16 CO3 2 – ⋅ [37], and the different stability of green rusts is not [Mg4Fe2(OH)10][Cl ) 2H2O], meixnerite – mentioned. The gleyed horizon may be either resistant [Mg6Al2(OH)16][OН )2 ⋅ 4H2O] [9, 32]. to oxidation, with the green color being preserved in Unlike lamellar aluminosilicates charged nega the soil profile for a long time without getting brown tively and manifesting a high cationexchange capac ish, or nonresistant, with the dovecolored horizon ity, LDHs are positively charged, and they possess a becoming brownish in less than an hour [15]. This high anionexchange capacity under neutral pH con points to different Fe(II) pigments. In this paper, we ditions. Sometimes, they are called anionic clays to consider only unstable ephemeral green rusts. emphasize their layered structure with a high anion The soil Fe(II)/Fe(III) mineral belonging to a large exchange capacity and specific electrochemical and group of layered double hydroxides (LDH), or ephem magnetic properties. eral green rust, was registered in 2004; it was called Fougerites with different anionic compositions fougerite after the town of Fougéres in Bretagne, have been studied in laboratories. Xray diffractome France, in the vicinity of which it was described in try, Mössbauer spectroscopy, and transmission elec detail [13, 32–34]. The ephemeral green rust is classi tron microscopy are the principle analytic methods for fied as an LDH with different types of interlayer model ephemeral green rusts (fougerites). However, 240 EPHEMERAL Fe(II)/Fe(III) LAYERED DOUBLE HYDROXIDES 241 the specimen shooting used upon routine analyses (a) appears to be inapplicable for ephemeral green rust. Structure of layers For example, the routine preparation of specimens on С a glass plate for Xray diffractometry leads to the quick c/3 loss of the green color in the green rust, which points Ba to its oxidation. Therefore, diffractometers are Interlayer space Bc equipped with special devices for shooting wet sus Hydroxide layer pended samples [24]. The same measures are under A taken when using Mössbauer spectroscopy [24]. These b Cl– methods permit studying the composition and chemi – cal formula of ephemeral green rusts. Transmission H2O OH Fe(III) Fe(II) electron microscopy reveals the unambiguously syn thesized green rust crystals according to their typical (b) hexagonal shape and their peculiar electron microdif Structure of layers fraction pattern. С c/3 The composition of the interlayer anions in natural Ba fougerite cannot be directly determined, because the Interlayer space Mössbauer spectroscopy focused on the study of iron Bc forms is unable to distinguish the anion type. At the Hydroxide layer same time, it provides data on the iron oxidation A b degree, which is very helpful, since the x parameter is OH– very sensitive to the seasonal variations in the pH–Eh Fe(II) OH– Fe(III) conditions in hydromorphic soils. Iron is known to be H2O Mg(II) the principle geochemical marker of mineral hydro morphic soils [4, 5]. Soil scientists are interested in (c) Fe(II)/Fe(III) LDHs as pigments responsible for the Structure of layers “cold” colors in gleyed soils, as well as the active inter A mediate phase controlling the trend of oxidogenesis c C development in hydromorphic soils. B The aim of this study was to systematize the data on the ephemeral Fe(II)/Fe(III) LDHs in soils with the variable moisture regime. A Interlayer space STRUCTURE, COMPOSITION, B Hydroxide layer AND PROPERTIES OF Fe(II)/Fe(III) LAYERED b DOUBLE HYDROXIDES S Diverse Fe(II) minerals occur in soils with variable H O – Fe(III) Fe(II) moisture regimes. Some Fe(II) minerals, in particu 2 O OH lar, ferruginated phyllosilicates (biotite, ferrous chlo rite, etc.), are inherited by soils from the parent rock Fig. 1. Crystalline lattice of green rust, the layer sequence, and, genetically, are not related to Fe(II) neoforma and the position of water molecules and anions in the tions (pedofeatures) in hydromorphic soils. Very interlayer space along the [001] direction: (A) hydroxyl chloride green rust of group I; (B) hydroxyl fougerite of diverse compounds are related to Fe(II) neoforma group I; (C) hydroxylsulfate green rust of group II [32]. tions (figure), i.e., phosphates, carbonates, oxides, and hydroxides (including Fe(II)/Fe(III) layered double hydroxides). vivianite Fe3(РO4)2 ⋅ 8Н2О particles and ephemeral Thus, it is convenient to subdivide all Fe(II) miner green rust. Vivianite, i.e., bivalent iron phosphate, was als into two groups according to their resistance to oxi discovered nearly 200 years ago (in 1817), and it has dation. One group is formed by the stable Fe(II) com been well studied [9]. The unoxidized vivianite is pounds suitable for routine mineralogical analysis. uncolored; however, it becomes blue due to the quick 2+ They include yellow carbonate siderite FeCO3, which partial oxidation of Fe , which is registered in gleyed often forms massive light yellow accumulations beneath soils containing iron phosphate. Ephemeral green the peat layer [7], and black oxide magnetite Fe3O4, rust, i.e., layered double hydroxides Fe(II) and which is able to accumulate in Fe–Mn concretions Fe(III), is less studied than vivianite. [10]. Stable green rusts are also included in this group. All ephemeral green rusts crystallize in a trigonal Another group comprises the nonresistant to oxi system. They are subdivided depending on the chemi dation Fe(II) minerals requiring reducing conditions cal composition of the interlayer anions. Fougerite, as to be preserved during the analysis. These are disperse one of the LDH types, is a later mineralogical name of EURASIAN SOIL SCIENCE Vol. 48 No. 3 2015 242 VODYANITSKII, SHOBA Table 1. Structural parameters of green rust [32] Type Interlayer spacing, Mössbauer spectral parameters: isomeric shift δ of interlayer anion nm and quadrupole splitting ΔE Green rust I Carbonate D1: δ = 1.27 mm/s; ΔE= 2.86 mm/s Hydroxyl D2: δ = 1.25mm/s; ΔE = 2.48 mm/s Chloride D3: δ = 0.46 mm/s; ΔE = 0.48 mm/s D4: δ = 0.46 mm/s; ΔE = 0.97 mm/s Carbonate d003 = 0.750 Hydroxyl d003 = 0.792 Chloride d003 = 0.797 Green rust II Sulfate d001 = 1.10–1.16 D1: δ = 1.27 mm/s; ΔE = 2.83 mm/s; D3: δ = 0.47 mm/s; ΔE = 0.45 mm/s The Mössbauer spectra were obtained at 77 K. D1 and D2 show Fe2+ doublets; D3 and D4 show Fe3+ doublets. ephemeral green rust. At the same time, as the term For anioncontaining ephemeral sulfate green rust green rust is widely used [37], we also address it in this with two layers, the interlayer distance d001 = 0.110– way when citing appropriate sources.