ISSN 10642293, Eurasian Soil Science, 2014, Vol. 47, No. 5, pp. 407–415. © Pleiades Publishing, Ltd., 2014. Original Russian Text © Yu.N. Vodyanitskii, S.A. Shoba, O.G. Lopatovskaya, 2014, published in Pochvovedenie, 2014, No. 5, pp. 553–562.
SOIL CHEMISTRY
Iron Compounds in SulfateCarbonate Soils on RedColored Cambrian Rocks in the Southern Angara Region Yu. N. Vodyanitskiia, S. A. Shobaa, and O. G. Lopatovskayab a
Faculty of Soil Science, Moscow State University, Moscow, 119991 Russia b Irkutsk State University, ul. K. Marksa 1, Irkutsk, 664011 Russia Email:
[email protected] Received June 6, 2013
Abstract—In some regions of Irkutsk oblast in the southern Angara region, brown carbonate–sulfate soils have been formed on redcolored Cambrian rocks. In the automorphic soils, even a low content of hematite strongly affects the soil color, and the increase in its content only slightly enhances the red color of the soil. The brown color of carbonate soil is due to the partial preservation of lithogenic hematite in the upper part of the soil profile. The abundant gypsum “preserves” the lithogenic hematite in the carbonate–sulfate soil; the oxidation of iron is also hampered in this soil. Important changes occur in the wetted dark solonchak: litho genic hematite is dissolved, the structure of iron chlorite loses order, and coarse and crystallized magnetite is formed in the humus horizon. Keywords: lithogenic hematite, iron chlorite, magnetite DOI: 10.1134/S1064229314050251
INTRODUCTION The development of soil largely depends on the lithogenic factor. Its effect is most manifested on red colored deposits under pedogenetic conditions [16, 19, 21]. Many authors have studied soils on redcol ored deposits. Foreign authors mainly studied humid tropical soils [33], and Russian authors focused on soils of European Russia [19, 21]. In Asian Russia, redcolored rocks also occur, but the soils developed on them are poorly understood. In the southern Angara region, including the Novonukutskii, Zalari, Balakansk, and Osa districts of Irkutsk oblast, many soils have been formed on red colored Cambrian rocks [14]. They are characterized by their carbonate–sulfate composition. In Novonukutskii district, gypsum is produced for indus trial purposes. Carbonaterich soils are confined to hill tops; sul fates also occur in soils on hill slopes. On the flood plain, the situation is complicated because of the ero sion on hill tops, which provides a pronounced binary soil structure; the wetting factor also significantly con tributes. Hematite, which was formed under dry and warm climatic conditions in the Cambrian period, can fail to stand up against overmoistening in the accumu lative positions of the recent topography under perco lative water conditions. Pedogenesis is accompanied by transformations of iron compounds, which proceed in two opposite directions: the oxidogenesis of iron (the term was pro posed by Glazovskaya [15]) and the dissolution of
lithogenic iron (hydr)oxides. In the automorphic soils, oxidogenesis prevails as one of the main elementary soilforming processes. Oxidogenesis is presently con sidered as a natural landscapegeochemical process involving the formation and accumulation of iron oxides/hydroxides, as well as manganese and other elements [5]. It is morphologically manifested in neo formations, but the chemical and mineralogical anal yses reveal and quantify the development of iron oxi dogenesis, e.g., using the iron oxidation index Ко = Fe3+ : (Fe3+ + Fe2+). In the soils on redcolored rocks containing hema tite, its complete or partial dissolution is recorded. An example is provided by the dissolution of hematite during the weathering of redcolored rocks in humid (sup)tropics [33]. In some ferrallitic soils (Oxisols and texturally differentiated Hapludalfs) containing both hematite and goethite, the microbiological reduction of hematite results in the yellowing of the horizon due to the accumulation of residual aluminogoethite [24, 31]. Lithogenic iron (hydr)oxides in (semi)hydromor phic soils are dissolved due to the energy of organic matter and at the active participation of ironreducing microorganisms [4, 28, 29]. The fate of iron com pounds under (semi)arid conditions is poorly under stood. Let us find answers to the following questions. What is the contribution of hematite to the color of carbonate–sulfate soils? After all, the brown color can be mineralogically determined by different ironcon taining pigments: residual red hematite or yellow goet
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hite [23]. How is goethite dissolved under automor phic and semihydromorphic conditions? How are strongly magnetic iron compounds distributed in the profile, and what ferrimagnets occur in these soils? What pathways are possible for the transformation of iron silicates? The aim of this work was to determine the transfor mation pathways of iron compounds in brown carbon ate–sulfate soils developed on redcolored Cambrian rocks in the Angara region. OBJECTS OF STUDY Three carbonate–sulfate soils developed on red colored Cambrian rocks in the foreststeppe zone of the southern Angara region were studied. Profile pits were established on the right bank of the Zalari River, a left tributary of the Angara, in the Novonukutskii district of Irkutsk oblast. The area is hilly; the slopes are eroded with abundant ravines. The soils are named brown arid soils according to the 2004 Classification [17]. Two brown soils were sub divided into carbonate and carbonate–sulfate soils. Brown arid carbonate soil (profile 2) was pene trated at the top of a slope. The profile coordinates are 53°41′28.16′′ N and 102°44′16.87′′ E. The profile includes the following horizons: AJ1–AJ2–BM– BCA–Cca. The soil is dry throughout the profile. The redness of the soil (a*) successively increases with depth from 4.1 to 8.2. Effervescence with HCl is observed throughout the profile. Brown arid carbonate–sulfate soil (profile 3) was penetrated 9 m from profile 2 in a saddle on the middle slope. The coordinates of profile 3 are 53°41′28.11′′ N and 102°44′16.66′′ E. The profile includes the follow ing horizons: AJ–AJs–BMs–BCAs–D1ca,s–D2ca,s. The soil is dry throughout the profile. The redness of the soil is high and almost constant throughout the profile: a* = 7.5–8.5. Effervescence with HCl is observed throughout the profile. A gley solonchak (profile 4) was penetrated in the Zalari River floodplain 190 m from profile 3 and 50 m from the river bed. The coordinates of the profile are 53°41′27.33′′ N and 102°44′14.96′′ E. The horizon system is as follows: AU–S–Cca,s–D1g,ca,s– D2g,ca,s–D3g,ca,s. The soil is wet throughout the profile. Visually, the soil is dark gray; redness a* < 2.2. Effervescence from HCl is observed in the upper part of the profile. METHODS OF STUDY The total iron was determined by Rray fluorimetry on a Tefa6111 analyzer. The free iron compounds were determined by the dithionite–citrate–bicarbon ate Mehra–Jackson method. The iron compounds considered as being poorly ordered were determined by the acid ammonium oxalate Tamm method. This interpretation of the oxalatesoluble iron compounds
is valid for automorphic soils. However, in (semi)hydromorphic soils, the content of Feox mainly depends on the content and composition of Fe(II) compounds [4, 13]. This also explains the paradox of hydromorphic soils with stagnant water conditions, where sometimes the Schwertmann criterion KSch = Feox/Fedit > 1 [4]. Möessbauer spectroscopy was performed using an Ms1104Em spectrometer in the constant accelera tion regime with a 57Co source in a chromium matrix at room temperature. The Möessbauer spectra were recorded for 256 channels and processed using Univem MS software. The contents and parameters of the hematite and magnetite were thus determined. The content of iron in the Fe2+ sublattices of the iron chlorite was found. From the obtained data, the con tents of Fe2+ and Fe3+ were determined, and the iron oxidation index Ko was calculated as the portion of Fe3+ in the total iron. As for goethites, their complete identification requires the cooling of the sample to 77 K; only their largest and crystallized particles are identified at room temperature. The specific magnetic susceptibility χ was mea sured using a KLY2 kappa bridge. In the humus hori zons of the automorphic soils, the magnetic suscepti bility increases due to the synthesis of dispersed strongly magnetic magnetite [1]. The magnetic sus ceptibility of hydromorphic soils is usually lower than that of automorphic soils, because the synthesis of magnetite is impossible, especially under acidic con ditions [1, 25, 26, 40]. The color of the soils was determined using a Pulsar spectrocolorimeter with an integrating sphere. The instrument determines the reflectivity at 24 fixed wavelengths in the visible spectral region (380– 720 nm) per lamp flash. The reflectivity and color hue of the soils were assessed using the CIE–L*a*b* color scale. The CIE–L*a*b* color system in Cartesian coordinates quantitatively reflects the contributions of the four main colors. The abscissa axis characterizes the degrees of redness (+a*) and greenness (–a*) of the soil; the ordinate axis characterizes the degrees of yellowness (+b*) and blueness (–b*). The origin of the coordinates corresponds to the gray color. The third axis perpendicular to the a*–b* plane deter mines the lightness of the soil L* from 0 to 100 [12, 24]. Many methods were proposed to express the redness index (RI) with a single number [24, 38]. We used the modified Barron–Torrent concept [24]; the RI value was determined from the empirical equation [12] RI = а(а2 + b2)1/2/7b. RESULTS AND DISCUSSION All three soils have a neutral–weakly alkaline reac tion: pH about 8. The soils are mainly of heavy loamy texture. They differ in their moisture, which is related to their positions in the relief. Two soils (the brown EURASIAN SOIL SCIENCE
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Table 1. Iron forms, optical properties (in the CIE–L*a*b* system), and magnetic susceptibility (χ) of soils Horizon
Depth, cm
Feox
Fedit %
AJ1 AJ2 BM BCA Cca
0–11 11–22 22–30 30–49 49–60
0.06 0.04 0.04 0.04 0.03
0.85 0.84 0.84 0.91 0.99
AJ AJs BMs BCAs D1ca,s D2ca,s
0–6 6–27 27–40 40–53 53–70 70–80
0.03 0.01 0.01 0.01 0.01 0.01
1.55 1.18 1.07 1.16 1.26 1.02
0.57 0.80 0.88 0.83 0.87 0.66
0.91 1.07 1.27 1.04 1.01 1.35
AU 0–12 S 12–32 Cca,s 32–61 D1g,ca,s 61–83 D2g,ca,s 83–108 D3g,ca,s 108–130
Feox/Fedit Fetot, % Fedit/Fetot
Brown arid carbonate soil, profile 2 0.07 3.08 0.28 48.9 0.05 3.16 0.26 52.1 0.06 3.02 0.28 54.5 0.05 2.51 0.36 55.8 0.03 2.88 0.34 57.9 Brown arid carbonate–sulfate soil, profile 3 0.02 4.72 0.33 51.2 0.01 4.05 0.29 54.2 0.01 3.56 0.30 55.5 0.01 4.33 0.27 51.9 0.01 4.48 0.28 51.9 0.01 4.07 0.25 53.0 Gley solonchak, profile 4 0.63 3.49 0.26 51.6 0.75 3.27 0.33 51.4 0.69 4.28 0.30 51.4 0.80 2.05 0.51 54.8 0.87 2.48 0.41 – 0.49 3.84 0.35 –
arid carbonate–sulfate soil and the gley solonchak) have binary structures. The binary structure of the brown arid carbonate– sulfate soil is manifested in the change of texture: at the depth of 53 cm, the content of clay decreases from 17 to 33%. In the gley solonchak, many properties of the soil below 60 cm differ from those of the upper layer. This is true not only for the lightened texture in the lower soil layer but also for the chemical composition, some properties of which do not agree with the lightened texture of the lower layer. In this layer, the content of SiO2 is decreased and that of P2O5 is increased [11]. In addition, an abrupt decrease in the magnetic suscepti bility of the soil from 32 to 9 × 10–8 m3/kg is noted at the layer boundary. These serious differences suggest that the upper layer was formed due to the eroded material washed from the hill top, and the lower layer consists of Cambrian deposits, whose original red color was lost because of the dissolution of hematite at the high water content. The soil is wet throughout the profile. Rusty and brown mottles and other hydromorphism signs are observed in the lower part of the profile. Data on the iron compounds (from the chemical analysis), magnetic susceptibility, and color of the soils are given in Table 1; the results of the Moessbauer analysis of the iron compounds are given in Table 2. EURASIAN SOIL SCIENCE
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a*
b*
RI
χ × 10–8, m3/kg
4.1 5.8 6.8 7.3 8.2
7.8 10.6 12.2 12.3 12.6
0.66 0.94 1.11 1.21 1.40
41 20 14 6 7
7.5 8.1 8.1 7.5 8.5 8.2
9.9 11.3 12.2 10.7 10.8 10.2
1.34 1.42 1.39 1.31 1.54 1.50
17 8 8 8 9 8
1.7 1.8 2.1 1.5 – –
7.0 6.5 6.9 6.2 – –
0.25 0.27 0.31 0.22 – –
56 31 32 9 5 14
Iron compounds in automorphic soils. Content and properties of hematite particles. Hematite, which imparts the red color to the parent rock, was formed in the Cambrian period, the second half of which was characterized by a warm and dry climate favorable for its formation. In the humid regions, the development of soils on redcolored deposits is manifested in brunification: browning of the upper profile because of the blooming of the original red lithogenic color [21]. The decrease in the brightness of the original redcolored material is a common phenomenon; e.g., in the soils on old allu vial deposits of the Kama River, the redness a* in the upper layer decreases to 4–5 compared to a* = 8–10 in the C and D horizons [8]. The development of brunification can follow dif ferent pathways. The browning of soils can be due to the accumulation of yellow goethite in some soils [23] and a decrease in the content of hematite in soils on redcolored deposits [19]. The Moessbauer analysis of brunified soils on redcolored deposits in the forest and foreststeppe zones showed a decrease in the con tent of hematite in the absence of largecrystalline goethite [8, 9, 21]. In the southern Angara region, only the carbonate soil is brunified, where the redness a* successively decreases from 8.2 in the BM3 horizon to 4.1 near the
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Table 2. Moessbauer data on iron compounds in soils Horizon, depth, cm
Spectrum component
Component area IS, mm/s QS, mm/s
Heff
Ko % of Fe
Brown arid carbonate soil, profile 2 –0.21 510 23 2.66 23 0.64 54
BCA 30–49
(Fe3+)
S1 D1 (Fe2+) D2 (Fe3+)
0.36 1.11 0.36
BMs 27–40
S1 (Fe3+) D1 (Fe2+) D2 (Fe3+)
0.38 1.14 0.37
D1ca,s 53–70
S1 (Fe3+) D1 (Fe2+) D2 (Fe3+)
0.37 1.13 0.38
–0.21 2.64 0.63
S1 S2(Fe3++ Fe2+) S1 (Fe3+) D1 (Fe2+) D2 (Fe2+) D3 (Fe3+)
0.27 0.67 0.37 1.12 1.06 0.37
Gley solonchak, profile 4 –0.02 489 2 0.00 459 4 –0.22 512 5 2.68 19 2.18 6 0.64 64
D1g,ca,s 61–83
D1 (Fe2+) D2 (Fe3+)
1.08 0.36
2.59 0.52
23 77
0.77
D3g,ca,s 108–130
D1 (Fe2+) D2 (Fe2+) D3 (Fe3+)
1.13 1.03 0.37
2.66 2.26 0.58
15 3 82
0.82
AU 0–12
(Fe3+)
0.82
Brown arid carbonate–sulfate soil, profile 3 –022 510 41 2.11 2.63 22 0.61 37 510
40 22 38
Phase
% of soil 0.77
Hematite Chlorite Fe2+ Fe hydroxides Silicate Fe3+
0.63
Hematite Chlorite Fe2+ Fe hydroxides, silicate Fe3+ Hematite Chlorite Fe2+ Fe hydroxides, silicate Fe3+
2.48
0.62
0.19
0.75
0.26
Magnetite Hematite Chlorite Fe2+ Fe hydroxides, silicate Fe3+ Chlorite Fe2+ Fe hydroxides, silicate Fe3+ Chlorite Fe2+ Fe hydroxides, silicate Fe3+
(IS) isomer shift (δ); QS quadrupole splitting (Δ); (Heff) effective magnetic field.
surface, probably because of the humification of the surface horizon. A significantly different situation is observed in the lowhumified carbonate–sulfate soil, in the upper layer of which hematite remains. Its con tent in this soil is double that in the carbonate soil, and the redness is higher and almost constant throughout the profile: a* = 7.5–8.5. The persistence of lithogenic goethite can be due to the impact of sulfates. The experimental simulation of the effect of phenols as natural reducers on the dissolution of iron oxides was performed earlier. It was found that the reduction of phenols is strongly hampered in the presence of sul fates [30]. This effect is due to the specific adsorption of sulfate on the surface of iron oxides, which preserves their particles from reductive dissolution.
Let us discuss the properties of hematite particles. It is known from the literature that the redness index of the mixture depends on the portion of hematite. The redness index of the redcolored precipitate is 1.4 at the hematite content of 1.7% and increases to 2.4 at the hematite content of 11.7% [12]. However, the rela tionship between the redness index of the soil and the content of hematite can vary from a linear to a more complex one when the linear increase remains only to a specific (critical) hematite content and is replaced by the saturation of the redness when this critical level is exceeded. The critical content of hematite depends on its particle size, because the effect of hematite (as well as of any other pigment) depends on the specific sur face of its particles: the larger the specific surface, the more efficient the pigment. Therefore, the critical EURASIAN SOIL SCIENCE
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3.5 3.0 2.5 2.0 1.5 1.0 0.5 Redness index
content of hematite is indicative of its average particle size: the lower the critical content of hematite, the finer its particles. The redness saturation at the high content of pigment particles is related to their aggrega tion. Thus, the saturation of redness is not reached at the low content of hematite in the soil (below the critical level). For example, in WestEuropean alfisols con taining up to 4% hematite, the redness index increases linearly with the increasing hematite content [38]. However, in Brazilian Oxisols and Ultisols with a high content of hematite, the redness index linearly increases only to 5% hematite; then, its saturation begins and continues up to 15% hematite [38]. The critical value of 5% is high and indicates the large size of the hematite particles in European alfisols and Bra zilian Oxisols and Ultisols. Coarse hematite particles are also present in the soils on redcolored deposits in the Cisural region [7], where the relationship between the redness index and the content of hematite is almost similarly approximated by a straight line (R2 = 0.870) or a polynomial (R2 = 0.885) (Fig. 1a). It can be seen that, when the content of hematite is lower than 3%, its effect on the redness of the soils in the Cisural region does not result in saturation, which indicates coarse hematite particles. A different situation is observed in the soils of the southern Angara region. The regression is better described by a polynomial (R2 = 0.931) than by a straight line (R2 = 0.833) (Fig. 1b). It can be seen that even a small amount of hematite (up to ~1.0%) strongly affects the redness index, and the further increase little increases it. This is indicative of finer hematite particles than those in soils on the redcol ored Permian deposits of the Cisural region. Iron oxidation degree. The soils on loose deposits are characterized by high degrees of iron oxidation. In the forestzone soils of the Russian Plain, the degree of iron oxidation reaches 0.9 and higher [1, 3, 6]. According to the Moessbauer spectroscopic data, the automorphic soils of the Angara region are oxi dized to a low or medium degree: Ko = 0.62–0.77. A similar degree of oxidogenesis is also typical to the mountain soils of Eastern Siberia. Nogina and Rozh kov [22] reported the contents of Fe2O3 and FeO in 20 samples of mountain cryogenic taiga soils of Trans baikalia. From these data, the iron oxidation index can be calculated: Ko ± σ = 0.74 ± 0.09. The lower oxida tion degree of iron than in the soils of the Russian Plain is related to the colder climate of Eastern Sibe ria. The active degradation of Fe silicates occurs with the participation of microorganisms during the warm season, which is shorter in Eastern Siberia than on the Russian Plain. Notable differences in the oxidation degree of iron in the automorphic soils of the southern Angara region are observed. The least oxidized iron occurs in the car bonate–sulfate soils (Ko = 0.62–0.63). The oxidation of lithogenic Fe(II) is strongly hampered in the sulfate
0
(a) 2
y = 0.1165x + 0.4541x + 0.3241 R2 = 0.8853
0.5
1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0
411
1.0
1.5
2.0
2.5
3.0
3.5
(b)
y = –0.3508x2 + 1.4079x + 0.1209 R2 = 0.9319 0.5
1.0
1.5
2.0
2.5 3.0 Hematite, %
Fig. 1. Redness index as a function of the hematite content in soils on (a) redcolored Permian deposits in the Cisural region and (b) redcolored Cambrian deposits in the southern Angara region.
environment under a water deficit. In the carbonate soil (in the absence of sulfate), the oxidation of iron(II) proceeds more successfully: Ko = 0.77, although its content of hematite is lower than that in the carbonate–sulfate burozem by 3 times. The disso lution of lithogenic hematite is obviously accompa nied by the oxidation of Fe(II) silicates. Increase of the magnetic susceptibility in the humus horizons. The autonomous soils are generally charac terized by increased values of their magnetic suscepti bility (χ) in the humus horizon. The increase in this parameter is related to the formation of a small amount of fine particles of strongly magnetic magne tite Fe3O4. Their formation is symbiotic with humus accumulation, especially at a high portion of humates [1]. An increase in χ is typical, e.g., for chernozems in the steppe part of the Russian Plain. In the lower layers of automorphic soils in the southern Angara region, the magnetic susceptibility is low: χ = (6–9) × 10–8 m3/kg, but it increases in the humus horizons in spite of the low content of organic carbon in the automorphic soils. In the humus horizon of the carbonate–sulfate soil, where Corg = 0.8%, the magnetic susceptibility increases to 17 × 10–8 m3/kg. In the carbonate soil, where Corg = 1.2% in the humus hori zon, the magnetic susceptibility reaches 41 × 10–8 m3/kg. The insignificant increase in the content of organic carbon (by 1.5 times) is accompanied by an increase in the magnetic susceptibility by 2.4 times.
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Transformation of iron minerals in the wetted solon chak. The overmoistening of soils radically changes the composition and content of iron minerals. Iron, as a redoxsensitive element, is reduced under an oxygen deficit and in the presence of organic matter (a source of energy necessary for the vital activity of ironreduc ing microorganisms [28, 29]. However, the effect of overmoistening can vary depending on the hydrological conditions. The two alternatives strongly differ: soils with stagnant and per colative water conditions. Under stagnant conditions, recently formed Fe2+ is accumulated and precipitated in the form of Fe(II) minerals: magnetite Fe3O4, sider ite FeCO3, iron sulfides, etc. [4]. Their formation affects the results of the chemical analysis of the iron compounds. The content of oxalatesoluble com pounds is augmented, and the Schwertmann criterion KSch increases, which reflects the increase in the degree of hydromorphism. The Schwertmann criterion. Under percolative con ditions, the content of Feox does not increase, and the Schwertmann criterion ceases to fulfill the function of a gleying criterion [4]. In addition, after the removal of the reaction product (i.e., Fe2+), the reduction of Fe(III) is activated, and the process frequently involves Fe(III) silicates, which are degraded. The case is that not only iron (hydr)oxides but also Fe(III) silicates act as electron acceptors in the humus enriched horizons [28, 36, 37]. There is a rule that the least stable particles (dispersed and disordered) are first reduced [4]. These can be both fine particles of iron (hydr)oxides (e.g., ferrihydrites and feroxyhyte) and disordered particles of clay minerals (e.g., nontro nites) [36, 37]. An example is provided by the reduc tive processes in the soil on redcolored deposits of the Cisural region, where unstable Fe(III)containing smectites are dissolved, while lithogenic hematite par ticles persist [7]. Thus, the water conditions in the accumulative positions of the landscape can be determined from the value of KSch obviously, the conclusion based on the chemical analysis should be confirmed by mineralogi cal studies. In the gley solonchak, the Schwertmann criterion significantly increases (compared to the automorphic soils) from 0.01–0.07 to 0.5–0.8. This is related to the effect of Fe(II): the dependence of the Feox value from Fe(II) minerals was demonstrated in some model experiments for magnetite, siderite, and pyrite [4]. The increase of the Schwertmann criterion is typical for stagnant water conditions, which are favored by the heavy loamy texture of the solonchak. Dissolution of hematite. In the wetted solonchak of the floodplain, hematite is dissolved under the effect of local overmoistening: it is not revealed by the Moess bauer analysis. Therefore, the redness of the solon chak (a*) decreases to 1.5–2.1 against 7–8 in the lower part of the automorphic soils. The value of the redness index decreases respectively to 0.22–0.31
against RI = 1.1–1.5 in the lower part of the automor phic soils. All this indicates the dissolution of litho genic hematite in the wet solonchak; the mineral ceases to act as a red pigment inherited from the Cam brian period. It is interesting that hematite is dissolved in the solonchak under weakly alkaline conditions (pH ~9). The stability of iron oxides generally increases with increasing pH, and their dissolution almost ceases in strongly alkaline soils with pH ~11 [39]. Only the addition of NaHCO3 to an alkaline soil changed the pH value to 9.3 and initiated the reduction of Fe(III). Thus, the reduction of hematite in gley hematite is developed near the limit of the permissible pH range. Disordering of iron chlorite. Only active Fe(II) min erals are capable of catalyzing the dissolution of iron compounds in an acid ammonium oxalate solution [28, 36]. The ordered iron chlorites, which include Fe2+ and Fe3+ within the additional octahedral layer, do not participate in the dissolution by ammonium oxalate. In the automorphic soils on the hill top and slope, the Schwertmann criterion does not exceed 0.07 in spite of the presence of Fe(II) chlorite. How ever, the chemical activity of the chlorites abruptly increases at the partial dissolution of an additional octa hedral layer [18]. Moessbauer spectroscopy identifies dis ordered iron chlorites by the appearance of extra doublets of Fe2+; their total number can reach six [2]. In the wetted solonchak, characteristic changes were noted in the Moessbauer spectrum of the iron chlorite. Along with the main doublet with quadrupole splitting Δ = 2.63–2.66 mm/s, which was single in the spectrum of the automorphic soils, an additional dou blet with Δ = 2.18–2.26 mm/s appears, which indi cates the disordering of the chlorite structure in the solonchak. In a sample, no doublet splitting is revealed; only a decrease in the doublet quadrupole is observed from Δ = 2.63–2.66 to Δ = 2.59 mm/s. Note that a similar splitting of the Fe2+ doublet into two components occurs not only at microbiological reduc tion under natural conditions but also at the chemical reduction of the soil with the use of dithionite in a lab oratory experiment [3]. Thus, the activity of the chlo rite is determined by the degree of its structural disor dering, which can be assessed from the quadrupole of its Fe2+ sublattice. A decrease in the ordering of iron chlorites is observed in other regions, e.g., in the gleyed horizons of soils on deluvium and eluvium of redcolored Per mian clays in the Cisural region. The quadrupole split ting of the Fe2+ sublattice was 2.62–2.71 mm/s in the P, BT, and C horizons, but it decreased to 2.57–2.59 mm/s in the G horizon. Thus, chlorite loses ordering under overmoistening conditions, which results in a decrease in the quadrupole of the Fe2+ sublattice [6]. Let us return to the soils of the southern Cisural region. To verify the relationship between the quadru pole splitting of the chlorites and their activity, the relationship between the quadrupole Δ of the Fe2+ sub lattice and the Schwertmann criterion was calculated. EURASIAN SOIL SCIENCE
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0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 –0.1 –0.2
y = –8.1341x + 21.575 R2 = 0.7563
Feox/Fedit
The single splitting value for a Fe2+ doublet of chlorite or the weighted average value (from doublet areas) for chlorites with two Fe2+ doublets was taken as the Δ value. An expected inverse relationship was found between these parameters, which is described by the linear equation Feox/Fedit = 21.57 –8.13(ΔFe2+ sublat tice) with the determination coefficient R2 = 0.756 (Fig. 2). The increase in the solubility of iron com pounds in the Tamm solution agrees with the increase in the disordering and the activation of iron chlorites. The transformation of stable chlorite to a disor dered chlorite–vermiculite interstratification proba bly occurs only after the degradation of less stable hematite particles. Really, in the carbonate automor phic soil, where hematite is partially dissolved, iron chlorites retain their ordering, as is confirmed by their single Fe2+ doublet in the Moessbauer spectrum. Magnetite formation. The magnetic susceptibility of soils usually decreases under overmoistening condi tions due to the dissolution of iron (hydr)oxides [1]. This process is enhanced under intense percolative conditions, when the reduced Fe2+ is leached from the profile. However, an unusual situation is observed in the humus horizon of the solonchak: the magnetic suscep tibility increases to 56 × 10–8 m3/kg. Moessbauer spec troscopy revealed 0.19% magnetite. From these data, the magnetic susceptibility of the magnetite can be approximately calculated without consideration for the contribution of the weakly magnetic dia and para magnets. The magnetite susceptibility is about 29000 × 10–8 m3/kg. This value is typical for relatively large and crystallized magnetite particles; it is close to the upper limit of the magnetite susceptibility in soddypodzolic soils (16000–30000) × 10–8 m3/kg [4] and approaches the value for the standard coarsegrained magnetite (39000–58000) × 10 ⎯8 m3/kg [34]. The degree of crystallinity of the magnetite parti cles can also be assessed from the values of the effective magnetic field Heff for the mineral sublattices deter mined by Möessbauer spectroscopy. Magnetite has two sublattices: (A) tetrahedral (Fe3+) and (B) octahe dral (Fe3+ + Fe2+) ones. For the magnetite in the humus horizon of the solonchak, Heff (tetra) = 498 kOe and Heff (octa) = 459 kOe. These are relatively high values typ ical for coarsecrystalline polydomain magnetite; e.g., for technogenic magnetite in aerially contaminated soils of the city of Chusovoi in the Cisural region, Heff (tetra) = 488–492 kOe and Heff (octa) = 447–459 kOe. The spectral area ratio of the A and B magnetite sublattices is also an important parameter. The distur bance of the stoichiometry, which is expressed by the inequation SB : SA ≠ 2, is explained by two reasons [20]. The first reason is the substitution of Fe atoms by other metals: Fe2+ is sometimes substituted by Mg2+ (in nat ural magnetites) and heavy metals (in technogenic magnetites). The second reason is the vacancies in the lattice related to the oxidation (maghemitization) of magnetite. In uncontaminated soils, the disturbance
413
2.56
2.58 2.60 2.62 2.64 2.66 Quadrupole splitting, mm/s
2.68
Fig. 2. Schwertmann criterion versus the quadrupole of Fe(II) chlorite in soils of the southern Angara region.
of the stoichiometry is most frequently explained by the formation of intermediate oxides in the following series: magnetite Fe3O4 to maghemite γFe2O3. The oxidation degree of magnetite is expressed through the index of vacancies ν, which varies from 0 for stoichiometric magnetite to 0.33 for maghemite. The ν value is calculated from the relationship SB : SA = (2 – 6ν) : (1 + 5ν) [20]. Solving the equation with respect to ν, we obtain ν = (2SA – SB) : (6SA + 5SB). The Moessbauer analysis showed that magnetite is com pletely or partially oxidized in many natural objects. In black sand near the town of Poti (Georgia), the oxida tion degree of magnetite is low (ν = 0.02); magnetite in alluvium from a brook in the region of the Indigirka River (Yakutia) is more oxidized (ν = 0.14). The mag netite in a chernozem (Kursk) is also partially oxi dized: ν = 0.10. Completely oxidized magnetite occurs in highmoor peat: ν = 0.33 [20]. The oxidation degree of magnetite in soils of differ ent genesis was determined using the magnetochemi cal method [11]. In hydromorphic steppe soils of Ciscaucasia, magnetite is less oxidized than in upland soils. Strongly oxidized magnetite is found in soils on redcolored Permian deposits of the Cisural region. On the contrary, for magnetite in the humus horizon of the studied dark solonchak, ν = 0, which corresponds to stoichiometric magnetite. Thus, a number of parameters confirm the forma tion of large and ordered particles of stoichiometric magnetite in the humus horizon of the solonchak. This can be favored by the reducing and weakly alkaline conditions of the wetted solonchak, which agrees with the stability diagram of magnetite in the pH–Eh coor dinates [4]. Thus, in the wetted solonchak with stag nant water conditions and a weakly alkaline reaction (pH ~8), conditions are created for the synthesis of magnetite of stoichiometric composition with coarse particles and a high degree of crystallinity.
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CONCLUSIONS (1) In some areas of Irkutsk oblast in the southern Angara region, many soils were formed on hematite containing redcolored Cambrian rocks. According to the effect on the soil color, the magnetite particles are relatively fine compared to those in the soils on Per mian deposits in the Cisural region. The brown color of carbonate soils is due to the partial retention of lithogenic hematite in the upper part of the profile. In the carbonate soil, the oxidation of iron(II) proceeds more rapidly, although the con tent of hematite is lower than in the carbonate–sulfate burozem by three times. The dissolution of lithogenic hematite is apparently accompanied by the strong oxi dation of Fe(II) silicates. In the carbonate–sulfate soil, sulfates are sorbed on the surface of hematite and protect its particles from dissolution by organic substance. (2) In the wetted gley solonchak, the lithogenic hematite, which was formed under the dry and warm climate during the Cambrian period, cannot stand up against overmoistening and is dissolved. This is detected from the changes in some parameters: the Moessbauer analysis does not reveal hematite, and the optical data also indicate an abrupt decrease in the redness index. The Schwertmann criterion KSch = Feox/Fedit in the gley solonchak significantly increases compared to the automorphic soils. This reflects the stagnant water conditions favored by the heavy loamy texture of the solonchak. Along with the dissolution of hematite, transfor mations of iron chlorite are observed in the solonchak. Along with the main doublet with quadrupole splitting Δ = 2.63–2.66 mm/s, an additional doublet with Δ = 2.18–2.26 mm/s appears in the Moessbauer spectrum, which indicates the disordering of the chlorite struc ture in the solonchak. An inverse relationship is found between the quadrupole of the chlorites and the Schw ertmann criterion KSch. This confirms that the disor dered Fe(II) chlorites in the solonchak favor the disso lution of iron compounds in the Tamm solution. Thus, a number of parameters confirm the forma tion of coarse and ordered particles of stoichiometric magnetite in the humus horizon of the solonchak, which is favored by the reducing and weakly alkaline conditions of wetted solonchak and agrees with the stability diagram of magnetite in the pH–Eh coordi nates [5]. Thus, in the wetted solonchak with stagnant water conditions and a weakly alkaline reaction (pH ~8), conditions are created for the synthesis of magnetite of stoichiometric composition with coarse and highly crystallized particles.
REFERENCES 1. V. F. Babanin, V. I. Trukhin, L. O. Karpachevskii, A. V. Ivanov, and V. V. Morozov, Soil Magnetism (Yaro slavl–Moscow, 1995) [in Russian]. 2. V. F. Belov, E. P. Val’yashikhina, E. A. Vlasova, E. I. Dolomanova, V. V. Korovushkin, et al., “The impact of thermal and mechanical loads on the compo sition and structure of iron chlorites (according to Mös bauer spectroscopy data),” Izv. Vyssh. Uchebn. Zaved., Geol. Razved., No. 8, 50–59 (1974). 3. G. N. Belozerskii, M. I. Kazakov, E. I. Gagarina, and A. A. Khantulev, “Application of Mössbauer spectros copy to study iron forms in forest soils,” Pochvovede nie, No. 9, 35–45 (1978). 4. Yu. N. Vodyanitskii, “Biogeochemical role of magnetite in urban soils (review of publications),” Eur. Soil Sci. 46 (3), 317–324 (2013). 5. Yu. N. Vodyanitskii, Diagnosis of waterlogged mineral soils (Pochven. Inst. im. V.V. Dokuchaeva, Moscow, 2008) [in Russian]. 6. Yu. N. Vodyanitskii, Iron Compounds and Their Role in Soil Protection (Pochven. Inst. im. V.V. Dokuchaeva, Moscow, 2010) [in Russian]. 7. Yu. N. Vodyanitskii, A. A. Vasil’ev, M. V. Vlasov, and V. V. Korovushkin, “The role of iron compounds in fix ing heavy metals and arsenic in alluvial and soddypod zolic soils in the Perm area,” Eur. Soil Sci. 42 (7), 738– 749 (2009). 8. Yu. N. Vodyanitskii, A. A. Vasil’ev, and V. Yu. Gilev, “Iron minerals in soils on redearth deposits in the Cis Ural region,” Eur. Soil Sci. 40 (4), (2007). 9. Yu. N. Vodyanitskii, A. A. Vasil’ev, A. V. Kozheva, E. F. Sataev, M. N. Vlasov, “Influence of ironcontain ing pigments on the color of soils on alluvium of the Middle Kama plain,” Eur. Soil Sci. 40 (3), 289–301 (2007). 10. Yu. N. Vodyanitskii, E. I. Gagarina, and S. N. Lesovaya, “The formation of iron oxides in soils developed from local moraines,” Pochvovedenie, No. 2, 67–77 (1994). 11. Yu. N. Vodyanitskii, E. G. Morgun, L. A. Obydenova, K. A. Rumyantseva, N. V. Chapygina, “Geochemistry of magnetite and maghemite in soils of European Rus sia,” Geochem. Intern., 47 (3), 297–310 (2009). 12. Yu. N. Vodyanitskii and L. L. Shishov, Study of Some Soil Processes as Indicated by the Soil Color (Pochv. Inst. im. V.V. Dokuchaeva, Moscow, 2004) [in Russian]. 13. Yu. N. Vodyanitskii and S. A. Shoba, “Biogeochemistry of iron in waterlogged soils,” Pochvovedenie, No. 9, 1047–1059 (2013). 14. G. A. Vorob’eva, Soils as a Record of Natural Events in the Baikal Region: Problems of Soil Evolution and Soil Classification (Izd. Irkut. Univ., Irkutsk, 2010) [in Rus sian]. 15. M. A. Glazovskaya, “Natural LandscapeGeochemical Processes in the USSR,” Vestn. Mosk. Univ., Ser. Geogr., No. 5, 3–9 (1988). 16. V. V. Dobrovol’skii, “Mantle redearth deposits in the southeastern Europe,” Pochvovedenie, No. 8, 15–27 (1987). 17. Classification and Diagnostic System of Russian Soils, (Oikumena, Smolensk, 2004) [in Russian]. EURASIAN SOIL SCIENCE
Vol. 47
No. 5
2014
IRON COMPOUNDS IN SULFATECARBONATE SOILS ON REDCOLORED 18. T. A. Sokolova, T. Ya. Dronova, and I. I. Tolpeshta, Clay Minerals in Soils (Izd. Mosk. Gos. Univ., Moscow, 2005) [in Russian]. 19. S. N. Lesovaya, Extended Abstract of Doctoral Disser tation in Geography (St. Petersburg, 2006). 20. V. V. Morozov, Extended Abstract of Doctoral Disserta tion in Mathematics and Physics (Moscow, 2006). 21. V. V. Morozov, V. F. Babanin, S. A. Shoba, A. V. Ivanov, E. I. Svetlova, “Forms of iron compounds in soils devel oping from the redearth Permian deposits according to Mössbauer spectroscopy,” Vestn. MGU , Ser. Poch voved., No. 4, 8–14 (1987). 22. N. A. Nogina and A. I. Rozhkov, “Dynamics of oxalatesoluble forms of iron and aluminum in moun tain taiga frozen soils of Transbaikal region,” Poch vovedenie, No. 10, 56–70 (1974). 23. Elementary SoilForming Processes. Conceptual Analysis, Characterization, Systematics (Nauka, Moscow, 1992) [in Russian]. 24. V. Barron and J. Torrent, “Use of the KubelkaMunk theory to study the influence of iron oxides on soil color,” J. Soil Sci. 37, 499–510 (1986). 25. J. M. Bigham, S. E. Heckendorn, W. F. Jaynes, and N. E. Smeck, “Stability of iron oxides in two soils with contrasting colors,” Soil Sci. Soc. Am. J. 55, 1485– 1492 (1991). 26. E. De Jong and P. A. Hestor, D. J. Pennock, “The use of magnetic susceptibility to measure longterm soil redistribution,” Catena 32, 23–35 (1998). 27. D. A. Grimley and M. J. Vepraskas, “Magnetic suscep tibility for use in delineating hydric soils,” Soil Sci. Soc. Am. J. 64, 2174–2180 (2000). 28. J. E. Kostka, J. W. Stucki, K. H. Nealson, and J. Wu, “Reduction of structural Fe(III) in smectite by a pure culture of the Fereducing bacterium Shewanella putre faciens Strain MR1,” Clays Clay Minerals 44, 522– 529 (1996). 29. J. E. Kostka, J. Wu, K. H. Nealson, and J. W. Stucki, “Effects of microbial reduction on physical and chem ical properties of clay minerals,” Geochim. Cosmo chim. Acta 63, 3705–3713 (1999).
EURASIAN SOIL SCIENCE
Vol. 47
No. 5
2014
415
30. J. S. LaKind and A. T. Stone, “Reductive dissolution of goethite by phenolic reductants,” Geochim. Cosmo chim. Acta 53, 961–971 (1989). 31. D. R. Lovley, “Organic matter mineralization with the reduction of ferric iron: a review,” Geomicrobiology J. 5 (3/4), 375–399 (1987). 32. D. R. Lovley, “Reduction of iron and humic substances in subsurface environments,” in Subsurface Microbiol ogy and Biogeochemistry, Ed. by J.K. Fredrickson and M. Fletcher, 193–217 (2001). 33. J. Macedo and R. B. Bryant, “Preferential microbial reduction of hematite over goethite in a Brazilian Oxisol,” Soil Sci. Soc. Am. J. 53, 1114–1118 (1989). 34. C. E. Mullins, “Magnetic susceptibility or the soil and its significance in soil science—a review,” J. Soil Sci. 28 (2), 223–246 (1977). 35. U. Schwertmann, “Occurrence and formation of iron oxides in various pedoenvironments,” Iron in Soils & Clay Minerals (NATO, Reidel, Dordrecht, 1988), pp. 267–308. 36. J. W. Stucki, D. C. Golden, and C. D. Roth, “Effects of reduction and reoxidation of structural iron on the sur face charge and dissolution of dioctahedral smectites,” Clays Clay Miner. 32, 350–356 (1984). 37. J. W. Stucki, K. Lee, B. A. Goodman, and J. E. Kostka, “Effects of in situ biostimulation on iron mineral speci ation in a subsurface soil,” Geochim. Cosmochim Acta 71, 835–843 (2007). 38. J. Torrent, U. Schwertmann, H. Fechter, and F. Alferez, “Quantitative relationships between soil color and hematite content,” Soil Sci. 136, 354–358 (1983). 39. R. A. Whittleston, D. I. Stewart, R. J. G. Mortimer, and I. T. Burke, “Enhancing microbial iron reduction in hyperalkaline, chromium contaminated sediments by pH amendment,” Appl. Geochem. 28, 135–144 (2013). 40. R. D. Williams and J. R. Cooper, “Locating soil bound aries using magnetic susceptibility,” Soil Sci. 150, 889– 895 (1990). Translated by K. Pankratova
