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Weathering of glauconite in soils of temperate climate as exemplified bya Luvisol profile from Góra Puławska, Poland
Michał Skiba a,⁎, Katarzyna Maj-Szeliga a, Wojciech Szymański b, Artur Błachowski c
a Jagiellonian University, Institute of Geological Sciences, Department of Mineralogy, Petrology and Geochemistry, 30-063 Kraków, ul. Oleandry 2a, Polandb Jagiellonian University, Institute of Geography and Spatial Management, Department of Pedology and Soil Geography, 30-387 Kraków, ul. Gronostajowa 7, Polandc Pedagogical University, Institute of Physics, Mössbauer Spectroscopy Division, 30-084 Kraków, ul Podchorążych St. 2, Poland
The aimof the research reported hereinwas to study theweathering of glauconite taking place in a Luvisol profiledeveloped on quartz-glauconitic sand in Góra Puławska (eastern Poland) using X-ray diffractometry, Fouriertransform infrared spectroscopy, Mossbauer spectroscopy, SEM–EDS, and an optical microscope. The bulk soilmaterial contains quartz, glauconite, glauconite–smectite mixed-layered minerals rich in smectite layers, andtraces of feldspars. When compared with the parent material (the Cg horizon), the Bt1 and Bt2 horizons areenriched in clayminerals while the Ap, AEg, and E horizons are depleted in clays. The observed mineral distribu-tion is most likely controlled by the weathering of clay minerals in the uppermost part of the profile and trans-location of clay fractions down the profile (lessivage). However contamination with glaciofluvial and/oraeolianmaterial has to be taken into consideration. According to themicroscopic observations of the thin sectionsclay is present in the BCg and Cg horizons in the formof clay coatings and clay infilings. This indicates that initiallythe parent sand did not contain clay fraction. The fraction was formed most likely in the upper part of the soilprofile and was deposited in the lower horizons by percolating atmospheric water. Green pellets separatedfrom all the soil horizons are composed of glauconite showing more or less uniform chemical composition,while clay fractions from all the horizons contain glauconite and glauconite–smectite. Fine clay fractions separat-ed from Ap and AEg horizons are enriched in glauconite–smectite relative to the fractions separated from thelower horizons. The presence of glauconite and glauconite–smectite minerals in the clay fractions studiedindicates that glauconite weathering in the soil studied involves formation of glauconite clay by the pellet disin-tegration and glauconite smectitization. Ferrous iron accounts only for b0.1 layer charge per half unit cell in theprimary glauconite. This indicates that ferrous iron oxidation is not the main mechanism leading to theglauconite smectitization. Because the glauconite–smectite appears to be depleted in magnesium and totaliron and enriched in aluminum and silicon relative to primary glauconite, leaching of magnesium and iron to-gether with possible reorganization of the structure appears to be the likely mechanism for the glauconitesmectitization.
Glauconite is a green, dioctahedral, iron-rich 2:1 phyllosilicate withidealized general chemical formula:
K; Nað ÞxþyVI Fe2þMg� �
x
VIAl; Fe3þ
� �2−x
□0:9Si4−yIV Al; Fe3þ� �
yO10 OHð Þ2
where:
□ stands for a vacant site within octahedral sheet,
K≫Na;x≫y; 0:6≤xþ yb0:8;andVIAl= VIAlþVIFe3þ
� �≤0:5
(Odom, 1984; Rieder et al., 1998). According to Rieder et al. (1998) theterm “glauconite” is a series name and refers to glauconite–nontronitemixed-layered minerals with composition defined by the generalformula. Glauconite end member is a dioctahedral interlayer deficientmica having the layer charge close to 0.9 per half unit cell. High
Fig. 1.Morphology of the soil profile studied (units on the band in cm).
Table 1Field description morphology of the studied soil profile.
Horizon Depth [cm] Color (moist) Structure Roots Mottles Clay coatings
Ap 0–25 10 YR 4/2 Sub- to angular blocky +++ Absence AbsenceAEg 25–35 2.5 Y 5/3 Angular blocky ++ Few AbsenceE 35–50 2.5 Y 5/3 Angular blocky + Absence AbsenceBt1 50–90 7.5 Y 4/3 Angular blocky + Absence +++Bt2 90–112 7.5 Y 4/3 Angular blocky + Absence +++BCg 112–130 5 Y 4/3 Massive + +++ ++Cg 130–160 5 Y 4/3 Massive Absence ++ +
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glauconite layer charge originates mainly from the presence of divalentcation in the octahedral sheet. The charge is compensated by K+ bound-ed within the interlayer space.
Glauconite forms in marine environments. Although the exactmechanism of glauconite formation is not well understood, many re-searchers (e.g. Baldermann et al., 2013; Moore and Reynolds, 1997;Odin, 1988; Odom, 1984) believe that the mineral forms from an iron-rich smectite (nontronite) by reduction of octahedral Fe3+, uptake ofMg2+, selective sorption of K+ from sea water, and fixation of the K+
within interlayer space. Glauconite has been described from sedimentsand sedimentary rocks of different ages (from Precambrian to Holocene)where it commonly occurs in green aggregates (pellets) usually 100 μm–
500 μm in diameter (e.g. Odom, 1984). Today glauconite appears to beforming during early diagenesis near the marine sediment–water inter-face between 50°S and 65°N close to the border between outer shelvesand continental slopes (i.e. at 50–300 m depth) (Moore and Reynolds,1997), however formation of glauconite in deep-sea (N1000 m waterdepth) environment has also been reported (e.g. Baldermann et al.,2013; Wiewióra et al., 2001). The most important conditions playing acrucial role in glauconite formation are: slow accumulation rate of detritalmaterial and redox potential (Eh)= 0mV (Odin, 1988). Glauconite com-monly is associated with upwelling zones rich in phosphate and organicmatter.
Glauconite rarely constitutes a major soil mineral, however it com-monly occurs in soils developed from sediments and sedimentary rocks.The mineral is also the main component of the so-called greensands —“organic” (i.e. natural) fertilizers which are used as a source of potassiumand iron easily accessible for plants (e.g. Thompson and Ukrainczyk,2002). Greensands are being commonly added to crop soils because ofthe current popularity of “organic” fertilizers. For these reasons, themech-anism of glauconite weathering is of great interest because it may poten-tially influence chemical properties of arable soils.
In general, the mechanism of glauconite weathering has notbeen studied extensively. According to available data from laboratory-induced weathering of glauconite (Abudelgawad et al., 1975; Derkowskiet al., 2009; El-Amamy et al., 1982; Robert, 1973) themineral is expectedto be very unstable and easilyweatherable in the soil environment. Potas-sium boundwithin the interlayer space of glauconite is expected to easilyexchange with other soil ions leading to the formation (at the expense ofglauconite) of a Fe-dioctahedral vermiculite-like or smectite-like phase.Additional oxidation of glauconite octahedral iron is believed to promotepotassium depletion and formation of more smectitic minerals. Dataconcerning natural weathering of glauconite within soil environmentsare scarce, and not very many papers dealing with the issue are available(Carlson and Kunze, 1967; Cloos et al., 1961; Courbe et al., 1981; Fanninget al., 1989; Gildersleeve, 1932; Hutcheson and Haney, 1963; Loveland,1981; Nash et al., 1988; Pestitschek et al., 2012; Van Ranst and DeConinck, 1983; Wolff, 1967; Wurman, 1960). According to Courbe et al.(1981), Hutcheson and Haney (1963), Loveland (1981), Nash et al.(1988), Pestitschek et al., 2012; Van Ranst and De Coninck (1983),Wolff (1967), and Wurman (1960) irrespective of the climate glauconiteweathers into vermiculite and/or smectite, via glauconite-expandablemixed-layered minerals. Courbe et al. (1981), Loveland (1981), Nashet al. (1988), Van Ranst and De Coninck (1983), andWolff (1967) report-ed formation of kaolinite due to glauconite weathering taking place
together with glauconite vermiculitization and/or smectitization. Trans-formation of glauconite into amorphous gel and co-precipitation ofkaolinite were reported by Carlson and Kunze (1967) from soils devel-oped in tropical climate and by Cloos et al. (1961) from soils developedin moderate climate. These different results and conclusions concerningtransformation of glauconite indicate the need for additional studiesinto glauconite weathering.
This paper reports the results of a mineralogical study of glauconiteweathering within a soil profile developed on quartz-glauconitic sandnear Góra Puławska in eastern Poland.
These materials provide the context for interpreting and discussingthemechanisms of weathering, using optical microscopy, X-ray diffrac-tometry, infrared spectroscopy,Mössbauer spectroscopy, scanning elec-tron microscopy, and energy dispersive spectrometry.
2. Materials and methods
The samples for the present study were collected from a CutanicLuvisol (Epidystric) according to WRB classification system (IUSSWorking GroupWRB, 2006) profile in eastern Poland in Góra Puławska(51°24′00″N; 21°55′17″E) at an altitude 151 m above sea level. Theprofile was developed on a gentle slope (5°) with southern exposure.In the study area the mean annual temperature ranges from 6 °C to7 °C and the mean annual precipitation ranges between 450 mm and500 mm. The studied soil profile shows udic moisture regime andmesic temperature regime according to Soil Taxonomy (Soil SurveyStaff, 2010). The parent material for the soil is Oligocene quartz-glauconitic sand containing single quartz gravel grains. The sand was
Table 2Physical and chemical properties of the investigated soil profile.
Horizon Depth pH SOCa SOMb USDA texture class Dbc Pd
a Soil organic carbon.b Soil organic matter.c Bulk density.d Total porosity.
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deposited inmarginal part of Oligocenemarine basin in highly energeticdepositional regime (Baraniecka, 1984). In Pleistocene the potentiallyexisting pre-glacial soils developed on the glauconitic sand werescoured away by continental glaciers and the study area was coveredwith glacial tills, glaciofluvial sands and gravels, and likely loess. Mostlikely in late Pleistocene/Holocene the Oligocene quartz-glauconiticsand was exposed by erosion. Though the profile appeared to be devel-oped from the Oligocene sand, single pebbles of crystalline Scandina-vian rocks in the nearest proximity of the profile were observed,indicating that the uppermost soil horizonmay contain some admixtureof Pleistocene glacial/glaciofluvial deposits.
The samples were collected from each of the genetic horizons withadditional subsamples from the E and Bt1 horizons. The detailed profileand sample descriptions are given in Table 1. The morphology of thestudied soil profile is presented in Fig. 1. The bulk samples wereair-dried at room temperature (~20 °C), gently crushed, and passedthrough a 2 mm steel sieve. The obtained b2 mm fractions (hereinaftercalled bulk soils) were used for determination of the soil properties.Texture (particle-size distribution)was determined by sieving and a hy-drometer method (Gee and Bauder, 1986). Organic carbon content wasdetermined by a rapid dichromate oxidation technique (Nelson andSommers, 1996). Soil pH was measured in distilled water and in 1 M
Abundance of sand fraction (%)
Depth(cm)
10 20 30 40 50 60 70 80 90 100
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
Abundance
Depth(cm)
10 20 30 40
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
Ap Ap
AEg AEgE E
Bt1 Bt1
Bt2 Bt2
BCg BCg
Cg Cg
Fig. 2. Depth plots showing abundan
KCl solution (1:2.5 soil/liquid ratio) (Thomas, 1996). The bulk densityand total porosity were determined by means of the core method(Blake and Hartge, 1986).
Mineral composition of the bulk soils and separated clay fractionswas determined by X-ray diffraction (XRD). A 2.7 g portion of eachbulk soil sample was mixed with 0.3 g of Baker ZnO (catalog no. 1314-13-2) and ground under ethanol in a McCrone micronizing mill for10 min. After drying, the ground mixtures were passed through a0.4 mm steel sieve and side-loaded to obtain random powder mountsaccording to the procedure described by Środoń et al. (2001).
Clay fractions (b2 μm and b0.2 μm) were separated from bulk soilsby centrifugation. Prior to the separation, the bulk soils were treatedwith Na acetate–acetic acid buffer to replace all the exchangeable cat-ions with Na+. The separated fractions were coagulated using NaCland each split into three portions, which were saturated with K+,Mg2+, and Ca2+ respectively. The time of contact between the fractionsseparated and the reagents used was kept to minimum to avoid alter-ation of the glauconite structure likely occurring during extensivechemical treatment (Abudelgawad et al., 1975; El-Amamy et al., 1982;Robert, 1973). Approximately 75–100 h in total was needed to performthe complete separation procedure. Oxidation and reduction treat-ments (organic matter removal and free iron oxide removal protocols),
of silt fraction (%)50 60 70 80 90 100
Abundance of clay fraction (%)
Depth(cm)
10 20 30 40 50 60 70 80 90 100
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
Ap
AEg
E
Bt1
Bt2
BCg
Cg
ce of sand, silt, and clay fraction.
Table 3Quantitative mineral compositions given in wt.% of the bulk soil materials (fractions b 2 mm) studied, based on the XRD data.
Ap 2.48 b1 5.50 1.19 b1 b1 89.44AEg 4.07 b1 5.16 ni b1 2.55 88.22E 6.02 b1 5.46 ni b1 2.84 85.68
14.74 b1 2.50 ni b1 2.07 80.69B1t 35.24 b1 2.02 ni b1 5.01 57.25
27.64 b1 1.77 ni b1 3.74 66.10B2t 22.40 b1 2.28 ni b1 7.17 67.91BCg 18.69 b1 1.62 ni b1 3.93 75.11Cg 16.89 b1 1.69 ni b1 3.29 77.39
ni— not identified.
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which are commonly applied before clay fraction separation (e.g.Jackson, 1969) were omitted because according to Komadel et al.(1995) the treatments strongly affect the structure of iron-rich clays.
To evaluate a potential influence of the pretreatments used in thisstudy on the mineralogy of the studied samples, a clay fraction fromthe soil parent material (the Cg horizon) was also separated withoutany pretreatments. Immediately after the separation the fraction wasCa-saturated.
Thin sections were prepared from undisturbed soil samples embed-ded in an Araldite 2020 epoxy resin. The sections were studied using aNikon Eclipse E 600 POL microscope equipped with a Nikon D5100digital camera. The obtained digital images were used for estimationof abundance of green pellets using JMicroVision 1.2.7 image analysissoftware.
Green pellets were separated from sand fractions (obtained from bulksoils by gentle wet sieving) using neodymium bar magnet. The pelletswere ground in an agate mortar and saturated with Mg and Ca.
Abundance of glauconite p
Depth(cm)
2 4 6 8 10 12 14
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
AEg
E
Fig. 3. Depth plot showing abun
Oriented mounts with an areal density of about 10 mg/cm2 wereprepared from Ca-saturated, K-saturated, and Mg-saturated clays andthe ground pellet portions by deposition of the material dispersed(using ultrasonic treatment) in deionized water on petrographic glassslides. Random mounts were prepared from fine clays using side-loading technique.
XRD patterns were recorded with a Philips X'Pert diffractometerwith a vertical goniometer PW3020. The instrument was equippedwith a 1° divergence slit, 0.2 mm receiving slit, incident- anddiffracted-beam Soller slits, a 1° anti-scatter slit, and a graphite mono-chromator positioned at the diffracted beam. CuKα radiation producedat 40 kV and 30 mA was used. The mounts were scanned from 2° to52° 2θ at a speed of 0.02°/2 s (oriented mounts) and 2–65° 2θ at aspeed of 0.02°/5 s (random mounts). The K-saturated, Ca-saturated,and Mg-saturated clays were analyzed in air-dried form at ambient rel-ative humidity (14–30%). K-saturated clays were also analyzed afterheating for 1 h at 330 °C and after heating for 1 h at 550 °C while
216 M. Skiba et al. / Geoderma 235–236 (2014) 212–226
Mg-saturated clays were analyzed after solvation with liquid glycerol.Quantitative mineral composition calculations for bulk soils and thefine clays were performed with the Rietveld AutoQuan/BGMN comput-er program (Taut et al., 1998) using the data registered for randommounts. For the calculation of the content of glauconite–smectitemixed-layeredminerals, a smectite structural model was used. Oper-ational definitions given by Środoń (2006) were used to identifyclay minerals.
Fourier transform infrared (FTIR) spectra for Ca-saturated fine clay(b0.2 μm) fractions dispersed in KBr pellets were obtained with a BioRad FTS 135 FTIR spectrometer. Thirty-two scans were collected foreach sample in the range 400 cm−1 to 4000 cm−1 with a resolution of2 cm−1. The pellets (1.6 mg clay in 300 mg KBr) were prepared fromclay samples that had been dried at 105 °C for 24 h and KBr powderthat was pre-heated at 550 °C for 24 h. After the collection of the FTIRspectra, the pellets were heated at 180 °C overnight and a second setof spectra was collected. The recorded transmission intensities weretransformed to absorbance values with WinIR BioRad software. TheOH-stretching region (3000 cm−1–3750 cm−1) was analyzed in detail.Positions of individual bands in the OH-stretching region were revealedfrom analysis of the first and the second derivatives of the absorbancefunctions. The obtained values were used in decomposition of the spec-trawithWinIR BioRad software. Lorentzian peak shapewas used for thedecomposition. The variable parameters were: position, half-heightwidth, and intensity of each band. The quality of the decompositionwas evaluated by the reduced χ2 values, by the correlation coefficientR, and by common agreement between the experimental spectrumand the one synthesized by summation of all extracted bands.
Fine clay (b0.2 μm) fractions from all soil horizons and green pelletsfrom two selected horizons were examined using Mössbauer spectros-copy. The pellets only from the two horizons were analyzed becauseexamination under a binocular microscope showed that the magneticfractions from the other horizons were contaminated with iron oxy-hydroxyoxides. Transmission Mössbauer spectra were collected usinga RENON MsAa-3 spectrometer with the velocity scale calibrated bythe Michelson–Morley interferometer. The spectra were collected atroom temperature for the 14.41-keV resonant transition in 57Fe apply-ing commercial 57Co(Rh) source. Mössbauer absorbers were prepared
Fig. 4. Photomicrographs showing structures of the soil studied. (a, b) Clay coatings bonding gpellets embedded in clay matrix in Bt1 horizon. (e) Glauconite pellets showing cracking and rfrom the AEg horizon.
in the powder form. Data was processed within transmission integralapproximation as implemented in the MOSGRAF suite. All shifts of thespectra are reported versus room temperatureα-Fe. The obtained spec-tra were used for calculation of Fe3+ and Fe2+ atomic percentages.
Chemical analyses for the fine clay fractions and for the green pelletswere performed under a Hitachi S-4700 scanning electron microscopeequipped with a Thermo Scientific Noran System 7 energy dispersivespectrometric (EDS) system. Small chips (few square mm) of orientedclay films previously used for XRD analysis were deposited on carbonsticky foil and coated with carbon using a Cressington 208 carboncoater. EDS spectra were collected three times for areas ~ 10,000 μm2
with the application of an acceleration voltage of 20 kV, an emissioncurrent of 10 mA, and a 60 s counting acquisition live time. Relativeconcentrations of the main elements were calculated by application ofstandardless analysis by NSS 2.3 software.
3. Results
3.1. Texture,morphology, and other physical, and chemical properties of thestudied soil profile
The top and bottom parts of the soil profile are characterized bysandy loam texture. The middle part of the profile (Bt1, Bt2, andBCg horizons) shows finer texture: sandy clay or sandy clay loam(Table 2). The upper part of the soil profile (the Ap, AEg, and E horizons)is relatively depleted in clay fraction and relatively enriched in silt frac-tion. The middle part of the profile (the Bt1, Bt2, and BCg horizons) isclearly enriched in clay fraction when compared with the parent mate-rial (the Cg horizon) (Fig. 2).
Field description and ofmorphology of the soil profile studied is pre-sented in Table 1. The uppermost Ap and AEg horizons are formed fromgrayish yellow brown (10 YR 4/2) or yellowish brown (2.5 Y 5/4) mate-rial containing glauconite pellets and scarce fragments of Scandinaviangranitoids. The lower soil horizons (i.e. E, Bt1, Bt2, BCg, and Cg) areformed fromgrayish olive to dark olive (5 Y 4/3–5/3, 7.5 Y 4/3)material.Structure of soil material within the profile studied is subangular blockyto angular blocky in theAp horizon and angular blocky in theAEg, E, Bt1,and Bt2 horizons. The lowermost horizons (BCg and Cg) show massive
lauconite pellets and quartz sand grains in Cg horizon. (c, d) Quartz sand and glauconiteusty rims developed along the cracks. (f). Glauconite pellets, quartz silt, and quartz sand
217M. Skiba et al. / Geoderma 235–236 (2014) 212–226
structure. Roots occur mainly in the upper part of the profile (Ap andAEg horizons), but in the lower part, few roots are also present. In theBCg and Cg horizons numerous rusty mottles occur. A few fine mottlesin the AEg horizon are also present, indicating periodic stagnation ofwater due to occurrence of plowpan (in the AEg horizon). The presenceof brownish-green clay coatings on ped faces in the Bt1 and Bt2 hori-zons, as well as the BCg horizon, indicates translocation of colloidalclay down the soil profile (lessivage).
Some physical and chemical properties of the soil profile studied arepresented in Table 2. The soil profile studied is characterized by acidicreaction (pH in distilled water below 5.3) and very low content of or-ganic carbon (0.5% in the Ap horizon and 0.2% in the AEg horizon).The Ap and AEg horizons show slightly lower pH than the E, Bt1, Bt2,BCg and Cg horizons which is most likely related to the presence ofsoil organic matter and leaching.
5 10 15 20 25 30
gl
gl
d: 5.0d: 4.5
d: 10.3
d: 10
glq
q
d: 3.33
d: 4.26
Fig. 5. XRD patterns of air-dry and glycerol-solvated, oriented mounts of the Mg-saturateglauconite (R N 1 glauconite–smectite).
3.2. Mineral composition of the samples studied
3.2.1. Quantitative mineral composition of the bulk soil material(fraction b 2 mm)
The soil material contains quartz, glauconite, glauconite–smectitemixed-layered mineral rich in swelling interlayers, and feldspars(Table 3). The lowermost horizons Cg and BCg contain approximately75–77% of quartz and 22–24% of clay minerals (glauconite andglauconite–smectite mixed-layered mineral). When compared with theparent material, the Bt1 and Bt2 horizons are enriched in clay mineralswhile the Ap, AEg, and E horizons are depleted in clays.
3.2.2. Thin section observationsThe material of the lowest Cg horizon is dominated by quartz
grains belonging to sand fraction. Glauconite pellets of green to
35 40 45 50
d: 2.00
d: 2.50d: 2.69
Mg-sat. air-dried
Mg-sat. glycerol - solvated
Mg-sat. air-dried
Mg-sat. glycerol - solvated
Ap
BCg
gl
glgl
d material of green pellets separated from the Cg, Bt2, Bt1 and the E horizons; gl —
218 M. Skiba et al. / Geoderma 235–236 (2014) 212–226
brownish-green color constitute b20% of the material (Fig. 3).Quartz grains and the pellets are embedded in clay matrix formingclay coatings and clay infillings (Fig. 4a, b). In the BCg, Bt2, and Bt1quartz sand grains also dominate. Clay matrix is much more abun-dant here when compared with those in Cg horizon. The coatingsand the infillings are composed of both yellowish and green tobrownish-green clay. The concentration of glauconite pellets inBCg, Bt2, and Bt1 horizons (Fig. 3) is most likely overestimated be-cause of the presence of the green and brownish-green clay matrix(glauconite micromass) (Fig. 4c, d). Some of the glauconite pelletsshow signs of disintegration.
In the E horizon quartz sand dominates, while the clay matrix is lessabundant when compared with those in the underneath horizons. Theconcentration of glauconite pellets is 12.3% (Fig. 3). The pellets aresmaller than the ones from the Cg, BCg, Bt2, and Bt1 horizons andsome of them are cracked and show rusty rims (Fig. 4e).
The material from AEg and Ap horizons is dominated by quartz finesand and silt with very little amount of clay fraction. Glauconite pellets
85
90
95
100
90
95
100
-3 -2 -1 0 1 2 3
85
90
95
100
90
95
100Fe3+ Fe2+
Transmission(%)
Velocity (mm/s)
AEg pellet
BCg pellet
AEg fine clay
BCg fine clay
Fig. 6. 57FeMössbauer spectra and their computerfittings collected for fine clays and glau-conite pellet samples from Cg and AEg horizons.
constituting ~10% of the material (Fig. 3) are smaller than the pelletsfrom the lower horizons (Fig. 4f).
3.2.3. Mineralogy of the green pelletsThe green pellets separated from all the horizons show very similar
XRD patterns (Fig. 5) with basal and higher order reflections from a ~10Å non-swelling phase. When examined in details the basal reflectionchanges the position from 10.3 Å in air-dry state to 10 Å after solvationthe mounts with glycerol. Also the intensity of the reflection slightlydecreases after solvation. The observed behavior indicates that the ana-lyzed material is glauconite containing small amount of smectite layers(i.e. R N 1 glauconite–smectite mixed-layered mineral) (Środoń, 2006).
3.2.4. Mössbauer spectroscopyThe spectra obtained were fitted with two to three quadrupole dou-
blets of Fe3+ and one doublet of Fe2+ (Fig. 6) (Table 4). The isomer shiftwas 0.31–0.68 mm/s for Fe3+ doublets and 1.19–1.22 mm/s for Fe2+
doublets while quadrupole splitting was 0.25–1.19 mm/s for Fe3+ and2.26–2.8 mm/s for Fe2+ doublets (Table 4). The ratios of Fe2+/Fe3+
Table 4Mössbauer spectroscopy parameters measured for the studied samples.
Sample A (%)a IS (mm/s)b QS (mm/s)c Iron valence state
PelletsAEg 57 0.33 0.4 Fe3+
23 0.28 0.9 Fe3+
14 0.63 0.7 Fe3+
6 1.22 2.3 Fe2+
BCg 55 0.32 0.4 Fe3+
24 0.27 0.9 Fe3+
14 0.63 0.7 Fe3+
7 1.22 2.3 Fe2+
Fine clays b 0.2 mmAp 68 0.34 0.5 Fe3+
16 0.29 0.9 Fe3+
13 0.59 0.6 Fe3+
3 1.19 2.5 Fe2+
AEg 68 0.34 0.4 Fe3+
20 0.3 0.9 Fe3+
9 0.67 0.6 Fe3+
3 1.19 2.5 Fe2+
E 61 0.35 0.4 Fe3+
27 0.31 0.8 Fe3+
10 0.68 0.6 Fe3+
2 1.2 2.8 Fe2+
E′ 42 0.37 0.3 Fe3+
46 0.35 0.7 Fe3+
10 0.39 1.2 Fe3+
2 1.2 2.7 Fe2+
Bt1 38 0.38 0.3 Fe3+
47 0.36 0.6 Fe3+
12 0.39 1.2 Fe3+
3 1.2 2.7 Fe2+
Bt1′ 38 0.38 0.3 Fe3+
46 0.36 0.6 Fe3+
13 0.39 1.2 Fe3+
3 1.2 2.7 Fe2+
Bt2 40 0.38 0.3 Fe3+
47 0.35 0.7 Fe3+
11 0.39 1.2 Fe3+
2 1.2 2.7 Fe2+
BCg 91 0.36 0.5 Fe3+
6 0.4 1.2 Fe3+
3 1.17 2.7 Fe2+
Cg 50 0.37 0.3 Fe3+
36 0.36 0.7 Fe3+
11 0.4 1.2 Fe3+
3 1.2 2.6 Fe2+
Errors for all values are of the order of unity for the last digit shown.a Contribution of the respective sub-profile to the total absorption profile equivalent to
content of Fe3+ and Fe2+.b Isomer shift versus room temperature α-Fe.c Quadrupole splitting.
219M. Skiba et al. / Geoderma 235–236 (2014) 212–226
were calculated from the measured integral intensities of the doublets,and are given in atomic percents (Table 4). In general the samplesstudied are dominated by Fe3+ which constitutes ~97% of the totalFe in fine clay fractions (b0.2 μm) and 93–94% of the total Fe in glauco-nite pellets.
3.2.5. XRD analysis of the oriented clay mountsA clay fraction separated from the parent material without chemical
pretreatments gave almost the same XRD pattern as the fraction sepa-rated after Na acetate–acetic acid buffer and NaCl treatment (Fig. 7).Both the bulk clay (b2 μm) and the fine clay (b0.2 μm) fractions gavesimilar results when analyzed by XRD method, however the bulk claycontained more non-clay minerals.
XRD patterns of clay fractions separated from Cg, BCg, and Bt2showed dominating basal (and higher order) reflections which origi-nate from a 10 Å non-swelling phase (similar to the one identified inthe green pellets) and lower angle reflections from swelling mixed-layered mineral. The mixed-layered mineral in Mg-saturated formgave d(001) values ~14 Å and ~18 Å in air-dried state (RH ~20%) andafter saturation with glycerol, respectively (Fig. 8). According to opera-tional definitions given by Środoń (2006) the ~10 Å reflection series in-dicates the presence of glauconite (with only minor amount of smectitelayers) while the ~14 Å vs. ~18 Å reflection can be assigned to the pres-ence of smectite-rich glauconite–smectite mixed-layered mineral orpure smectite. Weak ~7 Å and ~3.58 Å reflections disappearing afterheating at 550 °C indicate the presence of a trace amount of kaolinite.The amount of the swellingmixed-layered glauconite–smectitemineraldecreases up the profile, and only traces of the mineral are present inthe E horizon (Fig. 8). The clay fraction separated from the AEg horizoncontains glauconite and a swelling phase (also glauconite–smectitemixed-layered mineral), different (i.e. likely having different layercharge) from the swelling phase present in the parent material. Theswelling mineral from the AEg horizon in Mg-saturated form givesbasal reflections at ~14 Å and ~16.5 Å in the air-dried (RH ~20%) condi-tion and after glycerol saturation, respectively (Fig. 9). In addition, thisswelling phase does not collapse to 10 Å at room temperature when
5 10 15 20 25
d: 15
d: 10gl
gl
k
gl-sm
gl-sm
k
gl
d: 7.2
d: 5.0
d: 3.59
d: 3.33
separated without any chemicalpretreatment Ca - sat. RH 14%
treated with acetic buffer, washed with NaCl,
Fig. 7. XRD patterns of air-dry oriented mounts of the Ca-saturated b 0.2 μm fractions separatedsmectite mixed-layered mineral, k — kaolinite.
saturated with K+ while the glauconite–smectite mixed-layer mineralpresent in the parent material does (almost fully) (Fig. 10). In theclay fraction separated from the Ap horizon, in addition to the swelling14 Å phase, a 14 Å non-swelling phase (i.e. mica-vermiculite mixed-layered mineral) was identified (Fig. 9).
3.2.6. Quantitative Rietveld analysis of the fine clay fractionsThe fine clay fractions separated from the Cg, BCg, Bt2, and Bt1
contained ~80% of glauconite, 15–20% of smectite rich glauconite–smectite mixed-layered mineral, and b2% of kaolinite (Table 5). In thefraction separated from BCg ~1% of quartz was identified. Fine claysfrom the E horizon was slightly depleted in the glauconite–smectiteand enriched in quartz when compared with the clays from lower hori-zons. While the fine clays from AEg and Ap horizons were enriched inswelling phases and quartz and depleted in glauconite when comparedwith the lower horizons. In the 060 region of all the XRD patterns tworeflections at 1.514 Å and at 1.500 Å were observed (Fig. 11). Accordingto the obtained results the 1.514 Å reflection can be assigned to 1 Mglauconite, while the 1.500 Å reflection belongs to smectite-richglauconite–smectite and/or mica-vermiculite. This indicates that theglauconite-smectite minerals present in fine clays are likely enrichedin Al relative to glauconite.
3.2.7. FTIRS analysis of the fine clay fractionsDecomposition of the OH-stretching region of the FTIR spectra collect-
ed for the clay fractions separated from the Cg, BCg, Bt2, Bt1, and E hori-zons gave eight bands. Selected decomposed and fitted FTIR spectra aregiven in Fig. 12. The positions of the individual OH-band assigned to spe-cific cation pair types using the data presented by Besson and Drits(1997), Russell and Fraser (1994), and Zviagina et al. (2004) are givenin Table 6. The simplified relative integrated intensities of the OH bandsare presented in Table 7. The bands from AlMgOH at ~3604 cm−1,MgMgOH at ~3584 cm−1, Fe3+ MgOH at ~3562 cm−1, Fe3+Fe3+OH at3538 cm−1, and Fe2+Fe2+OHat 3520 cm−1 can be regarded as indicativefor the presence of glauconite. The band at ~3700 cm−1 arisingmost like-ly from internal surface OH groups is indicative for kaolinite. Relatively
30 35 40 45 50
glgl
d: 2.51d: 1.998
and Ca - sat. RH 23.2 %
from BCg horizon treated using different protocols; gl— glauconite, gl–sm— glauconite–
5 10 15 20 25 30 35 40 45 50
d: 18.1d: 14.5
d: 10.2
d: 7.23d: 4.98
d: 3.34
d: 3.58 d: 2.51d: 2.00
E
Bt1
Bt2
Cg
Mg-sat. air-dried
Mg-sat. glycerol - solvated
Mg-sat. air-dried
Mg-sat. glycerol - solvated
Mg-sat. air-dried
Mg-sat. glycerol - solvated
Mg-sat. air-dried
Mg-sat. glycerol - solvated
gl
gl
gl
gl, kgl
k k
gl-sm
gl-sm
Fig. 8. XRD patterns of air-dry and glycerol-solvated, orientedmounts of theMg-saturated b 0.2 μm fractions separated from the Cg, Bt2, Bt1 and the E horizons; gl— glauconite, gl–sm—
glauconite–smectite mixed-layered mineral, k — kaolinite.
220 M. Skiba et al. / Geoderma 235–236 (2014) 212–226
high intensity of the 3622 cm−1 band assigned to AlAlOH within 2:1phyllosilicates may also suggest the presence of aluminous dioctahedralminerals in the samples studied.
In the FTIR spectra of the clay samples separated from the AEg andAp horizons, appearance of additional AlAlOH bands at 3644 cm−1
and an increase in the intensity of AlAlOH bands at ~3700 cm−1 and3622 cm−1 relative to bands AlMgOH at ~3604 cm−1, MgMgOH at~3584 cm−1, Fe3+MgOH at ~3562 cm−1, Fe3+Fe3+OH at 3538 cm−1,and Fe2+Fe2+OH at 3520 cm−1 were observed indicating relativeenrichment in aluminous minerals (Fig. 13).
d: 16.9
d: 14.2
d: 10.0
d: 7.16d: 4.97
d: 4.25d: 3.58
d: 3.34
d: 2.72
5 10 15 20 25 30 35 40 45 50
d: 2.00d: 2.51
Mg-sat. air-dried
Mg-sat. glycerol - solvated
Mg-sat. air-dried
Mg-sat. glycerol - solvated
Ap
AEg
gl
v
gl
q, gl
qSi
glkk
gl-sm
gl-sm
Fig. 9.XRDpatterns of air-dry and glycerol-solvated, orientedmounts of theMg-saturated b 0.2 μmfractions separated from theAEg andAphorizons; gl— glauconite, gl–sm— glauconite–smectite mixed-layered mineral, k — kaolinite, q — quartz, Si— silicon, v — vermiculite.
221M. Skiba et al. / Geoderma 235–236 (2014) 212–226
The ratio of the integrated intensity of the Fe3+Fe3+OH band to theintegrated intensity of the Fe2+Fe3+OH band for the fine clay fractionsranges from 2.4 to 2.7. This indicates that there are likely not much dif-ferences in the Fe2+ content between fine clay fractions separated fromthe different soil horizons.
3.2.8. Chemistry of the fine clay fractions and the green pelletsThe fine clay fractions separated from Cg, BCg, Bt2, Bt1, and E hori-
zons show quite a similar chemical composition (Table 8), while thefine clay fromAEg horizon is slightly enriched in Al and Ca and depletedin K in relation to the lower horizons. Fine clay from Ap horizon is de-pleted in K, Mg, and Fe and enriched in Al and Ca in relation to theclays from lower part of the profile. Green pellets separated from allthe soil horizons show more or less similar chemical composition(Table 8) except for samples from Cg and Ap horizons. The samplefrom Cg horizon is enriched in Fe and slightly depleted in Mg, Al, Si,and K in relation to the samples from other horizons. Sample from Aphorizon is enriched in Ti when compared with the other samples. Ingeneral, the fine clays are enriched in Al, Si and Ca and depleted inMg, K, and Fe in relation to the green pellets.
4. Interpretation and discussion
4.1. Influence of chemical pretreatments on the structure of glauconite
The fact that a clay fraction separated from the parentmaterialwith-out chemical pretreatments gave similar XRD pattern as the fractionseparated after chemical pretreatments used in the present study indi-cates that the pretreatment had no or only little effect on the structureof glauconite. Thus the data presented in this study are meaningfuland can be used for evaluation of the mechanism of the soil weatheringof glauconite. This finding is in contradiction with the results presentedby El-Amamy et al. (1982), who stated that glauconite is very unstableand loses the interlayered potassium, transforming into a smectite-like phase when equilibrated for 70 h in dilute suspension with 1 MNaCl solution.
4.2. Texture
The observed difference in texture between the upper and the mid-dle parts of the profile studied is likely to be the effect of translocation ofclay fraction down the profile (lessivage), however it may also suggest
5 10 15 20 25 30 35
d: 10.1
d: 10.3
d: 3.33
d: 3.32
d: 7.2d: 5.03
d: 3.58d: 2.51
AEg
Cg
Fig. 10. XRD patterns of air-dry oriented mounts of the K-saturated b 0.2 μm fractions separated from the Cg and AEg horizons.
222 M. Skiba et al. / Geoderma 235–236 (2014) 212–226
contamination of the upper horizons (Ap, AEg)with glaciofluvial and/oraeolianmaterial. Aeolian additions are common in coarse-textured soilsdeveloped in the areas where loess deposition took place (e.g. Schaetzland Luehmann, 2013). The lessivage is indicated by the fact that BCg,Bt2, and Bt1 horizons are enriched in clay fraction, while the E, AEgand Ap horizons are depleted in clay fraction relative to Cg horizon.The presence of clay coatings and clay infillings in the Cg, BCg, Bt1,and Bt2 horizons also suggests the lessivage. According to e.g. Kühnet al. (2010) both clay coatings and clay infillings are formed in soilenvironment due to translocation of clay particles e.g. by percolatingatmospheric water. Orientation observed within the structures is aresult of clay flow through soil pores.
The cracking and disintegration of glauconite pellets observed inBCg, Bt, E, AEg and Ap horizons and the presence of the brownish rimsin the pellets indicate that the clay matrix present in Cg, BCg, Bt1, Bt2,and E horizons likely originates from disintegration and weathering ofglauconite pellets. In other words, the disintegration and the chemicalweathering (indicated by the presence of brownish rims) most likelylead to the formation of green and yellowish green to yellowishbrown micromass. This finding is in a good agreement with data pre-sented in previous studies (Courbe et al., 1981; Loveland, 1981)reporting disintegration of glauconite pellets in soil environment. The
Table 5Quantitative mineral compositions given in wt.% of the fine clays, based on the XRD data.
Ap 55.5 1.4 3.2 32.5 7.4AEg 70.2 2.3 ni 20.3 7.2E 79.6 1.6 ni 15.3 3.5E′ 80.4 1.2 ni 13.4 4.9Bt1 82.6 1.8 ni 15.6 niBt1′ 81.1 1.7 ni 17.2 niBt2 83.2 1.2 ni 15.5 niBCg 77.1 1.8 ni 19.7 1.4Cg 82.1 0.3 ni 17.6 ni
ni— not identified.
observed disintegration of glauconite pellets leading to formation ofsignificant amounts of fine fractions appears to be an important phe-nomenon taking place during the weathering of glauconite.
The enrichment of the Ap horizon in quartz silt relative to the lowerpart of the profile together with the presence of ~1% of albite, which oc-curs only in this horizon, suggests the admixture of extraneousmaterial.An admixture of Pleistocene glaciofluvial or aeolian material or a pres-ent day air-borne dust has to be considered. Pleistocene aeolian deposi-tion leading to the formation of thick (up to tens of meters) loess covershas been commonly described from the study area (e.g. Baraniecka,1984). According to Manecki et al. (1978) and Šucha et al. (2001), alsoa present day aeolian deposition seems to be commonly occurring phe-nomenon in this part of Europe, however noticeable deposition eventsoccur only sporadically. The influence of the process onmineral compo-sition of soils is neglected in most of the mineralogical studies despitethe fact that the deposition of air-borne dust may significantly affectmineral composition of the upper soil horizons (e.g. Šucha et al.,2001). The presence of Scandinavian rock pebbles observed within theAp horizon indicates the likely glaciofluvial origin of the albite.
4.3. Mechanism of glauconite weathering
The occurrence of smectite-rich glauconite–smectite mixed-layeredminerals in the soil clay fractions and the lack of the minerals in theseparated green pellets may suggest that glauconite–smectite rich insmectite layers is a product of glauconite weathering. One may arguethat the presence of the glauconite–smectite minerals in Cg horizoncould suggest the primary character of the smectite-richminerals. How-ever, the primary glauconitic sand is composed of rounded quartz sandmainly with admixture of glauconite pellets and rounded quartz gravelgrains. This suggests that the sand was deposited in highly energeticsedimentary environment where deposition of clay fraction would beunlikely. Aswas shown, glauconite–smectite minerals in the Cg horizonoccur only in clay fraction and do not occur in green pellets. At the sametime, the clay fraction occurring in the Cg horizon forms well orientedclay coatings and clay infillings. Taking those into account it can be
60 62
Ap
Cg
BCg
B2t
B1t
E
AEg
d: 1.514
d: 1.542q
d: 1.500
Fig. 11. XRD patterns of air-dry random mounts of the Mg-saturated b 0.2 μm fractionsseparated from the soil studied.
Table 6Positions of OH-stretching bands (cm−1) assigned to specific OH-bonded cation pairs.
223M. Skiba et al. / Geoderma 235–236 (2014) 212–226
concluded that the primary material, before the soil formation hasstarted, contained glauconite and did not contain glauconite–smectiteminerals. The glauconite–smectite minerals present in the Cg horizonwere likely formed somewhere above and deposited as clay matrix bypercolating atmospheric water. A part of clay fraction present in the
AEg
3000 3500 4000
Relativeabsorbance
Wavenumber (cm )-1
37013668
36443622
3604
3585
3562
3538
3520
water + organics
Fig. 12. Decomposition of the FTIR spectra of the Ca-saturated b
Cg horizon could have potentially been formed in situ by weatheringof the glauconite pellets, however not much signs of the pelletweathering were observed in thin sections prepared from soil materialfrom the Cg horizon. Because glauconite pellets from the BCg, Bt2, Bt1and E horizons show brownish rims indicating weathering, it is likelythat glauconite–smectite minerals present in these horizons were, atleast partially formed in situ. Enrichment in clay observed in BCg, Bt2,and Bt1 horizons relative to E, AEg, and Ap horizons and the presenceof clay coatings and clay infillings commonly occurring in BCg, Bt2,and Bt1, horizons indicate that a vast amount of clay present in the ho-rizons comes from the upper part of the profile. The lack of clay coatingsand numerous brownish rims in glauconite pellets fromAEg and Ap ho-rizons may suggest that a significant part of clay fraction present inthese horizons was formed in situ by glauconite weathering.
The difference in swelling properties and the degree of collapse aftersaturation with K+ between glauconite–smectite minerals present inAEg horizon and the glauconite–smectite from the lower part of the pro-file may suggest that the minerals represent different stages of glauco-nite weathering. The glauconite–smectite minerals present in Cg, BCg,Bt2, Bt1, and E horizons are likely to be a product of less advancedweathering of primary glauconite, while the glauconite–smectite fromthe AEg horizon is likely to be formed due to more advanced transfor-mation of the glauconite.
One of the mechanisms likely involved in the formation of smectiteat the expense of glauconite is oxidation of Fe2+ present in the glauco-nite octahedral layer causing layer charge reduction. This statement issupported by the Mössbauer spectroscopy data showing depletion inFe2+ relative to Fe2+ observed for fine clays in comparisonwith glauco-nite pellets. Assuming that concentration of Fe2+ in primary glauconitepellets from all the horizons is similar and constitutes ~7% of total Fe,the approximated chemical formula of the glauconite, calculated ac-cording to Moore and Reynolds (1997) is: K0.63Ca0.04Fe3+1.03Al0.47Ti0.03Mg0.37Fe2+0.08Al0.29Si3.71O10(OH)2 which is in a good agreement with
Cg
3000 3500 4000
Relativeabsorbance
Wavenumber (cm )-1
3604
3705
3653
3622
3583
3562
3538
3520water
0.2 μm fractions separated from the AEg and Cg horizons.
Table 7Relative integrated intensities of OH-stretching bands given as percent of the total OH-stretching region area.
224 M. Skiba et al. / Geoderma 235–236 (2014) 212–226
the data obtained by Kotlicki et al. (1981). The formula was calculatedwithout taking into account the results obtained for pellets from Apand Cg horizons, because those appeared to be significantly contaminat-edwith Timinerals and amorphous Fe oxyhydroxides, respectively. Theformula indicates that Fe2+ oxidation may account at best for layercharge reduction as low as 0.08 per half unit cell which would not beenough to transform the primary glauconite into smectite. As wasshown in Table 8 fine clays are depleted inMg and Fe relative to glauco-nite pellets and the depletion is higher in the samples with higherconcentration of glauconite–smectite minerals. This indicates that glau-conite–smectite minerals formed at the expense of primary glauconitecontain less Mg and Fe than the glauconite. Aluminous character ofthe glauconite–smectite minerals is also indicated by the position ofthe (060) XRD reflection at 1.500 Å. Also the increase in the intensityof AlAlOH absorption band at ~3620 cm−1 observed in FTIRS spectraof the fine clay showing higher concentration of glauconite–smectite
3450 3500 3550 3600
Relativeabsorbance
Wavenum
3604
36
35623538
Fig. 13. FTIR spectra of the Ca-saturated b 0.2 μm fractions sep
minerals relative to the spectra of the clays with lower concentrationof the glauconite–smectite supports this interpretation. The aluminouscharacter of glauconite–smectite minerals formed at the expense of Feand Mg-rich glauconite strongly suggests that selective leaching of Mgand Fe plays a crucial role in glauconiteweathering leading to formationof smectite. From the above data one cannot conclude what the exactmechanism of the glauconite smectitization is but solid-state transfor-mation involving selective leaching and likely reorganization of octahe-dral sheet and/or dissolution-precipitation is the most probable.
The origin of vermiculite (the 14 Å phase) present in the Ap horizonis not clear, however glaciofluvial or aeolian origin of themineral seemsto be likely. The presence of albite in the uppermost horizon, whichmost likely is of glaciofluvial or aeolian origin, suggests the externalorigin of other constituents such as vermiculite. The formation ofvermiculite at the expense of glauconite taking part in the Ap horizonis unlikely because weathering in Ap horizon is more intensive than in
3650 3700 3750 3800
ber (cm )-1
Cg
3705
22
BCg
Bt2
Bt1
E
AEgAp
arated from the individual horizons of the profile studied.
Table 8Chemical composition of glauconite pellets and b0.2 mm fractions.
Sample Fraction MgK* AlK* SiK* KK* CaK* TiK* FeK*
Ap Pellets 2.00 5.37 24.74 5.45 0.58 1.77 16.29b0.2 mm 1.33 9.68 27.90 3.07 1.34 0.27 9.62
AEg Pellets 2.21 5.54 25.79 5.97 0.38 0.58 15.40b0.2 mm 1.99 7.19 27.25 3.78 1.07 1.07 13.03
225M. Skiba et al. / Geoderma 235–236 (2014) 212–226
AEg horizon where glauconite transforms directly (i.e. without forma-tion of vermiculite) into glauconite–smectite. Potential formation ofvermiculite-like phase by deposition of hydroxy-interlayers withinsmectite interlayer spaces preventing the structure from swellingmight take place in the uppermost horizon. According to Bain et al.(1990) and Skiba (2007), formation of hydroxy-interlayers is pH-dependent and takes place when pH is higher than 4.4. The fact thatthe non-swelling 14 Å phase is present only in the uppermost Ap hori-zon and not in the AEg horizon having very similar pH does not supportthe possibility of the hydroxy-interlayer formation. The presence of the14 Å nonswelling phase in the Ap horizon enriched in organic matter(Table 2) may also be explained by the intercalation of organic materialwithin smectite interlayers. The process of organic intercalation waspreviously documented for acidic soils by Bain and Fraser (1994),Righi et al. (1995), and Skiba et al. (2011).
The lack of kaolinite in the primary glauconite pellets and its pres-ence in clay fractions separated from all the soil horizons indicates themineral formation within the profile. This statement is also supportedby the fact that the amount of kaolinite is slightly larger in ratio to theother clay minerals in the upper AEg and Ap horizons. As indicated bythe enrichment in glauconite–smectite minerals relative to glauconitein those horizons (the AEg and Ap) the weathering of glauconite ap-pears to be more intensive than in the other parts of the profile. Thissuggests that theweathering origin of kaolinite is likely. The enrichmentin kaolinite observed for the uppermost horizon may also be caused byadmixture of the mineral due to air-borne dust deposition. Kaolinitewas identified in air-borne dusts deposited in this part of Europe(e.g. Manecki et al., 1978; Šucha et al., 2001).
The data presented above indicated that in the profile studied glau-conite weathers into glauconite–smectite mixed-layer minerals rich insmectite layers. In addition, some evidence for kaolinite formation hasbeen observed in the present study. Those findings are in a good agree-mentwith the data presented by Courbe et al. (1981), Van Ranst and DeConinck (1983), and Wolff (1967) who reported formation of mica–smectite and kaolinite at the expense of glauconite. The lack ofglauconite-derived vermiculite and/or glauconite–vermiculite and/orglauconite–vermiculite–smectite mixed-layered minerals in the soilstudied may suggest that the glauconite smectitization is a one stepreaction leading to development of glauconite–smectitewithout forma-tion of transitional vermiculite phase. Similar results obtained byCourbe et al. (1981), Loveland (1981), Pestitschek et al. (2012), VanRanst and De Coninck (1983), and Wolff (1967) may suggest that inthe course of weathering glauconite undergoes direct smectitization,while the data presented by Hutcheson and Haney (1963), Nash et al.(1988), and Wurman (1960) indicate that weathering smectitizationof glauconite occurs via transitional vermiculite phase. The fact that
formation of glauconite–smectite at the expense of glauconite appearsto be themain weathering process in the soil studied is in contradictionwith the conclusion presented by Velde (2003), who stated that ingeneral alteration of glauconite present in sandstones is made in onestep with the formation of kaolinite and iron oxides. According toCourbe et al. (1981), Velde (2003), and Wolff (1967) formation of goe-thite or other unspecified iron hydroxides at the expense of glauconiteoccurs during glauconite weathering. According to the results of XRDanalyses presented in this study neither formation of “well crystallized”(i.e. giving recognizable diffraction pattern) iron hydroxides nor signif-icant portion of poorly crystallized/amorphous oxideswas observed as aresult of glauconite weathering. However, formation of small portion ofiron hydroxides in the soil studied would be expected as indicated bythe observation of rusty material under a polarizing microscope andrusty mottles in some horizons of the studied soil profile.
5. Conclusions
1. Chemical pretreatment used in the present study had no or only littleeffect on the structure of the primary glauconite.
2. Disintegration of glauconite pellets leading to the formation of clay-sized particles is an important phenomenon taking place in thecourse of glauconiteweathering. Texture and distribution ofmineralswithin the profile is most likely controlled by weathering of glauco-nite, lessivage, and, in the case of the Ap horizon, likely admixtureof aeolian and/or glaciofluvial material.
3. Smectite-rich glauconite–smectite mixed-layered minerals presentin the profile studied are formed at the expense of primary glauco-nite. The exact mechanism of the glauconite smectitization remainsobscure; however solid-state transformation involving the selectiveleaching of octahedral Mg and Fe and likely reorganization of thestructure is the most probable.
4. Kaolinite present in the soil studied is likely a product of glauconiteweathering, however aeolian origin of the mineral also has to beconsidered.
Acknowledgments
This study was supported by the Institute of Geological Sciences,Jagiellonian University (DS 811). The authors thank Edward E. Nater,Arek Derkowski, Marion Wampler and the two anonymous reviewersfor critical reading of the manuscript and for the constructive sugges-tions and comments.
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