Morphological characterization of soil clay fraction in nanometric scale

Powder Technology 241 (2013) 36–42

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Morphological characterization of soil clay fraction in nanometric scale Nivea M.P. Dias ⁎, Daniele Gonçalves, Wellington C. Leite, André M. Brinatti, Sérgio C. Saab, Luiz F. Pires Laboratory of Soil Physics and Environmental Sciences, Department of Physics, State University of Ponta Grossa (UEPG), Av. Carlos Cavalcanti, 4748, CEP 84.030-900, Ponta Grossa, PR, Brazil

a r t i c l e

i n f o

Article history: Received 20 July 2012 Received in revised form 24 February 2013 Accepted 2 March 2013 Available online 13 March 2013 Keywords: AFM Nanometric scale Image analysis XRD and RM

a b s t r a c t The atomic force microscopy (AFM) is a technique for direct three dimensional measurements of the mineral structure in nanometric scale. In the literature, there are studies which approach the characterization of surfaces in atomic scale through AFM, such as humic substances and minerals. However, the number of studies aiming to characterize the clay fraction minerals in soil using this technique is not representative. In this study, AFM was employed to characterize the clay fraction morphology and microtopography in the surface layer of a Rhodic Ferralsol in Brazil, and X-ray diffraction (XRD) together with the Rietveld Method (RM) to characterize and quantify the main minerals. Images analyzed presented particles from 3 to 25 nm. Through XRD and RM mineralogical analysis, the minerals found in higher amounts from the most to the least were, gibbsite, kaolinite, hematite, anatase, goethite, magnetite, calcite, vermiculite and rutile. AFM images made it possible to identify, by observing the height and shape, the particles that corresponded to kaolinite, goethite and gibbsite. This study shows the potential of the AFM technique to measure clay in nanometric scale and the possible identification of minerals present in the clay fraction with the use of XRD and RM. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The soil clay fraction is mainly constituted by minerals and organic matter, with particles of ≤ 2 μm diameter and the knowledge of its physical, chemical, mineralogical and morphological composition is essential to comprehend important processes that occur within this fraction. Examples of such processes are dissolution, precipitation, polymerization, adsorption/desorption, oxidation and reduction of chemical elements which are essential to plants, radionuclides, potentially toxic elements and pesticides. Certain techniques such as scanning electronic microscopy (SEM), transmission electronic microscopy (TEM) and atomic force microscopy (AFM) are used to analyze particles as the clay fraction in nanometric scale. While commercial electronic microscopes can provide a magnification of up to some hundreds of thousand times, AFM can obtain images with several tens of million times, with the advantage of presenting the same resolution at three dimensions (3D), which does not occur with the former equipment. Besides that, the high energy of the scanning microscope electron beam damages polymeric and organic samples limiting the use of this equipment, in practice, for increasing below 50,000 times for most of the materials analyzed. AFM employs a probe of atomic dimensions placed through a microcantilever which tracks the surface of the sample [1]. The basic principle of AFM functioning is the detection of interaction between the probe and the surface of a sample. The uses of such ⁎ Corresponding author. Tel.: +55 42 3220 3044; fax: +55 42 3220 3042. E-mail address: [email protected] (N.M.P. Dias). 0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.03.006

technique make it possible to obtain information about the shape, morphology and size of the clay particles. There are several studies in the literature that approach the characterization of surfaces in atomic scale through AFM, as river humic substances [2] and minerals such as illite and smectite [3], kaolinite [4–7], gibbsite [8], montmorillonite [9] and goethite [10,11]. However, the number of studies aiming to characterize the soil clay fraction minerals using this technique is still very low. Examples of studies using AFM technique to characterize the clay fraction are that of Vaz et al. [12] and Gélinas and Vidal [13]. In the former, the procedures to quantify shape, size and height of particles were carried out manually, while in the latter the authors have developed a direct imaging method for measuring particle shape distributions of 10 different clay pigments from Brazil and the United States. The automated method used by Gélinas and Vidal [13] allowed routine processing of images and consistent analysis of pigment shape in a short time. However, the present study proposes an alternative way to carry out the characterization of the clay fraction morphology and microtopography through the use of image analysis program and also presents the use of other analytical techniques to identify the minerals present in this soil fraction. Examples of analytical techniques which can be used to characterize minerals in the soil clay fraction are the spectroscopic techniques such as X-ray fluorescence (XRF) and Fourier transform infrared (IR) [15]. While XRF permits qualitative determination of elements present in the sample through measurements of their emitted characteristic wavelength [14], IR indicates the minerals present in the sample through the absorption bands related to the constituent element bonded to these minerals. However, X-ray diffraction (XRD) is the

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most commonly employed technique to qualitatively and quantitatively characterize the soil mineral composition, and more recently, together with the Rietveld Method (RM) [16–19] provides the fastest shape mineral quantification when compared to the conventional methods. In the present study, AFM technique was employed to characterize the morphology and microtopography of the main minerals present in the clay fraction from the superficial layer (0–0.20 m) of a soil in the South of Brazil. In the characterization, image analysis programs, XRF, IR spectroscopic techniques, and XRD were used together with RM to determine and quantify the main minerals identified. 2. Material and methods 2.1. Soil description The soil analyzed was collected at the surface layer (0–0.20 m) of a Rhodic Ferralsol according to FAO classification [20] in the South of Brazil (25° 13′ S, 50° 01′ W). In order to obtain the clay fraction, a 20 g soil sample was placed in an erlenmayer flask containing 200 mL of deionized water and 10 mL of NaOH — 1 mol/L for particle disaggregation. This suspension was manually shaken and put to rest for 24 h. Then, the sample in suspension was taken to an ultrasonic washer for 20 min to disaggregate the particles. After this time, the sample was sieved in a 53 μm (270 mesh) sieve with deionized water to separate the sand fraction from the remaining ones. The clay fraction was separated from the silt fraction through the particle sedimentation process using Stokes law. Once the clay fraction particles were aggregated after dried, this fraction was powdered and sieved in a 53 μm sieve to guarantee homogenization of sample particle size or aggregates for all analyses. 2.2. Characterization of clay fraction morphology and microtopography through AFM When preparing the samples for AFM analysis, 10 mg of clay were diluted in 1 L of deionized water. 1 cm 2 of mica surface was cleaved and introduced into a beaker containing 100 mL of the clay solution. After being shaken for 24 h, the sample was removed, washed with deionized water, placed on Petri dishes, dried in dissector for 24 h and analyzed through AFM. Three samples were prepared using the procedure described above. For each sample, images were produced in two different positions. An atomic force microscope, Shimadzu model SPM 9600, was used to obtain the images. The dynamic mode and “Silicon SPM — sensor” with constant 42 N m −1 power and resonance frequency of 285 kHz were employed. The clay sample scanning area was 5 × 5 μm. From each image, topography (2D and 3D) and phase were obtained. After producing the images in the dynamic mode, the “flattening” command in the “SPM 9600” program, which is part of the AFM package, was applied. The objective of this command was to correct artifacts occurred throughout the scanning for later “thresholding” procedures, carried out with the “Gwyddion v.2.19” program. The “thresholding” procedure can be carried out in three different ways: 1) height (“height” command), 2) slope (“slope” command), and 3) curvature (“curvature” command). This procedure was chosen according to the image quality and kind of sample analyzed using the “height” command. After the “thresholding”, the images were submitted to morphological analysis through the free program “Image J v.1.38x”. At this stage, the objective was to study the difference in object concentration (particles and/or particle agglomerates) in the image, evaluate the shape of such objects (circularity), area and perimeter. The first phase in this procedure is to carry out the image binarization using the “make binary” command in the “Image J”. Regarding the images

37

in this study, the particles were represented by the color black (1) and the sample void areas by the color white (0). After binarization, the samples were submitted for particle analysis. In this procedure, the “analyze particles” command was used and selected as output result of area, perimeter and circularity of particles. In order to assess each sample particle concentration the command “measure” was employed. The table with particle analysis results was used to construct the histograms using graphic programs. The height was determined by the “Gwyddion v.2.19” program through the command “extract profiles” and the determination of roughness was carried out through the surface analysis procedure available in the “SPM 9600” program. 2.3. XRF, IR and XRD-RM analyses The equipment used in the XRF analyses was a Philips X-ray fluorescence spectrometer, model XRF-PW2400. Pellets were prepared by using 2 g of dry samples. The analysis protocol, including statistical treatment, was based on the method described by Mori et al. [21]. The IR analysis was carried out with samples prepared with 1 mg of soil clay fraction sample and 100 mg of dry KBr. The sample and KBr mixture were homogenized in agate mortar. Spectra were obtained, in absorbance mode with 4 cm −1 resolution, from 16 scanning procedures at the interval of 4000 to 400 cm −1. The equipment used was a Shimadzu Fourier transform infrared spectrophotometer, model FTIR-8400. In the X-ray diffraction, an automatic horizontal X-ray diffractometer (Rigaku model Ru-200B) with a rotating anode generator (Rigaku Rotaflex — 12 kW) and goniometer (Rint 2000 Wide Angle Goniometer), with CuKα radiation, operating at 50 kV and 100 mA was employed. At aiming by using the RM, the XRD data was collected in the step by step mode at 5° ≤ 2θ ≤ 70° extension, step length of 0.02° in 2θ and counting time of 2 s per step, with divergence slit: 1.00°; scattering slit: 1.00°; and reception slit: 0.30 mm. Each sample was placed and slightly pressed in the standard powder front-loading sample holder. In order to identify diffraction peaks, the three most intense XRD peaks of each mineral present in the sample were used, whenever possible, based on the information contained in the Mincryst and/or Mineralogy databases [22,23]. For adjustments of diffraction pattern profile and refinement of each mineral crystalline structure in the sample under study, with XRD data, RM [16–18] was performed with the "DBWS-9807a" [24]. Crystallographic data of each mineral identified was considered according to the following literature: quartz; gibbsite; anatase; rutile; hematite; goethite; magnetite; kaolinite; halloysite; calcite; vermiculite [25–35]. Pseudo-Voigt function was used to model the profile shape. RM was performed with the following parameters adjusted: six terms of background function; 2θ–zero; sample displacement; scale factors; cell parameters; U, V, W parameters of the FWHM function; asymmetry; atomic displacement factors; NA and NB parameters, both referring to the η parameter of the pseudo-Voigt function; and for some cases, the preferential orientation [18,24]. The results were statistically treated as reported by Young [18] and implemented in the program "DBWS-9807a" [24]. 3. Results and discussion 3.1. Characterization of clay fraction morphology and microtopography through AFM The clay fraction images with 5 × 5 μm scanning area obtained through AFM in the dynamic mode after performing the flattening, thresholding, binarization, phase image, 3D image and height graph commands, are shown in Fig. 1.

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N.M.P. Dias et al. / Powder Technology 241 (2013) 36–42

a

b

c

d

e

f

Fig. 1. Clay fraction images with scanning area of 5 × 5 μm. (a) Height image obtained by AFM after the application of the flattening command by the program SPM 9600. (b) Image after the application of the thresholding command by the program Gwyddion. (c) Image after the application of the binarization command by the program Image J. (d) Phase image. (e) 3D image. (f) Graphic of height variation for transects 1, 2, and 3 presented in a.

In Fig. 1a, the lighter colors on the surface analyzed through AFM represent the higher heights and the dark colors the lower ones. It is qualitatively possible to verify, through the images, that the individual samples, regardless their size, present regular and irregular shapes (Fig. 1a–c). Such difference in shape indicates different mineral types in the clay fraction. The height graph (Fig. 1f) shows height values of the particles scanned along the three transects selected in the original image (Fig. 1a). It is possible to verify in the three transects selected that there is no homogeneity of height values. The height graph is also in accordance with the distribution of colors in the AFM image, that is, the lighter the colors are, the higher the height is, while the darker the colors are the lower the height is. It is possible to see through the graph that the particles presented heights which varied from 3 to 25 nm. The 3D graph (Fig. 1e) also helps to observe the variations

in particle height of the soil clay fraction analyzed and the heterogeneity of particle distribution. Images similar to the ones obtained in the present study were observed by Vaz et al. [12] in the clay fraction of an Oxisol in the Southeast of Brazil. These authors found heterogeneous distribution of clay particles on mica, and the smaller particles presented 14 nm height and the larger ones 124 nm. These height values were higher than the ones obtained in this study (Fig. 1f). This difference in height can be explained by the higher or lower concentration of the clay fraction during the sample preparation, and also because the sample came from an Oxisol collected in another region of Brazil, with different mineralogical characteristics from the soil investigated in this study. The soils located in the South of Brazil are in a sub-tropical region with climate conditions different from the rest of the country.

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Results from the particle analysis (morphological), which present the parameter area, circularity and perimeter of the clay fraction particles were obtained through the ‘Image J’ program and are shown in Fig. 2. This analysis was carried out for the sample analyzed in Fig. 1. Regarding particle area, it was possible to verify that most of them (81%) have areas smaller than 30,000 nm 2 and that the remaining 19% have areas situated in the interval from 30,000 to 170,000 nm 2 (Fig. 2a). This demonstrates that this sample is characterized by great number of particles with smaller sizes, uniformly distributed and approximately of the same area. In relation to circularity (Fig. 2b) most of the particles (73%) present circularity between 0.8 and 1.0. From these, 25% present circularity of 0.9 and 23% of 1.0, that is, perfectly circular. This information shows that the particles in general present more regular shape, that is, less complex. And that occurs both for the smaller particles and the larger ones. Other circularity values (27%) are related to the irregular particles. The highest frequency for irregular shaped particles occurs for the circularity values of 0.6–0.7. When comparing information about the particle area and circularity parameter to the quantitative analysis, the results of the image visual analyses are confirmed, that is, few large particles with circular shape or almost circular and a large number of small particles with this shape. When analyzing the soil image, 204 particles were identified, revealing the potential of the image analysis programs for the characterization of the clay fraction in nanometric scale via AFM. Regarding the particle perimeter (Fig. 2c), it was possible to see similar result to the one obtained for the area, which was expected, as these two parameters are related. Most of the particles (84%) presented perimeters smaller than 1000 nm and the remaining (16%) perimeters situated between 1000 and 3250 nm. For this soil, regarding the concentration used in the sample preparation, it was possible to verify the decrease in the frequency of particles as a function of area and perimeter (Fig. 2a and c) which might be adjusted from, for example, an exponential model. It is expected that an analysis in similar soil samples might present the same kind of behavior. Roughness results determined through the clay fraction sample surface analysis (Fig. 1a), through the SPM program, were Rq = 3.42 nm; Rp = 35.81 nm and Rv = 2.28 nm. Where Rq is the root mean-square roughness; Rp is the profile maximum height above the mean line within a sampling length; Rv is the maximum depth below the mean line within the sampling length.

4

5

5

5

5.0x10 1.0x10 1.5x10 2.0x10

b 99.9

0.2

0.4

Table 1 XRF data of the clay fraction studied. Al2O3

SiO2

Fe2O3

4

5

5

5

2

A (nm )

K2O

Na2O

0.25 0.19 0.02

1.48 15.70 4.30 1.20 0.65 11.22 3.57 0.89 0.03 0.10 0.10 0.20

DL — detection limit; LOI — loss on ignition (LOI: 246.20 g kg−1).

These analyses of the clay fraction particles based on the morphology of samples can be carried out in any kind of soil sample when studies in nanometric scale are relevant. Results obtained in this study show that the use of AFM technique together with image analysis programs can be a powerful tool to characterize the soil clay fraction.

3.2. XRF, IR, XRD-RM The sample oxide contents obtained through XRF are presented in Table 1. As it can be seen, the highest oxide contents are that of Al, Si, Fe, Ca, Ti and K. These are in accordance with the following ideal stoichiometry of minerals: quartz (SiO2), 100.00% (SiO2); gibbsite [Al(OH)3], 65.36% (Al2O3); rutile (TiO2), 100.00% (TiO2); hematite (Fe2O3), 100.00% (Fe2O3); goethite [FeO(OH)], 89.86% (Fe2O3); kaolinite and halloysite [Al2Si2O5(OH)4], 39.50% (Al2O3) and 46.55% (SiO2); vermiculite [(Mg,Fe,Al)3(Al,Si)4O10(OH)2•4(H2O)], 43.48% (Al2O3), 12.82% (FeO) and 11.92% (SiO2) [23]. The IR spectrum obtained for the clay fraction is presented in Fig. 3. The main mineral vibration bands: An (anatase); Ka (kaolinite); Gt (goethite); Ha (halloysite); He (hematite); Rt (rutile); Qz (quartz); Ve (vermiculite) were identified [15,36] and corroborated the results of elemental composition given by the XRF for the majority of elements. The XRD pattern fitted by the RM for the clay fraction analyzed is presented in Fig. 4. It was possible to verify the following minerals: Qz (quartz); Gb (gibbsite); An (anatase); Rt (rutile); He (hematite); Gt (goethite); Mg (magnetite); Ka (kaolinite); Ha (halloysite); Calc (calcite); and Ve (vermiculite) whose mineral composition and disagreement indices for the RM refinement are presented in Table 2. These results are in accordance with the XRF and IR.

0.6

0.8

1.0

1.2

99 95 80 60 40 20 5 1 0.1 0.01

c

99.9 0 99 95 80 60 40 20 5 1 0.1 0.01 150

1000

2000

3000

4000

2000

3000

4000

120

Ctg

40

5.0x10 1.0x10 1.5x10 2.0x10

MgO MnO P2O5 CaO

Oxides 400.50 166.50 143.90 13.49 2.90 Elements 211.96 77.83 100.65 8.09 1.75 DL 0.20 0.30 0.10 0.03 0.10

50

0.0

TiO2

g kg−1

Cum Ctg

0.0

Cum Ctg

99.9 99 95 80 60 40 20 5 1 0.1 0.01 140 120 100 80 60 40 20 0

Ctg

Ctg

Cum Ctg

a

39

30

90

20

60

10

30 0

0 0.2

0.4

0.6

C

0.8

1.0

1.2

0

1000

P (nm)

Fig. 2. Particle analysis for the evaluation of area (A), circularity (C), and perimeter (P) of clay fraction with scanning area of 5 × 5 μm. Ctg (counts) and Ctg Cum (cumulated counts). (a) Area. (b) Circularity. (c) Perimeter.

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N.M.P. Dias et al. / Powder Technology 241 (2013) 36–42 Table 2 Rietveld Method (RM) results of the mineral composition and disagreement indexes of the clay fraction studied. Phases

RB %

RF %

% Mass

General indexes

Qz Gb An Rt He Gt Mg Ka Ha Calc Ve

3.10 4.01 7.27 6.35 4.00 3.13 1.68 6.12 5.98 8.10 26.02

1.71 3.03 2.93 4.42 2.13 1.47 1.01 3.08 3.46 3.82 4.37

5 (2) 24 (7) 9 (2) 2 (4) 11 (2) 7 (1) 5 (2) 18 (1) 14 (3) 3 (2) 2 (5)

RP %

6.52

RWP %

8.39

REXP %

1.66

S

5.05

Background

5th order polynomial

RB, RF, Rp, Rwp, Rexp, S according to Young [18]. Standard errors are in parentheses.

Fig. 3. Minerals indicated by the vibration bands for the clay fraction sample. An (anatase); Ka (kaolinite); Gb (gibbsite); Gt (goethite); Ha (halloysite); He (hematite); Mt (montmorillonite); Rt (rutile); Qz (quartz); and Ve (vermiculite).

3.3. Identification of minerals through the morphology presented in the AFM images Clay fraction images obtained through AFM, particle height values and the identification of minerals are presented in Fig. 5. In the images analyzed through AFM it is possible to identify through the height and shape, the particles corresponding to the minerals kaolinite, goethite and gibbsite. The selection of particles was carried out through a preliminary scan of all particles (n = 204) present in the image obtained. Results of Fig. 5 present only

one sample of particles (n = 9) corresponding to the minerals kaolinite, goethite and gibbsite. Particles 1, 2 and 3 (Fig. 5a) presented euhedral shape and average height values of 13.9 nm (± 0.83) (Fig. 5b). These results are according the results found by Zbik and Smart [6] and Gupta et al. [4] and correspond to kaolinite. The particles identified by numbers 4, 5 and 6 (Fig. 5a) have long shapes with average height values of 22.05 nm (± 2.01) (Fig. 5c). Prélot et al. [11], when studying synthetic samples of goethite through AFM, obtained height values for those particles between 30 and 40 nm and Kosmulski et al. [10] found height values of 23 nm. The morphologies found in these studies are similar to those found in the particles numbered 4, 5 and 6 (Fig. 5a). The particles identified by numbers 7, 8 and 9 (Fig. 5a) presented indefinite shape (complex) and average height values of 7.82 nm (± 0.35) (Fig. 5d). These results are similar to the groupings found and identified by Nagy et al. [8] for the gibbsite.

2 (degree) Fig. 4. X-ray diffraction (XRD) pattern of the clay fraction fitted by the Rietveld Method (RM). Crosses indicate the observed profile, solid line indicates the calculated profile, and the residual curve is shown in solid line below the Bragg peak positions in vertical lines. Counts per second (c.p.s.).

N.M.P. Dias et al. / Powder Technology 241 (2013) 36–42

a

41

b 30

h (nm)

25 20

Particle 1 Particle 2 Particle 3

15 10 5 0 0.00

0.25

0.50

x (µm)

c 30

30 25

20

h (nm)

h (nm)

25

d

Particle 4 Particle 5 Particle 3

15 10

15 10 5

5 0 0.00

20

Particle 7 Particle 8 Particle 9

0.10

0.20

x (µm)

0 0.00

0.20

0.40

x (µm)

Fig. 5. (a) AFM image with identification of the minerals present in the sample. (b) Graphic of height for particles 1, 2 and 3. (c) Graphic of height for particles 4, 5 and 6. (d) Graphic of height for particles 7, 8 and 9.

4. Conclusions In this study, it was possible to identify clay particles with heights ranging from 3 to 25 nm, which demonstrates the potential of the AFM technique for the measurement in nanometric scale. The image analysis method employed revealed good performance when evaluating surface particles higher than 3 nm. Combining the XRF results, to obtain the elemental composition, IR, to help the qualitative identification of some minerals, XRD to identify and quantify data through RM, it was possible to determine the mineralogical composition of the sample with predominance of gibbsite, kaolinite, and halloysite, followed by anatase, hematite and goethite. From these minerals, and employing the AFM morphological analysis, it was possible to identify gibbsite, kaolinite and goethite. Acknowledgments Many thanks are owed to the Brazilian Federal Funding Agencies: CNPq for the provision of the productivity fellowship in the research of Profs. Luiz F. Pires and Sérgio C. Saab and CAPES for the PNPD Scholarship of Dr. Nivea M.P. Dias. References [1] P.A. O'Day, Molecular environmental geochemistry, Reviews of Geophysics 37 (1999) 249–274. [2] S.C. Saab, E.R. Carvalho, R. Bernardes Filho, M.R. Moura, L. Martin-Neto, L.H. Mattoso, pH effect in aquatic fulvic acid from Brazilian river, Journal of the Brazilian Chemical Society 21 (2010) 1490–1496. [3] H. Lindgreen, Ultrafine particles of North Sea illite/smectite clay minerals investigated by STM and AFM, American Mineralogist 76 (1991) 1218–1222. [4] V. Gupta, M.A. Hampton, A.V. Nguyen, J.D. Miller, Crystal lattice imaging of the silica and alumina faces of kaolinite using atomic force microscopy, Journal of Colloid and Interface Science 352 (2010) 75–80. [5] M. Zbik, R.St.C. Smart, Nanomorphology of kaolinites: comparative SEM and AFM studies, Clays and Clay Minerals 46 (1998) 153–160.

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