SOILS, SEC 5 • SOIL AND LANDSCAPE ECOLOGY • RESEARCH ARTICLE Characterization of kaolinite in the hardsetting clay fraction using atomic force microscopy, X-ray diffraction, and the Rietveld method Luis Valério Prandel 1 & Nívea Maria Piccolomini Dias 1 & Sérgio da Costa Saab 1 & André Maurício Brinatti 1 & Neyde Fabíola Balarezo Giarola 2 & Luiz Fernando Pires 1 Received: 21 July 2016 /Accepted: 7 January 2017 # Springer-Verlag Berlin Heidelberg 2017 Abstract Purpose Brazilian soils that present extremely hard sub-superficial horizons when dry and friable when humid are similar to the Australian and South African hardsetting horizons whose hardness can be mainly related to low crys- tallinity. Studies involving refinement by the Rietveld method with X-ray diffraction (RM-XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and their rela- tion have not been carried out in hardsetting horizon soils. Thus, the objective of this study is to obtain information about the kaolinite in the hardsetting horizon of a Yellow Argisol clay fraction, taking into consideration the results of isomor- phic substitution, crystallite average size, and microstrains, relating them to particle image analysis regarding their mor- phology and size. Materials and methods Soil samples were collected in the hardsetting horizon of a Yellow Argisol in the Coastal Tablelands region, which covers the whole Brazilian Northeast coast and part of the Southeast region. The sample was powdered, sieved, and submitted to dispersion and phys- ical fractioning process by sedimentation. The clay fraction was analyzed by RM-XRD, AFM, and SEM techniques. Results and discussion The RM-XRD provided improvement of indices with isomorphic substitutions in the goethite [Fe 0.70 Al 0.30 O(OH)], kaolinite [Al 1.44 Fe 0.56 Si 2 O 5 (OH) 4 ], and halloysite [Al 1.42 Fe 0.58 Si 2 O 5 (OH) 4 ]; 29 nm crystallite average size; 5 × 10 -3 microstrain; and 49.5% kaolinite. AFM analyses indicated particle average size from 80 to 250 nm and average height from 60 to 80 nm. By relating this data, it was possible to estimate that the particles under analysis are kaolinite com- posed of 3 to 9 crystallites and stacking of 88 to 112 layers. Conclusions The process, analyses, and comparisons such as crystallographic and morphologic information about the kao- linite mineral particles contribute to the comprehension of the hardsetting horizon soil nature as well as other soils that pres- ent minerals with a high degree of isomorphic substitution. Keywords Crystallite size . Crystalline structure . Isomorphic substitution . Microstrain . SEM 1 Introduction The term hardsetting is attributed to Brazilian soils that present extremely hard sub-superficial horizons when dry and friable when humid (Giarola et al. 2009). These soils are similar to the Australian (Franzmeier et al. 1996) and South African (Materechera 2009) hardsetting horizons found in the semi- arid environments. The high hardness degree of these hori- zons, which makes it difficult or even impossible for plants to grow, might be related to (1) soil pores filled with illuvial clay, (2) changes in the soil structure due to wetting and drying cycles (Pires et al. 2008), and (3) the presence of cementing agents constituted of low crystallinity oxides (Giarola et al. 2001, 2009). With the aim of evaluating soil mineralogical characteris- tics of hardsetting horizons, some techniques related to the Responsible editor: Richard K. Shaw * Luis Valério Prandel luisprandel@gmail.com 1 Laboratory of Soil Physics and Environmental Sciences, Department of Physics, State University of Ponta Grossa (UEPG), Av. Carlos Cavalcanti, 4748, Ponta Grossa, PR CEP 84.030-900, Brazil 2 Department of Soil Science and Agricultural Engineering, State University of Ponta Grossa (UEPG), Av. Carlos Cavalcanti, 4748, Ponta Grossa, PR CEP 84.030-900, Brazil J Soils Sediments DOI 10.1007/s11368-017-1654-z