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Induration of C Horizons by Allophane and Imogolite in Costa Rican Volcanic Soils

^a Dep. of Soil Science and Geology, Agricultural Univ., Wageningen, The Netherlands
^b Unité de Science du Sol, INRA, Versailles, France
^c REPOSA, Guapiles, Costa Rica

View larger version (55K): [in a new window]   Fig. 1 Location of the sample area

View larger version (155K): [in a new window]   Fig. 2 Selected micromorphological features. (A) Isotropic, fine textured coatings in the indurated horizon (80 cm) of soil H5. (B) Isotropic, fine textured coatings in the indurated horizon (1.8 m) of soil P2. Left images in crossed polarized light, right images in plane polarized light.

View larger version (89K): [in a new window]   Fig. 3 Scanning electron microscopy of rough surfaces from indurated horizons in the Holocene profiles (H4). (A) Coatings forming a three-dimensional framework between the sand grains. (B) Detailed view of A. (C) Qualitative chemical spectrum of the coatings

View larger version (87K): [in a new window]   Fig. 4 Scanning electron microscopy of undisturbed mineral surfaces from indurated horizons in the Pleistocene profiles (P2). (A) Coatings forming a three-dimensional framework between the sand grains. (B) Detailed view of A. (C) Qualitative chemical spectrum of the coatings

View larger version (113K): [in a new window]   Fig. 5 Transmission electron microscopy of coatings in the Holocene profiles (H7). (A) Micrograph made in the ultra-thin section of a coating, showing the presence of allophane and imogolite. (B) Micrograph made of a suspension. Overview of allophane and imogolite. (C) Detail of B showing the characteristic threads of imogolite. (D) Detail of B showing the characteristic spheres of allophane

View larger version (103K): [in a new window]   Fig. 6 Transmission electron microscopy of coatings in the Pleistocene profiles (P2). (A) Micrograph made in the ultra-thin section of a coating showing the presence of allophane, imogolite and pieces of volcanic glass (v). (B) Detail of A showing imogolite threads (i). (C) Micrograph made of a suspension showing allophane spheres and imogolite threads