Characterization of Mineral Forms in Andepts from Chile

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Abstract
The mineralogy of two undisturbed volcanic soils from Chile (Typic Dystrandepts) and of the sand and clay‐size particles of the soil residues was studied by using ultrasonic, scanning electron microscope, X‐ray diffraction, transmission electron microscope, infrared spectroscopy, differential thermal analysis, and electron microprobe techniques. The effects of removal of organic matter, free iron and aluminum oxides, and amorphous alumino‐silicates was evaluated in the X‐ray, DTA, and IR analyses. The soils are characterized by an amorphous inorganic matrix in a flocculated state with a sponge type structure. This matrix presents a high specific surface (225 m2/g), CEC of about 45 meq/100g at pH 7.0, low base saturation (< 20%) and bulk density of 0.9 to 1.1 (g/cm3) and SiO2/Al2O3 molar ratio of 2.0 to 4.0. Organic matter appeared in some well preserved structures, partially due to silica accumulation. The principal primary minerals are fresh volcanic glass and K‐feldspars and the principal secondary mineral is allophane or an amorphous analog. The removal of organic matter, free iron and aluminum oxides and amorphous alumino‐silicates did not alter the DTA, X‐ray and IR patterns.

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