Minerales de arcilla en fracciones limo de horizontes superficiales de suelos de diferente mineralogia.

Particle size fractions were separated by sedimentation from the A horizon of soils developed on two types of loess (II, IV), on an alluvial material (III) and on a basaltic rock (I). X-ray powder diffraction traces were used to identify the mineralogical composition of the isolated fractions and their CEC was determined. The obtained data showed the following clay type dominance for each soil: Kaolinite (I), illite (II), smectite (III) and illite-smectite interstratified (IV). Clay minerals could be detected in all the analized size-fractions decreasing their content as the fraction increases in size. The same behavior was found concerning charge and size of the fractions as demonstrated by the CEC values. The CEC of the silt fractions displayed the following order: Kaolinite < smectite < interstratified < illite. When considering the relationship between the clay content and the charges originated in the coarse fraction, it could be observed that this relationship remained constant in the soil where kaolinite prevails, increased in the illitic soil and decreased in the soil with smectite and interstratified. Therefore, it is concluded that in soil I kaolinite is responsible for the entirety of the charges generated in the silt fraction. In soil II, the charge surplus could be attributed to the illite along with other coarser soil minerals. In soils II and IV, the mentioned reduction could be explained through the aggregation of phyllosilicates forming pseudo-silts which probably makes part of the charges inaccessible as to participate in cation exchange reactions.