Kaolinite is commonly found in fine fractions of tropical and subtropical Brazilian soils. Kaolinite can be verified by an X-ray diffractogram, made of a series of 00l peaks, at the distance of 0.715 nm. However, in some soils, major peaks can be found at about d = 0.720 nm, indicating halloysite or kaolinite-smectite (K-S) interstratified minerals. The objectives of this study were to verify this hypothesis and subsequently discuss soil genesis. Parent material and soil samples from the horizons A1, A2, E and Bt were collected in a Red Yellow Acrisol, covered by native vegetation, on the Campus of the Universidade Federal de Santa Maria, RS, Brazil. The clay fraction was separated by sedimentation, Ca2+ saturated and X-ray diffracted after the following treatments: control, air drying, (N); ethylene-glycol saturation (EG); heating to 300 and 550 °C; and, after formamide saturation. The raw X-ray diffractograms (XRD) were modeled between 11 and 15 degrees 2 theta (using DecompRX). Two peaks were found in the XRD around this region. The first peak was sharp and intense at d = 0.717 nm, corresponding to kaolinite; the second, broader and less intense, was observed at d = 0.720 nm. The peak at d = 0.720 nm was sensitive to EG treatment; it varied from d = 0.720 nm to d = 0.730 nm and returned to about d = 0.720 nm after heating to 300 °C. This behaviour is typical of expansive layers. The presence of interstratified K-S minerals was inferred since halloysite clay was not identified by the formamide test. The XRD were analyzed and indicated interstratified K-S mineral at a proportion of 0.8_0.9 kaolinite layers and 0.1-0.2 smectite layers. Furthermore, the K-S covered about 90 % and kaolinite alone 10 % of the modelled peak, at d = 0.720 nm. K-S was also found in the bedrock, indicating that K-S was inherited from the parent material. The soil mineralogy indicates an intermediate weathering stage of the clay minerals with a 2:1 and 1:1 structure in the weathering process of this soil.
weathering; XRD analysis; Bt horizon
