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This study examined the utility of nonlinear regression as a means of developing a statistically-based function for the estimation of particle density (Dp) of clay-rich mineral soils where the <0.002 mm fraction was mineralogically diverse. Physical and chemical soil property data (including semi-quantitative clay mineralogy) were analyzed for 54 mineral soil horizons sampled from 18 soil profiles at seven different locations across North America. Plausible estimates of particle density for the mineral component (2.72 Mg m-3), and particularly for the humic component (1.48 Mg m-3), were obtained (r² = 0.856, root mean square error [RMSE] = 0.035 Mg m-3 P < 0.0001) even though the calibration data set had a limited range of soil organic matter content (<23%). Particle density was shown to vary with clay content (P < 0.05), and Dp of different mineral particle-size fractions could also he distinguished statistically. The convenient and traditional assumption of Dp = 2.65 Mg m-3 for silicate-based soils in the absence of Dp measurements is challenged, given the considerable observed variability in Dp with changes in soil organic matter content and texture. [ABSTRACT FROM AUTHOR]

Little is understood about chemical weathering processes in Alaskan arctic soils, where moisture is generally not limited but acidity varies and the average soil temperature is close to or below freezing. Weathering reactions in soil convert primary minerals into secondary clay minerals. Silty loam textured soils from three sites in moist acidic tundra (MAT) and three sites in moist nonacidic tundra (MNT) in the northern Arctic Foothills, Alaska, were characterized with emphasis on the origin of the clay minerals. The MNT soils had a discontinuous and thinner organic layer, which leads to a deeper summer thaw and greater cryoturbation than the MAT soils. The MNT had higher cation exchange capacity and base saturation than MAT. These buffer against acidification and account for the pH differences of MAT and MNT. Other chemical characteristics including C and N content as well as Fe and Al were similar (by horizon) across the MAT/MNT boundary. X-ray diffraction of coarse (0.0002-0.002 ram) and fine clay (<0.0002 mm) fractions indicate that illite, vermiculite, and kaolinite are the predominant clay minerals. Presumably, kaolinite is detrital and vermiculite is weathered from illite. The proportion of vermiculite to illite is higher in MAT and the illite to vermiculite proportion is higher in MNT. This shows that soil acidity does affect weathering processes despite the low soil temperature. [ABSTRACT FROM AUTHOR]

Although mineralogic quantitative phase analysis (QPA) of the soil clay fraction can provide useful information for the improvement of soil management practices, QPA often requires a combination of several analytical techniques, which can be expensive and time consuming. One alternative that involves a single analysis to give accurate QPA of soils is the use of the Rietveld method to analyze powder x-ray diffraction (XRD) data. In this study, we evaluated the accuracy of the XRD-Rietveld approach for mineralogic quantitative analyses of Oxisol clays when observed structure factors [F(hk[)] ofpedogenic minerals (i.e., kaolinite, hematite, and goethite) are used in the Rietveld analyses performed using Siroquant software. The results showed that although the structures of disordered clay minerals are especially difficult to incorporate in standard Rietveld refinement, which relies on ordered three-dimensional structure models, Mineralogic quantification can be accurately done for complex matrices having a large number of phases and various degrees of structural and coinpositional disorders when observed F(hkl) values are used. It is also possible to develop observed F(hkl) values for Al-hematite and Algoethite from impure samples of such oxides to use as starting structure models for Rietveld analysis. We believe that this approach can be successfully extended to other geologic materials. [ABSTRACT FROM AUTHOR]