| dc.description.abstract | Kaolinite is ubiquitous in soils and is the dominant clay mineral in highly weathered soils such as Oxisols and Ultisols. The determinative roles of mineral structures at the atomic level in their surface properties, their interactions with nutrients, contaminants, and biological compounds have been documented. Yet the structural details of clay minerals in soils are generally unknown or poorly characterized due to the complexity of mineral assemblage and often abundant structure disordering. In the past decades, there are ample studies on the nature (abundance and type) of structural disorder in geologic kaolinite, many computer programs have been developed to quantitatively describe the disordering. The objectives of this study were 1) to model disordering of soil kaolinite using the programs commonly employed in single-composition geological specimens, 2) to identify the factors or parameters limiting soil kaolinite structure modeling and the possible solutions for these limiting factors, and 3) to examine the effect of the disordering on the thermal stability of kaolinite.
Two pedogenic kaolinites, one from Brazil labeled BRZ and one from Hawaii label WAI were the focus of the study. One sedimentary kaolinite from Georgia labeled KGa was included for comparison and verification of accuracy of the modeling procedures.
The structural disorder was studied by modeling the X-ray diffraction (XRD) patterns of the samples using computer program FAULTS. The disorder model used assumes that a disorder-free kaolinite will be produced from a 1:1 sequence of either a B
layer (displacement vector ῑv1) or its enantiomorph (displacement vector ῑv2), while the random interstratification of the ῑv1 and ῑv2 vectors within the kaolinite structure causes disorder. A third layer displacement vector, ῑv0 , located along the empty B octahedral site may also exist. This model attempts to estimate Pvt1, Pvt2 and Pvt0 which are the proportions of ῑv1, ῑv2 , and ῑv0 layer displacement translations, respectively within the structure.
The modeling of the KGa kaolinite was only possible after using two phases — an
almost no disorder phase (NDP) and a highly disordered phase (HDP) both having
different Pvt1, Pvt2 and Pvt0. The soil kaolinite samples were both modeled with single-phase
and contained 43, 30 and 27% of a Pvt1, Pvt2 and Pvt0, respectively. High XRD profile factor
(Rp = 16 – 19%) observed for the soil samples was likely due to preferential orientation,
accessory minerals and amorphous phases.
The major limiting factor in modeling disorder in soil kaolinite was the presence
of crystalline and amorphous phases of other minerals which often share the same
diffraction peaks as kaolinite.
Thermal dehydroxylation experiments showed that the least disordered sample
(KGa) was most resilient to dehydroxylation while BRZ and WAI showed similar trends. | en |
