Clay mineralogy and related chemical properties of soils formed on Mazama pumice Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/8049g802j

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  • A sequence of soil profiles was sampled along a climate-vegetation transect on the dated Mazama pumice fall in central Oregon. The <2 μ size fraction from each sample was characterized with respect to the relative significance of amorphous and crystalline components, and the individual minerals of the latter group were identified. X-ray diffraction and differential thermal analyses and specific surface measurements by N₂ adsorption were used, each in conjunction with a differential dissolution technique. Data from these procedures showed that the amorphous component was predominant in the clay size fraction over the entire transect, especially in the less intensely weathered horizons. The relatively small percentages of crystalline minerals present were unexpectedly complex. These included, among the phyllosilicates: beidellite, montmorillonite, vermiculite, a micaceous mineral, chloritic intergrades, and chlorite. Non-phyllosilicate crystalline minerals of the suite were gibbsite, plagioclase feldspars, and quartz. The amount and distribution of the individual clay size minerals varied within and between profiles. The pH dependent ion exchange capacities of the clay samples were measured in order to determine the usefulness of that property in the evaluation of the presence of the amorphous component. Data indicated that the samples treated for dissolution of amorphous materials expressed a pH dependent charge equal to or greater than that measured for samples prior to such treatment. This behavior, attributed to the characteristics of the hydroxy interlayer material in the chloritic intergrades, confounded the interpretation of the data from samples comprised of both crystalline and amorphous components. The origin of the pH dependent charge, usually ascribed to the amorphous clay materials in young ash and pumice soils, was not discernible. Although the pH dependent ion exchange capacity measurements conveyed a significant characteristic of the samples, they were concluded to be of little use in assessing the significance of the amorphous component in soils of the present investigation. During the course of ion exchange procedure verification against standard montmorillonite samples it became apparent that the cation exchange capacity values obtained were lower than those accepted for the particular clays being used. An hypothesis was proposed in which the cause of the problem was attributed to the trapping of index ion against extraction during the ion exchange replacement wash. Data from an experiment in which the index ion solution was tagged with ³⁶Cl⁻ prior to sample saturation verified that the hypothesis was correct. It was reasoned that, since differences in ion exchange capacity measurements as a function of pH were of primary concern, any discrepancies between values measured by the proposed procedure and the true values should not invalidate interpretation of the data. Results of clay mineral identification were evaluated, both with respect to environmental data available for the transect sites, and data from other investigations of the weathering of volcanic ash and pumice soils. On these bases a weathering sequence which allowed for the formation of the existent phyllosilicate suite from the pumice parent material, through the mediation of amorphous precursors, was proposed for the transect soils. The factors primarily controlling the processes responsible for clay mineral genesis were thought to be the vesicular structure and chemical composition of the pumice material. Such a microenvironment as the pumice particle capillary space would provide conditions for the chemical reactions leading to the formation of the 2:1 phyllosilicate group identified in the transect soils. Other factors such as climate and vegetation were believed to govern local weathering intensity.
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