Phosphate Mineralogy and Equilibria in Two Kentucky Alfisols Derived from Ordovician Limestones
The phosphate mineral distribution as it relates to in situ soil solution composition of two Alfisols developed from phosphatic Ordovician limestones in the inner Bluegrass Region of Kentucky was investigated. At both sites, microcrystalline apatite appeared to be the primary source of P. Surface weathering under a moderate pH (5.7–6.7) and a carbonate-free environment has apparently induced apatite transformations to wavellite, crandallite-like, and barrandite-like minerals as indicated by x-ray diffraction (XRD), differential scanning calorimetry (DSC) or scanning electron microscopy (SEM) analyses. The possible stability of othe phosphate minerals such as millisite, montgomeryite, variscite, strengite and MnPO4·1.5H2O was supported by solution-composition data, but it was not confirmed by XRD, DSC, or SEM analyses. Sequential-fractionation extractions suggested the presence of significant amounts of discrete or occluded Al- and Fe-phosphates in soil and nodular materials, mostly in amorphous or poorly crystalline forms. The solution ion-activity products in most cases were consistent with amorphous Fe- or Al-phosphates approaching the stoichiometry, but exceeding the solubility, or strengite or variscite-like minerals. Solution data and SEM observations suggested that the formation of secondary Ca-Al-, Al-, and/or Fe-phosphates in these soils may occur through an apatite congruent-dissolution stage, which is followed by gradual adsorption and precipitation of P on chemically etched clay-mineral and amorphous-hydroxide surfaces.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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