The nature and properties of amorphous colloids formed from Mazama tephra Public Deposited

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  • Soil samples from the Mazama tephra deposit were collected from the AC and C2 horizons at five different locations along the axis of the main depositional lobe. The clay fraction from the samples was recovered and studied by means of chemical, x-ray, DTA and electron microscopy methods in order to characterize the nature and properties of amorphous clays forming in this relatively homogeneous parent material of known age. Dispersion of the amorphous clays from the pumice samples was found to be difficult unless samples were first pretreated to remove iron oxides and organic matter. The diothionite-citratebicarbonate (DCB) iron removal treatment, however, was found to significantly alter the morphology of a reference imogolite sample by leaving individual strands of mineral coated with a gel. The comparison of DTA data from the imogolite and synthetic allophanes before and after defferation treatment also shows that significant changes in the amorphous clays resulted from the clean up treatment. Alternate methods for amorphous clay dispersion were tried on a variety of ando soil samples. None of the methods were found to be effective for dispersing all samples. Several washings in distilled water adjusted to pH 5. 0 with HC1 resulted in the dispersion of most samples and was used in this study to separate the amorphous clays from the Mazama tephra. The clays removed from the pumice samples contained from 47 to 84 percent amorphous colloids. The molar ratio of SiO₂:Al₂0₃ for all the samples was generally between 1 and 2. The types of clays found at each site studied along the transect were rather varied. Halloysite was found at all locations except one and generally was more common near the base of the pumice profile. The mineral dominated the clay fraction at a poorly drained site near the source but was absent in another similar site about 400 km away. The latter contained abundant imogolite and allophane. This difference in secondary mineralogy from the same parent material under similar weathering regimes presents an enigma. One possible explanation is based on the observation that samples of undisturbed tephra at far distances from the source contain fewer feldspars than samples near the source. The halloysite is thought to be related to feldspar weathering. Samples from drier sites along the transect had clay fractions containing predominately allophane with some few crystals of halloysite and some imogolite All samples had appreciable quantities of clay size glass shards. Masses of silica bodies and oxides of iron in addition to halloysite and allophane were found in the C2 horizon at the driest site. The cation exchange capacities of the clay samples were highly pH dependent. All samples had appreciable anion exchange capacities at pH 4.0. The DCB treated samples had CEC values much higher than non-treated samples. The magnitude of CEC for the synthetic gels depended on their composition and was highest when the molar SiO₂ :A1₂O₃ ratio was about 4.0. The clays sorbed large amounts of phosphate. The samples highest in the amorphous gels generally sorbed more phosphate than samples containing halloysite. All the clays had weak acid buffering curves. Most samples had two distinct buffering ranges, one at low pH and one at high pH. The curves for the samples highest in halloysite were buffered at much higher pH values than samples containing mostly amorphous clays. The nature and properties of the amorphous clays in the Mazama tephra were found to be similar to those reported by Japanese and New Zealand workers for other volcanic tephra deposits.
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