Graduate Thesis Or Dissertation

Nature of interlayer material in silicate clays of selected Oregon soils

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  • A study was conducted to investigate the nature of hydroxy interlayers in the chlorite-like intergrade clays of three Oregon soils with respect to kind, amount, stability, and conditions of formation. The clays of the Hembre, Wren, and Lookout soils, selected to represent weathering products originating from basaltic materials under humid, subhumid, and semi-arid climatic conditions respectively, were subjected to a series of progressive treatments designed to effect a differential dissolution of the materials intimately associated with them. The treatments, chosen to represent a range of increasing severity of dissolution, were (1) distilled water plus mechanical stirring, (2) boiling 2% sodium carbonate, (3) buffered sodium citrate-dithionite, (4) boiling sodium hydroxide, and (5) preheating to 400 °C for 4 hours plus boiling sodium hydroxide. Extracts from the various steps of the dissolution procedure were chemically analyzed in order to identify the materials removed from the clays. X-ray diffraction analysis and cation exchange capacity determinations were made on the clays after each step, and any differences noted in the measured values were attributed to the removal of hydroxy interlayers from the clays. Hydroxy interlayers were found to occur more in the Hembre and Wren soils than in the Lookout soil, with the most stable interlayers occurring in the Wren. Soil reaction was one of the major differences between these soils. The pH values of the Hembre and Wren ranged between 4.5 and 5.0, while that of the Lookout soil was near neutral at the surface and 8.3 in the zone of calcium carbonate accumulation. It appeared from the data, that the parent material of the Wren soil had reached a more advanced stage of weathering than that of the other soils. The data from the Hembre clays showed that hydroxy interlayers formed to the greatest extent in the surface horizon and decreased with depth. Such interlayers occurred in the A and B horizons of the Wren clays in about equal amounts, but less in the C horizon. The zone of clay enrichment in the B horizon of the Wren soil suggests a downward movement of clay in the profile. Consequently, it is conceivable that some of the interlayer hydroxides formed in the A horizon would ultimately have reached the B horizon. There appeared to be as much interlayering in the C horizon clays of the Lookout soil as in the upper horizons. This seems to contradict the idea that interlayering develops most in the upper horizons where weathering is greatest. The apparent anomaly is resolved, however, if the lithologic discontinuity in the profile is taken into consideration. Evidence indicates that the present C horizon of the Lookout soil may have been a surface soil at one time. Chemical analysis of the treatment extracts suggests that iron and magnesium as well as aluminum probably occupy the interlayer positions of the clays studied. A part of the magnesium was presumed to have been residuum from brucite interlayers of degraded chlorite. The results indicated that intergrade clays may have formed both from chloritization, and from chlorite degradation. The high amount of chlorite in the Hembre parent materials together with the acid environmental conditions probably led to some chlorite breakdown. Somewhat similar conditions existed in the Wren soil. There is, however, a high amount of biotite in the parent materials of the Wren soil that could readily weather to an intergrade by being altered to smectite or vermiculite and undergoing subsequent chloritization. The intergrades in the Lookout soil clays apparently formed from chloritization of smectites in as much as no chlorite was observed in the parent materials.
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