ORIGIN OF CLAY-SIZE VERMICULITE INSANDY VOLCANIC ASH SOILS DERIVED FROM MODERN PINATUBO LAHAR DEPOSITS IN CENTRAL LUZON, PHILIPPINES

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Vermiculite (Vt) was found in the clay fraction of sandy volcanic ash soils around Mt.Pinatubo, Philippines. Origin of the clay-size Vt was examined selecting two horizons from two pedons with different land use, from a paddy rice and a sugarcane field. In order to concentrate Vt, 2-0.4 μm fraction was taken because thlloysite and a small amount of smectite and quartz were also included in these clay fractions. Regarding (060) diffraction spacing, two major peaks were found at 0.153 and 0.149 nm in both samples. With heating at 600℃, intensity of the diffraction peak at 0.149 nm was greatly reduced and that at 0.153 nm remained almost unaffected, indicating that the diffraction peak at 0.149 nm was mostly due to halloysite. Overlap between diffraction peaks at 0.154 nm of quartz and that at 0.153 nm was only partial in these samples. Thus, it was concluded that the diffraction peak at 0.153 nm was assigned to trioctahedral Vt. Weathered particles of biotite were found with the naked eye and under an optical microscope in the sand fractions of these soils. In the silt fractions, Vt and biotite were suggested by X-ray diffraction (XRD) analysis. XRD peak intensity indicated that vermiculitization of biotite in the sand fractions was more advanced in the paddy field than in the sugarcane field. However, Vt was dominant in the silt and clay fractions of both soils. From these results, it was strongly suggested that ciaysize Vt was inherited from biotite in these soils.
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