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This article in SSSAJ

  1. Vol. 39 No. 3, p. 561-566
    Received: Nov 29, 1973
    Accepted: Jan 28, 1975

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Weathering and Authigenic Halloysite in Soil Developed in Mazama Ash1

  1. M. J. Dudas and
  2. M. E. Harward2



Hydrated halloysite is the dominant clay mineral formed by weathering of Mazama dacitic tephra. Allophane was also identified as a weathering product in some soils developing in this volcanic deposit. The amount of weathering and clay formation is related to soil moisture contents. In well drained upland soils, localized formation of clay occurred in pumiceous ash and lapilli near the contact with the palesols. Alteration of tephra in lower portions of these profiles was related to an area of higher moisture content above the less permeable paleosols. In poorly drained areas, considerable weathering of the Mazama tephra has occurred. This is evidenced by relatively high clay contents (15–20%) and easy physical disintegration of particles to a slippery paste. The Mazama tephra often appears relatively fresh even though deposition occurred 6,600 years ago. Limited weathering in upper horizons of well-drained soils is attributed to inadequate moisture when temperatures are favorable for alteration processes.

The authigenic halloysite in Mazama ash soils has a low degree of crystallinity suggesting early stages of formation. Low degree of order is indicated by loss of diffraction peaks after heating to 300C for 2 hours and low endothermic peak temperatures during differential thermal analysis.

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