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Scanning Electron Microscopy of Hydrous Metal Oxide Crusts Intercalated in Naturally Weathered Micaceous Vermiculite¹

ABSTRACT

Scanning electron microscopy (SEM) of the weathering products of trioctahedral mica, chemically weathered in nature to micaceous vermiculite (14 and 10Å spacings by X-ray diffraction), revealed occasional deposition of hydrous metal oxide crusts on intermittent cleavages interspersed at intervals among chemically clean (001) surfaces. Examination by SEM of layers through pores or holes, and along (hkO) fractures through the (001) planes, revealed that the intercalated crusts were 1–15µm thick with sequences of hundreds of clean 14Å crystallographic layers between each crust layer. The pores may represent weathering out of ²³⁸U fission tracks resulting from impurities in the original micas.

Treatment of a micaceous vermiculite flake with citratebicarbonate-dithionite (CBD) produced a large number of smooth, thin phyllosilicate flakes which had planar areas at least one-half that of the original untreated flake; hundreds of smaller flakes were released by this treatment. Natural weathering apparently formed cracks in the large flakes both in the (001) and (hkO) crystallographic directions. The metal oxides removed by CBD cemented the mica pseudomorphs in the untreated sample. The holes and fractures along (hkO) planes apparently permitted a diffusion of Fe³⁺ and Al³⁺ from the octahedral positions into the crusts during the geochemical conversions of biotite to micaceous vermiculite and may relate to the associated lowering of layer charge.

¹ Research supported in part by the Dep. of Soil Science, College of Agr. and Life Sci., Univ. of Wisconsin, Madison; in part by the Engineering Exp. Sta., Georgia Institute of Technology, Atlanta; in part by the Forest Products Laboratory, USDA, Madison; and in part by the Ecological Sciences Branch, Division of Biomedical and Environmental Research, US Atomic Energy Commission Contract AT (11-1)-1515-Jackson (Paper COO-1515-34); through an International Consortium for Interinstitutional Cooperation in the Advancement of Learning (ICICAL). Presented before a meeting of Div. S-2 and S-4, Soil Science Society of America, Aug. 24, 1970, Tucson, Arizona.

² Professor and Graduate Research Assistant (UW), Director Engineering Analytical Instrumentation Laboratory (GIT), Cytologist (USDA), and Associate Professor (UW), respectively.

Received for publication May 3, 1972. Accepted for publication September 29, 1972.