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Origin of Allophane and Retardation of Pebble Weathering in Quaternary Marine Terrace Deposits

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—Quaternary marine terrace deposits consisting of gravels interbedded with thin sandy gravel layers have been subjected to subaerial weathering. Restricted to the sandy gravel layers, allophane gel either replaced bytownite sands to form a pseudomorph or coated the pebbles. The allophane has an average Al/Si atomic ratio of 1.5 with 45% H2O. The sandy gravels were originally rich in bytownite (av. An86) sands derived from underlying Tertiary basaltic lapilli tuff. The highly soluble and aluminous bytownite favored the formation of allophane. In the sandy gravel layers, pebbles coated with allophane gel were almost fresh whereas those in the gravel layers were highly weathered to form halloysite-rich clays. Allophane gels acted as a somewhat impermeable geochemical barrier impeding a mineral-water reaction in the bytownite-rich sandy gravel layers and thus significantly retarding pebble weathering, while prolonged weathering in the gravel layers resulted in the severe decomposition of pebbles. Bytownite protected the pebbles against weathering, implying that minor soluble minerals might be one of the factors in the natural variation of the weathering rates of rocks and sediments.
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Document Type: Research Article

Publication date: 2002-04-01

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  • The JOURNAL publishes articles of interest to the international community of clay scientists, including but not limited to areas in mineralogy, crystallography, geology, geochemistry, sedimentology, soil science, agronomy, physical chemistry, colloid chemistry, ceramics, petroleum engineering, foundry engineering, and soil mechanics. Clays and Clay Minerals exists to disseminate to its worldwide readership the most recent developments in all of these aspects of clay materials. Manuscripts are welcome from all countries.

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