UNUSUAL ACCUMULATION OF GIBBSITE AND HALLOYSITE IN THE KITAKAMI PUMICE BED, WITH A NOTE ON THEIR GENESIS

Abstract

Unusual accumulation of yellowish gray clay materials has been noticed at places in the lower part of the Kitakami pumice bed. Field survey and laboratory examinations have revealed that this accumulation is mainly a secondary deposition of the clays carried down by percolating water. The deposition was apparently accelerated by water stagnation caused by an underlying Pleistocene clay bed as a slowly permeable layer. Gibbsite, halloysite, and allophane were the dominant clay minerals in the accumulated material with some imogolite and free sesquioxides as accessory constituents. The sum of the content of the former two minerals amounted to 35 to 40 per cent, and was higher than that in the clay fraction of the upper part of the pumice layer where no accumulation occurred. The relative concentration of gibbsite and halloysite was accounted for by their high dispersibility relative to allophane in the percolating water. Chemical analyses have shown that a considerable amount of Al together with Si is liberated rather continually from the upper pumice bed. The concentration of Al in water extracts of a moist sample taken from the field was evidently higher than that expected from its solubility in the same pH range probably owing to the presence of organic matter, but decreased to a few-hundredths upon mild air-drying of the sample. These results suggested strongly that aluminum hydroxides and silica-alumina coprecipitates form rather readily, and evolve to gibbsite, halloysite, imogolite, or possibly allophane in the pumice bed.