The structure of “imogolite,” first described in 1962, was studied with the type specimens formed by weathering of pumice and glassy volcanic ash. Chemical analyses give the formula 1.1 SiO2· A12O3·2.3-2.8 H2O (+) without correction for allophane as an impurity. Diffraction and electron microscopic studies suggested that “imogolite” appears in a para-crystalline state forming threads with diameter 100 to 300 A, which consist of the chain units with the repeat distance, 8.4 Å and with the mean interchain separation, 17.7 Å. The density of the imogolite film measured by displacement with water is 2.63 to 2.70, while that measured by suspension in heavy liquids is 1.70 to 1.97. The presence of many micropores between the chain units, about 55% is estimated from the difference between the two density values and from the maximum adsorption of water vapor. The progressive change of the X-ray pattern and weight loss by heating suggests rearrangement and linking of the chain units at 50 to 250°C, and breakdown of their structure by dehydroxylation above 250°C.

“Imogolite” is structurally distinct both from allophane and from known layer or chain clay minerals. Preliminary analyses indicate a 1:2 chain structure unit for “imogolite” with the structural formula (Al8O4·OH20·4H2O)2(Si8O10·OH4).

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