The chemical composition and structure of a 14 Å intergradient mineral in a Korean Ultisol

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Abstract
An intergradient 14 Å mineral showing X-ray diffraction features of “chloritized” vermiculite in a Korean Ultisol was studied. The structural formula of the whole 2:1 layer-silicate (14 Å mineral and mica) in the Na+-saturated 0·2–0·5 µm fraction was obtained by elemental, thermogravimetric and 29Si and 27Al nuclear magnetic resonance spectroscopic analyses in combination with extraction of the interlayer material from the 14 Å mineral by hot 1/3 m sodium citrate treatment. This formula showed: (1) the 2 : 1 layer contains nearly one Al3+ in four tetrahedral positions, and Al3+ is the dominant cation in the octahedral sheet; (2) K+, exchangeable Na+ and sodium citrate extractable 1 : 1 layer occupy the interlayer space of the 14 Å mineral in similar proportions. Very little interlayer K was replaced by Na+ during the citrate treatment. Possible schemes of alteration of the 2 : 1 layer to the 1 : 1 layer as its interlayer material are discussed.