Chemical composition of soil vermiculite clays as related to their genesis

Chemical and mineralogical analyses of the clay fraction of eleven soils containing a large amount of vermiculite clay and representing a wide range of parent materials revealed that two types of vermiculite clays exist: (1) An aluminous type in which Al³⁺ substitutes for Si⁴⁺ in tetrahedral positions in the same order of magnitude as in the coarse grained vermiculites and micas, and with Al³⁺ as the dominant octahedral ion. (2) A silicious type in which only Si⁴⁺ occupies the tetrahedral positions, and with Fe³⁺ and Mg²⁺ as the dominant octahedral ions. The aluminous vermiculite clay was found to occur in soils derived from acid igneous rocks and is usally associated with mica, whereas the silicious type was found to occur in soils derived from basic igneous rocks which do not contain mica. Because of this close association of these two types to their parent material, it was concluded that the aluminous vermiculite is a product of alteration of mica whereas the silicious type is a product of synthesis from primary oxides of silica, alumina, iron, and magnesium. Both types of vermiculite clays tend to be dioctahedral in contrast to the trioctahedral nature of the coarse-grained vermiculite.