The Origin of Vermiculite in Soils Developed from Lower Palaeozoic Sedimentary Rocks in Mid-Wales
1983
Adams, W. A. | Kassim, J. K.
Vermiculite occurs frequently in mid-Wales' soils derived from Lower Palaeozoic sedimentary rocks though illite and chlorite are the only clay minerals in the parent rock. Vermiculites were produced experimentally from rock illite by fusion with LiNO₃ and from chlorite by oxidation at 400°C in the presence of Na₂Cr₂O₇. The proportion of vermiculite in illitic clay treated with LiNO₃ was closely correlated with K or Na content whereas no such relationship was apparent in soil clays which contained vermiculite. The cation exchange capacity (CEC) of experimentally altered illite was closely correlated with vermiculite content and although the CEC of soil clays was also correlated with vermiculite content the relationships were quite different. The susceptibility to chemical dissolution of soil vermiculite was found to be more akin to that of vermiculite produced from chlorite than vermiculite derived from illite. A positive relationship was found between the vermiculite content of soil clays and total free iron content. Vermiculite was low or absent from highly weathered, gleyed eluvial horizons of stagnopodzols. Laboratory and field evidence support the conclusion that the vermiculite which occurs in the relatively young soils examined is predominantly a product of chlorite alteration under aerobic conditions. The evidence suggests that there has not been a significant alteration of illite to vermiculite in soils developed in postglacial deposits.
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