Abstract: The weathering of micas to form the dioctahedral analogue of vermiculite is a common process in acid soils. Laboratory weathering of the micaceous 5–2µ fraction of parent materials of certain Virginia soils resulted in a product similar in character (whether mixtures of vermiculite and regularly and randomly interstratified mica-vermiculite, or vermiculite only) to those formed by natural weathering processes. The formation of vermiculite rather than (or, in addition to) kaolinite is attributed to substitution of Na and other ions for K, other structural imperfections, and the small size of the original mica flakes.
The low cation exchange capacity and stability, in contraction and expansion, of the 14 Å product is attributed partly to retention of hydroxy-Al and Fe groups in the interlayer spaces. The removal of these interlayers by natural or laboratory processes results in a product that contracts easily but may or may not expand to 18 Å on glycerol solvation. This may be associated with the original charge of the mica as evidenced by its K and Na content.