Abstract: Electron optical observations on Marblehead illite showed the presence of twinned aggregates of lath-shaped crystallites. The selected area diffraction patterns of these aggregates indicate a strict orientational relationship between them.
Original twinned mica flakes display all possible stages of the transformation of these micas into lath-shaped illites, where the a and b dimensions of layers do not show any noticeable changes, but c-dimension becomes shortened in the illites. The transformation seems to involve parting along (110) of the micas, subsequent H2O and OH inclusion in the structure and other possible chemical changes resulting from the hydration. The morphology and the size of lath-shaped crystallites displaying (001) and (110) forms seem to be responsible for the excess of water and K-deficiency in the Marblehead illite compared to the micas.
Individual illite laths commonly have a length 0·1–4 µ, a width of 0·01–0·1 µ and a thickness varying between 10 and 50 Å.