ATOMIC-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPE EVIDENCE FOR THE MECHANISM BY WHICH CHLORITE WEATHERS TO 1 1-SEMI-REGULAR CHLORITE-VERMICULITE/

Atomic-resolution transmission electron microscope (TEM) images reveal that IIbb (beta = 97 degrees) Mg,Al,Fe-chlorite from Koongarra, Austr alia, transforms to vermiculite via a range of intermediate chemical a nd structural states. Semi-quantitative analysis of contrast in atomic -resolution images of layer silicates with similar to 1.4 nm basal spa cings indicate that the interlayers range from brucite-like to having similar to 0.3-0.6 interlayer cations per formula unit. Octahedral cat ions (predominantly Mg and Fe) tend to be removed from every second in terlayer, leading to semi-regular 1:1 interstratifications of chlorite -vermiculite. Further loss of interlayer cations is accompanied by par tial to complete interlayer collapse in the vacuum of the TEM. Resulti ng intergrowths of chlorite and semi-regular 1:1 chlorite-vermiculite retain the primary chlorite orientation, morphology, and sense of octa hedral tilt in 2:1 layers. Although vermiculitization is a continuous process that occurs by a solid-state mechanism, the reaction involves important structural modifications. Atomic-resolution [010] images ind icate initial loss of interlayer cations is accompanied by similar to a/3 shifts of 2:1 layers and cations in brucite-like interlayers. Disp lacements of interlayer cations change the interlayer stacking from Il bb to Iab and shift of the following 2:1 layer converts it from Iab to the Iaa. Displacements are driven by the lower energy of a-type inter actions when vacancies occur in sites above tetrahedral cations. Shift of a 2:1 layer alters the subsequent interlayer from Ilbb to IIab. St abilization of every slightly altered second interlayer by introductio n of a-type stacking explains development of semi-regular 1:1 chlorite -vermiculite interstratifications. Displacements occur before signific ant modification of interlayer electron density can be detected in hig h-resolution images. This observation is consistent with previously re ported inhibition of layer shifts by low interlayer charge. Layer disp lacement may occur by an elastic process (no rupture of bonds within t he 2:1 layer) at the tip of the growing vermiculite portion of the int ergrowth. Removal of cations from the chlorite-vermiculite junction ma y be facilitated by rapid diffusion along the vacancy-rich interlayer. Mg is removed in solution, Fe is precipitated locally in aggregates o f nanocrystalline Al-, Si-, and P-bearing goethite.