TRANSFORMATION OF SMECTITE TO ILLITE IN BENTONITE AND ASSOCIATED SEDIMENTS FROM KAKA-POINT, NEW-ZEALAND - CONTRAST IN RATE AND MECHANISM

Smectite and mixed-layer illite/smectite (I/S) in Triassic heulandite- rich bentonite from Kaka Point, New Zealand, have been investigated by scanning electron microscopy (SEM), transmission electron microscopy/ analytical electron microscopy (TEM/AEM) and X-ray diffraction (XRD) f or comparison with matrix phyllosilicates in closely associated siltst ones and analcimized tuff. Samples that were treated to achieve perman ent expansion showed that some smectite in bentonite occurs as curved packets of wavy 10.5- to 13-Angstrom layers enveloping relict glass sh ards, the centers of which consist of an amorphous clay precursor. The dominant clay minerals in bentonite are smectite-rich randomly disord ered (R0) I/S with variable proportions of 10-Angstrom illite-like int erlayers, only locally organized as 1:1 ordered (R1) I/S. R0 I/S was a lso observed in separate packets retaining the detailed texture of pac kets that replaced shards. Such relations are consistent with a ''soli d-state''-like, layer-by-layer replacement of original smectite layers by illite-like layers with partial preservation of the primary smecti te texture, in contrast to textures observed elsewhere, such as in Gul f Coast mudstones. The smectite, as in other examples in marine sedime nts, has K as the dominant interlayer cation, suggesting that precurso r smectite may be a major K source for reaction to form illite. Only a small proportion of illite (35%) occurs in mixed-layer smectite-rich I/S in bentonite and the dominant trioctahedral phyllosilicate is diso rdered high-Fe berthierine, implying that little mineralogical change has occurred with burial. This contrasts with observations of closely associated siltstones and analcimized tuff, which contain well-defined packets of illite and chlorite but which have no detectable matrix sm ectite component. These data imply that the rate of transformation of smectite to illite is much slower in bentonites than in associated sed iments of the same burial depth and age. Such relations emphasize the significance of factors other than temperature, (e.g., organic acids, permeability and pore fluid compositions) in affecting the rate and de gree (and perhaps mechanism) of transformation of smectite to illite.