STACKING DISORDER IN A SEDIMENTARY KAOLINITE

Although structural disorder in kaolinite has been investigated extensively, it is still not understood properly. To investigate the problem, a kaolinite specimen of sedimentary origin from Capim, Brazil, was examined, mainly by transmission electron microscopy (TEM). Selected-area electron diffraction (SAED) along the X-i ([100], [110], and [1 (1) over bar0]) directions shows various features, from completely discrete patterns to heavily streaked ones along the c* direction, suggesting that the degree of stacking disorder is variable among individual grains. High-resolution TEM images indicate that stacking faults are mainly caused by disorder of alternating t(1) (similar to-a/3) and t(2) (-a/3 +b/3) layer displacements. Furthermore, stacking faults have been observed (1) as isolated stacking faults (e.g. insertion of an isolated t(2) 'fault' in an ordered sequence with t(1) layer displacement) and (2) as interstratification of two kinds of multilayer blocks having regular t(1) and t(2) layer displacements. A mixture of grains with various degrees and modes of disorder with alternating t(1) and t(2) layer displacements may explain the experimental profile of the 02, 11 X-ray diffraction band. Faults related to displacement of the octahedral vacancy and/or to layer rotation were also observed in HRTEM images. The SAED patterns along the Y-i ([010], [310], and [3 (1) over bar0]) directions occasionally have extra spots and/or streaks, suggesting the presence of stacking sequences with (+/-60 degrees, 180 degrees) mutual layer rotation and/or with (0, +/-b/3) layer displacements. The local dickite or nacrite-like fragments formed by these faults are in qualitative agreement with recent low-temperature FTIR results from this sample, where distinct v(OH) absorption bands reflect multiple interlayer O-H center dot center dot center dot O environments that are possibly ascribed to dickite and nacrite.