The ultra-structure of kaolin
Creators: name: Ma, Chi
Publisher: name: The Australian National University
Year: 1996

description: Kaolin samples obtained from a wide range of sources and origins in eastern Australia (ie, Pittong, Lal Lal, Meredith, Vic.; Woodside, SA; Swan Bay, Bexhill, NSW; Weipa, Tarong, Cooyar, Nanango, Pierce's Creek, Mt Morgan, Qld.) were examined using Xray diffraction (XRD), infrared (IR), scanning electron microscopic (SEM) and transmission electron microscopic (TEM) techniques. Kaolinite as the dominant kaolin mineral occurs in most samples of weathered, hydrothermal and secondary origins, whereas halloysite appears mainly in samples derived from the weathering of basalts. Kaolinites exhibit structural differences that range from the perfectly tri-periodic minerals in most samples from the weathering and hydrothermal profiles of granite (eg, Pittong, Lal Lal) to the highly disordered materials in sedimentary kaolin deposits (eg, Swan Bay). Electron microscopic studies indicated some close relationships between texture, morphology and genesis of the kaolins. Primary kaolinites show no orientation and no particle size fractionation. Book-shaped and vermiform kaolinites are present in nearly all primary kaolins but are uncommon in transported kaolins. In sedimentary kaolins the arrangement of crystals seems to be tight, controlled by the effects of particle size fractionation and sedimentation. Halloysite tubes with an internal tunnel show great morphological variation in the weathered basalt samples. Parallel halloysite tubes occurring in samples from Tarong, together with kaolinite and smectite, are believed to have formed by rolling parallel to the y-axis of broken kaolinite plates. Based on the theoretical and experimental study of kaolin CECs, it is concluded that for kaolinite the exchangeable cations occur mostly on the edges and on the basal hydroxyl surface of the octahedral units, and therefore its cation exchange capacity strongly depends on the particle size (both thickness and diameter in ab-plane) and pH value. Particle size plays a more important role than crystallinity in affecting the CEC of kaolinite. Calculations of edge and surface CEC conform closely to observed CECs. High-resolution TEM (HRTEM) studies revealed the structural details of kaolin. Although HRTEM imaging of kaolinite is very difficult due to rapid electron beam damage, lattice fringes showing a 7.1-A periodicity and less commonly, 3.6-A and 3.5-A sub-periodicities were obtained. Computer simulations of HRTEM images were carried out to ascertain the electron optical conditions needed to obtain interpretable images, and these have allowed reliable TEM image interpretations. The 7-A lattice fringes of kaolinite crystals always show mottled contrast. The selected area electron diffraction (SAED) pattern of the [001] zone indicates the structure of kaolinite from all sites is C-centered. No kaolinite was found to have turbostratic stacking. Defects within the layer structure are common in both well-ordered kaolinite and poorly-ordered kaolinite. All halloysite HRTEM images show a 7-A fringe spacing, evidence that halloysite has lost all of its interlayer water and has collapsed under the high-vacuum conditions of TEM. Therefore, distinction between kaolinite and halloysite (7-A or 10-A) by the different spacing of the lattice fringes is impossible under the TEM. Based on SAED analysis, all tubular halloysite crystals examined (10 samples) are elongated along theyaxis and show a two-layer periodicity. The distinctive two-layer structure may be used as a diagnostic feature for the identification of tubular kaolin particles, particularly when halloysite constitutes only a very low amount in a mixture with kaolinite. Kaolinite [Al2Si20s(OH)4] is defined as a 1:1 di-octahedral clay mineral. Its surface layers (or cover layers) along the [001] direction are ideally a 1:1 TO layer on both sides, yielding a layer sequence TOTOTO ... TOTOTO, where T stands for a tetrahedral sheet and 0 for an octahedral sheet. Three types of surface layer were discovered in natural kaolinites. Type 1 has the expected 7-A surface layer as terminations. Type 2 has one 10-A pyrophyllite-like layer as the surface layer on one side of a kaolinite particle (ie, the layer sequence is TOTOTO ... TOTOTOT). Some industrialgrade highly-ordered kaolinites (eg, Weipa and Pittong deposits) have such a 10-A 2:1 surface layer on one side of the crystal. The spacing between the 1 0-A layer and the adjacent 7-A layer is not expandable. Type 3 kaolinite has one or several 10-A smectitelike layers at one or both sides of a stack, ie, (TOT)TOTO ... TOTOTOT(TOT), forming a special kind of kaolinite/smectite interstratification. These smectite-like layers were not detectable by XRD. This type has only been recognised in some poorly-ordered kaolinites from Bunyan and Andoom sediments. The presence of such 2: 1 surface layers on kaolinite may affect its physical and chemical properties and hence its industrial use. The surface smectite layer(s) contribute to higher CEC values. Chemical compositions of kaolins, in terms of Fe &lt;=&gt; AI octahedral substitution, were found to vary among individual kaolin particles with different or the same morphology. In general, larger kaolinite particles have less structural Fe. A negative correlation exists between structural Fe and kaolinite crystallinity. Kaolinite derived from the weathering of granites has the lowest structural Fe content (Fe203 &lt; 0.3 wt% ), compared with kaolinite of hydrothermal or transported origins. In weathering profiles, structural Fe in kaolinite from the mottled zone is higher than that from the pallid zone. Differential loss of not only alkali elements (eg, K, Na, Mg) and low-atomicnumber elements (eg, AI) but also higher-atomic-number elements (eg, Fe, Ti) in kaolins and other phyllosilicates during AEM analysis was revealed. The loss of AI in kaolin minerals is particularly severe. Kaolinite whose crystal structure can be damaged by electron irradiation (judged by diffraction) over several seconds is the most sensitive clay to the electron beam. Generally, phyllosilicates have progressively greater stabilities under AEM in the order kaolin&lt; smectite&lt; pyrophyllite