The ultra-structure of kaolin
Date
1996
Authors
Ma, Chi
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
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 <=> 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 < 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< smectite< pyrophyllite <mica. A clear dependence of
element loss on the crystallographic orientation of layer silicates has been observed. An
exponential correlation between the i/Si intensity ratio (i: elements other than Si) and
analytical measuring time was discovered.
Glauconitic minerals occurring in unweathered marine sediments at Weipa were
found to consist mainly of glauconitic vermiculite' and 'glauconitic smectite'. They
belong to the end member of expandable glaucony which has not been well documented
elsewhere.
Description
Keywords
Citation
Collections
Source
Type
Thesis (PhD)
Book Title
Entity type
Access Statement
License Rights
Restricted until
Downloads
File
Description