The most fundamental clay mineral. This mineral is found in nature in its purest form as kaolin. However it makes up at least part of all ceramic clays. The purest deposits are created as it is weathered from rock and deposited, mixed with other products of the weathering process, in layers and lenses in nearby valleys. Thus kaolins (also called China Clays) are whiter and cleaner than other clays because they are mined near the site where they were weathered and altered (rather than being contaminated when transported and sedimented by water). Because kaolin ore is mixed with other rocks and impurities, it has to be separated using various wet and dry processing methods. Kaolin can be found is some sedimentary deposits where it is mixed with quartz and feldspar particles, it can be separated by wet processing methods.
Kaolinite is a pure clay mineral crystal of one part alumina and two parts silica. Most other clay minerals are derivatives of kaolin. Kaolin particles are flat and comparatively the largest of all clay minerals, this makes them among the least plastic. They have a surface chemistry that gives them an affinity for water. The attached water both 'glues' particles together and acts as a 'buffer' to lubricate particle-against-particle movement in the plastic matrix.
In it purest form, kaolin melts at 1770C, thus it is quite refractory. However this temperature drops quickly as impurities increase (like feldspar). Kaolin particles have an inner crystalline structure of alternating flat Al2O3, SiO2 and hydroxyl layers. The Al2O3 and SiO2 bind firmly but the hydroxyl layer does not. This structure accounts for the flat shape of kaolin particles and the way they stack and separate in reacting to physical forces. It also accounts for the affinity for water that the particles have and the difference in charge between the flats and edges. In addition, this structure shapes what happens as heat is applied: the hydroxyl is driven off breaking the bonds between the layers and freeing most of the SiO2 and Al2O3 react on their own. Some of the SiO2 combines to form quartz micro-crystals and some of the Al2O3 forms alumina micro-crystals. Both can dissolve in the feldspar glass. Some of the SiO2 and Al2O3 can remain connected to preserve flat kaolinite particles. Of course the way it which this all plays out is dependent on many factors, especially the firing curve.
What is clay? How is it different that regular dirt? For ceramics, the answer lies on the microscopic level with the particle shape, size and how the surfaces interact with water.
|Materials||Standard Porcelain Kaolin|
|Materials||Super Standard Porcelain|
|Materials||Calcined Topaz Kaolin|
Kaolinite on WikiPedia
One of a number of micrographs of kaolinite at minersoc.org
The property of plasticity is evident in a wide array of materials of different mineralogy. In addition, many minerals are considered parent minerals to clays, they weather down to become these clays.