From a purely physical properties point-of-view, halloysite is a clay mineral similar to kaolin in firing and raw appearance. In fact, it normally occurs with kaolin deposits because kaolin is the precursor mineral. Applied Minerals describes it this way: "Halloysite is chemically identical to kaolin clay (Al2Si2O5(OH)4 x nH2O) with one layer of water molecules existing between layers of alumina and silica. Formation occurs when kaolin sheets roll into tubes due to the strain caused by a lattice mismatch between the adjacent silicon dioxide and aluminum oxide layers. This is a process that occurs over millions of years under extremely rare conditions."
Halloysites can often be employed in high quality porcelain recipes interchangeably (depending on purity and amount of iron). Some clays referred to as halloysite just contain a small percentage of the actual mineral (Helmer Kaolin is an example). It seems obvious that kaolin, having a flat particle shape, should theoretically be much more plastic. But in practice the plasticities of various halloysites are comparable with kaolins of the same iron content. While it is also intuitive that halloysites would have lower shrinkages, they actually shrink more, both in drying and firing.
Commercial halloysites of exceptional purity and fired whiteness are available. The unique particle shape of the material make it useful for a wide range of non-ceramic uses, in fact only a small percentage is used in ceramics.
Halloysite forms over long periods as kaolin sheets roll into tubes.
These three materials also fire to a similar color. Grolleg is the most plastic, Dragonite the least.
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.
|Materials||Plainsman Troy Clay|
|Materials||New Zealand Halloysite|
Halloysite mineral data
Halloysite at Wikipedia
One of many halloysite micrographs at minersoc.org
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.