Kaolin is the most abundant clay mineral and follows the elemental abundances of elements in the earth’s crust. (link 1)

Kaolin is the term for the crude mineral in the ground and can comprise kaolinite, quartz, mica, iron oxides etc. The latter minerals are usually very coarse and a simple particle size fractionation by sedimentation begins a refining process. Further mineral processing using magnetic separation, froth flotation, selective flocculation and reductive or oxidative bleaching further purifies the kaolinite.

Kaolinite primarily occurs as vermiform stacks of pseudo-hexagonal platy crystals. The crystal shape is dictated by the layered structure comprising aluminum hydroxide in an octahedrally-coordinated state bonded to silicate in a tetrahedrally coordinated state. It is essentially a 2D structure.

Each layer is hydrogen bonded to successive layers to form a crystal ranging from 20 to 200 nm thick that are loosely associated in vermiform stacks found in the crude clay in the ground. When testing prospective kaolin deposits, water containing a low level of dispersant is added to the clay to the point of incipient wetness; that is when the mixture begins to flow. This measurement gives an indication of the rheological properties of a suspension and the likely highest solids level a fluid slurry can achieve. This as been important for paper coating grades (the dominant high volume usage of kaolinite)  As a result thicker crystal kaolins are selected as they provide the highest solids slurries .  

These stacks can be broken down to the individual crystals by liquid working ( high solids makedown) and particularly through the use of a media mill. The latter provides what are termed delaminated clays.

The delaminated clays can have aspect ratios around 30:1

Differences in stacking due to shifts from layer to layer leads to the different polymorphs for kaolinite and designated as Dickite and Nacrite.

Halloysite: tubular Kaolin

When the layers are thin enough the mismatch in dimensions between the alumina and silicate layers results in the layers rolling up to form tubes, usually 50 nm in diameter. This form occurs naturally and is called Halloysite.