Chapter 6. The Defect Structure of Kaolinite

Summary

Kaolinite, At₂Si₂O₅(OH)₄, is a layer silicate mineral of the 1:1 variety. The layers are trioctahedral and have one surface formed ofhydroxyl groups while the other surface consists of oxygen atoms. Stacking of these layers to form a crystal places the hydroxyls opposite to the oxygen atoms. The interlayer bonding is a combination of nonspecific Lifshitz-van der Waals attraction and specific Lewis acid-base interactions (hydrogen bonding). Both layer surfaces are approximately closest packed oxygen atoms, so there are a large number of ways geometrically to stack the kaolin layers so that hydrogen bonding is possible. Many of these result in unfavorable superpositions of cations and thus do not occur.

Actual X-ray diffraction patterns of kaolinite samples show a remarkable variety ranging from sharp, well-separated peaks to broad and diffuse bands. Several attempts have been made to quantify these observations, the most commonly used is the Hinckley index. The diffraction effects are the result of stacking faults in the succession of kaolin layers. After nearly 50 years of effort, it is now possible to quantify the types of defects present in a specific kaolinite sample and to calculate a theoretical diffraction pattern for the defect structure. What is not clear presently is what effect the defect structure has on the physical, chemical, and colloidal properties of kaolinite samples.