This document discusses clay minerals and their properties. It begins by defining clay minerals as hydrous aluminium phyllosilicates that occur with varying amounts of calcium, magnesium, iron, sodium and potassium. It notes that clay minerals are ultrafine grained and require special techniques like X-ray diffraction for study. The document then covers the chemical composition, physical properties, optical properties, structures, and classification of different clay minerals - including kaolinite, smectite, illite, and vermiculite. It concludes by discussing the paragenesis, uses, and references.