Clay mineral X-ray diffraction

Clay minerals are one of the more diverse minerals but all have a commonalty of crystal or grain sizes below 2 μm. Chemically, clays are defined by crystal structure and chemical composition, often determined by clay mineral X-ray diffraction. Sometimes fine grain sediments are mistakenly described as clays; this is actually a description of the "clay-size fraction" rather than the mineralogy of the sediment. There are three crystallographic clay groups: platy clays (phyllosilicates), fibrous clay minerals, and amorphous clay. Phyllosilicates are the more abundant clays and are categorized based on the layering of a tetrahedral and octahedral layer. For most clays, the octahedral layer is centered with Al³⁺, Fe³⁺, or Mg(OH)₂, but sometimes Zn²⁺, Li⁺, and Cr³⁺ can substitute as well. Si⁴⁺ is normally the center of the tetrahedral layer but Al³⁺ will often partially substitute and create a charge imbalance. Two-layer clays are composed of a tetrahedral layer and an octahedral layer (T-O) while three-layer clays contain an octahedral layer sandwiched by two tetrahedral layers (T-O-T). When substitution of Al³⁺ for Si⁴⁺ creates a charge imbalance, an interlayer cation will fill in between tetrahedral layers to balance the charge of the clay.