In the Mineral Science section we consider how atoms are arranged in crystalline materials and how this ultimately governs the nature of geomaterials. Interaction of crystalline materials with light, X-rays and electrons are used to introduce the theoretical and practical basis behind the polarising microscope, X-ray diffraction and electron microscope/microprobe. In Composition of the Earth we review the main groups of Earth Materials, considering (1) how structure, chemistry, physical properties, and occurrence are interrelated, (2) how earth materials are used in modern research as information sources to reveal the nature of Earth processes, and (3) introduce theoretical aspects of modern Earth Materials research (e.g. phase stability and transitions). In the final section Chemical Equilibria we consider how the stability and occurrence of geomaterials can be predicted and determined numerically using thermodynamics, and consider factors governing the rates of Earth processes at variable depths.