Crystalline materials consist of periodically repeating arrays of atoms with compositions that can be represented by definite chemical formulas (which may be variable within certain compositional limits). Crystal chemistry addresses the interdependence of crystallography and chemistry, and the attendant consequences and phenomena. Thus crystal chemistry is concerned with the symmetry and dimensions of the unit cell, the positions of atoms within the unit cell, the kinds of atoms found at each crystallographic site, and relationships with similar phases through such phenomena as solid solution, exsolution, polymorphism, and so forth. For earth scientists, the crystals of interest are minerals. Although the boundaries between crystal chemistry, materials science, and certain areas of solid state physics and solid state chemistry are quite blurry, we shall approach the subject from the view point of geology - that is, mineralogical crystal chemistry.