IV.

Crystallography

The majority of minerals occur in crystal form when conditions of formation are favorable. Crystallography is the study of the growth, shape, and geometric character of crystals. The arrangement of atoms within a crystal is determined by X-ray diffraction analysis. Crystal chemistry is the study of the relationship of chemical composition, arrangement of atoms, and the binding forces among atoms. This relationship determines minerals' chemical and physical properties. Crystals are grouped into six main classes of symmetry: isometric, hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic. See Crystal.

The study of minerals is an important aid in understanding rock formation. Laboratory synthesis of the high-pressure varieties of minerals is helping the understanding of igneous processes deep in the lithosphere (see Earth). Because all of the inorganic materials of commerce are minerals or derivatives of minerals, mineralogy has direct economic application. Important uses of minerals and examples in each category are gem minerals (diamond, garnet, opal, zircon); ornamental objects and structural material (agate, calcite, gypsum); abrasives (corundum, diamond, kaolin); lime, cement, and plaster (calcite, gypsum); refractories (asbestos, graphite, magnesite, mica); ceramics (feldspar, quartz); chemical minerals (halite, sulfur, borax); fertilizers (phosphates); natural pigments (hematite, limonite); optical and scientific apparatus (quartz, mica, tourmaline); and the ores of metals (cassiterite, chalcopyrite, chromite, cinnabar, ilmenite, molybdenite, galena, and sphalerite).