Mineralogy is the branch of geology that studies the chemical composition, crystal structure and physical, including optical, properties of minerals. Its specific fields include the origin and formation of minerals, classification, geographical distribution and utilisation.
Mineralogy is closely linked to petrology, geochemistry and crystallography. Petrology concerns the bulk composition and origin of rocks, geochemistry concerns the distribution and migration of elements, and crystallography concerns the geometry of atomic arrangements, while mineralogy combines these approaches to study minerals as the basic units of Earth and planetary materials.
Early writings on mineralogy, especially gemmology, can be traced to ancient Babylon, the Greek and Roman world, ancient and medieval China, and Sanskrit texts of ancient India and the Islamic world. They include Pliny the Elder's Natural History, which described many minerals and their properties, and the Persian scientist al-Biruni's Book of Gems.
The German Renaissance scholar Georgius Agricola wrote De re metallica in 1556 and De natura fossilium in 1546, establishing scientific methods for the field. Post-Renaissance Europe developed systematic mineral and rock research. Modern mineralogy rests on crystallographic principles and on microscopic study of rock thin sections after the invention of the microscope in the 17th century.
Nicolas Steno first observed the law of constancy of interfacial angles in quartz crystals in 1669, also called the first law of crystallography. The law was generalised and experimentally confirmed by Jean-Baptiste Romé de l'Isle in 1783. René Just Haüy, the father of modern crystallography, demonstrated the periodicity of crystals and established that crystal-face orientations can be expressed by rational numbers, the law of rational indices, later encoded as Miller indices.
In 1814 Jöns Jacob Berzelius introduced a mineral classification based on chemical composition rather than crystal structure. William Nicol developed the Nicol prism for producing polarised light while studying fossil wood in 1827–1828, and Henry Clifton Sorby showed that thin sections could be identified by optical properties under a polarising microscope. James Dana published the first edition of his System of Mineralogy in 1837 and introduced a chemical classification that remains standard. Max von Laue demonstrated X-ray diffraction in 1912, and William Henry Bragg and William Lawrence Bragg developed it into a tool for analysing crystal structures.
The first step in mineral identification is to examine physical properties, many of which can be measured on a hand specimen. They include density, usually expressed as specific gravity, measures of mechanical cohesion such as hardness, tenacity, cleavage, fracture and parting, macroscopic visual properties such as lustre, colour, streak, luminescence and diaphaneity, magnetic and electrical properties, radioactivity and solubility in hydrochloric acid.
Hardness is determined by comparison with other minerals. In the Mohs scale, a set of standard minerals is numbered from 1, talc, to 10, diamond, in order of increasing hardness. Harder minerals scratch softer ones, so an unknown mineral can be compared with the scale to see which minerals it scratches and which scratch it. Some minerals, such as calcite and kyanite, show significant directional hardness. Hardness can also be measured on an absolute scale with a hardness tester, and the Mohs scale is nonlinear relative to absolute hardness.
Tenacity describes how a mineral behaves when broken, crushed, bent or torn. Minerals may be brittle, malleable, sectile, ductile, flexible or elastic. An important factor is the type of chemical bonding, such as ionic or metallic.
Cleavage is the tendency to break along certain crystallographic planes, described by quality such as perfect or good and by crystallographic orientation. Parting is the tendency to break along weak planes caused by pressure, twinning or exsolution. When neither occurs, fracture is a more disordered break, which may be conchoidal, fibrous, splintery, jagged or uneven.
A well-crystallised mineral also has a characteristic crystal habit reflecting its structure or atomic arrangement, such as hexagonal prismatic or botryoidal. Habit is also affected by crystal defects and twinning. Many crystals are polymorphic and have more than one possible structure depending on pressure and temperature.
Crystal structure is the arrangement of atoms in a crystal. It is represented by a lattice that repeats a basic pattern, the unit cell, in three dimensions. The lattice is characterised by its symmetry and unit-cell dimensions, expressed by three Miller indices.
The lattice is invariant under symmetry operations including reflection about any point, rotation, inversion and rotation-inversion. These operations together form a crystallographic point group or crystal class, of which there are 32. There are also operations that displace all points, including translation, screw axes and glide planes. Combined with point symmetry, these give 230 possible space groups.
Most geology departments have X-ray powder diffraction equipment for analysing crystal structures. X-ray wavelengths are comparable to interatomic distances. Diffraction, the constructive and destructive interference of waves scattered at different atoms, produces a pattern of intensities that depends on crystal geometry. In a powdered sample, X-rays sample a random distribution of all crystal orientations.
Mineralogy can be divided into descriptive, genetic, experimental, applied and physical mineralogy. Descriptive mineralogy covers identification, classification and naming, genetic mineralogy studies the physicochemical conditions of growth, experimental mineralogy simulates high-temperature and high-pressure reactions, applied mineralogy concerns industrial, environmental and materials uses, and mineral physics studies elastic, thermal, electrical and optical properties.
Modern mineralogy also intersects environmental mineralogy, biomineralogy, planetary mineralogy and mineral informatics. Environmental mineralogy examines contaminant-mineral interactions, biomineralogy studies biologically controlled mineralisation, planetary mineralogy uses meteorite and spacecraft data to study extraterrestrial minerals, and mineral informatics applies databases and machine learning to large mineral datasets.
In recent years, experimental techniques such as neutron diffraction and advances in computing have extended the field into broader problems of inorganic chemistry and solid-state physics. It nevertheless remains focused on crystal structures common in petrogenetic minerals, such as perovskites, clay minerals and framework silicates.
The field has made major progress in understanding the relationship between atomic-scale structure and function. Important natural examples include precise measurement and prediction of mineral elastic properties, which provide new insight into the seismic behaviour of rocks and the depth discontinuities in seismic images of the Earth's mantle. In its focus on the link between atomic-scale phenomena and macroscopic properties, what is now often called mineral science perhaps overlaps more with materials science than any other discipline.