The study of the crystalline phases (chemical compounds or pure elements) that make up the Earth and other rocky bodies in the solar system—that is, the terrestrial planets (Mercury, Venus, and Mars), meteorites, asteroids, and planetary satellites. In universities, nearly all mineralogists are found in earth science or geology departments.
Before the twentieth century, the science of mineralogy was concerned with identifying new minerals and determining their chemical compositions and physical properties. Investigations of the chemical compositions of minerals played a central role in the early development of chemistry and atomic theory. One of the most striking aspects of minerals is that they often occur as well-formed crystals. This motivated the hypothesis that the external symmetry reflected an internal symmetrical arrangement of atoms. The development of x-ray diffraction in 1915 enabled mineralogists to determine the internal crystal structures of minerals (that is, how the atoms are arranged). Once the chemical compositions and crystal structures of minerals were known, it became possible to systematically determine the thermodynamic properties of minerals and understand their stability as a function of pressure, temperature, and composition. This enabled geologists to understand the stabilities of mineral assemblages and the origins of the different kinds of igneous and metamorphic rocks. See also: Crystal structure; Geology; Mineral; Petrology; X-ray diffraction
By the 1960s, the crystal structures of most of the rock-forming minerals had been determined by x-ray diffraction. New techniques, however, were used in pursuit of a deeper understanding of the structural chemistry of minerals. Transmission electron microscopy, developed in 1961, has revealed the nanoscale (10–1000 Å) structures of minerals that result from twinning, dislocations, and phase transformations. Starting in the 1960s, a variety of spectroscopic methods (such as Mössbauer, nuclear magnetic resonance, infrared, and Raman) were used to understand how atoms are ordered among the crystallographic sites in minerals and to identify the nature of defects and impurities. See also: Electron microscope; Spectroscopy
With the advent of increasingly powerful computers, mineralogists began investigating the structures and properties of minerals using methods in computational chemistry. In the 1980s and 1990s, these calculations were largely based on classical models for interatomic interactions. In recent years, calculations of mineral stabilities and properties have been done using first-principles calculations based on either the Hartree-Fock approximation or density functional theory. See also: Computational chemistry; Quantum chemistry
Current research areas
Today, mineralogy is concerned with understanding the physics and chemistry of minerals insofar as they control processes in geochemistry and geophysics. Indeed, the boundaries between mineralogy, petrology, and geochemistry have largely disappeared. See also: Geochemistry; Geophysics
Phase equilibria and thermodynamic properties of minerals
The important minerals that occur deep in the Earth's crust have been discovered; however, their thermodynamic properties are often unknown. Laboratory experiments using high-pressure apparatus (the multianvil press and the diamond-anvil cell) can be used to determine the stable phases at the conditions of the Earth's deep interior. Laboratory experiments show that, in the extreme pressures of the Earth's lower mantle, the dominant mineral phases appear to be (Mg,Fe)SiO3 and CaSiO3 with a perovskite structure, along with (Mg,Fe)O with the rocksalt structure. There are still many other minerals, composed of minor chemical components, which may also be stable in the Earth's mantle. Although these phases may not contribute much to the seismic profile of the Earth, they may be geochemically important as reservoirs of trace components used to understand the chemical evolution of the Earth. The crystal structures of minerals allow a variety of solid solutions and order–disorder transformations. As we understand the thermodynamics and kinetics of such processes, we can develop tools to infer the temperatures and pressures at which a rock formed. Order–disorder transformations can give clues on the rate at which rocks cooled after their formation. These geothermometers, geobarometers, and geospeedometers are invaluable tools for geologists seeking to unravel the processes of mountain building and metamorphism. See also: Geologic thermometry; High-pressure mineral synthesis; Metamorphism; Orogeny
Physical properties of minerals at high pressure and temperature: understanding the composition of the Earth from seismology
A major goal in the earth sciences is to determine the chemical composition of the Earth. An important constraint is provided by seismic data on the density and seismic velocities of the Earth as a function of depth. To derive compositional information from such data, we need to know what mineral phases are stable at high pressures and temperatures and their densities and elastic properties. The multianvil press and the diamond-anvil cells also allow in-situ measurements using x-ray diffraction of minerals confined at high pressure and heated to high temperature. From such experiments, we can determine structural changes and densities of minerals under the conditions of the deep Earth. Elastic properties can be measured using Brillouin scattering; because such experiments are difficult, theoretical calculations based on first principles (quantum mechanics) are also being used. See also: Seismology
Water in nominally anhydrous minerals at high pressure
Some high-pressure minerals such as wadsleyite, β-Mg2SiO4, have defect structures that allow some oxygen atoms to be replaced by OH. Many investigators are proposing that a significant amount of water is chemically stored in the Earth's mantle by this mechanism.
Colloid and interface chemistry of minerals: controls on aqueous and environmental geochemistry
As rocks weather, the minerals that formed at high temperature will dissolve and reprecipitate to form a variety of clay minerals along with several oxide and hydroxide minerals. These secondary minerals are important phases in soils and sediments. Because they occur as colloidal-sized (nanocrystalline) particles, they have a very high surface area. The electrostatic charge of the mineral surfaces promotes the sorption of dissolved ions from aqueous solutions. Consequently, clay minerals and authigenic oxide and hydroxide minerals exert major controls on the chemistry of natural waters. In particular, chemical processes at the mineral–water interface control the fate of toxic metals in soil and ground water and the availability of important micronutrients in the oceans. Understanding these processes at a molecular level is an active area of research. The interfacial properties of sulfide minerals are investigated for the development of flotation techniques to separate ore minerals from their host rocks. Reactions at the surfaces of sulfide minerals also are responsible for the formation of acid mine drainage. A variety of analytical techniques are being used to investigate the mineral–water interface. The availability of synchrotron radiation sources has enabled mineralogists to characterize the coordination chemistry at the mineral–water interface using a variety of spectroscopic techniques. Scanning tunneling microscopy and atomic force microscopy are unraveling the atomic structures of mineral surfaces. See also: Authigenic minerals; Clay minerals; Oxide and hydroxide minerals; Scanning tunneling microscope; Sugarcane
Current research is investigating the mechanisms by which organisms form inorganic minerals. Biomineralization processes may provide important clues to Earth's history: variations in trace metals and isotopic compositions are found in the shells of marine planktonic organisms, such as foraminifera. It is believed that these proxies may be giving a record of past seawater temperatures.