Mineralogy studies the composition, structure and properties of minerals. A chemical analysis identifies which elements are present, while X-ray diffraction constrains their periodic atomic arrangement. These measurements answer different questions: substances with the same overall composition can have different crystal structures.
Microscopy and physical properties provide complementary evidence. Mineral assemblages and textures help reconstruct rock formation, deformation and fluid interaction. Identification must allow for solid solutions, in which composition varies within one structural family, and for alteration that changes only part of a grain.
A mineralogist studies minerals through their composition, crystal structure, properties and formation conditions. Identifying a specimen requires distinguishing observable appearance from the underlying atomic arrangement: different minerals can look similar, while one mineral can occur in many colors.
X-ray diffraction constrains repeating crystal structure, spectroscopy probes composition and bonding, and controlled heating or pressure experiments test stability ranges. Combining these measurements connects an individual mineral grain with the physical and chemical history of the rock or planetary material in which it formed.
A small trapped fluid pocket in a mineral, preserving evidence of fluid composition and trapping conditions, subject to later modification.
A mineral or fluid trapped within a diamond. Its formation time can differ from diamond growth and from later transport to the surface.
A zirconium silicate mineral widely used in uranium-lead dating and studies of hafnium isotope ratios and oxygen isotope ratios.
A titanium dioxide mineral that can strongly influence the partitioning of some trace elements during melting.
Silicon dioxide, including crystalline and amorphous forms.
Fine-grained hydrous silicate minerals with structures and surface properties important to adsorption and water-rock interaction.
A clay mineral with composition . Relative to gibbsite, its greater silica content allows higher dissolved silica thermodynamic activity to favor its formation, as quantified by the appropriate equilibrium constant.
A naturally occurring solid substance with a characteristic composition range and internal structure. Synthetic analogues can be engineered as hosts for selected waste constituents.
An aluminium hydroxide mineral with composition . In a simplified mineral activity diagram, it is favored over kaolinite at sufficiently low dissolved silica thermodynamic activity.
A specified limiting composition used to describe a solid solution in a mineral. Its thermodynamic activity depends on the chosen standard state and activity model. For example, pyrope is a magnesian garnet endmember, while phlogopite is a magnesian biotite endmember.
A mineral whose structure contains silicon–oxygen units. Different linkages give isolated groups, chains, sheets or extended frameworks. Refractory silicate grains are an important solid component of a protoplanetary disk.
A group of sheet-silicate minerals; an idealized Mg-rich composition used in weathering models is . Its alteration to gibbsite or kaolinite depends on dissolved silica and the magnesium-to-proton thermodynamic activity ratio.
A high-temperature potassium feldspar mineral, with idealized composition . Substitution on its K and Al sites changes the thermodynamic activity of that mineral endmember and therefore the chemical equilibrium of reactions involving it.
A mica mineral with an idealized endmember composition . Its formula contains one tetrahedral Al and two octahedral Al per formula unit. Under ideal independent mixing on crystallographic sites, an endmember thermodynamic activity includes all mixed sites rather than only a bulk mole fraction.
A family of commonly hydrous silicate minerals with double-chain structures, important in amphibolite and many igneous rocks.
An aluminium-bearing silicate mineral, one of several minerals sharing this composition with different crystal structures.
A silicate mineral represented by its sodium endmember in simplified aqueous chemical equilibrium models. Natural compositions can include potassium substitution.
Feldspar is a group of framework silicate minerals important in planetary crust. Common ideal endmembers include albite, , and calcium- and potassium-bearing compositions. Coupled substitutions preserve electrical charge while changing elemental proportions, so a natural feldspar need not have one composition shared by every specimen.
Plagioclase denotes the sodium-calcium solid-solution series. Composition and crystal structure influence a rock's physical properties and the interpretation of mineral observations. In chemical equilibrium calculations, an endmember formula is a useful component definition; it should not automatically be substituted for the full composition of a natural mixed crystal.
A group of silicate minerals with several cation positions that permit extensive solid solutions. Pyrope is the magnesian aluminium-bearing endmember relevant to a simple iron–magnesium exchange model.
The magnesian aluminium-bearing garnet mineral endmember. In an ideal Mg–Fe site model with other positions fixed, its thermodynamic activity is because three magnesium positions occur per formula unit.
A uranium titanate with ideal composition . It occurs in natural actinide-bearing minerals and can appear in titanate ceramic waste forms.
A calcium zirconium titanate with ideal formula . Its structural variants and substitution mechanisms are important in actinide-bearing ceramic waste forms.
A monoclinic zirconolite polytype with a four-layer repeat, observed in some actinide-substituted composition ranges.
