Accretion. Accumulation of materials in space to form larger objects. This process may occur homogeneously (producing bodies with uniform composition) or heterogeneously (producing layered bodies).
Activation energy. The amount of energy that must be provided to overcome some kinetic barrier.
Activity. Concentration of a component, adjusted for any effects of nonideality; also, a measurement of the number of radioactive decay events per unit of time.
Activity coefficient. Ratio of the activity of a species to its concentration; a measure of the degree of nonideal behavior of a chemical species in solution.
Adiabatic process. A process that occurs without exchange of heat with the surroundings.
Advection. Transport of ions or molecules within a moving medium.
Alkalinity. Charge deficit between the sum of dissolved conservative cations and anions in an electrolyte solution.
Alkane. Saturated aliphatic hydrocarbon with general formula CnH2n+2; alkanes with at least 16 carbons are solids, usually found in primary producers (photosynthesizing organisms).
Alloy. A nonstoichiometric combination of metals.
Amino acid. Organic compound containing an amino (NH2) and a carboxyl (COOH) group. There are 20 amino acids used in protein synthesis.
Assimilation. Incorporation of solid material into a magma.
Atomic number. The number of protons in an atomic nucleus.
Authigenesis. Mineral formation from dissolved or solid constituents already present at the site, as opposed to constituents transported from elsewhere.
Biomolecule. Organic compound, such as a protein or carbohydrate, that is a component of an organism.
Biopolymer. Complex organic molecule synthesized by plants or animals. Examples of these molecules include carbohydrates, proteins, lignin, and lipids.
Bitumin. Generic term applied to naturally occurring, flammable organic substances consisting of hydrocarbons. Bitumin is soluble in organic solvents.
Buffer. Assemblage of chemical species whose coexistence allows a system to resist change in some intensive property, such as pH or oxygen fugacity.
Calorimetry. Experimental measurement of heat evolved or absorbed during a specific reaction.
Carbohydrate. Organic compound present in living cells with general formula CH2O. It is the most abundant class of organic compounds and includes sugars, starches, and cellulose.
Carbonate compensation depth (CCD). Depth in the oceans at which the downward flux of carbonate minerals is balanced by their rate of dissolution.
Chalcophile. An element with an affinity for sulfide phases.
Chelate. Large molecule or complex that can enclose weakly bound atoms or ions.
Chemical potential (μ). Partial molar free energy, which describes the way in which total free energy for a phase responds to a change in the amount of component i in the phase [=(∂G/∂ni)P,T,nj≠i].
Chondrite. A common type of meteorite, with nearly cosmic elemental abundances, thought to be a sample of the earliest Solar System material.
Closed system. A system that can exchange energy, but not matter, with its surroundings. Compare: Isolated system, Open system.
Colloid. Stable electrostatic suspension of small particles in a liquid.
Components. Abstract chemical entities, independently variable within a system, which collectively describe all of the potential compositional variations within it.
Compressibility. Measure of the relationship between phase volume and lithostatic pressure, [βT = −1/V(∂V/∂P)T; βs = −1/V(∂V/∂P)S].
Condensation. The formation of solids or liquids from a gas phase during cooling.
Congruent reaction. Reaction in which one phase melts or dissolves to form another phase of the same composition. Compare: Incongruent reaction.
Conservative species. Any dissolved species whose concentrations are not affected in solution in any way, other than dilution, by variations in the abundance of other species in solution.
Cosmic abundance of the elements. Relative elemental abundance pattern in the Sun.
Cosmogenic nuclide. Any isotope formed by interaction of matter with cosmic rays.
Cotectic. Boundary curve on an n-component phase diagram, along which two or more phases crystallize simultaneously. Compare: Peritectic.
Delta notation. Notation used for stable isotope data, equal to [(Rsample − Rstandard)/Rstandard] × 1000, where R is the ratio of heavy to light isotope of the element of interest.
Diagenesis. The set of processes, other than weathering or metamorphism, that change the texture or mineral composition of sediments; these include compaction, cementation, recrystallization, and authigenesis.
Differentiation. Separation within a homogeneous planet of crust, mantle, and core; also, any process by which magma can give rise to rocks of contrasting composition.
Diffusion. Dispersion of ions or molecules through a medium that is not moving, due to a gradient in some intensive property across the system.
Diffusion coefficent. Proportionality constant relating flux to gradient in a diffusion equation.
Distribution coefficient (KD). Ratio of concentrations of a component in two coexisting phases.
e-folding time. Time required for a compositional variable to return to within a factor of 1/e of its steady state value following some perturbing event, numerically equal to the inverse of the kinetic rate constant.
Eh. Redox potential of a cell, expressed in terms of voltage.
Electrolyte. Any substance that dissociates into ions when dissolved in an appropriate medium.
Electron. Subatomic particle, found in a region surrounding the nucleus of an atom, with a unit negative electrical charge.
Electron affinity (EA). Amount of energy released when an electron is added to an atom, creating an anion.
Elementary reaction. Any chemical reaction that occurs at the molecular level among species as they appear in the written representation. Compare: Overall reaction.
Enantiomer. One of two compounds with the same composition but having structures that are mirror images of each other.
Endothermic reaction. Reaction that absorbs heat, resulting in a decrease in enthalpy. Compare: Exothermic reaction.
Enthalpy. Thermodynamic function of state, H; a measure of the energy that is irretrievably converted to heat during any natural process.
Entropy. Thermodynamic function, S, the change of which is defined as the change in heat gained by a body at a certain temperature in a reversible process [dS = (dQ/T)rev].
Equation of state. Function that defines interrelationships among intensive properties of a system.
Equilibrium. Condition in which the properties of a system do not change with time.
Equilibrium constant (Keq). Constant relating the composition of a system at equilibrium to its Gibbs free energy.
Eutectic. Point on a phase diagram at which the liquidus touches the solidus, where two or more solids crystallize simultaneously from a melt.
Exothermic reaction. Reaction that evolves heat, resulting in an increase in enthalpy. Compare: Endothermic reaction.
Exsolution. Physical unmixing of two phases.
Extended defects. Dislocations and planar defects in crystals that provide effective pathways for diffusion.
Extensive property. Variable whose value is a measure of the size of the system. Compare: Intensive property.
Extinct radionuclide. Any unstable nuclide that decays so rapidly that virtually none remains at the present time.
Fluid inclusion. Fluid trapped within crystals. These may be primary (trapped as the crystal grew) or secondary (introduced at some later time).
Flux. Amount of material moving from one location to another per unit time.
Fractional crystallization. Physical separation of crystals and liquid, preventing equilibrium between phases.
Fractionation. Separation of any two entities, such as isotopes (by mass), elements (by geochemical or cosmochemical properties), or crystals and melt (by fractional crystallization or fractional melting).
Fractionation factor. Ratio of a heavy to a light isotope of an element in one phase divided by the same ratio in another coexisting phase.
Fugacity. Gas partial pressure that has been adjusted for nonideality.
Geobarometer. Calibrated mineral exchange reaction that is a strong function of pressure but not temperature.
Geochronology. Determination of the ages of rocks using radiometric dating techniques.
Geopolymer. Any of several complex organic compounds of high molecular weight, formed from biopolymers during diagenesis; these include kerogen and bitumen.
Geothermal gradient. Rate of change of temperature with depth in the earth; also called a geotherm.
Geothermometer. Calibrated mineral exchange reaction that is a strong function of temperature but not pressure.
Gibbs free energy. Thermodynamic function of state, G, which is a measure of the energy available for changing chemical bonds in a system.
Half-life. Time required for one-half of a given number of atoms of a radionuclide to decay.
Half-reaction. Hypothetical reaction that illustrates either gain or loss of electrons, useful for understanding redox equilibria.
Heat. Transfer of energy that results in an increase in temperature.
Heat capacity. Functional relationship between enthalpy and temperature [CP = (∂H/∂T)P; Cv = (∂H/∂T)V].
Helmholtz free energy. Thermodynamic function of state, F, which, for an isothermal process, is a measure of the energy transferred as work; also called the work function.
Henry’s law. Expression of solution behavior in which the activities of dissolved species are directly proportional to their concentrations at high dilution.
Humic substance. Altered complex organic substance derived from the remains of terrestrial plants and phytoplankton.
Ideal solution. Any solution in which the end-member constituents behave as if they were independent.
Immiscibility. Condition that results in spontaneous phase separation.
Incongruent reaction. Reaction in which one phase melts or dissolves to form phases of different composition. Compare: Congruent reaction.
Intensive property. Variable whose value is independent of the size of the system. Compare: Extensive property.
Internal energy. Thermodynamic function of state, E, the change of which is a measure of the energy transferred as heat or work between the system and its surroundings.
Ionization potential (IP). Amount of energy needed to remove an electron from an atom, creating a cation.
Isochron. Line on an isotope plot whose slope determines the age of a rock system. Isochrons may be defined by the isotopic compositions of constituent minerals or whole rocks.
Isolated system. A system that cannot exchange either matter or energy with the universe beyond the system’s limits. Compare: Closed system, Open system.
Isotopes. Atoms of the same element but with different numbers of neutrons and hence different atomic weights. Isotopes may be stable or unstable; also called nuclides.
Kerogen. Insoluble, high-molecular-weight organic matter derived from algae and woody plant material and found in sedimentary rocks. This material may yield petroleum when heated.
Kinetics. Description of a system’s behavior in terms of its rates of change. Compare: Thermodynamics.
Lignin. A complex polysaccharide that provides structural support for many plants and is one of the chief organic substances in wood.
Lipid. Class of aliphatic hydrocarbons containing the fatty acids, fats, waxes, and steroids. These compounds are energy sources for the host organism.
Liquidus. Boundary curve on a phase diagram representing the onset of crystallization of a liquid with lowering temperature. Compare: Solidus.
Lithophile. Element with an affinity for silicate phases.
Local equilibrium. Equilibrium that is attained on a small scale, but not on a larger scale; also called mosaic equilibrium.
Lysocline. Depth in the oceans at which the effects of carbonate dissolution are first discernable.
Mass number. The number of protons plus neutrons in a nucleus.
Mean residence time. Average residence time for a chemical species in a system before it is removed by some loss process.
Metamorphism. Collection of processes that change the texture or phase composition of a rock through the action of temperature, pressure, and fluids.
Metasomatism. Metamorphism with accompanying change in chemical composition, usually as a result of fluid migration.
Metastable system. A system that appears to be stable because it is observed over a short time compared to the rates of reactions that alter it.
Mobile component. A component free to migrate into or out of a system.
Monolayer. An organic coating with a thickness of only one molecule.
Monolayer equivalent. The mass of organic matter that is theoretically present over a square meter if all surfaces of the sediment have a monolayer coating.
Neutron. Subatomic particle, found in the nucleus of an atom, with no electrical charge.
Nuclear statistical equilibrium. The steady state in which disintegrative and constructive nuclear reactions balance in an evolved star.
Nucleation. The initiation of a small volume of a new phase. This process may occur homogeneously (without a substrate) or heterogeneously (on a substrate).
Nucleosynthesis. The production of nuclides, primarily in stars.
Nuclides. See Isotopes.
Open system. A system that can exchange matter and energy with its surroundings. Compare: Closed system, Isolated system.
Overall reaction. Chemical reaction that describes a net change involving several intermediate steps and competing pathways. Compare: Elementary reaction.
Partial melting. Fusion of something less than an entire rock, a process that may occur under equilibrium or fractional (continuous separation of melt and crystals) conditions.
Periodic table. A tabular arrangement of the elements, in order of their increasing mean atomic weight, in such a way that elements with similar chemical properties related to their electronic configuration are grouped together.
Peritectic. Point on a phase diagram at which an incongruent reaction occurs. Compare: Cotectic.
pH. Measure of acidity in solution [pH = −log10aH+].
Phase. Substance with continuous physical properties.
Phase diagram. Graphical summary of how a system reacts to changing conditions or composition.
Point defects. Crystal imperfections caused by the absence of atoms at lattice sites; such defects may be intrinsic or extrinsic.
Proton. Subatomic particle, found in the nucleus of an atom, with a unit positive electrical charge.
Quantum. A discrete quantity or level of energy.
Racemic. An equal mixture of D- and L- enantiomers.
Racemization. Transformation of optically active compounds, such as amino acids, from the D-form to the L-form or vice versa.
Radioactive decay. Spontaneous transformation of one unstable nuclide into another isotope; this occurs by alpha or beta decay, positron decay, electron capture, or spontaneous fission.
Radiogenic nuclide. Isotope formed by decay of some parent radionuclide.
Raoult’s law. Expression of solution behavior in which the activities of dissolved species are equal to their concentrations.
Refractory. Having a high melting temperature or condensing from a gas at a high temperature.
Salinity. The total dissolved salt content.
Saturated hydrocarbons. Organic molecules consisting of carbon and hydrogen atoms linked only by single bonds; these may form chain structures called aliphatic compounds, or cyclical structures called alicyclic compounds.
Secular equilibrium. Condition in which the decay rate for daughter radionuclides in a decay series equals that of the parent.
Siderophile. An element with an affinity for metallic phases.
Solidus. Boundary curve on a phase diagram representing complete solidification of a system with lowering temperature. Compare: Liquidus.
Solubility product constant (Ksp). Equilibrium constant for a dissolution reaction.
Solute. The dissolved species in a solution.
Solvent. The host species in a solution.
Solvus. Region of unmixing on a phase diagram.
Spinodal. Region of unmixing inside the solvus on a phase diagram; such unmixing involves no nucleation barrier.
Standard state. Arbitrarily selected state of a system used as a reference against which changes in thermodynamic properties can be compared.
Steady state. Condition in which rates of change within a system balance one another, so that there is no net change in its appearance for an indefinite time period, even though various parts of the system are not in thermodynamic equilibrium.
Sterol. A group of solid, mainly unsaturated steroid alcohols, including cholesterol and ergosterol, present in plant and animal tissues.
Stoichiometric. Summing to the whole, as in stoichiometric equations or coefficients.
Supernova. Massive stellar explosion, important for nucleosynthesis.
Surface tension. Tensional force applied perpendicular to any line on a droplet surface.
System. That portion of the universe that is of interest for a particular problem; a system can be open, closed, or isolated.
Thermal expansion. Measure of the relationship between phase volume and temperature [αP = −V(∂V/∂T)P].
Thermocline. Transitional range between the warm surface zone and the cold deeper zone in the oceans.
Thermodynamics. Set of laws that predict the equilibrium configuration of a system and how it will change if its environmental parameters are changed. Compare: Kinetics.
Titration. Experiment in which a reaction is allowed to proceed incrementally, so that proportions of reactants can be determined.
Trace elements. Elements that occur in rocks with concentrations of a few tenths of a percent or less by weight; these may exhibit compatible or incompatible behavior.
Transition elements. Elements with inner orbitals incompletely filled by electrons.
Troposphere. Lower portion of the atmosphere that contains the bulk of its mass.
Uncompressed mean density. Mean density of a planet, corrected for the effects of gravitational self-compression.
Undercooling. Difference between the equilibrium temperature for the appearance of a phase and the temperature at which it actually appears.
Unsaturated hydrocarbons. Organic molecules with double or triple bonds between carbon atoms; these may form chain structures, called aliphatic compounds, or cyclic structures, called aromatic compounds.
Variance. Number of independent parameters that must be defined in order to specify the state of a system at equilibrium; also called degrees of freedom.
Volatile. Term indicating that a given element or compound commonly occurs in the gaseous state at the temperature of interest, or indicating that a given element condenses from a gas at low temperature.
Weathering. Those processes occurring at the Earth’s surface that cause decomposition of rocks.
Work. Transfer of energy that causes a mechanical change in a system or its surroundings; the integral of force × displacement.