Geochemistry Definitions
Terms
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- redox reaction
- A reaction featuring the gain or loss of electrons.
- tie-line
- In a phase diagram, connects two phases of different chemical compositions that are in mutual chemical equilibrium at a certain temperature.
- brine
- Trapped seawater, highly saline, trapped in pore spaces, may concentrate and transport metals
- corona
- area between two mineral grains that contains their reaction product (evidence of disequilibrium)
- common ion effect
- Caused by contribution of the same ion by many different salts. Increases the IAP of the ion which may cause it to precipitate if its IAP is higher than the Ksp for the ion.
- kinetic fractionation
- in unidirectional reactions, due to reaction rates of different isotopes being slightly different
- incompatible element
- elements whose ions are not easily accommodated in the crystal structures of the principal igneous/metamorphic minerals. They tend to remain in a magmatic melt as it cools, and subsequently the melt becomes enriched in incompatibles.
- lysocline
- the depth at which carbonate dissolution increases
- reduction
- the gain of electrons
- exsolution
- the unmixing of a homogenous phase into two different immiscible phases, usually occurs as exsolution lamellae
- Nernst equation
- Determines voltage of an electrolytic cell from activities of involved elements and the number of electrons transferred during a reaction.
- lysocline
- the depth at which carbonate dissolution increases
- solvus
- in a limited solid solution phase diagram, the line below which a homogenous solid phase will exsolve into two different phases.
- coordination number
- In an ionic crystal, the number of anions surrounding a cation (or vice versa). Equivalent to the number of corners in a crystal's shape.
- reaction rim
- area around a mineral grain that indicates interaction between an early-formed crystal and a later, more evolved melt (evidence of disequilibrium)
- oxidation
- the loss of electrons
- zoning
- compositional gradient across a mineral crystal that indicates diffusion was unable to keep up with external system changes
- eutectic
- a point on a phase diagram where degrees of freedom is zero
- activity coefficient
- Relates activity to concentration, accounts for non-ideal behaviour
- concordia
- A plot of 207Pb/235Pb (xaxis) against 206Pb/238U (yaxis) with a curve connecting all concordant ages.
- fugacity
- For gases: partial pressure times activity coefficient.
- ionic strength
- Represents the total amount of ions in a solution and their charges. Less than 0.01 mol/kg for freshwater, 0.07 for seawater, and higher for brines.
- partition coefficient
- The ratio between the concentration of an element in a mineral and the melt, dependent on T,P and composition of melt.
- lithosphere
- The crust and uppermost mantle, the part of the Earth that is rigid and transmits stress. A mechanical division.
- Ion activity product (IAP)
- In a saturated solution: the product of the dissolved ions activity products, an equilibrium constant. If it is higher than the normal Ksp, solutes will crystallize. If it is lower, solutes will dissolve.
- phase
- a part of a system with uniform physical and chemical characteristics which distinguish it from all other parts of the system
- reaction rim
- An area between two mineral grains containing the product(s) of their mutual reaction. Evidence for dis-equilibrium.
- Eh
- Oxidation potential. Relative oxidizing or reducing character of a system. Measured with pH. Relates oxygen fugacity and pH
- electronegativity
- The ability of an atom to attract electrons
- initial ratio
- Ratio of radiogenic daughter element to parent element at the time a mineral passed through the blocking temperature, the ratio before decay products accumulated.
- Arrhenius plot
- Can be used to determine activation energy, a plot of inverse-temperature versus K.
- equilibrium
- a state with no change in its properties over time, a steady state; after a
- mineral
- A naturally occurring, inorganic solid with crystal structure that is orderly over long distances
- equilibrium fractionation
- Separation of isotopes due to their mass difference and the corresponding difference in reactivity, occurs in equilibrium reactions
- model age
- Uses assumed initial isotopic ratios for dating, not isochrons. Caveat mensor!
- meteoric water line
- Relates delta O-18 values to latitude (more negative = higher latitude), for meteoric waters only!
- activation energy
- The minimum energy required for molecules to react.