Radiogenic A daughter isotope that results from the
decay of a radioactive parent isotope, and whose
abundance increases with time. The proportion of
such an isotope that is the product of the decay of a
Radionuclide A nuclide consisting of unstable
nuclei, undergoing radioactive decay.
RAM Abbreviation of relative atomic mass .
Random error an error that varies randomly (in
magnitude and sign) about a mean value when a
measurement is repeated under identical conditions.
The random error of a measurement can be reduced
by repeating it several times and taking the mean.
Rare earth element Alternative name for lanthanide
elements, but embracing lanthanum (La) too.
Rare gas See inert gas.
Reactant A chemical species participating in a chemi-
cal reaction, appearing on the left-hand side of the
Reaction rim An overgrowth of new minerals rim-
ming an early-formed crystal owing to its reaction
with later-stage melt. See Plate 1.
Redox A term embracing both red uction and ox ida-
Redox potential Eh See Box 4.7.
Reduction Original meaning: a chemical reaction
resulting in the removal of oxygen from a com-
pound. Current meaning: any chemical reaction
resulting in the addition of one or more electrons to
an atom or ion (Box 4.7), cf. oxidation .
REE Abbreviation for rare earth elements .
Refractory (of elements or their compounds) Having
particularly high melting and vaporization tempera-
tures, cf. volatile .
Relative atomic mass ( A r ) Mass of an atom, expressed
on a scale in which 12 C = 12.0000.
Relative molecular mass, relative molar mass ( M r )
Mass of a molecule expressed on scale in which 12 C =
Salt A class of compounds that result from reactions
between acids and bases, in which one or more
hydrogen ions of the acid have been replaced by
metal ions from the base - see Appendix B.
Saturated Describes (1) a solution containing the
maximum solute content, that coexists stably with
excess solute in solid or gas form; (2) an organic
compound containing only single C-C bonds.
Screened, -ing Lessening of the electrostatic inter-
action between two charges owing to another charge
interposed between them. Outer electrons in an
atom are screened from the nucleus by inner electron
shells, making them easier to detach.
Secular (of variations in the value of a physical quan-
tity) Non-cyclic, changing in a consistent direction
over a period of time.
SI Abbreviation for the Système International d'Unités
(International System of Units), the currently accepted
metric system used in this topic. See Appendix A.
Sialic Describes rock compositions rich in Si and Al
(e.g. continental crust).
Silica Silicon dioxide (SiO 2 ), in any crystalline or
Silicate Compound in which one (or more than one)
metal is combined with silicon and oxygen. See
Solidus Temperature at which the last fraction of
melt crystallizes, or the temperature at which a sub-
stance begins to melt. (Cf. liquidus .)
Solute The dissolved species in a solution.
Solvation The formation of a sheath of polar solvent
molecules around a dissolved ion, inhibiting reac-
tion with other ions and stabilizing the ion in solu-
tion. See hydration and Box 4.1.
Solvent The dominant component of a solution, the
medium in which dissolved species are dispersed.
Solvus A line or surface in a phase diagram indicat-
ing the compositions of immiscible phases in mutual
equilibrium at each temperature.
Speciation The identification of the different chemi-
cal forms in which an element or compound occurs
naturally (e.g. mercury metal as distinct from
Specific heat The enthalpy required to raise the
temperature of 1 kg of a substance by one kelvin
(J kg −1 K −1 ).
Abbreviation of relative molecular mass .
Salinity 'Saltiness' of a water sample, expressed
most simply as total grams of dissolved salts per kg
solution. 2 Seawater salinity averages 35 g kg −1 .
Salinity defined in this simple way is difficult to measure in prac-
tice, and other definitions are used today.