Spectrum The various wavelength components pre-
sent in a beam of light or other electromagnetic radia-
tion (such as an X-ray beam), displayed in order of
their wavelength or photon energy. The emission
spectrum of an element consists of a series of lines or
peaks, representing the specific wavelengths (or pho-
ton energies) that are characteristic of that element
(e.g. Figure 6.5).
Stoichiometry, stoichiometric The proportions,
determined by valency, in which elements combine
in a compound.
Strong acid (or base) An acid (or base) - such as HCl
(or NaOH) - that is completely ionized in aqueous
solution (see Appendix B).
Strong force (or strong nuclear interaction) The
powerful short-range force that binds nucleons
together in atomic nuclei. See Box 11.2.
Sublimate A solid crystallized directly from the
vapour, e.g. frost. Sublimation is the vaporization of
a solid without the intermediate formation of a melt.
Supercritical Describes a fluid whose temperature
exceeds its critical point . See Box 2.2.
Supersaturated (of a solution or solute) Having a
molality (or activity) product exceeding the solubil-
ity product of that solute at that temperature.
Metastable with respect to precipitation , tending to
precipitate solute until saturation is attained.
System Any part of the world to which we wish
to confine attention. May refer to a specific domain
of composition space, e.g. 'the system NaAlSi 3 O 8 -
CaAl 2 Si 2 O 8 '.
Systematic error A deviation between a measured
value and the true value of a quantity that is not
reduced by repeated measurement (cf. random
error ), but introduces a consistent bias.
Tracer A geochemical variable (e.g. 18 O/ 16 O) whose
value is altered by a specific geochemical process,
and therefore records the operation of that process.
Triple point An invariant point where three phase
boundaries (and the three phase-fields they sepa-
rate) meet in a phase diagram.
Trivalent (of atom, element) Having a valency of 3.
Twin, twinned Describes a crystal comprising two or
more domains with distinct crystal lattice orienta-
tions (illustrated by the dark and light grey fields in
Fig. 2.7) related by a symmetry operation (e.g. a mir-
Unsaturated Describes an organic compound con-
taining one or more double C=C bonds or triple
C ≡ C carbon-carbon bonds.
Valence electron An electron occupying an orbital in
the valence (highest energy) shell, and available for
Valency The number of chemical bonds that an
atom (element) can make in forming a molecule
(compound). Some elements have more than one
Vapour (vapor in N. America) Refers to any gas
phase present during a geochemical reaction. 3
Volatile Describes an element or compound readily
converted to the gaseous state (at relatively low tem-
perature), cf. refractory .
Weak acid An acid (e.g. carbonic acid, H 2 CO 3 ) that
exhibits only slight ionization in aqueous solution
(see Appendix B).
World Health Organization.
Zoned, zoning Continuous or abrupt (or sometimes
oscillatory) changes in composition between the
core and rim of a single crystal (see rear cover image)
or between neighbouring sectors.
Zwitterion (from German zwitter = hybrid) An ion
carrying both a positive and negative charge (a
property of amino acids, Chapter 9).
Texture Describes the geometrical relationships
between the constituent mineral grains in a rock.
Thermal energy Total kinetic energy possessed by a
substance by virtue of individual molecular motions.
Thermonuclear Describes energy released by nuclear
fusion reactions (in stars and the hydrogen bomb, etc.).
Tie-line Isothermal line in a phase diagram linking
two phases of different chemical compositions that
are in mutual chemical equilibrium at the tempera-
ture concerned. See Box 2.3.
Chemists reserve the term 'vapour' for a gas that is below its criti-
cal temperature (Box 2.2) and which can therefore be liquified by
compression, but this restriction in meaning is rarely adhered to
in geochemical usage.