GLOSSARY - Chemical Fundamentals of Geology and Environmental Geoscience - page 250

Divalent (of an atom or element) Having a valency Eutectic An invariant point in a phase diagram

where the melt field projects to the lowest tempera-
ture (e.g. E in Figure 2.4; E in Fig. 2.8). A eutectic
marks the final destination of a melt undergoing
fractional crystallization, or the initial melt composi-
tion during partial melting (Box 2.4).
Evaporite A sedimentary rock consisting of mineral
salts deposited from supersaturated brine as a result
of evaporation.
Exothermic (of a reaction) Giving out heat (Δ H
negative).
Exsolve, exsolution (-lamellae) The unmixing of a
homogeneous phase into two immiscible phases,
commonly as a lamellar intergrowth (e.g. perthite).
Extensive (of variables) Depending in magnitude on
the size of the system being considered (e.g. mass).

of 2.
dm − 3 'Per cubic decimetre' = 'per litre'. See Appendix A.

Electrolysis The extraction of an element (e.g. Cl)
from a solution containing its ions (Cl − ) by passage
of an electric current.
Electrolyte A compound which (as solid, melt or
solution) conducts electricity. See Appendix B.
Electron (e − or β − ) A negatively charged atomic parti-
cle responsible for bonding between atoms. See
Table 5.1.
Electron core The tightly held inner electron shells
not participating in bonding.
Electron-volt (eV) The unit of energy used to meas-
ure quantum energies and energy differences within
atoms. The kinetic energy possessed by an electron
that has been accelerated from rest by an electro-
static field of one volt: 1 eV = 1.6021 × 10 −19 J.
Element A substance consisting of a single type of
atom (all atoms having the same atomic number Z ).
Empirical Determined by experiment or observa-
tion, not by theoretical reasoning or calculation.
End-member One of two or more chemical compo-
nents or formulae in terms of which the composition
of a solid solution may be expressed.
Endothermic (of a reaction) Absorbing heat (Δ H
positive).
Energy See Box 1.1.
Energy level One of a small number of permitted (=
'quantized') energy values that an electron in an
atom may possess according to wave mechanics.
Enthalpy ( H ) Thermodynamic variable representing
the total kinetic energy of the atoms or molecules in
a substance.
Entropy ( S ) Thermodynamic variable representing
the internal disorder of a substance (Box 1.4).
Equilibrium constant A parameter of a particular
chemical equilibrium, expressing the relative pro-
portions of reactants and products at which the for-
ward and reverse reaction rates are equalized under
specific conditions - see Equation 4.8.
Equivalent, - mass The mass of a substance that will
neutralize one mole of H + in an acid-base reaction.
For a cation, a mass equal to molar mass divided by
the charge on ion. See also milliequivalent .

Ferromagnesian (of silicate minerals) Containing
essential Mg and Fe: olivine, pyroxene, amphibole
and dark mica.
Fissile Describes a nuclide capable of fission - split-
ting into two lighter nuclei when irradiated with
neutrons (Box 11.2).
Fission product A radioactive nuclide formed by
nuclear fission. Most fission products are neutron-
rich (Box 11.2) and prone to β-decay (Box 10.1).
Fluid Either liquid or gas. The term is applied to any-
thing that can flow. In geology, it often connotes a
supercritical aqueous fluid (see Box 2.2).
Fluorescence The excitation of light emission by
absorption of shorter-wavelength light.
Foliation Rock texture in which platy crystals tend to
lie parallel to each other.
Fractionated, -ation The progressive partial separa-
tion of one element (or isotope) from another owing
to chemical differences between them. (Hence 'frac-
tionated', the antonym of primitive .)
Fraunhofer lines Dark lines on the optical spectrum
of white light from a star (Plate 4), caused by absorp-
tion of light by electronic transitions in atoms of ele-
ments present in the cool outer envelope of the star.
Free energy ( G ) See Chapter 1.
Free radical A temporarily unattached atom (e.g.
Cl •) or group of atoms (e.g. CH 3 •) having an
unbonded valence electron (symbolized by '•') that
makes it highly reactive.