Functional group A peripheral group of atoms
attached to the basic -C-C-C- skeleton of an
organic molecule, that bestows a particular chemi-
waveform with a different shape and energy struc-
ture, e.g. the tetrahedral sp 3 hybrid characteristic of
many carbon compounds (Figures 7.5a and d) by
combining one 2s and three 2p orbitals.
Hydration (a) Incorporation of water or OH − , cf.
dehydration . (b) The stabilization of an ion in aque-
ous solution by the gathering of polar water mole-
cules around it. See Box 4.1.
Hydrothermal Refers to hot saline aqueous fluids
circulating in the crust, or to veins and ore deposits
crystallized from such fluids.
Gas constant R A fundamental physical constant.
R = 8.314 J mol −1 K −1 .
Geothermometry Science of estimation of a rock's
temperature of formation (the temperature at which
its mineral assemblage equilibrated) from the com-
positions of the equilibrated minerals.
Glass A state of matter having the disordered struc-
ture of a melt but the mechanical properties of a
The OH group or OH − ion.
Ideal solution A solution sufficiently dilute for ion-
ion interactions to be insignificant.
Idiochromatic Colour of a crystal that is derived
from one of its major elements (Gr. idios = own).
Incompatible element See Box 9.1.
Incongruent melting Melting of a mineral where a
new mineral is formed in addition to melt. See Box 2.6.
Inert gas The family of gaseous elements helium (He),
neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and
radon (Rn) on the extreme right of the Periodic Table.
Half-life t 1/2 The characteristic time it takes for a
radioisotope to decay to half its original concentra-
tion or radioactivity.
N taken at
Describes something that is extremely
Integral The graph shows y varying as a function of
x . The integral of y is an algebraic function express-
ing the area beneath the y curve between any speci-
fied limits x 1 and x 2 . It is written ∫ 1
0.5 N o
x y d x .
Hard water, hardness Describes fresh water with a
relatively high natural content of Ca 2+ and Mg 2+
leached from aquifer minerals (usually carbonates)
by the reverse of reaction 4.2.1 (Box 4.2). Hardness is
the molar concentration of Mg 2+ + Ca 2+ in mol l −1 .
Heterogeneous (of a reaction or equilibrium)
Involving the participation of more than one chemi-
cal phase (e.g. reaction 4.12), cf. homogeneous .
Homogeneous (of a reaction or equilibrium) Taking
place within a single chemical phase (e.g. reaction
3.1), cf. heterogeneous .
Hybrid, -ization (of electron orbitals in atoms) The
merging of the wave functions of several valence
electrons in an atom to generate a multi-electron
Integrate, -ation The mathematical process of calcu-
lating the integral of, or the area under, a curve.
Integration can be considered the reverse of differen-
tiation: thus the integral of velocity as a function of
time indicates the distance travelled.
Intensive (of a variable) Having a magnitude inde-
pendent of the size of the system considered (e.g.
Interfere, -ence Term in optics describing interac-
tions between superposed waves (e.g. light rays) of