GLOSSARY - Chemical Fundamentals of Geology and Environmental Geoscience - page 248

the Al 2 SiO 5 polymorphs kyanite, andalusite and silli-
manite (Figure 2.1). Some or all of the aluminium
occurs in octahedral co-ordination (Figure 8.2 and
Table 8.1).
Amorphous (of a solid) Lacking crystalline structure
and morphology.
Amphoteric Describes an oxide that can behave as an
acidic oxide in reaction with a strong base, or as a basic
oxide in reaction with a strong acid - see Box 8.1.
Anaerobic In the absence of oxygen (= anoxic).
Anhydrous Incorporating no water or OH − . See
dehydration.
Anion A negatively charged ion, produced when a
neutral atom accepts one or more additional elec-
trons ( an - comes from the Greek meaning 'up', refer-
ring to the increased number of electrons).
Anisotropy, -ic (of a material, e.g. a crystal) Having
physical properties that vary according to the direc-
tion of measurement.
Anoxic Describes an environment devoid of oxygen.
Antimatter Matter consisting of antiparticles .
Antiparticle Any subatomic particle of antimatter ,
having the same mass as the equivalent conven-
tional particle but the opposite charge; e.g. a posi-
tron (a contraction of 'positive electron') has the
same mass as an electron but a positive charge.
Atomic number ( Z ) The number of protons in the
nucleus of an atom. Also the total number of elec-
trons in the neutral atom.
Atomic weight See relative atomic mass.

Birefringence (a) A property of a crystal whereby the
refractive index varies according to the vibration
direction of the incident polarized light. (b) The
numerical difference between the maximum and
minimum refractive indices of the crystal.
Blocking temperature See closure temperature.
Buffer, -ing A chemical equilibrium that has the capac-
ity to resist externally induced change in key proper-
ties of a system (e.g. pH, Equation 4.24; f O 2 Figure 9.5).

Catalyst A substance that accelerates a chemical reac-
tion without being consumed by it.
Cation A positively charged ion, produced by the
loss of one or more electrons from a neutral atom
( cat - comes from the Greek meaning 'down', refer-
ring to the reduced number of electrons compared
with the neutral atom).
CFC Abbreviation for chlorofluorocarbon: class of
inert derivatives of simple hydrocarbons formerly
used as refrigerants, foam expanders and aerosol
propellants. See Chapter 9.
Chalcophile Cosmochemical category of elements
with an affinity for sulfide minerals - see Figure 11.4.
Closure temperature A temperature (specific to a
particular reaction or diffusion process) below which
diffusion rate becomes slower than cooling rate, and
the reaction effectively ceases.
Cogenetic Describes a suite of rocks of the same age
that share a common origin (e.g. crystallized from
the same parent magma).
Complex, -ing (Formation of) a cluster of ligands
around a central metal atom or ion, held together by
co-ordinate bonds (Chapter 7) that involve the over-
lap of ligand lone pairs with empty orbitals of the
central atom.
Component The chemical components of a system
comprise the minimum number of chemical (atomic,
isotopic or molecular) species required to specify com-
pletely the compositions of all the phases present.
Compound A substance in which different elements
are combined in specific proportions.
Concentration A parameter indicating how much of
a particular chemical species (component) is present
in a unit amount of the medium (phase) in which it
resides (which may be a gas, a liquid or a solid). See
Chapter 4.

β -particle (beta-particle, β − ) A high-energy electron
released by a nuclear reaction.
Bar A unit of pressure widely used in petrological lit-
erature. It approximates to one atmosphere. It has
been superseded in the SI system by the pascal (Pa)
= N m −2 . 1 bar = 10 5 Pa.
Baryon A term (from the Greek for 'heavy') embrac-
ing both protons and neutrons . Baryonic matter is
matter consisting of atoms and molecules (and ulti-
mately of protons, neutrons and electrons) as distinct
from more exotic forms of dark matter (Chapter 11).
Base, basic See Appendix B.
Biolimiting Describes essential nutrient elements
(N, P, Si) whose low concentration may limit biologi-
cal productivity.