GLOSSARY - Chemical Fundamentals of Geology and Environmental Geoscience - page 249

Condensate Liquid or solid phase(s) that condense
from a saturated gas or vapour.
Condense, -ate To form a liquid or solid from a cool-
ing gas; product of condensation.
Condensed phases Liquid and solid phases, in
which the atoms or molecules are in mutual contact.
See Box 1.3.
Connate (of waters) Seawater trapped in the inter-
stices of a sedimentary rock at the time of
deposition.
Co-ordination polyhedron The hypothetical three-
dimensional shape obtained by joining the centres
of the anions immediately surrounding a cation (or
vice versa).
Core electron An electron in an inner shell, too
tightly bound to participate in bonding.
Corona A rim of different minerals surrounding a
crystal, resulting from incomplete metamorphic
reaction between the crystal and surrounding min-
erals (see Box 3.1).
Cosmogenic nuclide A nuclide/isotope formed by
cosmic-ray bombardment of certain elements in mete-
orites, the solid Earth and the Earth's atmosphere.
Cotectic One of several liquidus phase boundaries
(thermal valleys) in a ternary phase diagram.
Critical temperature The temperature above which
the liquid and gas states of a given substance merge
into one homogeneous phase, i.e. a supercritical
fluid (Figure 2.2.2).
Cryptocrystalline Consisting of crystals too small to
be readily seen under the optical microscope.

Diagenesis A term embracing post-depositional chemi-
cal, mineralogical and sometimes microbiological
changes that take place during burial of a sediment
(Box 4.7).
Diatomic (of a molecule of gas) Consisting of two
atoms.
DIC Dissolved inorganic carbon.
Differential equation An equation in which one or
more terms is a derivative .
Differentiation Mathematics (calculus): the process of
calculating the gradient or derivative of a depend-
ent variable with respect to another variable
(Appendix A).
Geochemistry: the physical or chemical segregation
of one type of material from another. A homogene-
ous or intimately mixed body becomes segregated
into two bodies or phases of differing composition,
hence differentiated .
Diffract, -ion Optical interference phenomenon
observed when electromagnetic rays interact with a
regularly spaced pattern, e.g. of atoms in a crystal.
See Box 5.3.
Diffusion The dispersal of one substance in another

by atom-by-atom (or molecule-by-molecule)

migration.
Dilute Describes a solution having a low solute
concentration.
Dimensionless number A number expressing the
magnitude of a quantity that has no associated
dimensions or units. An example is π, the ratio of the
circumference of a circle to its diameter: the circum-
ference and diameter both have dimensions of
length, expressed in units of metres, which therefore
cancel each other out when ratioed. The value of a
dimensionless number is independent of the system
of units adopted.
Dimer A polymer consisting of only two basic units.
Dipole A system of two electrostatic charges of equal
but opposite magnitude, held a specific distance
apart. Molecules may be permanent dipoles (owing
to internal differences in electronegativity between
atoms, as in water) or induced dipoles, caused by
the polarizing effect of an electrostatic field.
Dissociation A reaction in which a compound
decomposes into two or more simpler species. See
Chapter 4.

Dehydration The removal of water from a substance.
In mineralogy, a dehydration reaction is one in
which a hydrous (water-bearing) mineral such as
mica breaks down at high temperature to form an
assemblage of anhydrous (water-free) minerals plus
water vapour. (See Figure 2.3.)
Derivative An expression giving the rate of change
of one variable in relation to another upon which it
depends. A body's speed, the rate at which its posi-
tion x changes with respect to time t , can be regarded
as the derivative of x with respect to t written d
d

x
t

.
See differentiation below, Appendix A and Waltham
(2000).