Environmental Engineering Reference
In-Depth Information
MINERAL STRUCTURE
We need to consider atomic structures in order to understand minerals further. Atoms
comprise a nucleus of protons and neutrons inside an electron shell. Protons and electrons
have positive and negative electrical charges respectively and, when balanced, give atoms
electrical neutrality. However, atoms exchanging electrons create an electrical imbalance
and become known as
ions
. Net loss of negatively charged electrons leaves a smaller,
positive
cation
and net gain leaves a larger, negative
anion
. Mineral structure is created
by three-dimensional arrangements of suitable anions and cations in a repeated geometric
pattern. They are held together primarily by
electrostatic
attraction in
ionic bonds
or the
sharing of electrons by two atoms by
covalent bonds
. Precise patterns depend largely on
the size and internal packing of the anions and the presence of suitably sized cations in
the spaces between. A snug, compact fit restores electrical equilibrium at the points of
contact. Electron sharing provides a symmetrical framework which provides minerals
with distinct, crystalline structures. The geometric arrangement of cations and anions is
imparted faithfully to each crystal, providing three-dimensional symmetry and a crystal
lattice
.
When minerals grow freely from a melt or solution, rather than competing with
surrounding minerals for space, crystal lattices become visible to the eye, whereas
naturally their atomic structure is not. Each lattice is unique to a particular mineral,
although
ionic substitution
occurs in some minerals and others are
polymorphic
. The
former refers to substitution of an ion by one of similar size and charge, therefore fitting
physically and electrically into the atomic structure without altering the crystal structure.
Iron and magnesium, for example, may substitute for each other in
olivine
and its crystals
are referred to as a
solid solution
. Polymorphism, on the other hand, refers to identical
chemical composition expressed in two mineral and crystalline forms and is indicative of
adjustments in crystal structure between lithospheric and denser, deep mantle minerals.
Calcite/ aragonite and graphite/diamond are alternative polymorphs of calcium carbonate
and carbon respectively, formed under different temperature and pressure conditions. The
great hardness of diamond, formed at depths of approaching 150 km, is diagnostic of
depth/pressure influences on crystal structure.
MINERAL CHEMISTRY
Minerals such as lead, gold and carbon are single elements but the majority are
compounds. Fortunately, a short list of mineral groups from our cast of 2,000 accounts
for virtually all rock mass. One group - the
silicates
- not only forms by far the greatest
bulk and diverse range of minerals but also is an outstanding illustration of linkage
between textural and chemical properties. This centres on SiO
4
4-
the
silicate
tetrahedron
, an anion with four negative charges composed of four large oxygen anions
(4 × O
2−
) clustered around a small silicon cation (Si
4
+). Covalent bonding leaves four
surplus oxygen electrons, giving the silicate compound a negative charge. Tetrahedral
structure provides a large range of potential linkages in which electrical equilibrium can
be reached, either by
polymerization
through sharing oxygen atoms or by the addition of
other suitable cations, primarily from the above short list of elements. Aluminium may
also replace silicon, converting Si
4
to AlSi
3
or Al
2
Si
2
to form
alumino-silicates
requiring
further, balancing ions.
