Geoscience Reference
In-Depth Information
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 replacement 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 sub-
stitute 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.
electrical and elemental associations. For example, each
tetrahedron shares two common oxygen atoms to create
the basic formula (SiO
2
)
n
2-
of
chain silicates
. A divalent
cation such as Mg
2
+ or Fe
2
+ would then form the
pyroxene-group mineral, hypersthene (Mg,Fe)SiO
3
.
This also happens to be a solid solution! Mineral density
and hardness are greatest in the
single
and
ring
sili-
cates, decreasing as the number of shared oxygen atoms
increases and the framework expands with larger
balancing cations. The tetrahedral complex is based on
strong, covalent Si-O (anion) bonds and weaker ionic
(cation) bonds with the associated elements. This renders
crystal strength
anisotropic
; crystals are not uniformly
strong in all directions and cleave more readily through
the ionic bonds.
Cleavage
develops across columnar
crystals in ring silicates and between the bands and sheets
of double-chain and sheet silicates. Mica, for example, has
a particularly flaky structure. Generally, the strongest
common minerals are three-dimensional tetrahedra or
framework silicates
such as quartz and feldspar. Quartz is
the purer silicate, formed solely of SiO
2
with all oxygen
anions shared, but aluminium replaces some silicon in
feldspar and is balanced by potassium (K
1+
), sodium
(Na
1+
) or calcium (Ca
2+
) to form orthoclase, albite or
anorthite solid solution feldspars. Quartz and feldspars
form over 70 per cent of continental lithosphere.
Other principal mineral groups are built of simpler
oxides and anion complexes (
Table 12.2
).
Oxides and
sulphides are important metallic minerals, with sulphur
replacing oxygen as the anion in the latter, and salt cations
form halides with fluorine and chlorine. Oxygen
associated with the carbonate, sulphate, phosphate and
hydroxyl anion complexes (CO
3
)
2-
, (SO
4
)
2-
, (PO
4
)
3-
and
OH
-
forms carbonates, sulphates, phosphates and
hydroxides respectively. Most of these minerals do not
form directly from melts but are dependent instead on
metamorphic
,
metasomatic
and
sedimentary
processes
described below.
Fractionation changes other important physicochem-
ical properties. Silicate percentage rises steadily from
45-54 per cent (ultramafic) to 55-64 per cent (interme-
diate) and 65-78 per cent in felsic rocks. This causes a
corresponding increase in viscosity and progressively
slower flow rates. Feldspar minerals also become less
alkaline as the plagioclase-orthoclase series shows. Early
settling of denser, ultrabasic minerals and the continued
rise of lighter fractions combine to create a
layering
effect
in igneous rocks. We have seen this already in layered
oceanic crust (
Chapter 11)
and mineral distinctions
between upper (felsic) and lower (mafic) continental crust
but it can also be present within subsurface
plutons
.The
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 most 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.
The principal silicate tetrahedra and minerals they
construct are illustrated in
Figure 12.2
.
The
valency
,or
combining potential based on the number of electrons lost
or gained by the atoms, is the key to interpreting their
