Environmental Engineering Reference
In-Depth Information
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 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
Figure 12.2
The structure and characteristics of silicates and
some representative silicate minerals. The silicate
tetrahedron of four large oxygen anions and a single small
silicon cation and its simplified form are shown in the first
panel. Subsequent structures are shown with the number of
shared oxygen anions and their silicon-oxygen formulae.
also happens to be a solid solution! Mineral density and hardness are greatest in the
single
and
ring
silicates, 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
; they 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
doublechain 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
