Chemistry Reference
In-Depth Information
3.5.5.3 Other Chain Silicates
There exist a number of other chain silicates for which the chains are not linked in
the way as they are in pyroxenes or amphiboles. A first example is babingtonite in
which two chains of SiO
4
tetrahedra are joined by groups of four edge-sharing
octahedra. The ideal formula of the iron member is Ca
2
Fe
2+
Fe
3+
Si
5
O
14
(OH) in
which Fe
2+
and Fe
3+
each occupy a distinct site. Calcium occupies two types of
large interstices. The spectra are consequently quite simple, consisting of narrow
ferric and ferrous doublets [
232
]. Another particular chain silicate is deerite which
has a structure based on a hybrid single-double Si
6
O
17
chain. There is a strip of
edge-sharing octahedra with nine crystallographically distinct sites, which are
classified
into
three
groups
of
each
three
sites.
The
ideal
formula
is
(Fe
2+
)
6
(Fe
3+
)
3
O
3
Si
6
O
17
(OH)
5
.
The
spectra
are
complicated
by
the
effects
of
electron hopping [
233
]. There are signs of a preference of Fe
3+
for M1 and M3
positions and of Fe
2+
for the others.
3.5.6 Phyllosilicates
Phyllosilicates are the most important minerals of the silicate group. They are
either inherited from parent rocks (detrital minerals), reflecting a chemical relation
to their environment, or they are secondary minerals. i.e. modified by strong
external conditions or transported from other places. Species from the latter group
usually show a rather small-particle morphology and are accordingly divided into
silt ([2 lm) and clay (\2 lm) fractions. Particularly the clays have ever since
ancient times been important minerals for industrial uses. Microcrystalline phyl-
losilicates were formerly referred to as ''clay minerals'', but nowadays also fine-
grained oxides and oxyhydroxides occurring in soils and sediments are also termed
as clay minerals.
The phyllosilicates are composed of sheets of SiO
4
tetrahedra, which can be
divided into two groups: the 1:1 layer and the 2:1 layer minerals. The 1:1 layer
silicates are composed of alternating tetrahedral Si
4
O
10
and octahedral Al
4
(OH)
12
layers. The octahedra are formed by two oxygen and four hydroxyl anions
(Fig.
3.29
). The layers themselves are electrically neutral and the stacks are held
together by van der Waals or hydrogen bonding. The repeat distance is about
0.7 nm. The general formula is
Si
2
O
5
O
ð
4
. According to the filling
of the octahedra the silicate can either be dioctahedral with 2/3 of the octahedral
sites filled or trioctahedral in which all octahedra are filled.
The basic block of a 2:1 layer silicate is a sandwich layer of one octahedral
sheet between two tetrahedral ones. The octahedral sheet contains now two apices
of two sheets of tetrehedra, so four of the ligands are now oxygen and two are
hydroxyl ions (Fig.
3.29
). The hydroxyls may be at opposite or adjacent corners of
the octahedron, giving trans and cis coordination, respectively, mostly denoted as
M
3
2
or M
2
þ
3
