Page 448 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Inorganic Polymers 411

12.4.1 NETWORK

Quartz is an important network silicate (Section 12.10). A number of additional tetrahedral silicate-
like materials possess some AlO tetrahedra substituted for the SiO tetrahedra. Such structures
4
4
offer a little larger “hole” in comparison to the entirely SiO structures, allowing alkali and alka-
4
line-earth cations to be introduced. Feldspar (orthoclase) is such a mineral. The aluminosilicate
networks are almost as hard as quartz. For feldspar and other tetrahedral networks, the number of
oxygen atoms is twice the summation of silicon and other MO cations.
4
The feldspars are widely distributed and comprise almost two-thirds of all igneous rocks.
Orthoclase and albite, NaAlSi O , are feldspars, where one-fourth of the silicon atoms are replaced
8
3
by aluminum and anorthite, CaAl Si O , has one-half of the silicon atoms replaced by aluminum.
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2
2
+2
+
Because the ionic radius of Na (0.095 nm) and Ca (0.1 nm) are about the same, solid solutions
are often formed between albite and anorthite. Good stones of albite and orthoclase are known as
moonstones.

Some of the network structures exhibit a framework sufficiently “open” to permit ions to move
in and out. The zeolite minerals used for softening water are of this type.
Ultramarines are three-dimensional cage-like structures. They differ from feldspars and zeolites
because of the large spaces within the structures that can contain cations and anions but no water
illustrating a natural “buckeyball-like” structure and cavity and a diversity of environment between
the internal cage and external. Ultramarines can act as ion exchangers for both anions and cations.
−
−
The blue color of ultramarines is due to the presence of the S ion although a yellow ion S also
2
3
exists in the same structure.
12.4.2 LAYER
–4
–2
Layered structures typically conform to the approximate composition Si O or Si O . For most of
4
10
2
5
these, three of the oxygen atoms of each tetrahedron are shared by other tetrahedra, and the fourth
oxygen is present on one side of the sheet.
In talc and kaolinite, the layers are neutral. Thus, the layers slide over one another easily
imparting to these minerals a softness and ease in cleavage. In other minerals, the layers are
charged and held together by cations. In mica, the aluminosilicate layers are negatively charged
+
and cations, generally K , are present between the layers giving the entire system of layers elec-
tronic neutrality. The ionic attractive forces between the layers result in mica being much harder
than talc and kaolinite. Even so, these intersheet bonding forces are less than the “within-the-
sheet” bonding forces, permitting relatively easy and clean cleavage of mica. Mica is used as
an insulator for furnaces and electric equipment. Montmorillonite is an important ingredient in
soils and is employed industrially as a catalyst in the conversion of straight-chain hydrocarbons
to more branched hydrocarbons and more recently as a sheet or clad material in the manufacture
of tires.
Vermiculites are formed by the decomposition of mica. They contain layers of water and mag-
o
o
nesium ions in place of the potassium ions. When heated to 800 C–1,100 C, vermiculite expands
because of the conversion of the water to a gas. The expanded vermiculite has a low thermal con-
ductivity and density and is used as a thermal and sound barrier and as an aggregate in light-weight
concrete. It is also used as a moisture-retaining soil conditioner in planting.

A number of clays are layered silicate-like materials. Most clays contain finely divided quartzs,
micas, and feldspars. Iron oxide-rich clays are employed to make pottery and terra cotta articles.
Clays containing iron oxide and sand are used to make bricks and tiles. Clays rich in calcium and
magnesium carbonate are known as marls and are used in the cement industry (Section 12.2).
Kaolinite is the main constituent in china clay used to make porcelain. The layers are largely
held together by van der Waals’ forces. Bentonite is used in cosmetics, as a fi ller for soaps, and as
a plasticizer, and it is used in drilling muds as a suspension stabilizer. Bentonite and kaolinite clays

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