Page 205 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN009F-398 July 6, 2001 20:34

14 Main Group Elements

Concentrated aqueous solutions of Na 2 SiO 3 are often sold minerals if quartz (SiO 2 ) is considered a silicate. The basic
in hobby shops under the name of water glass. structural feature of silicates is a silicon atom tetrahedrally
When white sand is fused at 1500 C with a mixture of surrounded by four oxygen atoms. These SiO 4 tetrahedra
◦
4−
sodium and calcium oxides and carbonates, the result is a canexistasdiscreteions(asintheorthosilicateion,SiO )
4
molten mixture of silicates, Na 2 SiO 3 , CaSiO 3 , and silica. or can share oxygen atoms at a corner to build up larger,
Cooling this liquid produces ordinary soda glass, an amor- more complex entities.
phous transparent substance that gives the appearance of
O 4
a solid. 6
Variations in the proportions of the three basic start- Si O O
O O O Si O Si O
ing materials and additions of a few other ingredients will O
4
produce a variety of glasses with modified properties. If SiO 4 O O
calcium is replaced with lead, a flint glass with a higher re- Orthosilicate ion Pyrosilicate ion
in nesosilicates in sorosilicates
fractive index results; SiO 2 partially replaced with P 4 O 10
yields crown glass. The addition of boron oxides to soda The classification of silicates is based on the number of
glass yields a borosilicate glass (Pyrex) with greatly im- shared oxygen atoms per tetrahedron and the nature of the
proved thermal-shock properties that result from the re- arrangements of tetrahedra into rings, chains, or sheets.
duced coefficient of expansion. Thus, in addition to the nesosilicates and sorosilicates
illustrated, there are cyclosilicates (closed ring struc-
E. Silicones tures), inosilicates (continuous chain or ribbon struc-
tures), phyllosilicates (continuous sheet structures), and
The simple organosilicon derivative (CH 3 ) 2 SiCl 2 , which
tectosilicates, (continuous three-dimensional structures).
can be prepared by several methods, serves as a starting
Quartz is an example of a tectosilicate, that is, a silicate
point synthesis of an important class of polymers called
in which all four oxygen atoms at the corners of the tetra-
silicones. Hydrolysis of (CH 3 ) 2 SiCl 2 followed by
hedron are shared, to give a three-dimensional structure
condensation–polymerization results in long-chain mole-
reminiscent of that in diamond.
cules with a host of valuable and interesting properties:
(CH 3 ) 2 SiCl 2 2H 2 O (CH 3 ) 2 Si(OH) 2 2HC l
Hydrolysis G. Asbestos
Defining asbestos is difficult, for the term is an imprecise
CH 3 CH 3 CH 3 CH 3 CH 3
H 2 O H 2 O
HO Si OH HO Si OH HO Si O Si O Si OH commercial designation referring to several fibrous inor-
ganic silicates with a high tensile strength and consider-
n
CH 3 CH 3 CH 3 CH 3 CH 3
Condensation-polymerization
able heat resistance. Various silicate minerals that usually
Cross-linked polymers can also be made: crystallize in nonfibrous rock forms can, under special
geological conditions, crystallize in bundles of tough,
R O R flexible fibers—the so-called asbestiform habit. Two
Si O Si O Si mineral groups are recognized: the serpentines and the
R R amphiboles. Serpentines have the generalized chemi-
O
R R cal formula Mg 6 (SiO 4 O 10 )(OH) 8 , and one serpentine
asbestiform variety called chrysotile (white asbestos)
Si O Si O Si
represents ∼95% of world production. The amphiboles
R O R
with asbestiform habit include a blue asbestos (crocido-
2+
3+
lite), Na 2 Fe Fe Si 8 O 22 (OH, F), and a brown asbestos
Variations in the length of chains, the organic groups, 3 2
2+
(cummingtonite–grunerite), (Mg ,Fe ) 7 Si 8 O 22 (OH) 2 .
2+
and the degree of cross-linking produce silicones that vary
In the mid-1980s, panic regarding the toxicity of as-
widely in their physical properties. Silicones may be fluids
bestos caused the demand to plummet to less than a third
(oils), greases, or even rubberlike solids. All possess a
host of desirable traits: resistance to temperature extremes, of that in 1973, the peak year. The demand now is for
asbestos abatement services to remove safely and quickly
water repellency, antistick properties, chemical inertness,
the huge quantities of asbestos that were employed as fire-
and lack of physiological activity.
proofing and insulation in the thousands of office build-
ings, schools, and municipal buildings constructed in the
F. Silicate Minerals
1960s and early 1970s.
Silicon has often been referred to as the “earth maker,” High exposure to asbestos causes pulmonary fibrosis
for nearly 95% of the earth’s crust is composed of silicate (asbestosis), lung cancer, and gastrointestinal cancers.