–185°C; b.p. –112°C. Silane is pro-
duced by reduction of silicon with
lithium tetrahydridoaluminate(III). It
is also formed by the reaction of
magnesium silicide (Mg 2 Si) with
acids, although other silicon hydrides
are also produced at the same time.
Silane itself is stable in the absence
of air but is spontaneouslyÛamma-
ble, even at low temperatures. It is a
reducing agent and has been used for
the removal of corrosion in inacces-
sible plants (e.g. pipes in nuclear re-
actors). 2.(or silicon hydride) Any of
a class of compounds of silicon and
hydrogen. They have the general for-
mula SinH 2 n+2. TheÜrst three in the
series are silane itself (SiH 4 ), disilane
(Si 2 H 6 ), and trisilane(Si 3 H 8 ). The com-
pounds are analogous to the alkanes
but are much less stable and only
the lower members of the series can
be prepared in any quantity (up to
Si 6 H 14 ). No silicon hydrides contain-
ing double or triple bonds exist (i.e.
there are no analogues of the alkenes
and alkynes).
silica See silicon(iv) oxide.
silica gelA rigid gel made by coag-
ulating a sol of sodium silicate and
heating to drive off water. It is used
as a support for catalysts and also as
a drying agent because it readily ab-
sorbs moisture from the air. The gel
itself is colourless but, when used in
desiccators, etc., a blue cobalt salt is
added. As moisture is taken up, the
salt turns pink, indicating that the
gel needs to be regenerated (by heat-
ing).
silicaneSee silane.
silicate Any of a group of sub-
stances containing negative ions
composed of silicon and oxygen. The
silicates are a very extensive group
and natural silicates form the major
component of most rocks (see sili-
cate minerals). The basic structural
unit is the tetrahedral SiO 4 group.
This may occur as a simple discrete
SiO 4 4–anion as in the orthosilicates,
e.g. phenacite(Be 2 SiO 4 ) and willemite
(Zn 2 SiO 4 ). Many larger silicate species
are also found (see illustration).
These are composed of SiO 4 tetrahe-
dra linked by sharing oxygen atoms
as in the pyrosilicates, Si 2 O 7 6–, e.g.
Sc 2 Si 2 O 7. The linking can extend to
such forms as bentonite, BaTiSi 3 O 9 ,
or alternatively inÜnite chain anions,
which are single strand (*pyroxenes)
or double strand (*amphiboles). Spo-
dumene, LiAl(SiO 3 ) 2 , is a pyroxene
and the asbestos minerals are amphi-
boles. Large two-dimensional sheets
are also possible, as in the various
*micas (see illustration), and the
linking can extend to full three-
dimensional framework structures,
often with substituted trivalent
atoms in the lattice. The *zeolites
are examples of this.
silicate mineralsA group of rock-
forming minerals that make up the
bulk of the earth’s outer crust (about
90%) and constitute one-third of all
minerals. All silicate minerals are
based on a fundamental structural
unit – the SiO 4 tetrahedron (see sili-
cate). They consist of a metal (e.g.
calcium, magnesium, aluminium)
combined with silicon and oxygen.
The silicate minerals are classiÜed on
a structural basis according to how
the tetrahedra are linked together.
The six groups are: nesosilicates (e.g.
olivine and *garnet); sorosilicates
(e.g. hemimorphite); cyclosilicates
(e.g. axinite, *beryl, and *tourma-
line); inosilicates (e.g. *amphiboles
and *pyroxenes); phyllosilicates (e.g.
*micas, *clay minerals, and *talc);
and tektosilicates (e.g. *feldspars and
*feldspathoids). Many silicate miner-
als are of economic importance.
silicideA compound of silicon with
a more electropositive element. The481 silicide
s