All these minerals cleave easily along the chain direction, but
the covalent Si-O bonds within a chain are not easily broken. This
is why asbestos (an amphibole) is fibrous and stringy.
Silicate tetrahedra also can be linked into endless sheets, with
three of the four oxygen atoms shared, and only one O atom per Si
left with a negative charge (bottom right).
This negative O is fully owned by one Si, while the other three
are shared; thus the overall ratio of O to Si is 1+ 1/2
+ 1/2 =
to one. The silicate sheet has the composition SiO2-
Even fewer metal ions are required to balance the negative charges
than in olivines or pyroxenes, so micas and clays with sheet structures
are lighter yet. They are believed to be present only in is the
crust of the Earth.
The familiar flaking of mica arises because it i easy to separate
silicate sheets, but much harder to break bonds within the sheets.
(Recall the similar behavior of graphite.)
The most common materials in the crust of the Earth are the framework
silicates: quartz and feldspars.
In these, all four corners of each silicate tetrahedron are linked
in a three-dimensional framework. In common hexagonal quartz, the
tetrahedra are linked in a six-fold helix, a spiral staircase with
six silicate steps per turn.
These helices then are packed parallel to one another in the quartz
crystal and connected by sharing oxygen atoms in the silicate tetrahedra.
Hence quartz is held together completely by covalent Si-O bonds,
and is a hard mineral.
There are left-handed and right-handed quartz crystals, depending
on the direction or "handedness" of the helices.
If quartz is heated above its melting point of 1610°C and then
cooled rapidly, the silicate chains do not have time to return to
a perfectly crystalline array, so they harden into a disordered
silicate glass instead.
Glass really is a liquid, although an extremely viscous one. Given
enough time, glass will flow, as the thickening of the bottom of
panes of glass in very old European buildings demonstrates.