I mentioned previously that ions can be pulled through materials such as glass by
the application of a direct current. Like glass, most rocks are made from silica
and are not crystalline. This is because silica is formed from the covalent bonding
of silicon atoms with atoms of oxygen. In covalent bonding electrons are shared between
two or more atoms and a fixed directional relationship between the atoms is required.
Silica has a tetrahedron configuration.
Silicon has a valence of four and shares electrons with four oxygen atoms giving
a total of eight electrons for each silicon atom. Oxygen has a valence of six and
shares electrons with two silicon atoms giving it a total of eight electrons. One
or more of the oxygen atoms in the silica arrangement can share electrons with silicon
atoms in other silica units and this means that polymers can be formed.
In this way a double unit (pyrosilicate), a chain, a ring structure, or even a complex
network can form. The spaces between chains or rings or sheets can be filled by cations
such as sodium and magnesium and these ions determine the properties of the mineral
or rock. For example, rings can be linked together in platelets or sheet structures
- as in kaolinite.
The mineral olivine is an example of the configuration of single silica units with
magnesium and iron cations held between them. As one would expect, olivine with its
simple structure is present in amorphous mafic rocks such as basalt and gabbro.
When glass is rrapidlycooled a network of silica forms. Modifiers such as sodium
oxide can be added to molten glass so that the network formed is less complex.
Glass can form naturally as obsidian when lava is quenched rapidly. Tektites are
glass beads which are formed when molten silica is rapidly cooled in the atmosphere
after an extremely violent volcanic event.
Aluminium atoms are also covalently bonded to oxygen atoms and form a range of equivalent
structures to silica. In hydrothermal conditions aluminium oxide (alumina) can be
hydrated to the hydroxide, gibbsite which is a component of bauxite.
Alumina and silica units can together form minerals such as andalusite.
Tension on all of these structures causes their chains to align in the plane of stress.
At extreme strain conditions they can form fibres or leaves. One would expect micas
to be formed in areas where there has been maximum strain. Indeed micas are to be
found in areas parallel with some coastlines - and these will have been zones of
maximum tension before continents split away from each other.
In previous chapters I suggested that a toffee bar being stretched apart as an analogy
for the way continental crust stretches, necks and finally fails. Toffee with its
chains of polysaccharides, randomly arranged in an amorphous structure, is very similar
to many rocks. If a toffee bar is pulled apart, the chains of polysaccharides will
align in the plane of the strain; creep between chains occurs, and the most strained
area in the middle of the bar becomes translucent prior to its breaking apart. Temperature
increase also assists this
process - warm toffee stretches much more easily than cold toffee.