SILICATES
General
infromation
Silicates
can either emerge through felsic (feldspar + silica) or mafic (magnesium +
ferric (=iron)) formation. The first occures in granites and is usually lighter
in weight and in colour due to the low percentage of iron and magnesium. Good
examples would be the quartz or the k-feldspar. Whereas the
second group is formed in upcoming lava that fills the gaps between the
tectonic plates and tends to be dense and dark. Basalt, gabbro and pyroxene
belong to that category.
Approximately
2000 (30%) of all known minerals are silicates. It is therefore the largest and
most important class of rock-forming mineral. Moreover nearly 95% of the
Earth’s crust is composed of silicates.
Silicon, Silica,
Silicate
Silicon is a chemical
element of the periodic table with the atomic number 14. Its symbol is Si.
However it shouldn’t be mistaken for the chemical compund SiO2 called silica which can be found as quartz or in living
organisms. The last term is silicate which is a class of compounds that contain
silicon in their anion. (An anion is a negatively charged ion contrary to the
positively charged cations.. The
charge is due to the majority, or respectively minority of electrons.)
Classification
Formerly
silicates were divided into the felsic and the mafic group, due to their
formation. Nowadays the classification has evolved and silicates are arranged
by their structure (the ratio of silicon and oxygen).We therefore now have six
main- and various subcategories. This categorization is known as the
Nickel-Strunz-Classification.
Nickel-Strunz-Classification
It is a scheme for
categorizing minerals after their chemical composition developed by the German
mineralogist Karl Hugo Strunz. It was first published in 1941 as the
Mineralogische Tabellen and has since then been modified through the following
years (most recently in 2001). Silicates have the group-number 9. Hence a
silicate can be recognised by that particular number in its Strunz
Classification. (e.g.
asbestos - 09.ED.15).
How
silicates break
Since the
covalent bond between two silica tetraheder, when sharing an oxygen atom is
very strong, these connections tend to break last. This means that a silicate
will have certain places where breaking it will be easier, since not every bond
will be between silica tetraheders.
Double
chain silicates
This sort
of silicate is formed by one long and thin chain of two interlinked rows of
bonded tetraheder. Due to its negative charge it bonds together with cations to
obtain a neutral charge. So the cations bond together above and below the
chains, thus linking them together. Because of the above named tendency, double
chain silicates will break between chains.
Hornblende
is a good example for this kind of silicate.
Sheet
silicates
Basically
the theory is the same as the one described above. The broad and flat sheets of
silica tetraheder which are linked on all sides need cations to be neutrally
charged. Thus the sheets get bonded together through the cations. Therefore
sheet silicates tend to break like “sheets”. Muscovite is a good example.
Quartz
Its formula
is SiO2 and it has the
Strunz-Classification
04.DA.05. It is furthermore the most abundant mineral in the Earth’s crust (The
most abundant is feldspar). Quartz exists in five varieties in colour (citrine
- pale yellow/brown, rose quartz - pale pink/rose red, amethyst- purple, smoky quartz
- transperent/gray, milky quartz - white). And is also used for
time-measurement (Quartz is piezoelectronic, so it vibrates in regular
intervalls when an electric current passes through), optics, circuit boards and
many more.
Unfortunately there
exist some hazards, too. The most common have to do with the lungs (silicosis,
silicotuberculosis, cancer) and immunological problems.
Asbestos
It has the following
formula: Mg3Si2O5(OH)4 and the followig Stunz-Classification: 09.ED.15.
Asbestos is
an all present silicate, since its air-concentration is about 0.00001-0.0001
fibres per millilitre and very useful mineral. Due to its resistance against
heat and chemicals it was used as an insulator. Otherwise we find it in car
brakes and clutches, railways and many more.
Through inhalation one
might get respiratory diseases, such as mesothelioma, asbestosis or lung
cancer. They cause about 4000 deaths a year. However all
these illnesses are latent which means that the effects first appear after some
15 or even 60 years.
Phenacite
- Hazards
Phenacite
(Be2Si02) was formerly used in fluorescent light tubes and bulbs, but because
of the Berrylium-containing dust (highly poisonous) it is no longer used in
technological products. It can still be found naturally in Austria, Canada,
France and Italy, though.