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Zusammenfassung

Silicates: A brief and understandable summary

719 / ~2½ sternsternsternsternstern_0.25 Alfred K. . 2013
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Zusammenfassung
Chemie

Gymnamsium Sacre Coeur Vienna

º2013

Alfred K. ©

0.15 Mb
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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.

 

 

 


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