2 minute read

Silicon - Silicates

Science EncyclopediaScience & Philosophy: Semiotics to SmeltingSilicon - Silicon Is An Abundant Element, Silicates, Silicones, Other Uses Of Silicon

Silicates

The atoms of carbon can bond to each other to make long chains that include branches, and rings of carbon atoms onto which atoms of hydrogen and several other elements (including oxygen) can bond. The entire field of organic chemistry, with its millions of different organic compounds, is based on this ability of the carbon atom.

Silicon also has increased bonding abilities. On the periodic table, silicon is directly beneath carbon in group 14, which means that it, like carbon, has four electrons in its outermost shell that are available to share in chemical bonds with other elements. Like carbon, it can share those electrons with other silicon atoms. Because silicon atoms are about one and a half times larger in diameter that carbon atoms, however, the atoms can not pack as tightly and therefore can not to bond into long -Si-Si-Si-Si-chains that allow as much access as do carbon chains. Oxygen atoms can act as separators, or bridges, between the Si atoms to make -Si-O-Si-O-Si-O-Si-chains. Oxygen has a valence of two, and it can bond to two silicon atoms to bridge a chain. Such bridged structures open up the possibility of vast networks of silicon and oxygen based silicates.

The network in a quartz crystal consists of silicon and oxygen atoms. Each silicon atom is bonded to four oxygen atoms. Each silicon atom has only half possession of the four oxygen atoms surrounding it, so the overall formula is SiO2, not SiO4. Half of four oxygen atoms per silicon atom equal two oxygen atoms per silicon atom. In other silicate minerals, this network incorporates the presence of other atoms such as aluminum, iron, sodium, and potassium, that allow crystals to take on different shapes and properties.

Talc is a silicate mineral whose silicon and oxygen atoms are bonded together in sheets rather than in quartz-like three-dimensional solid crystals. These thin sheets can slide over one another. The low friction of talcum powder (ground-up talc) results from this sheet like configuration. Asbestos is a silicate mineral with silicon and oxygen atoms are bonded in long strings. Asbestos is therefore a mineral rock that can be shredded into fibers.

A silicate material widely used in industry is cement. Recent estimates place use of this cement at more than 100 million tons of in the United States each year. Cement is manufactured from two minerals: clay or shale (both aluminum silicates) plus limestone (calcium carbonate, CaCO3). These minerals are mixed, then heated together at a temperature of 2,732°F (1,500°C). At this temperature the limestone converts to lime, CaO. The mixture is then cooled and ground to a very fine, gray powder. When this cement powder is mixed with sand, gravel, and water, it sets into concrete. Accordingly, although the terms are sometimes inappropriately used synonymously, concrete is actually an aggregate material containing cement. Concrete is a very hard and strong material, largely because strong Si-O-Si bridges in the clay.


Additional topics