Silica and Silicates

Silicon, Silica, Silicates and Silicone


People get confused about the differences between silicon, silicate, silica and even silicone. What is it exactly that we collect, cut and polish??

Silicon is a chemical element, one of the 97 natural building blocks from which our minerals are formed. A chemical element is a substance that can't be subdivided into simple substances without splitting atoms. Silicon is the second most abundant element in the earth's crust, making up about 27% of the average rock. Silicon links up with oxygen (which makes up 55% of the earth's crust) to form the most common suite of minerals, called the silicates. Quartz, feldspars, olivine, micas, thomsonite, jadeite, and prehnite are all silicates. There is so much oxygen around that pure native silicon is almost never found naturally.

Silica is a bit trickier concept. It refers the combination of silicon plus oxygen. The mineral quartz is silica. But so are the minerals tridymite, coesite, cristobalite and stishovite which are mineral forms of silica that are stable at high temperatures and pressures. All these minerals are also silicates. In other words, quartz is a silicate made of pure silica. But feldspars contain sodium, aluminum, potassium and calcium in addition to silicon and oxygen. Thus feldspars are silicates but they aren't pure silica.

Geochemists also use the term "silica" to refer to the overall silicon and oxygen content of rocks. This is confusing, but stems from the fact that in rock analysis and sample is dissolved, the solution treated, and the amount of silicon present is determined by precipitating it as silica. So a geologist may say "This rock is 48% silica". A rockhound will look at the rock and say "How can that be? I don't see any quartz in it!" Both are right. The rock will not have the mineral quartz because the silicon and oxygen are tied up with other elements to make silicate minerals like feldspar. Its a bit like looking at a cake and saying "I don't see any eggs in there!" The eggs are cake ingredients but are present now in different forms.

Now, what is silicone? Its a synthetic polymer of silicon with carbon and oxygen that could be in solid, liquid or gel form. It has all kinds of medical uses, such as in antacids, artificial joints, pacemakers and implants of various notoriety, but is not, as far as anyone knows, found in rocks.

Can pure silicon be found in Nature? Yes, rarely. Recently Russian geologists were sampling gasses from Kudriavy volcano on the Kamchatka Peninsula. Here they drove quartz tubes into vents jetting out gases of over 900 degrees C. Their tubes filled with minerals precipitating from this gas. Among them were pure silicon metal embedded in masses of salts such as halite. The silicon formed crystals up to 0.3 mm across. It was associated with pure aluminum metal, Si-Al alloys and other rare minerals. This find was unusual enough to warrant a note in the prestigious science journal, Nature.

So unless you are in Russia sampling hot volcanic gases, you can be sure that what you are finding are silica and silicates, but not silicon or silicone.


-Dr. Bill Cordua, U. Wisconsin- River Falls


Reference:

Korzhinsky, M.A., et. al., 1995, "Native Al and Si Formation", Nature, vol. 375, p. 544.

Notes

Silicon is not found free in nature, but occurs chiefly as the oxide, and as silicates. Sand, quartz, rock crystal, amethyst, agate, flint, jasper, and opal are all silicon oxides. Granite, hornblende, asbestos, feldspar, clay, mica are a few of the many silicate minerals. Silicon makes up 25.7% of the earth's crust by weight and is the second most abundant element in the earth's crust. Silicon is present in the sun and stars and is a principal component of a class of meteorites known as aerolites.

Silicon was first isolated and described as an element in 1824 by Jöns Jacob Berzelius, a Swedish chemist. Silicon does not occur uncombined in nature; but it is found in practically all rocks as well as in sand, clays, and soils, combined either with oxygen as silica (SiO2, silicon dioxide) or with oxygen and other elements (e.g., aluminum, magnesium, calcium, sodium, potassium, or iron) as silicates. Its compounds also occur in all natural waters, in the atmosphere (as siliceous dust), in many plants, and in the skeletons, tissues, and body fluids of some animals.

Pure silicon is a hard, dark gray solid with a metallic lustre and with a crystalline structure the same as that of the diamond form of carbon, to which silicon shows many chemical and physical similarities. A brown, powdery form of silicon has been described that also has a microcrystalline structure. The element is prepared commercially by reducing (removing the oxygen from) the oxide by its reaction with coke in electric furnaces. On a small scale, silicon can be obtained from the oxide by reduction with aluminum.

Silicon, like carbon, is relatively inactive at ordinary temperatures; but when heated it reacts vigorously with the halogens (fluorine, chlorine, bromine, and iodine) to form halides and with certain metals to form silicides. It is unaffected by acids except hydrofluoric. At red heat, silicon is attacked by water vapour or by oxygen, forming a surface layer of silicon dioxide. When silicon and carbon are combined at electric furnace temperatures (2,000°–2,600° C [3,600°–4,700° F]), they form silicon carbide (Carborundum, SiC), which is an important abrasive. With hydrogen, silicon forms a series of hydrides, the silanes. When combined with hydrocarbon groups, silicon forms a series of organic silicon compounds.

Silicon's atomic structure makes it an extremely important semiconductor; highly purified silicon, doped (infused) with such elements as boron, phosphorus, and arsenic, is the basic material used in computer chips, transistors, silicon diodes, solar cells, liquid crystal displays, and various other electronic and switching devices. Silicon of lesser purity is used in metallurgy as a reducing agent and as an alloying element in steel, aluminum, brass, and bronze. The most important compounds of silicon are the dioxide (silica) and the various silicates. Silica in the form of sand and clay is used to make concrete and bricks as well as refractory materials for high-temperature applications. As the mineral quartz, the compound may be softened by heating and shaped into glassware. Silicates, most of which are insoluble in water, are employed in making glass as well as in the fabrication of enamels, pottery, china, and other ceramic materials. Sodium silicates, commonly known as water glass, or silicate of soda, are used in soaps, in the treatment of wood to prevent decay, for the preservation of eggs, as a cement, and in dyeing. Silicones are synthetic organosilicon oxides composed of the elements silicon, oxygen, carbon, and hydrogen; they are used as lubricants, hydraulic fluids, waterproofing compounds, varnishes, and enamels because, as a class, they are chemically inert and unusually stable at high temperatures.

Three stable isotopes of silicon are known: silicon-28, which makes up 92.21 percent of the element in nature; silicon-29, 4.70 percent; and silicon-30, 3.09 percent. Five radioactive isotopes are known.


atomic number: 14
atomic weight: 28.086
melting point: 1,410° C (2,570° F)
boiling point: 2,355° C (4,270° F)
density: 2.33 g/cm3
oxidation state: -4, (+2), +4
electron config.: 1s22s22p63s23p2

SILICON
Silicon is the most abundant element on the earth's surface and in the form of silica is consumed in a great number of industrial uses.

Silica has many industrial applications in abrasives and polishes, in glass manufacture, fillers and extenders, silica brick manufacture, as a catalyst, in specialty coatings, cleansers, ceramics, electronics, optics and refractories, in ferro-silicon manufacture, rubber, and as a frac sand.

Processing of silica of specific quality yields several types of specialty silicas. These include colloidal, fumed, fused, high purity ground, silica gel, and precipitated silica.

Colloidal silica is mainly used as a high temperature binder for silicon wafer polishing and carbonless paper. Fumed silica, because of its unique strength, thixotropic properties and flatting properties, is a valuable ingredient in rubber, plastics, specialty coatings, adhesives, cement and sealants; and aids in manufacturing pesticides, cosmetics, pharmaceuticals, defoamers, inks, abrasives and batteries.

Fused silica is produced by fusion of very high quality silica sand feedstock in electric arc and resistance furnaces. The resulting product is very different from the original silica sand due to the extremely high quality, consistent chemistry, high resistance to thermal shock and low thermal conductivity. It is therefore suitable as a filler for electronic encapsulants, refractory and investment castings and specialty coatings.

High purity ground silica is produced from silica sand or soft, friable rocks and is often referred to as amorphous silica. It is used mainly in specialty coatings, plastics, rubber, electronics, abrasives, refractories and optics depending on average particle size, distribution, hardness and refractive index.

Silica gel is an extremely pure, porous, and amorphous form of silica known for a high degree of internal surface area, hardness, uniformity and chemical inertness. Its ability to absorb liquids makes it valuable in a wide variety of applications including catalysts, anticaking agents in food and pharmaceutical products, desiccants, cosmetics, plastics, specialty coatings, paper and adhesives.

Precipitated silica is another type of specialty silica which is mainly used as a reinforcing agent in tire compounds. Its properties also provide anticaking, conditioning, suspension, and viscosity control in products like tooth pastes, deodorants, nail polishes and vitamin tablets.

SILICA SAND IN TANZANIA

High Purity (>98 per cent SiO2) Silica sand is suitable for manufacturing glass bottles, fibreglass insulation and sand blasting occurs in many locations in Dar es Salaam Region and Bagamoyo plus also Bukoba there is a huge deposits of Complete clean Silica Sand. Before in Bukoba Region during the colonial rule there was an in industry which was manufacturing Glasses in the back 1950s the Industry after the Tanzania Independence it was closed. In Dar es Salaam Region there is also a Glass Industry called Kioo Ltd, this industry produces all its glass products with the Silica Sand from Dar es Salaam Region.

In order for the Silica sand to produce High Quality Glass the Silica Sand must have a higher Silicon Oxide Content and the content should be more than 98% of SiO2 and should have a lower Iron Oxide (FeO2).If the sand will have a higher Iron (Fe) then the end product will have dark impurities.


SOURCE: Internet
Reference:*http://www.seabgems.com/silica_sand.htm