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Clear quartz crystals or high-class silica sand are fused for the manufacture of ophthalmic glasses and lenses.

The History Says
Silicon in the amorphous form was prepared by Berzelius in 1823AD. Silicon in the crystalline form was prepared by H St Cdeville in 1854AD.

The Present Scenario
Quartzite, sandstone, quartz and other siliceous rocks like mica schists are used in the manufacture of silica bricks. Quartz and quartzite are used in making ferro-silicon for the manufacture of silicon steel.

SILICA is the most abundant mineral found in the crust of the earth. It forms an important constituent of practically all rock-forming minerals. It is found in a variety of forms, as quartz crystals, massive forming hills, quartz sand (silica sand), sandstone, quartzite, tripoli, diatomite, flint, opal, chalcedonic forms like agate, onyx etc., and in with numerous other forms depending upon colour such as purple quartz (amethyst), smoky quartz, yellow quartz or false topaz (citrine), rose quartz and milky quartz. Only pure quartz crystal or rock crystal, untwinned, clear, free from any inclusion, has an important property:

It expands (mechanically) under the influence of electric current and conversely pressure induces a measurable electric current. This property is known as piezoelectricity. The current thus developed is called piezoelectric current.

This property resulting from the asymmetry of its atomic groups makes quartz an effective transducer for coverting electrical energy into mechanical energy and vice-versa. This property in quartz crystals was discovered in 1880-82 by Pierre and Jacques Curie and remained a laboratory curiosity till in 1921 when W.G. Cady, a physicist, discovered that quartz plates could be used to control the frequency of wireless transmission circuits.

This discovery marked the dawn of quartz crystal application in modern communication equipments. A very thin plate of quartz is so cut that the frequency of the oscillating circuit corresponds with the quartz plate and when such plate is inserted in a radio receiving set or radio transmitter it prevents frequencies from wandering and deviation and greatly reduces interference.

Quartz plate is used in controlling frequencies in air and water media as well. It is largely used in radio circuit, radar, ultrasonic and in multiple telephone lines. Quartz plates keep the bradcast on the right beam.

Quartz cystals cut into prisms, wedges and lenses are used for microscopes and other optical instruments. Quartz wedge is the commonest accessory which students use in the petrological microscope.

A number of other crystals giving piezoelectricity are known but none compares with quartz. Chemically prepared Rochelle salt and Barium titanate have been found good substitutes for piwzoelectric quartz.

However, the crystal - quartz because of its chemical and physical stability and high elasticity has remained indispensable so far. The consumption of quartz plate pieces has tremendously increased with the increase in the manufacture of modern receiving sets.

Occurrences and Consumptions
USA alone uses a few million pieces every year for use in receiving sets for combat forces.

Clear quartz crystals or high-class silica sand are fused for the manufacture of ophthalmic glasses and lenses. Most of the world supply of flawless, untwinned radio-grade quartz comes from Brazil. It is found occurring in thousands of square km. in the States of Minas Gerais, Goiaz, and Bahia.

Quartz deposits are found in the form of veins, pipes, pockets, stock works and vugs. All crystals are not of radio grade. Clear crystals are rare.

Only clear pyramidical crystals are picked up for tests, Grey or milky quartz being unsuitable. Small production of piezoelectric quartz crystals is also reported from Mozambique.

Common Uses
The commonest use of quartz and glass-sand, also referred to as silica-sand, is in the manufacture of glass. Great advancement has been made in the manufacture of translucent, transparent, coloured and clear glass in sheets or in glassware.

The size of the sand grains is important in glass industry. It should be between 40 to 80 mesh (BSS). It should be of high purity containing a minimum of 98% SiO₂. In the manufacture of colourless glass the iron content (Fe₂O₃) should not exceed 0.04%. for optical purposes, the presence of iron oxide (Fe₂O₃) more than 0.008% is not tolerated.

Iron and chromium are both objectionable impurities in glass-sand. The minutest presence of these impurities gives colour effect in the glass melt. Glass is manufactured by melting a mixture in suitable proportion of felspar, dolomite, limestone and soda ash together with glass-sand at 1400º - 1500ºC in the furnace when clear molten glass is formed.

The average composition of the mixture by parts is: soda ash 1; silica sand 2½; limestone / dolomite ¼; borax 1/320; saltpetre 1/160; felspar 1/80 and selenium 1/36000. Selenium is added to neutralise the colour effect imparted by the presence of iron oxide.

Twelve grams of selenium are sufficient to neutralise the colour effect of one tonne glass containing 0.15% Fe₂O₃. Borax is used to manufacture borosilicate glass of low alkali content including laboratory wares and optical glass. Boron confers a low co-efficient of expansion, increases resistance to mechanical and thermal shocks and gives a bright and pleasing appearance to glass.

Abrasives
Glass-sand free from organic and clayey impurities is used in the manufacture of sand-paper, abrasive cloth etc. generally sands crushed from sandstone and quartzite are used. River-borne sands are unsuitable as they do not possess the angular faces.

Refractory
Quartzite, sandstone, quartz and other siliceous rocks lime mica schists are used in the manufacture of silica bricks. Quartzite contains mainly silica and has high refractoriness. The purity of raw material i.e., the high silica content is essential with least possible Al₂O₃. Phase diagram study has sown that even about 5% Al₂O₃ present brings down the refractoriness from 1728ºC to 1545ºC when eutectic is formed. The presence of 0.01% Al₂O₃ lowers the refractoriness of silica by 4.8ºF and 0.4% of Al₂O₃ by 140ºF. Silica-rock of metamorphic origin is better than that of igneous origin because silica grains cemented with cristobalite and tridymite are stable phases of silica. Silica bricks are used in the steel industry. The importance of silica bricks is fast losing ground because of a gradual change in the practice of manufacturing steel by basic hearth furnace. L.D. process also eliminates much use of silica bricks as instead dolimite bricks are used. At present the estimated rate of consumption of silica bricks in the domestic steel plants in open hearth process using silica roof is 8 kg. per ingot tonne of steel as against 2.8 kg. consumed in the advanced countries.

Metallurgical
Quartz and quartzite are used in making ferro-silicon for the manufacture of silicon steel. The specification of quartz and quartzite provided by the industry manufacturing ferro-silicon is as follows:

Ball mill use
Rounded pebbles of chalcedony are used in ball mills for finer crushing of minerals like felspar, calcite, and barytes. Chalcedony being the cryptocrystalline variety of silica is very hard and has good abrasive effect on the material to be pulverized. Some of the ball mills use steel shots for this purpose. Chalcedony pebbles are used to advantage where product absolutely free from iron contamination is required.

The agate pieces after cutting to requisite sizes and shapes are utilized in the manufacture of fulcra of scientific balances and making edges, planes and bearing of precision instruments. Cambay and Varanasi are important agate cutting centres. Big pebbles are used for making mortar and pestle.

Glass-sand in large quantities is used as moulding sand in the foundry industry, as body constituent in the ceramic industry in the preparation of glazes and sometimes added to the raw material for cement manufacture to balance SiO₂, Al₂O₃ and Fe₂O₃ percentages. Silica flour made by grinding silica sand is used in paints. Paint manufacturers generally prefer diatomite powder which has a wide covering powder.

Take Note
Silica is a specialist semiconductor distribution business offering an extensive technology and product portfolio from a highly focused linecard. Serving most Western and Eastern European countries, Silica markets through pan-European agreements from the leading suppliers of semiconductors.

Silica offers one of the most comprehensive product portfolios available from any distributor. With local teams of application engineers and technology specialists, it is dedicated to supporting its OEM customers technically and to providing the design-in expertise needed for customers to compete successfully.

As a division of Avnet Electronics Marketing (EM) in Europe, Silica is supported by Avnet Logistics for warehousing, programming and other value added services, including laser marking, tape & reel and inspection, fast turn-round prototyping, dry packaging and baking. The Avnet Logistics Programming Service Centre, one of the largest of its kind in Europe, has a capacity of more than 30 million units a year.

A full suite of value added supply chain management services, such as buffer stock management, replenishment services (Kanban), EDI and supply chain consulting, is available to Silica's European customers in order to reduce inventory, to lower costs and reduce time to market cycles.

Silica is a division of Phoenix-based Avnet Inc. (NYSE:AVT), a Fortune 500 company with annual sales exceeding $ 10,2 billion (fiscal year 2004). The world's largest distributors of semiconductors, interconnect, passive and electromechanical components from the leading manufacturers, Avnet EM markets, inventories and adds value to these products and provides world-class supply-chain management services. Avnet EM serves customers in 68 countries.