Impure corundum

Aluminum is a constituent of many minerals, including clay (ka-olinite), mica, feldspar, sillimanite, and the zeolites. Some of these minerals are discussed under the chemistry of silicon, in Chapter 31. Aluminum oxide (alumina), occurs in nature as the mineral corundum. Corundum is the hardest of aU naturally occurring substances except diamond it scratches all other minerals, but is itself scratched by diamond, and also by the artificial substances boron carbide, and silicon carbide, SiC. Corundum and impure corundum (emery) are used as abrasives. 

SYNONYMS aluminum oxide, aluminum trioxide, corundum, impure corundum, natural aluminum oxide. 

ISynonyms/Trade Names Aluminum oxide, Aluminum trioxide, Corundum, Impure corundum. I ... 

Embossing. The decoration of pottery by means of a raised pattern (flowers, figures, etc.) the effect is usually obtained by depressions in the plaster mould in which such ware is made (Cf. sprigging). Emery. A naturally occurring, impure, corundum (q.V.) used as an abrasive. Emissivity. A surface property, being the ratio of its emissive power for heat to that of a BLACK-BODY (q.V.) for a given wavelength and at the same temperature. Some reported values for refractory materials are ... 

Aluminium oxide occurs naturally as emery (an impure form) and as corundum. Corundum is a crystalline form which may be coloured by traces of impurity, for example as ruby (red) and sapphire (blue). Small synthetic rubies and sapphires have been made by heating alumina with the colouring oxide in an oxy-hydrogen flame. 

Corundum. Comndum [1302-75-5] (see Aluminum compounds) is a naturally occurring massive crystalline mineral composed of aluminum oxide. It is an impure form of the gems mby and sapphke. Prior to 1900 comndum was an important abrasive for the production of grinding wheels. Today it is mainly employed as a loose abrasive for grinding and polishing optical lenses. Almost all the world s supply of comndum now comes from Africa, primarily from Zimbabwe. 

Aluminum oxide, A1203, is known almost universally as alumina. It exists with a variety of crystal structures, many of which form important ceramic materials (see Section 14.22). As a-alumina, it is the very hard, stable, crystalline substance corundum impure microcrystalline corundum is the purple-black abrasive known as emery. Some impure forms of alumina are beautiful, rare, and highly prized (Fig. 14.25). A less dense and more reactive form of the oxide is y-alumina. This form absorbs water and is used as the stationary phase in chromatography. 

A colorless mineral known as corundum (composed of aluminum oxide) is colorless. A red variety of corundum known as ruby, a precious stone, owes its color to impurities of chromium within the crystal structure of corundum. Blue and violet varieties of corundum are classified as sapphires, the blue being the result of iron and titanium impurities, and the violet of vanadium impurities within the corundum crystal structure. Another colorless mineral is beryl (composed of beryllium aluminum silicate) but blue aquamarine, green emerald, and pink morganite, are precious varieties of beryl including different impurities aquamarine includes iron, emerald chromium and vanadium, and morganite manganese. 

Ruby and Sapphire. Ruby and sapphire are "sister stones" both are gemstone forms of the mineral corundum (composed of aluminum oxide). Pure corundum is colorless, but a variety of trace elements cause corundum to exhibit different colors. Ruby is red corundum, while sapphire is corundum in all colors except red. The red in rubies is caused by trace amounts of chromium the more intense the red color of a ruby, the more chromium it contains. The blue in sapphires is caused by titanium and/or iron impurities (Garland 2002 Hughes 1997). 

Color can also be induced into colorless crystals by the incorporation of impurity atoms. The mineral corundum, 01-AI2O3, is a colorless solid. Rubies are crystals of A1203 containing atomically dispersed traces of Cr203 impurity. The formula of the crystal can be written (CrvAli r)203. In the solid the Al3+ and Cr3+ cations randomly occupy sites between the oxygen ions, so that the Cr3+ cations are impurity substitutional, CrA1, defects. When x takes very small values close to 0.005, the crystal is colored a rich ruby red. 

The first laser produced was the ruby laser, invented in 1960. Rubies are crystals of aluminum oxide (corundum, AI2O3), containing about 0.5% chromium ions Cr3+, as substitution impurities, CrA, and laser action, as well as color, is entirely due to these... 

These materials are normally colored by low concentrations of 3d transition-metal ions or more rarely by lanthanide ions. The pale green color of ordinary window glass is due to the presence of Fe2+ impurities and small amounts of doping of Cr3+ into AI2O3 (corundum) creates ruby. 

It is possible to make mantles using 1 to 2 molar aluminium or galhum nitrate, presumably consisting of corundum-tjrpe AI2O3 and GaaOs. When impurities are carefully avoided, these mantles produce an rmusual, pale blue thermolumine-... 

Ruby is corundum (one form of AI2O3) with 0.04 to 0.5% Cr " ions as an impurity replacing aluminum ions. The aluminum ions, and hence the chromium ions, occupy... 

Several gemstone species occur in various colors, depending on the presence of impurities or irradiation-induced color centers. Examples are the beryl, corundum, and quartz families. Quartz has poor optical properties (RI = 1.55, DISP = 0.013), but becomes of gemological interest when it exhibits attractive colors. Any material can have its color modified by the addition of various impurities synthetic mby, sapphires, and spind are produced commercially in over 100 colors (2). Synthetic cubic zirconia has been made in essentially all colors of the spectrum (11), but only the colorless diamond imitation is produced commercially in any quantity. 

A1F3 is typically purified by sublimation in a corundum crucible with a platinum lid in a vertical vacuum furnace [32]. The bottom of the crucible is heated to 1000°C and the temperature of the platinum lid becomes about 750°C. As a result, the A1F3 is evaporated from the crucible and deposited on the inside of the platinum cover. The alkali metal fluorides are typically recrystallized from their melts in platinum crucibles the impure core of the solidified melt is discarded [32]. 

Several crystal structures of the oxide are known. The most common one is a-Al203 which is made by the calcination of A1205.3H20. Natural corundum is found as semi-translucent masses with a white or greyish colour. It resembles quartz. Chemically seen, corundum is related to the much more beautiful sapphire and ruby. These have the same formula, A1203, but in their cases impurities result in the colour. 

Sapphire, like ruby, is made of corundum (A1203) and has a hardness of 9. However, instead of Cr impurities, the crystal contains iron and titanium... 

Because of its high abundance, iron is often found as an impurity in other materials. For example, corundum (y-Al203) of gem quality is sapphire, and its colors are caused by small amounts of FeIv. 

Aluminum is the second most abundant metal on earth s crust. It is a common metal in tropical soils called laterites (red soils). It is extracted from bauxite that is a rich laterite by Bayer process that involves dissolution and separation of the oxide in caustic soda solution between 150 and 250°C and 20 atm of pressure. Though abundant and inexpensive, alumina based CBPCs are difficult to form because even in an acid solution the solubility of alumina is very low. This solubility, however, can be enhanced by a mUd thermal treatment and suitable CBPCs can be formed. Alumina is available commercially as calcined alumina called corundum, or as its hydrated forms such as aluminum hydroxide (Al(OH)3), as bohmite, (A1203-3H20), gibbsite (AI2O3 H2O) or in impure forms as in kaolin clay. These mineral forms and their use in ceramic formation are discussed in Chapter 11. 

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