White corundum

Aluminum Oxide (Alumina, oAlumina, Corundum, Alandum). A1203, mw 101.96, v hard white cryst powd, mp 2045°, bp 2980°, d 3.965g/cc. Insol in w, v si sol in aq alkaline solns. Found in nature as a mineral such as bauxite. Lab prepn is by heating aluminum hydroxide to above 1100°. 

Corundum Aluminum oxide White 4.0 9 Crystalline Abrasive... 

White crystals exists in three crystalline modifications alpha-, beta-, and gamma-Ga203 while the alpha-form is analogous to the corundum form of alumina, the beta-Ga203 is isomorphous with theta-alumina alpha-form converts to beta-modification on calcination at high temperatures (600°C) gamma form is stable at low temperatures density 6.44 g/cm (alpha-Ga203),... 

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. 

Crystal field spectral measurements of transition metal ions doped in a variety of silicate glass compositions (e.g., Fox et al., 1982 Nelson et al., 1983 Nelson and White, 1986 Calas and Petiau, 1983 Keppler, 1992) have produced estimates of the crystal field splitting and stabilization energy parameters for several of the transition metal ions, examples of which are summarized in table 8.1. Comparisons with CFSE data for each transition metal ion in octahedral sites in periclase, MgO (divalent cations) and corundum, A1203 (trivalent cations) and hydrated complexes show that CFSE differences between crystal and glass (e.g., basaltic melt) structures,... 

Table 10.1 gives typical chemical compositions of commercially produced calcium aluminate cements. The essential compound in all of them, because it develops the main hydraulic activity and is consequently responsible for the strength development, is monocaicium aluminate, CA. In white calcium aluminate cements, it can occur with various combinations of the other binary phases of the CaO-Al20j system and corundum. The crystal structures of these compounds and the phase equilibria relating to their formation were considered in Chapter 2. In the sintering process by which these cements are made, the reaction conditions are of the utmost importance, as... 

For a large number of cases and applications, a metal oxide is more important than the metal itself. Many oxides are found native in nature or are formed spontaneously (e.g., Ti02, which is used in white paint and in the whitening of paper), while others are artificially produced from the metal itself (e.g., AI2O3, which protects aluminum from surface mechanical damage and corrosion it is produced by an electrolytic industrial process, and it also occurs in nature as the mineral corundum). 

AHA peels have been used unsuccessfully in the treatment of stretch marks, both as monotherapy and in combination with corundum crystal microdermabrasion before the peel. Treating the stretch marks with microdermabrasion beforehand enhances AHA penetration, but does so linearly, in the form of claw marks (Figure 8.1). The results are poor, even on fine, white, superficial stretch marks. 

White bauxites are chiefly used for the production of aluminium sulphate and the alums. Red bauxites form the raw material for the preparation of alumina, and therefore of aluminium. Intermediate or refractory bauxites, fused in an electric furnace, give artificial corundum. 

Aluminum oxide (uh-LOO-min-um OK-side) is white crystalline powder that occurs in nature in a variety of minerals, including boehmite, bayerite, corundum, diaspore, and gibb-site. Corundum is second hardest naturally occurring mineral. Only diamond is harder. Aluminum oxide occurs in a variety of chemical forms in a variety of gemstones, including chryso-beryl, ruby, sapphire, and spinel. The color of these gemstones is a result of impurities, such as chromium (in the case of ruby) and iron and titanium (in the case of sapphire). The colors may also vary depending on the kind and amount of each impurity. 

PHYSICAL PROPERTIES mixture of aluminum oxide (AI2O3) and magnetite, with or without hematite varies in hardness according to iron oxide present solid medium brown color (corundum) or white crystalline powder (emery) odorless insoluble in water MP (2072°C, 3761.6T) BP (2980 C, 5396T) DN/SG (4.0) CP (79.0 J/K-mol crystal at 25 C)(corundum) VD (NA) VP (approximately 0 mmHg at 20°C, 0.750 mmHg at 2158°C). 

Natural corundum or a-AlgOa is a water-white, very hard, crystalline substance. Gem stones such as sapphire and ruby consist chiefly of corundum, with traces of other oxides. 

The white calcium aluminate cements, with special aggregate (for example corundum) can be used for production of refractory concretes resistant even up to 2,000 °C (Super Secar 250). Refractory castables have also good resistance to thermal shock. 

The flame fusion technique (see Figure 7.20b) was originally devised in 1904 by Verneuil for the manufacture of artificial gemstones, such as corundum (white sapphire) and ruby. This method is now used for the mass production of jewels for watches and scientific instruments. A trickle of fine alumina powder plus traces of colouring oxides is fed at a controlled rate into an oxyhydrogen flame. Fusion occurs and the molten droplets fall on to a ceramic collecting rod. A seed crystal cemented to the rod is fused in the flame and the rod is lowered at a rate that allows the top of the growing crystal (known as a boule) to remain just molten. Renewed interest has recently been shown in this method for the production of rubies for lasers. 

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