Corundum oxide

Fig. 17). This occurs on the rocksalt oxide (111) surfaces (/ = 1/2), the ZnO(OOOl) surface (/ = 1/4), the oxygen termination of corundum oxide (0001) surface (/ = 1.5), etc. These partial fillings were indeed found in the quantum calculations. For some other polar surfaces, / is integer, and the surface can remain insulating. This takes place for example, on the (111) or (110) polar surfaces of SrTiOs (/ = 1). 

Aluminium is not found free but its compounds are so widespread that it is the most abundant metal in the earth s crust. Aluminosilicates such as clay, kaolin (or china clay), mica and feldspar are well known and widely distributed. The oxide. AI2O3. occurs (anhydrous) as corundum and emery, and (hydrated) as bauxite. Cryolite. Na,AlF. (sodium hexafluoroaluminate). is found extensively in Greenland. 

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. 

The compounds of greatest importance are aluminum oxide, the sulfate, and the soluble sulfate with potassium (alum). The oxide, alumina, occurs naturally as ruby, sapphire, corundum, and emery, and is used in glassmaking and refractories. Synthetic ruby and sapphire are used in lasers for producing coherent light. 

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. 

Corundum (AljOj) Vegetable oil mists (except castor cashew nut, or similar irritant oils) Sucrose, Tin Oxide, Titanium Dioxide Silicoh Carbide... 

The binary oxides and hydroxides of Ga, In and T1 have been much less extensively studied. The Ga system is somewhat similar to the Al system and a diagram summarizing the transformations in the systems is in Fig. 7.13. In general the a- and y-series have the same structure as their Al counterparts. )3-Ga203 is the most stable crystalline modification (mp 1740°) it has a unique crystal structure with the oxide ions in distorted ccp and Ga " in distorted tetrahedral and octahedral sites. The structure appears to owe its stability to these distortions and, because of the lower coordination of half the Ga ", the density is 10% less than for the a-(corundum-type) form. This preference of Ga "... 

Aluminum oxide. Al3+ ions are present in corundum Heft), ruby (top right), and sapphire (bottom right). The anion in each case is 02-. 

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°. 

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. 

Trivalent Oxides.—It is probable that the rhombohedral crystal corundum, AI2O3, consists of ions that is, the bonds are probably electrostatic. The oxygen ions are arranged in layers parallel to the base (111) within each layer the distances between adjacent ions are 2.50 and 2.89 A., the values being somewhat uncertain because of the difficulty in the accurate determination of the parameter. These values are probably compatible with the oxygen radius 1.40 A., calculated for crystals with bivalent cations. The aluminum-oxygen distances are 1.85 and 1.99 A., in satisfactory agreement with the calculated radius sum 1.90 A. 

The Fe—O distances in hematite are 1.99 and 2.06 A. The (Mn,Fe)—O distances in bixbyite are expected to be the same in case that (Mn, Fe) has the coordination number 6, and slightly smaller, perhaps 1.90 A, for coordination number 4. The radius of 0= is 1.40 A, and the average O—O distance in oxide crystals has about twice this value. When coordinated polyhedra share edges the O—O distance is decreased to a minimum value of 2.50 A, shown by shared edges in rutile, anatase, brookite, corundum, hydrargillite, mica, chlorite, and other crystals. Our experience with complex ionic crystals leads us to believe that we may... 

Alumina - Alumina forms a variety of oxides and hydroxides whose structures have been characterized by X-ray diffraction (16). From the catalytic viewpoint y-alumina is the most important. This is a metastable phase that is produced from successive dehydration of aluminum trihydroxide (gibbsite) to aluminum oxide hydroxide (boehmite) to y-alumina, or from dehydration of boehmite formed hydrothermally. y-alumina is converted into a-alumina (corundum) at temperatures around 1000 C. 

Oxides and hydroxides Cuprite, uraninite, baddeleyite, corundum, haematite, rutile, cassiterite, brucite, diaspore, goethite, limonite... 

Corundum Aluminum oxide White 4.0 9 Crystalline Abrasive... 

Oxides Metal ion(s) + oxygen ion Haematite (composed of iron oxide), a red pigment and an iron ore Corundum (composed of aluminum oxide), an abrasive silica (composed of silicon dioxide), common sand... 

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). 

The growth of vanadium oxide overlayers on Rh(l 11) converges after a number of intermediate stages to the formation of a three-dimensional bulk-like epitaxial V203 film [90], which is oriented with the (0 0 01) plane of its corundum structure parallel to the Rh(l 1 1) substrate surface. The V203 phase is the thermodynamically stable... 

The number of oxide type minerals is quite large. Rostov (1956) has identified 160 specific minerals, grouped them into classes (chrysoberyl, spinel, corundum, periclase, etc.), and proposed a classification system. Only a few examples will be discussed here. 

W. A. Roth, U. Wolf, and O. Fritz, The Heat of Formation of Aluminum Oxide (Corundum) and of Lanthanum Oxide, Zeit. Elektro. u. Angew. Physik. Chemie, 46,42 (1940). 

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