Minerals corundum

A homogeneous mixture of two or more components, whether solid, liquid, or gaseous, is called a solution. Solutions have variable composition while pure substances do not. That is, the relative amounts of the various components in a solution can vary. Thus, air, salt water, and sixteen carat gold are each solutions. The gemstone, ruby, is also a solution since it consists of the mineral corundum (AI2O3) with some of the aluminum replaced by chromium to give the crystal its characteristic color. Since the amount of chromium present can be varied, ruby is a solution. 

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. 

In 1960 the first ruby laser was made from a ruby crystal of aluminum oxide (Al Oj). These crystals contain only a small amount of chromium, which stores the energy and is responsible for the laser action. A small amount of chromium found in the mineral corundum is responsible for the bright red color of the ruby gemstone. 

Occurs in nature in abundance the principal forms are bauxites and lat-erites. The mineral corundum is used to produce precious gems, such as ruhy and sapphire. Activated aluminas are used extensively as adsorbents because of their affinity for water and other polar molecules and as catalysts because of their large surface area and appropriate pore sturcture. As adsorbents, they are used for drying gases and liquids and in adsorption chromatography. Catalytic properties may be attributed to the presence of surface active sites (primarily OFT, 02, and AF+ ions). Such catalytic applications include sulfur recovery from H2S (Clauss catalysis) dehydration of alcohols, isomerization of olefins and as a catalyst support in petroleum refining. 

Elemental composition A1 52.91%, 0 47.08%. A1 may be anlayzed by atomic absorption or emission spectrophotometry or by colorimetric methods after acid digestion. Different forms of alumina may be identified by x-ray diffraction analysis. The X-ray crystallogaphic data for the mineral corundum are as follows ... 

Rubies and sapphires are color varieties of the mineral corundum, which is simple aluminum oxide. This mineral is second only to diamond in hardness, but is much tougher and harder to break. Rubies are, by definition, red or purplish red. The term sapphire is used for every other color. 

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). 

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. 

The mineral corundum provides a good example. Corundum is an extremely hard substance. Small bits of corundum are part of the rock called emery which has been used since ancient times as an abrasive, to cut and grind metal and stone. Pure corundum is still used for this purpose today. Another property of corundum is that it remains solid and stable at very high temperatures, well past the melting point of iron. Therefore, masses of small corundum crystals pressed together are shaped into alumina firebricks, crucibles, and other apparatus to use in furnaces. Corundum is also the basis of several gemstones. 

Alumina in the pure form is found in nature as the mineral corundum (i.e. a--AI2O3). In ceramics, use is made of chemically produced oxide or hydroxide, and in some instances even of purer bauxites. 

Aluminum oxide (AI2O3), often called alumina, is the principal raw material for the production of aluminum. Alumina occurs in the minerals corundum and bauxite. Determine the moles of aluminum ions (Al +) in 1.25 moles of aluminum oxide. 

The work conducted by Yamaguchi [28] deals in part with the effects of sodium carbonate vapor on the minerals corundum and mullite. In the case of corundum, the major reaction product was apparently sodium aluminate, whereas with mullite, carnegieite solid solution was observed. 

CAS 1344-28-1. A Oj. The mineral corundum is natural aluminum oxide, and emery, ruby, and sapphire are impure crystalline varieties. The mixed mineral bauxite is a hydrated aluminum oxide. 

Anhydrous AI2O3 occurs naturally as the extremely hard, high-melting mineral corundum, which has a network structure. It is colorless when pure, but becomes colored when transition metal ions replace a few Al + ions in the crystal. Sapphire is usually blue and contains some iron and titanium. Ruby is red due to the presence of small amounts of chromium. 

The structural relations between the many crystalline forms of aluminium oxide and hydroxide are exceedingly complex but they are of exceptional scientific interest and immense technological importance. The principal structural types are listed in Table 7.8 and many intermediate and related structures are also known. AI2O3 occurs as the mineral corundum (ot-Al203, cJ 4.0gcm ) and as emery, a granular form of corundum contaminated with iron oxide and silica. Because of its great hardness (Mohs... 

In most of the chemical reactions we will be considering, a combination of minerals, or minerals plus liquids or gases, reacts to form some different minerals under some given conditions. For example, the mineral corundum (AI2O3) is stable, considered by itself (i.e., there is no other form of AljOj that is more stable), but in the presence of water it reacts to form gibbsite (AI2O3 3H2O). The reaction is... 

Gemstones. The most important gems are diamonds, sapphires, rubies, and emeralds. Gems must be harder than most other minerals, and they must be beautiful. Diamonds are composed of the element carbon, and rubies and sapphires are gem-quality varieties of the mineral corundum. Emeralds are gem-quality varieties of the mineral beryl. 

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