Naturally Occurring Abrasives

Nonsilica mineral sands, such as magnetite, staurolite, and olivine, are tough (5 to 7 Mohr) and fairly dense (2.0 to 3.0 specific gravity) but are generally of finer particle size than silica sand. These heavy mineral sands — as opposed to silica sand — do not contain free silicates, the cause of the disease silicosis. In general. 

Olivine ([Mg,Fe]2[Si04]) has a somewhat lower efQciency than silica sand [2] and occasionally leaves white, chalk-hke spots on the blasted surface. It leaves a profile of 2.5 mil or finer, which makes it less suitable for applications where profiling the steel surface is important. 

Staurolite is a heavy mineral sand that has low dust levels and, in many cases, can be recycled three or four times. It has been reported to have good feathering and does not embed in the steel surface. 

Zircon has higher specific gravity (4.5) than any other abrasive in this class and is very hard (7.5 Mohr). Other good attributes of zircon are its low degree of dusting and its lack of free silica. Its fine size, however, limits its use to specialty applications because it leaves little or no surface profile. 

Novaculite is a siliceous rock that can be ground up to make an abrasive. It is the softest abrasive discussed in this class (4 Mohr) and is suitable only for specialty work because it leaves a smooth surface. Novaculite is composed mostly of free silica, so this abrasive is not recommended unless adequate precautions to protect the worker from silicosis can be taken. For the same reason, flint, which consists of 90% free silica, is not recommended for maintenance painting. 

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Naturally occurring abrasives are still an important item of commerce, although synthetic abrasives now fill many of thek former uses. In 1987 about 156 million metric tons of natural abrasives were produced in the United States. Production was up from 1986 because of increased nonabrasive uses and increased use of garnet in sandblasting (4). 

They also examined a large number of authentic cut crystal and hard-stone artifacts from documented Olmec, Maya, Aztec, and Mixtec contexts for comparative purposes. The SEM results showed for the British Museum skull unambiguous evidence of high-speed rotary disc cutting and the use of exU emely hard abrasives. Although diamond and corundum are two naturally occurring abrasives of adequate hardness, they were not in use in ancient Mesoamerica and the authentic artifacts showed no such SEM evidence. 

Natural abrasives can be sand, walnut shells, etc. Artificial abrasives include silicon carbide, aluminum oxide, boron carbide, and boron nitride. Artificial abrasives are generally superior in uniformity to naturally occurring abrasives and provide more consistent abrasive finishing processes. 

Garnet A naturally occurring abrasive material that is composed of metal silicates. 

An abrasive is a substance used to abrade, smooth, or polish an object. If the object is soft, such as wood, then relatively soft abrasive materials may be used. Usually, however, abrasive coimotes very hard substances ranging from naturally occuring sands to the hardest material known, diamond. 

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. 

An abrasive is usually chemically inert, neither interacting with other dentifrice ingredients nor dissolving in the paste or the mouth. Substances used as dentifrice abrasives include amorphous hydrated silica, dicalcium phosphate dihydrate [7789-77-7] anhydrous dicalcium phosphate [7757-93-9] insoluble sodium metaphosphate [10361-03-2], calcium pyrophosphate [35405-51-7], a-alumina trihydrate, and calcium carbonate [471-34-1]. These materials are usually synthesized to specifications for purity, particle size, and other characteristics naturally occurring minerals are used infrequently. Sodium bicarbonate [144-55-8] and sodium chloride [7647-14-5] have also been employed as dentifrice abrasives. 

In 1885, Charles Martin Hall invented his aluminum process and Hamilton Young Castner in 1890 developed the mercury-type alkali-chlorine cell, which produced caustic (sodium hydroxide) in its purest form. Edward G. Acheson in 1891, while attempting to make diamonds in an electric furnace, produced silicon carbide, the first synthetic abrasive, second to diamond in hardness. Four years later, Jacobs melted aluminum oxide to make a superior emeiy cloth. Within two decades, these two abrasives had displaced most natural cutting materials, including naturally occurring mixtures of aluminum and iron oxides. 

Aluminum is produced commercially by the electrolysis of cryolite, Na3AlF6, but bauxite, A1203, is the usual naturally occurring source of the metal. The oxide is a widely used catalyst which has surface sites that function as a Lewis acid. A form of the oxide known as activated alumina has the ability to adsorb gases and effectively remove them. Other uses of the oxide include ceramics, catalysts, polishing compounds, abrasives, and electrical insulators. 

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. 

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 ... 

Magnesium hydroxide is a white crystalline material. Its specific gravity is 2.4. Like ATH, it is soft and non-abrasive with a Mohs hardness about 3. It starts to decompose at about 300 °C. The naturally occurring mineral is known as brucite. The natural crystal form is as a flat hexagonal shaped plate. 

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