Microsoft word - 20130514_annexe_1_mineral ressources.docx

SECTION 5 - ANNEXE 10

1. Energy Resources
1.1 Fossil fuels
Coal, oil, natural gas, peat, oil shale and tar sands are called fossil fuels because they consist of plant and animal remains, which have been preserved in rocks. Organic material is changed easily by geological processes. Most organic matter decomposes by oxidation and are recycled in the atmosphere and hydrosphere, but a small amount is preserved due to burial beneath other sediments and becomes fossil fuels. Coal is a sedimentary rock made up largely of altered cellulose, lignin and other plant remains. Coal begins as peat, an accumulation of partly decomposed brownish plant remains. As peat is buried beneath sediment, the increasing temperature and pressure cause it to release water and other gases, gradually increasing the proportion of carbon. Crude oil and natural gas consist largely of chains and other forms of carbon and hydrogen atoms, known as hydrocarbons. In natural gas, which consists largely of methane (CH4), with smaller amounts of ethane (C2H6) and propane (C3H8), the chains are very short. In crude oil, the chains contain four to 30 carbon atoms and are more complex. Crude oil and natural gas are derived from fats and other lipids in marine algae and other aquatic plants that were buried with sediment Tar is commonly known as oil that will not flow. Although tar can be found in all types of rocks, most attention has been given to the large accumulations in sandstones, which are known as tar sands. The reservoir should be mined entirely to separate the tar from the rocks. Tar is less desirable than crude oil because they cannot be converted to gasoline and they yield a larger fraction of heavy oil products. They also contain more sulfur and nitrogen and have high metal concentration, eg Ni and V. Tar sands appear to be the remains of typical oils that were altered by reacting with groundwater and bacteria at relatively shallow depths. Oil shale is shale from which oil can be obtained by processing. 1.2 Other Energy resources
Nuclear Energy serves us in the form of heat produced by fission reactions. In present applications, the naturally occurring isotope, 235U is bombarded with slow neutrons, which increases the rate at which it splits into smaller fission products. During these reactions, some matter is converted in energy. Uranium forms an unusually complex range of deposit types that reflects its tendency to occur in nature in two oxidation states. The uranic ion (U6+) is stable in oxidized environments and forms in the presence of carbonate ions a wide range of different minerals. The uranous (U4+) is less soluble and forms the common uranium mineral (uraninite – UO2), also called pitchblende. Geothermal energy is the small fractions of earth’s thermal heat that we are able to use, largely in the form of natural hot water and steam in porous permeable reservoirs. Where the water is warm enough (> 150°C), enough of it will be flashed to steam when it is pumped upward to run turbines that generate electricity. When water is not that hot, it can still be used for residential and industrial heating. Hydropower or water power is power derived from the energy of falling water and running water, which may be harnessed for useful purposes. Since ancient times, hydropower has been used for irrigation and the operation of various mechanical devices, such as watermills, sawmills, textile mills, dock cranes, domestic lifts, power houses and paint making.
2. Iron and the Ferroalloy metals
Iron and steel from the framework around which we have built our civilization. Steel is a
major component of cars, ships, cans, bridges, etc. Even energy minerals are useless without
furnaces, pipelines, engines, etc, which are usually made of steel. This wide range of uses
reflects the relative ease with which it can be converted to steel. The simplest form, carbon
steel, usually contains less than 1% carbon and 0.5% manganese. Alloy steel is steel in which
other metals (Cr, Mn, Ni, Si, Co, Mo, V, W, Nb, Ta, etc) have been mixed with iron. They
permit steel to be used in a wide variety of applications (Kessler, 1994 and references
therein).
Common iron ore minerals such as hematite and goethite are stable in the presence of abundant oxygen, while magnetite and siderite are more stable in a more reducing, oxygen poor environment. The common iron mineral, pyrite (FeS2), is not usually mined for iron because of difficulties in disposing in sulfur. Sedimentary deposits, the largest and most important iron deposits, are chemical sediments. Most of them consist of alternating iron-rich and silica rich layers that has given them the name banded iron formation. Magmatic iron deposits formed by the separation of an immiscible iron oxide melt from silicate magmas. Other types of iron deposits exist, but are in general of no importance to the world market. 2.2 Ferroalloy metals
Manganese is geochemically similar to iron. Under reducing conditions, where it forms Mn2+, manganese remains in solution unless it encounters enough dissolved carbonate or silica to form rhodochrosite (MnCO3) or a manganese-bearing silicate mineral. In oxidized solutions, where Mn3+ and Mn4+ occur, oxide minerals including pyrolusite, psilomelane (romanechite) and braunite form. Manganese forms a wide range of deposits, including the manganese nodules of the modern sea floor, but land-based sedimentary and supergene deposits are the only current producers. Nickel is produced largely from laterite and magmatic bodies. Nickel-rich laterites form by weathering of ultramafic rocks in tropical climates. Typical minerals, like garnierite, i.e. a complex nickel silicate mineral, occur. Nickel reacts with sulfur in silicate magmas and an immiscible metal-sulfide magma rich in iron, nickel, cobalt and other metals will appear as droplets in the cooling ultramafic magma. Most nickel in these deposits is found in the iron-nickel sulphides, pentlandite and pyrrhotite. Chromium comes from chromite, a member of the spinel mineral group. Due to extensive substitution, the chromium content in chromite can vary largely. Chromite is mined from two principal types of deposits, stratiform and podiform. Stratiform chromite deposits are found in layered igneous complexes, while podiform deposits occur in (ultra-)mafic rocks from the base of the ocean crust and the top of the underlying mantle that have been obducted onto the continent. Silicon production comes largely from quartz, which is found in a wide variety of environments. Most common are quartz-rich clastic sediments, but others include coarse-grained igneous rocks and hydrothermal veins, consisting almost entirely of quartz. Co-production comes mainly from sedimentary deposits. Copper is actually the dominant mineral in these deposits, but cobalt is so highly concentrated that it is a major co-product. Much of the cobalt is in linnaeite, carrolite. Smaller amounts of cobalt are largely by-products from sulphide and laterite nickel deposits and from hydrothermal vein deposits. Molybdenum is a very scarce element. The most common molybdenum mineral is molybdenite, which is recovered largely from porphyry-type deposits. Vanadium minerals are rare because V+3, the form in which it is found in much of the crust, is geochemically similar to Fe3+, an abundant ion that is part of many common minerals. Thus, V+3 generally substitutes for Fe3+ rather than forming separate vanadium minerals. The most common vanadium-bearing mineral is magnetite, which contains as much as 3% V2O5. Vanadium is recovered from magmatic and hydrothermal deposits, but is also concentrated in many forms of organic material. The main sources of tungsten are the minerals scheelite and wolframite, which are deposited by hydrothermal solutions. Most tungsten deposits are closely related to granitic intrusions from which the metal-bearing solutions were probably derived. There are two main types of hydrothermal tungsten deposits. Scheelite-bearing skarns are found where granitic rocks are in contact with limestones. Wolframite-bearing veins occur in swarms of veins that cut granite and nearby non-carbonate sedimentary rocks. Tantalum is found in the minerals microlite, which is the Ta-rich equivalent of the niobium mineral pyrochlore and tantalite, the Ta-rich version of columbite. Tantalum deposits are largely found in pegmatites and veins associated with granite intrusions. Ta is also recuperated from the slag of tin-operations. Niobium occurs in nature largely as the oxide mineral pyrochlore. Much smaller amounts are found in columbite. Although tantalum is found in bother minerals, its concentrations are much lower. The largest niobium deposits consist of veins and disseminations in carbonatites and quartz-poor, felsic intrusive rocks that were emplaced as shallow intrusions in rifted areas of continents. Much smaller columbium deposits take the form of columbite in veins and pegmatites associated with granites. Tellurium forms a large family of minerals known as the tellurides, in which tellurium takes the places of sulfur. Although telluride minerals are found in many precious metal deposits, most of the tellurium production comes from the refining of copper metal. 3. Non-Ferrous metals
3.1 Light Metals
The light metals are so named because they have lower densities than most metals. In contrast to densities of 7.87 g/cm³ for iron, densities of light metals are only 1.74 for magnesium, 2.7 for aluminium and 4.51 for titanium. Aluminium is a major constituent of many common minerals. Although it can be recovered from common minerals, most production comes from the aluminium oxide minerals diaspora, boehmite and gibbsite, which are the major constituents of bauxite, the ore of aluminum. Bauxite is a special aluminum-rich laterite. Magnesium is produced from a wide range of natural resources, which have been exploited in order of declining grade. First to be used were deposits of magnesite, which are found in slabs of the mantle that are exposed to obduction zones along convergent margins. Magnesite is also found where magnesium-rich hydrothermal solutions replaced limestone or dolomite in sedimentara bassins. Attention has also given to the possibility of producing magnesium from tailings of asbestons deposits, which are found in ultramafic rocks. Small amounts of magnesium were also produced from evaporite minerals, which shifted attention to dissolved sources of magnesium (brine or seawater). Titanium forms two very common minerals, rutile and ilmenite, as well as leucoxene, a form of ilmenite from which the iron has been removed by the weathering and alteration. Rutile and ilmenite are heavy and resistant by weathering and erosion. Therefore, most of the production is recovered from placer deposits, in which they are combined with other heavy minerals such as monazite, garnet and zircon. A smaller amount of titanium minerals is produced by bedrock deposits and are related to mafic intrusive rocks. Beryllium is produced largely from the minerals beryl and bertrandite, which form in pegmatites and hydrothermal veins around felsic intrusions and volcanic rocks. Beryl is most common in pegmatites, where particularly good crystals qualify as the precious gems emerald and aquamarine. Bertrandite is more common in veins and disseminations in shallow rhyolitic intrusions and tuffs. 3.2 Base metals
Copper occurs in nature in a wide variety of minerals, of which the sulphide chalcopyrite is most common. It is found largely in hydrothermal deposits, although magmatic and supergene deposits are significant locally. The most important hydrothermal deposits are porphyry copper deposits, which formed around intrusions that fed volcanoes. Other important copper deposits are volcanogenic massive sulfides (VMS), sediment and volcanic-hosted copper deposits in sedimentary basins and associated rifts. Magmatic copper deposits consist of immiscible metal sulphide magmas that separated from mafic and ultramafic silicate magmas. Most copper-sulfides are not stable at the earth’s surface and dissolve during weathering. Where weathering acts on hydrothermal and magmatic deposits, secondary and supergene copper deposits are formed. Lead is usually obtained from the common ore minerals galena and zinc comes from sphalerite. They are found almost entirely in hydrothermal deposits, which are formed in three major geological environments. Basinal hydrothermal systems formed the most important lead-zinc deposits, including Mississippi Valley-type (MVT) and sedimentary Exhalative deposits (sedex). Lead and zinc are found in large vein deposits. Chimney-manto and skarn deposits are related to vein deposits and appear to form where the country rock is limestone or dolomite. The most important tin ore mineral is cassiterite. The most common deposits, known as lode deposits, are in pegmatites, quartz veins, stockworks and disseminations clustered around protrusions known as cupolas at the top of these intrusions. Tin is an important by-product from tungsten vein and stockwork deposits and porphyry molybdenum deposits, where it probably has the same magmatic origin. It is also found as a trace constituent ins ome VMS and sedex-deposits. Cassiterite is highly resistant to weathering, forming placer deposits during the erosion of tin deposits. 3.3 Chemical and Industrial metals
These metals are largely used in chemical and industrial applications and are not as widely known as the more abundant metals The rare earths are a group of closely related metals, from lanthanum to Lutetium in the periodic table. Yttrium and scandium are commonly included with the rare earths because of their chemical similarities and tendency to occur in the same deposits. The rare earths form two main minerals, bastnaesite and monazite. Xenotime is less common. Bastnaesite is usually found in hydrothermal deposits associated with alkaline intrusive rocks or carbonatites. Monazite is a common magmatic trace mineral in some granitic rocks and is concentrated in gravels due to weathering. It is a usually a by-product in gold, ilmenite, rutile, cassiterite or zircon placer mines. Although cadmium forms the mineral greenockite, most cadmium substitutes for zinc in sphalerite, which can have Cd concentrations as high as 1.3% Antimony forms the sulfide mineral, stibnite, as well as the copper-lead-antimony sulfides tetrahedrite and jamesonite, all of which are precipitated from hydrothermal solutions. Although stibnite is the dominant mineral in a few deposits, usually with arsenic and mercury, it is more commonly a by-product of lead-silver mining. Tetrahedrite and various antimony minerals are present in smaller amounts in Kuroko-type VMS deposits, chimney-manto Pb-Zn deposits, Sn-W veins and W skarn deposits. Most germanium is recovered as a by-product of zinc smelting, largely from MVT deposits. It is also found in certain copper deposits. Arsenic deposits are rare, although it forms several important sulphide minerals: arsenopyrite, realgar, orpiment, enargite and tennantite. It is remarkably common in almost all types of hydrothermal gold deposits. Enargite and tennantite are common in the upper part of some porphyry copper deposits. Because of its widespread occurrence with other metals, arsenic recovered largely as by-product. Rhenium does not form a common mineral, but substitutes for molybdenum in molybdenite. Molybdenite in porphyry copper deposits can contain up to 2000 ppm of rhenium. Mercury occurs as the sulfide mineral cinnabar. It occurs in deposits on the margin of larger hydrothermal systems, reflecting its relatively high solubility in low-temperature fluids, especially those that are alkaline. Mercury is also recovered as a by-product of gold and some base metal ores. The two metals are recovered from zircon, a heavy mineral that accumulates in placer deposits with the titanium minerals, ilmenite and rutile. Indium rarely occurs in mineral form, but substitutes for zinc, tin and tungsten in their minerals. Most indium production comes from sphalerite, which contains 10 to 20 ppm In. Selenium production comes largely from the residues from electrolytical refining of blister copper. Although Se-rich ore deposits exist, none are mined exclusively for this element. Most bismuth is a by-product of lead, molybdenum and tungsten mining and is usually recovered during smelting. Bi grades are too low to have any effect on mining.
Although thallium forms sulfide and oxide minerals, they are rare and most thallium occurs in
solid solution or as small inclusions of rare thallium minerals in sphalerite. Thallium has an
abundance of 10 to 40 ppm in most sphalerite.
4. Precious metals and gems
Gold is found largely in the native state or in combination with silver in electrum. If there is enough tellurium present, it can form also telluride minerals. It can substitute in small amounts for other metals in sulfide minerals, particularly in pyrite if arsenic is also present. Gold has been produced from hydrothermal deposits, from gold-bearing veins, veinlets and disseminations. However, production mainly comes from (paleo-) placer deposits. Gold is a very important by-product of many base-metal mines, especially porphyry copper deposits. 4.2 Silver
Silver occurs in electrum, the silver sulphide argentite, and several complex sulfide minerals containing lead, copper, antimony and arsenic, such as tennantite-tetrahedrite. It is found in a wide variety of hydrothermal deposits, as well as in a few placer deposits. More commonly, it is a by-product. The platinum-group elements (PGE) include platinum, palladium, rhodium, ruthenium, iridium and osmium that have similar chemical properties and occur together in nature. Minor PGE concentration comes from placer deposits, most comes from magmatic deposits associated with mafic igneous rocks. Although diamonds are the best known of the world’s gemstones, over 150 natural compounds have been used as gems (Kesler, 1994 and references therein). Of these, diamond, emerald and ruby consistently sell for the highest prices, with alexandrite and sapphire only slightly behind. Amber, aquamarine, jade, opal, pink topaz, spinel and tourmaline have intermediate values. Many other minerals, from agate and amethyst to zircon are usually inexpensive, although special examples can be quite expensive. Diamonds are metastable at the earth’s surface and were formed from carbon at high temperature and pressure. Diamonds are found as xenocryts in an usual potassium-rich ultramafic rock, known as kimberlite. Diamond-bearing kimberlite is found almost always in thick continental crust of Archean age. Diamond placer deposits have a wider geographical distribution than diamondiferous kimberlite pipes. Beryl a relatively common Be-Al silicate forms several important gemstones, when it develops crystals with good color and few imperfections, emerald and aquamarine. Gem beryls are found in beryllium deposits. The best gems come from pegmatites and quartz veins in which slow cooling probably permitted the growth of near-perfect crystals.
Corundum forms gems when it occurs in well-developed transparatn crystals with good color.
Red corundum is knowns as ruby, while bleu corundum is known as sapphire. Corundum
generally forms in low-silica mafic rocks and in limestones that have been altered by
hydrothermal solutions near the contact of silica-poor igneous intrusions, like syenites.
5. Fertilizer and Chemical Industrial Minerals
5.1 Limestone, dolomite and lime
Limestone is a rock made up of calcite and aragonite, which have the same composition (CaCO3), and dolomite ((CaMg(CO3)2), with lesser amounts of chert, apatite, pyrite, hematite and clastic sand, silt or clay. Most limestones are the direct or indirect product of organic activity, although small amounts of calcite will precipitate from seawater as it evaporates. Lime is made by calcining, the process in which CO2 is driven off by heating limestone to temperatures of 700 to 1100°C. 5.2 Phosphate
Phosphorus is found largely in the calcium phosphate apatite, which is a common constituent of most rocks, as well as skeletal material in many organisms. Phosphate deposits are found as sedimentary deposits, which consist of accumulation of apatite formed by biological activity. It occurs in igneous deposits, in rocks that are poor in silica, such as syenites and carbonatites. It is, however, also found as guano deposits, which is formed by bird and bat excrements. Salt is extremely abundant. Dissolved sodium and chlorine make up about 3.5% of the ocean and the mineral halite is found in many natural deposits. Most halite is found in evaporite deposits that occur in two main forms, bedded salts and salt domes. 5.4 Potash
Potash is an industry term that refers to a group of water-soluble salts containing the element potassium, as well as the ores containing these salts. The most common mineral in potash is sylvite (KCl). Modern potash production comes from evaporate deposits known as sylvinite, which consists of a mixture of halite, sylvite, caranllite and other K, Mg and Br minerals. Potash can also be recovered from brines that fill pores in (non-) marine evaporates. 5.5 Sulfur
Sulfur is produced from a wide array of sources, including mined sulfur, which comes from conventional solid mineral deposits, and recovered sulfur, which is a by-product of other mineral deposits. Three sources of mined sulfur are generally mentioned, sulfur deposited in sedimentary rocks, native sulfur in volcanic rocks and sulfur from metal sulfide minerals. 5.6 Nitrogen compounds and nitrate
Almost all of nitrogen world production comes from the Haber-Bush process in which air reacts with hydrogen from natural gas to make ammonia. Before this process was commercially feasible, nitrogen was obtained from natural deposits, including guano, and non-marine evaporite deposits. 5.7 Fluorite
Fluorite is the mineral from which the element fluorine is obtained. Most fluorite deposits are deposited by low to medium temperature hydrothermal solutions. The source and the nature of these solutions and the nature of the deposits vary greatly. 5.8 Iodine
Iodine, like bromine occurs in brines and evaporates. Iodine is also a by-product of nitrate evaporates. 5.9 Sodium sulphate
Natural sodium sulfate comes largely from non-marine brines and evaporates that are found in playas. It is also a by-product from the manufacturing of rayon, cellulose, lithium carbonate, borid acid and paper. 5.10 Bromine

Bromine has been recovered from seawater, brines and evaporates.
6. Construction and Manufacturing Industrial Minerals
6.1 Construction minerals
Cement is produced from cement rock, a silty limestone consisting of calcite, clay minerals and small amounts of iron oxide. When cement is heated, CO2 is driven off, leaving the desired mixture of calcium, aluminium, iron and silica. The preferred cement raw material is formed in areas where clastic sedimentation and carbonate-producing organic activity took place together. 6.1.2 Construction Aggregate – crushed stone and sand and gravel Construction aggregate is fragmental rock and mineral material that is used alone as fill and in combined form with concrete, asphalt and plaster. The favorite type of aggregate is sand and gravel, most of which comes from stream, beach and glacial deposits. Dimension stone includes decorative slabs, large blocks, monuments and tombstones. The most common types of rocks used are granites and related igneous rocks, limestones and its metamorphosed equivalent marble, sandstone and slate. 6.1.4 Lightweight aggregate and slag Lightweight aggregate and slag are produced in smaller volumes than natural aggregate and have special markets. Lightweight aggregate is any aggregate that has a relatively low weight, but retains the strength of natural rock, like volcanic rocks and manufactured light weight aggretes from clay-rich shales, vermiculite and perlite. Slag comes largely from blast furnaces and steel mills. Gypsum is the main raw material from which plaster is manufactured. Smaller amounts are made from anhydrite, the anhydrous equivalent of gypsum. Gypsum deposits are almost entirely marine evapoirtes that formed from evaporation of seawater or in sabkhas. 6.2 Fillers, extenders, pigments and filters
Clay is a term with many meaning. Clay is any fine-grained material that becomes plastic when mixed with water. The name clay is also used for a layered silicate mineral, as the finest grain size for clastic sediments. But is also used for any saleable clay mineral that can be used in the filler, extender and related markets. Clay minerals from where water leaches cations from feldspars and other minerals, leaving aluminium, silicon and oxygen. This occurs in the weathering zone and in hydrothermal systems. The clay trade can be broken down in three groups, the kaolinite, the smectite group and the common clays. Asbestos is the general term for any fibrous mineral with a thread-like or acicular shape, with a length greater than three times its width. Commercial asbestos comprises six distinct mineral groups, including the serpentine asbestos mineral, chrysotile, and five amphibole asbestos minerals of which only crocidolite and amosite are of economic significance. Almost all asbestos deposits form by hydrothermal alteration of rock that is rich in magnesium and/or iron. Diatomite is a rock consisting of tiny siliceous fossils known as diatoms, which are single-celled aquatic plants similar to algae. Diatomite deposits form as sediments in all types of water, from marine to fresh. Talc is a hydrous magnesium silicate that forms by hydrothermal alteration of magnesium-rich rock. Talc deposits are most common where fault zones cut mafic and ultramafic igneous and metamorphic rocks or where silica-bearing hydrothermal solutions flowed through magnesium-rich dolomite. Pyrophyllite deposits are found in similar hydrothermal settings. Barite deposits are of three different types, all of which are hydrothermal in origin. Smaller deposits precipitated from hydrothermal solutions that circulated through veins and open spaces. The really large deposits consist of layers and lenses of barite in shale, with or without the associated lead and zinc. A third type of deposit formed when cool reducing pore waters dissolved barium from the sediments and then vented into seawater containing sulphate. The barite is not associated with lead and zinc. Most commercial mica is muscovite that comes from aluminum rich metamorphic rocks that were derived from shales. Coarser grained mica comes largely from granitic pegmatites. Significant fine-grained mica is produced as by-product from kaolin and feldspar mining. Zeolites are a family of hydrous silicate minerals with peculiar crystals structures that allow them to absorb or trap other atoms or molecules. Natural zeolites from in low-temperature hydrothermal and sedimentary environments. 6.3 Glass raw materials
Glass is an amorphous solid without a well-defined crystal structure. Most glass is made by melting quartz and other minerals and rocks and then cooling the melt in a way that prevents it from crystallizing. Quartz sand deposits suitable for glass manufacture are much scarcer than common sand and gravel deposits used for construction. Elements such as iron that would impart color the glass must be avoided, as must refractory minerals like cassiterite, corundum; kyanite, chromite or zirocon, which would remain unmelted, creating imperfections in the glass. Soda ash is the industry term for sodium carbonate. Natural soda ash comes from extensive deposits of complex sodium carbonate minerals such as trona, which are found in lacustrine evaporate deposits. It is also recovered from playa brines and from playas and springs along the east African rift system. Boron comes almost entirely from lacustrine evaporates. The largest of these deposits consist of hydrous sodium borate minerals. Kernite, which contains only four water molecules in its structure, is stable at higher temperature than borax, which has ten. The boron in these deposits is thought to have come from hot springs that flowed into the lakes during evaporation. 6.3.4 Feldspar, Nepheline syenite and aplite Feldspar and the feldspar-rich rocks nepheline syenite and aplite, are used almost exclusively in glass and ceramics, where they are used as a flux and source of aluminium. Although feldspar are found in almost all types of igneous rocks, they are mined largely form special rock types that are depleted in mafic and other minerals, which would not melt easily during glassmaking or might add iron or other undesirable elements to the mix. The most common feldspar-rich rocks of this type are aplite, alaskite and pegmatite. Lithium comes from two very different types of deposits. The main lithium mineral that is recovered from pegmatites, spodumene, can be used directly in glass or other ceramics, but it must be processed into another form such as lithium carbonates for other markets. Lacustrine brines and playa evaporates also contain lithium. Deposits of strontium consists of layers and disseminations of the strontium sulphate mineral, celestite, almost always in limestone. In the best deposits, celestite forms extensive, nearly pure layers, known as mantos, that have apparently replaced layers of limestone. The replacement appears to have taken place when groundwaters containing dissolved strontium came into contact with sulphate-rich waters. 6.4 Abrasive and refractory Minerals
The most valuable natural abrasive material is industrial diamond, the hardest substance known, with silica sand a distant second and other products such as silica stone, garnet, Tripoli, emery, feldspar and diatomite bringing up the rear. The abrasive market is considerably larger, since so many synthetic abrasives are available. The most important are synthetic industrial diamonds. The other major synthetic abrasives are fused aluminium oxide, which is prepared from bauxite, and silicon carbide, which is prepared from quartz and coke. Refractory materials provide heat resistant bricks and blocks that are used in a wide range of industrial applications. 6.4.1 Industrial and synthethic diamond Industrial diamonds are those that cannot be use as gems. They range from imperfect stones of several carats to very fine grained diamond powder. Silica sand comes largely from industrial sand deposits. Because of the corrosion of metals by salt, sand used for abrasive purposes is mined from deposits that formed in fresh water. Silica stone and Tripoli are forms of chert, flint and other amorphous to microcrystalline forms of silica that are largely found in sedimentary rocks. Some of these deposits formed as accumulations of siliceous organisms such as diatoms in deep marine environments that were not contaminated by clastic sediments. Others formed by the redistribution of silica shortly after the deposition of the sedimentary rocks. Garnet is a common metamorphic mineral that becomes abundant enough to mine in a few rocks. Graphite is the crystal form of carbon in which the atoms form hexagonal plates. Graphite deposits form layers, disseminations or veins in metamorphic rocks or contact-localized skarn deposits near granitic intrusions. Some of this graphite is the remains of the decomposed original organic sediments, while other directly formed by deposition from hydrothermal solutions containing dissolved carbon. 6.4.4 Kyanite and related Minerals Kyanite, sillimanite and andalousite are different crystal structures of the aluminum silicate Al2SiO5. These minerals form by the metamorphism of aluminum-rich rocks that are poor in other cations such as sodium, calcium or iron. The three minerals form under different temperature and pressure conditions. Because these minerals are resistant to erosion, they can be mined from weathered overburden or from beach sands derived from the erosion of the overburden.

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