Adapted from 3rd edition, Encyclopaedia of Occupational Health and Safety. Revision includes information from A. Bruusgaard, L.L. Cash, Jr., G. Donatello, V. D’Onofrio, G. Fararone, M. Kleinfeld, M. Landwehr, A. Meiklejohn, J.A. Pendergrass, S.A. Roach, T.A. Roscina, N.I. Sadkovskaja and R. Stahl.
Minerals are used in ceramics, glass, jewellery, insulation, stone carving, abrasives, plastics and numerous other industries in which they present primarily an inhalation hazard. The amount and type of impurities within the minerals may also determine the potential hazard associated with inhalation of the dust. The major concern during mining and production is the presence of silica and asbestos. The silica content in different rock formations, such as sandstone, feldspars, granite and slate, may vary from 20% to nearly 100%. It is therefore imperative that worker exposure to dust concentrations be kept to a minimum by the implementation of strict dust-control measures.
Improved engineering controls, wet drilling, exhaust ventilation and remote handling are recommended to prevent the development of lung disease in mineral workers. Where effective engineering controls are not possible, workers should wear approved respiratory protection, including the proper selection of respirators. Where possible, industrial substitution of less hazardous agents can reduce occupational exposure. Finally, the education of workers and employers regarding the hazards and proper control measures is an essential component of any prevention programme.
Regular medical examinations of mineral-dust-exposed workers should include evaluations for respiratory symptoms, lung function abnormalities and neoplastic disease. Workers showing the first signs of lung changes should be assigned to other jobs entailing no dust hazards. In addition to individual reports of illness, data from groups of workers should be collected for prevention programmes. The chapter Respiratory system provides more detail on the health effects of several of the minerals described here.
Apatite (Calcium Phosphate)
Occurrence and uses. Apatite is a natural calcium phosphate, usually containing fluorine. It occurs in the earth’s crust as phosphate rock, and it is also the chief component of the bony structure of teeth. Deposits of apatite are located in Canada, Europe, the Russian Federation and the United States.
Apatite is used in laser crystals and as a source of phosphorus and phosphoric acid. It is also employed in the manufacture of fertilizers.
Health hazards. Skin contact, inhalation or ingestion may cause irritation of skin, eyes, nose, throat or gastric system. Fluorine may be present in the dust and may cause toxic effects.
Occurrence and uses. Asbestos is a term used to describe a group of naturally occurring fibrous minerals which are widely distributed throughout the world. The asbestos minerals fall into two groups—the serpentine group, which includes chrysotile, and the amphiboles, which include crocidolite, tremolite, amosite and anthophyllite. Chrysotile and the various amphibole asbestos minerals differ in crystalline structure, in chemical and surface characteristics, and in the physical characteristics of their fibres.
The industrial features which have made asbestos so useful in the past are the high tensile strength and flexibility of the fibres, and their resistance to heat and abrasion and to many chemicals. There are many manufactured products which contain asbestos, including construction products, friction materials, felts, packings and gaskets, floor tiles, paper, insulation and textiles.
Health hazards. Asbestosis, asbestos-related pleural disease, malignant mesothelioma and lung cancer are specific diseases associated with exposure to asbestos dust. The fibrotic changes which characterize the pneumoconiosis, asbestosis, are the consequence of an inflammatory process set up by fibres retained in the lung. Asbestos is discussed in the chapter Respiratory system.
Occurrence and uses. Bauxite is the principal source of aluminium. It consists of a mixture of minerals formed by the weathering of aluminium-bearing rocks. Bauxites are the richest form of these weathered ores, containing up to 55% alumina. Some lateritic ores (containing higher percentages of iron) contain up to 35% Al2O3. The commercial deposits of bauxite are mainly gibbsite (Al2O3 3H2O) and boehmite (Al2O3 H2O), and are found in Australia, Brazil, France, Ghana, Guinea, Guyana, Hungary, Jamaica and Surinam. Gibbsite is more readily soluble in sodium hydroxide solutions than boehmite, and is therefore preferred for alumina production.
Bauxite is extracted by open-cast mining. The richer ores are used as mined. The lower-grade ores may be upgraded by crushing and washing to remove clay and silica waste.
Health hazards. Severe pulmonary disability has been reported in workers employed on smelting bauxite that is combined with coke, iron and very small amounts of silica. The affliction is known as “Shaver’s disease”. Because silica contamination of aluminium-containing ores is common, the health hazards associated with the presence of free crystalline silica in bauxite ores must be considered an important causal factor.
Clays (Hydrated Aluminium Silicates)
Occurrence and uses. Clay is a malleable plastic material formed by the weathered disintegration residues of argillaceous silicate rock; it usually contains 15 to 20% water and is hygroscopic. It occurs as a sediment in many geological formations in all parts of the world and contains in varying amounts feldspars, mica and admixtures of quartz, calcspar and iron oxide.
The quality of clay depends on the amount of alumina in it—for example, a good porcelain clay contains about 40% alumina, and the silica content is as low as 3 to 6%. On average the quartz content of clay deposits is between 10 and 20%, but at worst, where there is less alumina than usual, the quartz content may be as high as 50%. Content may vary in a deposit, and separation of grades may take place in the pit. In its plastic state, clay can be moulded or pressed, but when fired it becomes hard and retains the shape into which it has been formed.
Clay is often extracted in open-cast pits but sometimes in underground mines. In open-cast pits the method of extraction depends on the quality of the material and the depth of the deposit; sometimes the conditions require the use of hand-operated pneumatic tools, but, wherever possible, mining is mechanized, using excavators, power shovels, clay cutters, deep digging machines and so on. The clay is taken to the surface by truck or cable transport. The clay brought to the surface may be subjected to preliminary processing before dispatch (drying, crushing, pugging, mixing and so on) or it may be sold whole (see the chapter Mining and quarrying). Sometimes, as in many brickyards, the clay pit may be adjacent to the factory where the finished articles are made.
Different types of clay form the basic material in the manufacture of pottery, bricks and tiles, and refractories. Clay may be used without any processing in dam construction; in situ, it sometimes serves as a cover for gas stored in lower stratum. Appropriate ventilation and engineering controls are required.
Health hazards. Clays usually contain large amounts of free silica, and chronic inhalation can cause silicosis. Skin contact with wet clay may cause skin drying and irritation. There is a silicosis risk to underground workers where there is mechanized mining of clay with a high quartz content and little natural moisture. Here the decisive factor is not merely the quartz content but also the natural dampness: if the moisture level is less than 12%, much fine dust must be expected in mechanical extraction.
Occurrence and uses. Coal is a natural, solid, combustible material formed from prehistoric plant life. It occurs in layers or veins in sedimentary rocks. Conditions suitable for the natural formation of coal occurred between 40 and 60 million years ago in the Tertiary Age (brown-coal formation) and over 250 million years ago in the Carboniferous Age (bituminous-coal formation), when swampland forests thrived in a hot climate and then gradually subsided during ensuing geological movements. The main deposits of brown coal are found in Australia, eastern Europe, Germany, the Russian Federation and the United States. Major reserves of bituminous coal are located in Australia, China, India, Japan, the Russian Federation and the United States.
Coal is an important source of chemical raw materials. Pyrolysis or destructive distillation yields coal tar and hydrocarbon gases, which can be upgraded by hydrogenation or methanation to synthetic crude oil and fuel gas. Catalytic hydrogenation yields hydrocarbon oils and gasoline. Gasification produces carbon monoxide and hydrogen (synthetic gas), from which ammonia and other products can be made. While in 1900, 94% of the world’s energy requirements were met by coal and only 5% by petroleum and natural gas, coal has been increasingly replaced by liquid and gaseous fuels throughout the world.
Health hazards. Hazards of mining and of coal dust are discussed in the chapters Mining and quarrying and Respiratory system.
Corundum (Aluminium Oxide)
Occurrence and uses. Corundum is one of the principal natural abrasives. Natural corundum and artificial corundum (alundum or artificial emery) are usually relatively pure. The artificial material is produced from bauxite by smelting in an electric furnace. Because of its hardness, corundum is used to shape metals, wood, glass and ceramics, by a process of grinding or polishing. Health hazards are discussed elsewhere in this Encyclopaedia.
Diatomaceous Earth (Diatomite, Kieselguhr, Infusorial Earth)
Occurrence and uses. Diatomaceous earth is a soft, bulky material composed of skeletons of small, prehistoric aquatic plants related to algae (diatoms). Certain deposits comprise up to 90% free amorphous silica. They have intricate geometric forms and are available as light-coloured blocks, bricks, powder and so on. Diatomaceous earth absorbs 1.5 to 4 times its weight of water and has a high oil absorption capacity. Deposits occur in Algeria, Europe, the Russian Federation and the western United States. Diatomaceous earth may be used in foundries, in paper coating, in ceramics and in the maintenance of filters, abrasives, lubricants and explosives. It is used as a filtering medium in the chemical industry. Diatomaceous earth also finds use as a drilling-mud thickener; an extender in paints, rubber and plastic products; and as an anti-caking agent in fertilizers.
Health hazards. Diatomaceous earth is highly respirable. For many industrial purposes diatomaceous earth is calcined at 800 to 1,000 ºC to produce a greyish-white powder called kieselguhr, which may contain 60% or more crystobalite. During mining and processing of diatomaceous earth, the risk of death from both respiratory diseases and lung cancer has been related to the inhalation of dust as well as to cumulative crystalline silica exposures, as discussed in the chapter Respiratory system.
Occurrence and uses. Erionite is a crystalline, fibrous zeolite. Zeolites, a group of alumino-silicates found in the cavities of volcanic rocks, are used in the filtration of hard water and in the refining of oil. Erionite occurs in California, Nevada and Oregon in the United States, and in Ireland, Iceland, New Zealand and Japan.
Health hazards. Erionite is a known human carcinogen. Chronic inhalation may cause mesothelioma.
Occurrence and uses. Feldspar is a general name for a group of sodium, potassium, calcium and barium aluminium silicates. Commercially, feldspar usually refers to the potassium feldspars with the formula KAlSi3O8, usually with a little sodium. Feldspar occurs in the United States. It is used in pottery, enamel and ceramic ware, glass, soaps, abrasives, cements and concretes. Feldspar serves as a bond for abrasive wheels, and it finds use in insulating compositions, tarred roofing materials and fertilizers.
Health hazards. Chronic inhalation may cause silicosis due to the presence of substantial amounts of free silica. Feldspars may also contain irritating sodium oxide (soda spars), potassium oxide (potash spars), and calcium oxide (lime spars) in insoluble form. See the section “Silica” below.
Occurrence and uses. Flint is a crystalline form of native silica or quartz. It occurs in Europe and the United States. Flint is used as an abrasive, a paint extender and a filler for fertilizer. In addition, it finds use in insecticides, rubber, plastics, road asphalt, ceramics and chemical tower packing. Historically, flint has been an important mineral because it was used to make some of the first known tools and weapons.
Health hazards are related to the toxic properties of silica.
Fluorspar (Calcium Fluoride)
Occurrence and uses. Fluorspar is a mineral that contains 90 to 95% calcium fluoride and 3.5 to 8% silica. It is extracted by drilling and blasting. Fluorspar is a principal source of fluorine and its compounds. It is used as a flux in open hearth steel furnaces and in metal smelting. In addition, it finds use in the ceramics, paint and optical industries.
Health hazards. The hazards of fluorspar are due primarily to the harmful effects of the fluorine content and its silica content. Acute inhalation may cause gastric, intestinal, circulatory and nervous system problems. Chronic inhalation or ingestion may cause loss of weight and appetite, anaemia, and bone and teeth defects. Pulmonary lesions have been reported among persons inhaling dust containing 92 to 96% calcium fluoride and 3.5% silica. It appears that calcium fluoride intensifies the fibrogenic action of silica in the lungs. Cases of bronchitis and silicosis have been reported among fluorspar miners.
In the mining of fluorspar, dust control should be carefully enforced, including wet drilling, watering of loose rock, and exhaust and general ventilation. When heating fluorspar, there is also the hazard of hydrofluoric acid being formed, and the relevant safety measures should be applied.
Occurrence and uses. The coarse-grained igneous rock granite consists of quartz, feldspar and mica in shapeless interlocking grains. It finds use as crushed granite and as dimension granite. After it is crushed to the required size, granite may be used for concrete aggregate, road metal, railroad ballast, in filter beds, and for riprap (large chunks) in piers and breakwaters. The colors—pink, grey, salmon, red and white—are desirable for dimension granite. The hardness, uniform texture and other physical properties make dimension granite ideal for monuments, memorials, foundation blocks, steps and columns.
Large production of crushed granite comes mainly from California, with substantial amounts from the other US States of Georgia, North Carolina, South Carolina and Virginia. Major production areas of dimension granite in the United States include Georgia, Maine, Massachusetts, Minnesota, North Carolina, South Dakota, Vermont, and Wisconsin.
Health hazards. Granite is heavily contaminated with silica. Therefore, silicosis is a major health hazard in granite mining.
Occurrence and uses. Graphite is found in almost every country of the world, but the majority of production of the natural ore is limited to Austria, Germany, Madagascar, Mexico, Norway, the Russian Federation and Sri Lanka. Most, if not all, natural graphite ores contain crystalline silica and silicates.
Lump graphite is found in veins which cross different types of igneous and metamorphic rock containing mineral impurities of feldspar, quartz, mica, pyroxine, zircon, rutile, apatite and iron sulphides. The impurities are often in isolated pockets in the veins of ore. Mining is commonly underground, with hand drills for selective mining of narrow veins.
Deposits of amorphous graphite are also underground, but usually in much thicker beds than the veins of lumps. Amorphous graphite is commonly associated with sandstone, slate, shale, limestone and adjunct minerals of quartz and iron sulphides. The ore is drilled, blasted and hand-loaded into wagons and brought to the surface for grinding and impurity separation.
Flake graphite is usually associated with metamorphosed sedimentary rock such as gneiss, schists and marbles. The deposits are often on or near the surface. Consequently, normal excavating equipment such as shovels, bulldozers and scarifiers are used in open-cast mining, and a minimum of drilling and blasting is necessary.
Artificial graphite is produced by the heating of coal or petroleum coke, and generally contains no free silica. Natural graphite is used in the manufacture of foundry linings, lubricants, paints, electrodes, dry batteries and crucibles for metallurgical purposes. The “lead” in pencils is also graphite.
Health hazards. Inhalation of carbon, as well as associated dusts, may occur during the mining and milling of natural graphite, and during the manufacture of artificial graphite. X-ray examinations of natural and artificial graphite workers have shown varying classifications of pneumoconioses. Microscopic histopathology has revealed pigment aggregates, focal emphysema, collagenous fibrosis, small fibrous nodules, cysts and cavities. The cavities have been found to contain an inky fluid in which graphite crystals were identified. Recent reports note that the materials implicated in exposures leading to severe cases with massive pulmonary fibrosis are likely to be mixed dusts.
Graphite pneumoconiosis is progressive even after the worker has been removed from the contaminated environment. Workers may remain asymptomatic during many years of exposure, and disability often comes suddenly. It is essential that periodic analyses are made of the raw ore and airborne dust for crystalline silica and silicates, with special attention to feldspar, talc and mica. Acceptable dust levels must be adjusted to accommodate the effect these disease-potentiating dusts may have on workers’ health.
In addition to being exposed to the physical hazards of mining, graphite workers may also face chemical hazards, such as hydrofluoric acid and sodium hydroxide used in graphite purification. Protection against the risks associated with these chemicals should be part of any health programme.
Gypsum (Hydrated Calcium Sulphate)
Occurrence and uses. Though it occurs throughout the world, gypsum is rarely found pure. Gypsum deposits may contain quartz, pyrites, carbonates and clayey and bituminous materials. It occurs in nature in five varieties: gypsum rock, gypsite (an impure, earthy form), alabaster (a massive, fine-grained translucent variety), satin spar (a fibrous silky form) and selenite (transparent crystals).
Gypsum rock may be crushed and ground for use in the dihydrate form, calcined at 190 to 200 ºC (thus removing part of the water of crystallization) to produce calcium sulphate hemihydrate or plaster of Paris, or completely dehydrated by calcining at over 600 ºC to produce anhydrous or dead-burned gypsum.
Ground dihydrate gypsum is used in the manufacture of Portland cement and artificial marble products; as a soil conditioner in agriculture; as a white pigment, filler or glaze in paints, enamels, pharmaceuticals, paper and so on; and as a filtration agent.
Health hazards. Workers employed in the processing of gypsum rock may be exposed to high atmospheric concentrations of gypsum dust, furnace gases and smoke. In gypsum calcining, workers are exposed to high environmental temperatures, and there is also the hazard of burns. Crushing, grinding, conveying and packaging equipment presents a danger of machinery accidents. The pneumoconiosis observed in gypsum miners has been attributed to silica contamination.
Dust formation in gypsum processing should be controlled by mechanization of dusty operations (crushing, loading, conveying and so on), addition of up to 2% by volume of water to gypsum prior to crushing, use of pneumatic conveyors with covers and dust traps, enclosure of dust sources and provision of exhaust systems for kiln openings and for conveyor transfer points. In the workshops containing the calcining kilns, it is advisable to face the walls and floors with smooth materials to facilitate cleaning. Hot piping, kiln walls and drier enclosures should be lagged to reduce the danger of burns and to limit heat radiation to the work environment.
Occurrence and uses. Limestone is a sedimentary rock composed mainly of calcium carbonate in the form of mineral calcite. Limestones may be classified either according to the impurities they contain (dolomitic limestone, which contains substantial amounts of magnesium carbonate; argillaceous limestone, with a high clay content; siliceous limestone, which contains sand or quartz; and so on) or according to the formation in which they occur (e.g., marble, which is a crystalline limestone). Limestone deposits are widely distributed throughout the earth’s crust and are extracted by quarrying.
Since early times, limestone has been used as a building stone. It is also crushed for use as a flux in smelting, in refining, and for the manufacture of lime. Limestone is used as hardcore and ballast in road and railway construction, and it is mixed with clay for the manufacture of cement.
Health hazards. During extraction, the appropriate quarrying safety measures should be taken, and machinery-guarding principles should be observed on crushers. The main health hazard in limestone quarries is the possible presence, in the airborne limestone dust, of free silica, which normally accounts for 1 to 10% of limestone rock. In studies of limestone quarry and processing workers, x-ray examinations revealed pulmonary changes, and clinical examination showed pharyngitis, bronchitis and emphysema. Workers dressing stone for construction work should observe the safety measures appropriate to the stone industry.
Marble (Calcium Carbonate)
Occurrence and uses. Marble is geologically defined as a metamorphosed (re-crystallized) limestone composed primarily of crystalline grains of calcite, dolomite, or both, having a visible crystalline texture. Long usage of the term marble by the quarry and finishing industry has led to the development of the term commercial marble, which includes all crystalline rock capable of taking a polish and composed primarily of one or more of the following minerals: calcite, dolomite or serpentine.
Marble has been utilized throughout historic time as an important construction material because of its strength, durability, ease of workability, architectural adaptability and aesthetic satisfaction. The marble industry comprises two major branches—dimension marble and crushed and broken marble. The term dimension marble is applied to deposits of marble quarried for the purpose of obtaining blocks or slabs that meet specifications as to size and shape. The uses of dimension marble include building stone, monumental stone, ashlar, veneer panelling, wainscotting, tiling, statuary and so on. Crushed and broken marble ranges in size from large boulders to finely ground products, and products include aggregates, ballast, roofing granules, terrazzo chips, extenders, pigments, agricultural lime and so on.
Health hazards. Occupational diseases specifically connected with the mining, quarrying and processing of marble itself have not been described. In underground mining there may be exposure to toxic gases produced by blasting and some types of motor-driven equipment; adequate ventilation and respiratory protection are necessary. In abrasive blasting there will be exposure to silica if sand is used, but silicon carbide or aluminium oxide are equally effective, carry no silicosis risk, and should be substituted. The large quantities of dust generated in processing marble should be subject to dust control, either by the use of moist methods or by exhaust ventilation.
Occurrence and uses. Mica (from the Latin micare, to gleam or sparkle) is a mineral silicate which occurs as a primary constituent of igneous rocks, particularly granites. It is also a common component of such silicate materials as kaolin, which are produced by the weathering of these rocks. In the rock bodies, particularly in the pegmatite veins, mica occurs as lenticular masses of cleavable sheets (known as books) of up to 1 m in diameter, or as particles. There are many varieties, of which the most useful are muscovite (common, clear or white mica), phlogopite (amber mica), vermiculite, lepidolite and sericite. Muscovite is generally found in siliceous rocks; there are substantial deposits in India, South Africa and the United States. Sericite is the small plate variety of muscovite. It results from the weathering of schists and gneisses. Phlogopite, which occurs in calcareous rocks, is concentrated in Madagascar. Vermiculite has the outstanding characteristic of expanding considerably when quickly heated to around 300 ºC. There are large deposits in the United States. The main value of lepidolite lies in its high content of lithium and rubidium.
Mica is still used for slow-combustion stoves, lanterns or peep-holes of furnaces. The supreme quality of mica is that it is dielectric, which makes it a top-priority material in aircraft construction. Mica powder is used in the manufacture of electric cables, pneumatic tyres, welding electrodes, bituminized cardboard, paints and plastics, dry lubricants, dielectric dressings and flameproof insulators. It is often compacted with alkyd resins. Vermiculite is widely used as an insulating material in the building industry. Lepidolite is used in the glass and ceramic industries.
Health hazards. When working with mica, the generation of static electricity is possible. Straightforward engineering techniques can harmlessly discharge it. Mica miners are exposed to the inhalation of a wide variety of dusts, including quartz, feldspar and silicates. Chronic inhalation may cause silicosis. Exposure of workers to mica powder may cause irritation of the respiratory tract, and, after several years, nodular fibrotic pneumoconiosis can occur. It was long considered to be a form of silicosis, but it is now believed not to be, because pure mica dust contains no free silica. The radiological appearance is often close to that of asbestosis. Experimentally, mica has proved to possess a low cytotoxicity on macrophages and to induce only a poor fibrogenic response limited to the formation of thick reticulin fibres.
Chronic inhalation of vermiculite, which often contains asbestos, may cause asbestosis, lung cancer and mesothelioma. Ingestion of vermiculite is also suspected in stomach and intestinal cancer.
Occurrence and uses. Pumice is a porous rock, grey or white, fragile and of low specific gravity, coming from recent volcanic magma; it is composed of quartz and silicates (mainly feldspar). It is found either pure or mixed with various substances, chief among them obsidian, which differs by its shiny black colour and its specific gravity, which is four times greater. It occurs principally in Ethiopia, Germany, Hungary, Italy (Sicily, Lipari), Madagascar, Spain and the United States. Some varieties, such as Lipari pumice, have a high content of total silica (71.2 to 73.7%) and a fair amount of free silica (1.2 to 5%).
In commerce and for practical uses, a distinction is made between pumice in blocks and in powder. When it is in block form the designation differs according to the size of block, colour, porosity and so on. The powder form is classified by numbers according to grain size. Industrial processing comprises a number of operations: sorting to separate the obsidian, crushing and pulverizing in machines with stone or metal grinding wheels, drying in open kilns, sifting and screening using hand-operated flat and open sieves and reciprocating or rotating screens, the waste matter generally being recovered.
Pumice is used as an abrasive (block or powder), as a lightweight building material, and in the manufacture of stoneware, explosives and so on.
Health hazards. The most dangerous operations involving exposure to pumice are kiln drying and sifting, because of the large amount of dust produced. Apart from the characteristic signs of silicosis observed in the lungs and sclerosis of the hilar lymphatic glands, the study of some fatal cases has revealed damage to various sections of the pulmonary arterial tree. Clinical examination has revealed respiratory disorders (emphysema and sometimes pleural damage), cardiovascular disorders (cor pulmonale) and renal disorders (albuminuria, haematuria, cylindruria), as well as signs of adrenal deficiency. Radiological evidence of aortitis is more common and serious than in the case of silicosis. A typical radiological appearance of lungs in liparitosis is the presence of linear thickening due to lamellar atelactasis.
Occurrence and uses. Sandstone is a siliciclastic sedimentary rock consisting primarily of sand, usually sand that is predominantly quartz. Sandstones often are poorly cemented and can be easily crumbled into sand. Yet, strong, durable sandstone, with tan and grey colours, is used as dimension sandstone for exterior facing and trim for buildings, in houses, as curbstones, in bridge abutments and in various retaining walls. Firm sandstones are crushed for use as concrete aggregate, railroad ballast and riprap. However, many commercial sandstones are weakly cemented and therefore are crumbled and used for moulding sand and glass sand. Glass sand is the main ingredient in glass. In the metalworking industry, sand with good cohesiveness and refractoriness is used for making special shaped moulds into which molten metal is poured.
Sandstone is found throughout the United States, in Illinois, Iowa, Minnesota, Missouri, New York, Ohio, Virginia and Wisconsin.
Health hazards. The primary risks are from the silica exposure, which is discussed in the chapter Respiratory system.
Occurrence and uses. Silica occurs naturally in crystalline (quartz, cristobalite and tridymite), cryptocrystalline (e.g., chalcedony) and amorphous (e.g., opal) forms, and the specific gravity and melting point depend on the crystalline form.
Crystalline silica is the most widely occurring of all minerals, and it is found in most rocks. The most commonly occurring form of silica is the sand found on beaches throughout the world. The sedimentary rock sandstone consists of grains of quartz cemented together with clays.
Silica is a constituent of common glass and most refractory bricks. It is also used extensively in the ceramic industry. Rocks containing silica are used as common building materials.
Free and combined silica. Free silica is silica which is not combined with any other element or compound. The term free is used to distinguish it from combined silica. Quartz is an example of free silica. The term combined silica originates from the chemical analysis of naturally occurring rocks, clays and soils. The inorganic constituents are found to consist almost always of oxides bound chemically, commonly including silicon dioxide. Silica so combined with one or more other oxides is known as combined silica. The silica in mica, for example, is present in the combined state.
In crystalline silica, the silicon and oxygen atoms are arranged in a definite, regular pattern throughout the crystal. The characteristic crystal faces of a crystalline form of silica are the outward expression of this regular arrangement of atoms. The crystalline forms of free silica are quartz, cristobalite and tridymite. Quartz is crystallized in the hexagonal system, cristobalite in the cubic or tetragonal system and tridymite in the ortho-rhombic system. Quartz is colourless and transparent in the pure form. The colours in naturally occurring quartz are due to contamination.
In amorphous silica the different molecules are in a dissimilar spatial relationship one to another, with the result that there is no definite regular pattern between molecules some distance apart. This absence of long-range order is characteristic of amorphous materials. Cryptocrystalline silica is intermediate between crystalline and amorphous silica in that it consists of minute crystals or crystallites of silica which are themselves arranged in no regular orientation one to another.
Opal is an amorphous variety of silica with a varying amount of combined water. A commercially important form of amorphous silica is diatomaceous earth, and calcinated diatomaceous earth (kieselguhr). Chalcedony is a cryptocrystalline form of silica which occurs filling cavities in lavas or associated with flint. It is also found in the annealing of ceramics when, under certain temperature conditions, the quartz in silicates may crystallize out in minute crystals in the body of the ware.
Health hazards. The inhalation of airborne dust of silica gives rise to silicosis, a serious and potentially fatal fibrotic disease of the lungs. The chronic, accelerated, and acute forms of silicosis reflect differing exposure intensities, latency periods and natural histories. Chronic silicosis may progress to progressive massive fibrosis, even after exposure to silica-containing dust has ceased. Hazards of silica are discussed in more detail in the chapter Respiratory system.
Occurrence and uses. Slate is very fine-grained, sedimentary argillaceous or schisto-argillaceous rock, easily split, of a leaden-grey, reddish or greenish colour. The principal deposits are in France (Ardennes), Belgium, the United Kingdom (Wales, Cornwall), the United States (Pennsylvania, Maryland) and Italy (Liguria). With a high calcium carbonate content, they contain silicates (mica, chlorite, hydrosilicates), iron oxides and free silica, amorphous or crystalline (quartz). The quartz content of hard slates is in the region of 15%, and that of soft slates, less than 10%. In North Wales quarries, respirable slate dust contains between 13 and 32% of respirable quartz.
Slate slabs are used for roofing; stair treads; door, window and porch casements; flooring; fireplaces; billiard tables; electricity switch panels; and school blackboards. Powdered slate has been used as a filler or pigment in rustproofing or insulating paints, in mastics, and in paints and bituminous products for road surfacing.
Health hazards. Disease in slate workers has attracted attention since the early nineteenth century, and cases of “miner’s phthisis” uncomplicated by tubercle bacilli were described at an early date. Pneumoconiosis has been found in a third of workers studied in the slate industry in North Wales, and in 54% of slate pencil makers in India. Slateworkers’ pneumoconiosis may have features of silicosis due to the high quartz content of some slates. Chronic bronchitis and emphysema are frequently observed, especially in extraction workers.
The replacement of the hand pick by low-velocity mechanical equipment considerably reduces dust generation in slate quarries, and the use of local exhaust ventilation systems makes it possible to maintain airborne dust concentrations within acceptable limits for 8-hour exposure. Ventilation of underground workings, drainage of groundwater into pits, lighting and work organization are improving the general hygiene of working conditions.
Circular sawing should be carried out under water jets, but planing does not usually give rise to dust provided the slivers of slate are not allowed to fall to the ground. Larger sheets are usually wet-polished; however, where dry-polishing is carried out, well-designed exhaust ventilation should be employed since slate dust is not easily collected even when using scrubbers. The dust readily clogs bag filters.
Workshops should be cleaned daily to prevent accumulation of dust deposits; in certain cases, it may be preferable to prevent deposited dust in gangways from becoming airborne again by covering dust with sawdust rather than by wetting it.
Occurrence and uses. Talc is a hydrous magnesium silicate whose basic formula is (Mg Fe+2)3Si4O10 (OH2), with theoretical weight percentages as follows: 63% SiO2, 32% MgO and 5% H2O. Talc is found in a variety of forms and is frequently contaminated with other minerals, including silica and asbestos. Talc production occurs in Australia, Austria, China, France and the United States.
The texture, stability and fibrous or flaky properties of the various talcs have made them useful for many purposes. The purest grades (i.e., those which most nearly approximate the theoretical composition) are fine in texture and colour, and are therefore widely used in cosmetics and toilet preparations. Other varieties, containing admixtures of different silicates, carbonates and oxides, and perhaps free silica, are relatively coarse in texture and are used in the manufacture of paint, ceramics, automobile tyres and paper.
Health hazards. Chronic inhalation may cause silicosis if silica is present, or asbestosis, lung cancer, and mesothelioma if asbestos or asbestos-like minerals are present. Investigations of workers exposed to talc without associated asbestos fibres revealed trends for higher mortality from silicosis, silicotuberculosis, emphysema and pneumonia. The major clinical symptoms and signs of talc pneumoconiosis include chronic productive cough, progressive shortness of breath, diminished breath sounds, limited chest expansion, diffuse rales and clubbing of the finger tips. Lung pathology has revealed various forms of pulmonary fibrosis.
Wollastonite (Calcium Silicate)
Occurrence and uses. Wollastonite (CaSiO3) is a natural calcium silicate found in metamorphic rock. It occurs in many different forms in New York and California in the United States, in Canada, Germany, Romania, Ireland, Italy, Japan, Madagascar, Mexico, Norway and Sweden.
Wollastonite is used in ceramics, welding-rod coatings, silica gels, mineral wool and paper coating. It is also used as a paint extender, a soil conditioner, and as a filler in plastics, rubber, cements and wallboard.
Health hazards. Wollastonite dust may cause skin, eye and respiratory irritation.