The forms in which metals are found in the crust of the earth and as seabed deposits depend on their reactivity with their environment, particularly with oxygen, sulphur and carbon dioxide. Gold and the platinum are found principally in the native and metallic form. Silver, copper and mercury are found native, as well as in the form of sulphides, carbonates, and chlorides. The more reactive metals are al ways in compound form, such as the oxides and sulphides of iron and the oxides and silicates of aluminium and beryllium. The naturally occurring compounds are known as minerals, most of which have been given names according to their composition ( e. g. galena-lead sulphide, PbS; sphalerite-zinc sulphide, ZnS; cassiterite-tin oxide, Sn02).
Minerals by definition are natural inorganic substances possessing definite chemical compositions and atomic structures. Some flexibility, however, is allowed in this definition. Many minerals exhibit isomor’phism, where substitution of atoms within the crystal structure by similar atoms takes place without affecting the atomic structure. The mineral olivine, for example, has the chemical composition (Mg, Fe)iSi04, but the ratio of Mg atoms to Fe atoms varies in different olivines. The total number of Mg and ,Fe atoms in all olivines, however, has the same ratio to that of the Si and O atoms. Minerals can also exhibit polymorphism, different minerals having the same chemical composition, but markedly different physical properties due to a difference in atomic structure. Thus the two minerals graphite and diamond have exactly the same composition, being composed entirely of carbon atoms, but have widely different properties due to the arrangement of the carbon atoms within the crystal lattice. The term “mineral” is often used in a much more extended sense to include anything of economic value which is extracted from the earth. Thus coal, chalk, clay and granite do not come within the definition of a mineral, although details of their production are usually included in national figures for mineral production. Such minerals are, in fact, rocks, which are not homogeneous in chemical and physical composition, as are minerals, but generally consist of a variety of minerals and form large parts of the earth’s crust. Granite, for instance, which is the most abundant igneous rock, i.e. a rock formed by cooling of molten material, or magma within the earth’s crust, is composed of three main mineral constituents, feldspar, quartz and mica. These three homogeneous mineral components occur in varying proportions in different granites, and even in different parts of the same granite mass.
An ore can be described briefly as an accumulation of mineral in sufficient quantity as to be capable of economic ext.raction. This establishes the market price of the metal as a critical criterion in the definition, and this will vary according to the commercial demands. With the passage of time and depletion of richer or more readily accessible material, a mineral deposit may be updated to an ore. Improvement in metallurgical extraction and the introduction of new methods also become factors in making available deposits hitherto regarded as uneconomic. Thus the introduction of the flotation process in mineral .processing permitted the extraction of copper from material containing less than 0. 5 % of the metal and formerly regarded as waste, Similarly the introduction of solvent extraction has enabled Nchanga Consolidated Copper Mines in Zambia to treat 9 million tonnes per year of flotation tailings, produ.cing 80,000 tqnnes of finished copper from what was previously regarded as waste material.
Ores are frequently classed according to the nature of the valuable mineral. Thus in native ores the metal is presented in the elementary form; sulphide ores contain the metal in the form of a sulphide, and in oxidised ores the valuable mineral may be present as oxide, sulphate, silicate, carbonate, or some hydrated from of these. Complex ores are those containing profitable amounts of more than one valuable mineral. Metallic minerals are often found in certain associations, within which they may occur as mixtures of a wide range of particle sizes or as single-phase solid solutions or compounds. Galena and sphalerite, for example, commonly associate, as do copper sulphide minerals .and sphalerite to a lesser extent. Pyrite (FeSz) is very often associated with these minerals.
Ores are also classified by the nature of their gangues, such as calcareous or basic ( lime rich), or siliceous, or acidic (silica rich) .
T·he minimum metal content required for a deposit to qualify as an ore varies from metal to metal. Many nonferrous ores contain, as mined, as little as 1 % metal, and often much less. Gold may be recovered profitahly in ores containing only 5 parts per million (ppm) of the metal, whereas iron ores containing less than about 20 % metal are regarded as low grade.
There are many plants_ where minerals are recovered in secondary circuits, treating tailings, where the feed grades are much low·er than would be economic on a mined ore. Typical ore grades for tungstel’) ores are in the range of 0. 5 ½ ~ 1. 5 ½ WQ3 , but the Climax Molybdenum plant in the United States treats 45, ·ooo tonnes per day of tailings, containing less than 0.· 1 % W03 , and is one of the two major producers of tungsten concentrate in the United States.
Ores of.economic value can be classed as metallic o.r non-metallic, according to the use of the mineral. Certain minerals may be mined and processed for more than one purpose. In one category the mineral may be a metal ore, i.e. when it is used to prepare the metal. as when bauxite (hydrated aluminium oxide). is used to make aluminium. The alternative is for the compoup.d to be classified as a non-metallic ore, i. e. when bauxite or natural aluminium oxi’de is used to make material for refractory bricks or abrasives.
Many non-metallic ore minerals associate with metallic ore minerals and are mined and processed together, e. g. galena, the main source of lead often associates with fluorite (CaF2 ) and barytes (BaS04 ), both important non-metallic minerals.