Minerals, Rocks and Ores – Their mining and environmental issues
Definition of mineral, rock and ore –
A mineral is a naturally occurring, inorganic solid with a definite chemical composition and a specific crystalline structure; where as a rock is an aggregate of one or more minerals. As rock consists of number of minerals, one or few minerals may be major constituents in a particular rock. When a particular rock formation, as per major minerals present, has economic value for extraction of a certain metal or group of metals, that rock is called ‘ore’ of that metal. These ores can be later undergoing metallurgical processes for extraction of metal. For example, Iron ore - a rock formation that has minerals compound of iron element, and iron (Fe) content is such that can be economically extracted in a steel plant.
Types of rock –
Rocks have been classified in three groups; these are (a) igneous, (b) sedimentary and (c) metamorphose.
Again, igneous rock is separated into two groups- extrusive and intrusive. Extrusive rocks are formed when magma flows onto the surface of the earth as lava or on to the ocean floor and cools, harden and crystalised. Lava reaches the earth's surface along fissures (deep cracks) and at volcanic vents. Extrusive rock formation varies from broad flat sheet to as massive shape. Intrusive rocks are formed when magma solidifies beneath the earth's surface. These are mostly found while mining or tunneling or at a surface where they are exposed by erosion or geological uplifting. The formation of intrusive rocks varies from thin sheet shape / vein to uneven and huge masses. Examples of igneous rocks are granite, basalt, feldspar, quartz etc.
Industrial minerals and ores –
Commercially valuable minerals and rocks are referred to as industrial minerals. Therefore, ores (i.e., commercially valuable minerals and rocks are referred to as industrial minerals or ore) must have economic value for extraction of a metal or group of metals or for a certain product. As these ores / rocks are obtained naturally on the earth crust, for ultimate use these are mined, i.e., taken out from earth – by opencast or underground process, as per the mode of availability.
Mineralogy –Mineralogy is a part of geology. It is the study of various properties, origin, classification, composition of minerals and their usages. Mineralogist identifies the minerals as well. Identification of minerals is made according to their chemical, physical and crystallographic characteristics. Minerals are the source of valuable metals, most frequently mined in the form of ore.
Some of the minerals are very colorful. In ancient time colors are made by grinding colorful minerals. Many colorful minerals are used as gems stones. Gemstones are treasured for their beauty and durability. Their value depends on beauty, hardness, its rarity and types of cutting & polishing made on the stone. Diamond, rubies, emerald, sapphire etc., attract greatest value.
Identification of minerals –
(a) A mineral can be identified by several physical properties. Most common physical properties or structure used for identification purpose is:
(i) Crystal structure - When minerals form in environments, they commonly develop into regular geometric shapes, called crystals, bounded by smooth crystal faces. The crystal form for a given mineral is governed by the mineral's internal structure, and may be distinctive enough to help identify the mineral. Therefore, it is nothing but an orderly geometric spatial arrangement of atoms in the structure of a mineral.
(ii) Transparency - Transparency refers to the way light passes through a mineral sample. It varies, depending on the way the mineral atoms are bonded together.
(iii) Physical hardness - Hardness is the measurement of how resistant the mineral is to being scratched. On the Mohs' scale of hardness, the softest mineral is talc to the hardest mineral, diamond.
(iv) Luster - Luster defines the way light is reflected off a mineral's exterior. The kind and intensity of luster vary according to the nature of the mineral surface and the amount of light absorbed. Generally, luster of a mineral is described as dull, metallic, pearly, glassy, greasy, and silky.
(v) Color - The color of a mineral is a useful identification feature. Although it helps to identify minerals, color identification can trick you. Many minerals, like quartz, occur in a lot of different colors, and many minerals are boring white or even colorless.
(vi) Streak - The color of a mineral's powder is called streak. The streak of a mineral can be seen when it is rubbed across the surface of an unglazed porcelain tile. Streak is quite reliable in identification as it is more consistent than color.
(vii) Cleavage - Cleavage is the way that a mineral breaks along well defined planes of weakness. The planes are between layers of atoms or other places where the atomic bonding is the weakest. Most cleavage surfaces are not always perfectly smooth like crystal faces, although they are very consistent and reflect light evenly.
(viii) Specific gravity - Comparing the weight of a mineral with the weight of an equal volume of water gives a mineral's specific gravity. This is shown in numbers.
More complex way and accurate way to determination of minerals is X-ray diffraction.
(b) Chemical composition of minerals is the most important property for identifying and distinguishing them. They generally form
(i) as elements, such as for gold, diamond, graphite, sulfur;
(ii) sulphides, such as galena and sphalerite ore of lead, copper, or silver;
(iii) oxides, such as haematite (Fe2O3), bauxite (Al2O3· H2O);
(iv) halides, such as halite (NaCl) ;
(v) carbonates, calcite (CaCO3);
(vi) phosphate, such as apatite;
(vii) sulphate, such as barite (BaSO4);
(viii) silicate, such as quartz, feldspar, mica, silica (SiO4).
Major economic minerals for production of metals -
Earth crust consists of rocks, and all the rocks have minerals. An ore is a rock containing metalliferous minerals of economic value, are mined for profitable extraction of metals. Ore is formed by concentration of low-abundance elements. 99% of the Earth’s crust is made up of oxygen, silica, aluminum, iron, calcium, sodium, potassium, magnesium and titanium. These are called major elements.
Many other elements are useful in modern society. Concentration of these elements in average crust is very small (all together they’re only 1%). Geologic processes are must to concentrate these elements hundreds to thousands of times to make ore. Most ore minerals belong to these four non-silicate mineral groups - (i) Native Elements (none); (ii) Sulfides (S); (iii) Oxides (O2); (iv) Hydroxides (OH).
Examples are given below:
(i) Native Elements:
• Metals - metallic bonds, metallic luster
• Semimetals - have some, but not all, properties of metals: arsenic, antimony and bismuth
• Non-metals - covalent bonding, lack metallic properties
Principal Native Metals
• Gold (Au)
• Silver (Ag)
• Copper (Cu)
• Platinum (Pt, rarest and most valuable)
Native Non-metals
• Sulfur (S)
• Graphite (C) – Low pressure polymorph of Carbon
• Diamond (C) – High pressure polymorph of Carbon >30kbar, – Formed deep in the Earth’s mantle >90km, – Brought to the surface by violent, explosive igneous eruptions of magmas called kimberlites
(ii) Common Sulfide Minerals (reduced, formed in low oxygen environments)
• Pyrite (FeS2) Grows in cubes.
• Chalcopyrite (CuFeS2) is the most important ore mineral of Copper.
• Sphalerite (ZnS) is the most important ore mineral of zinc.
• Galena (PbS) is the most important ore mineral of Pb (lead).
(iii) Common Oxide Minerals (oxidized, formed in hi oxygen environments)
• Magnetite (Fe3O4)
• Hematite (Fe2O3)
• Zincite (ZnO)
• Franklinite (ZnFe2O4)
• Pyrolusite (MnO2)
• Chromite (FeCr2O4)
• Cassiterite (SnO2)
Hematite (Fe2O3) and Magnetite (Fe3O4) are Fe (iron) ores.
Hematite and Magnetite occur together with red chert in BIFs
(iv) Common Hydroxide Minerals
• Goethite FeO(OH)
• Al hydroxides - gibbsite Al(OH)3, boehmite AlO(OH), diaspore AlO(OH),
Together these make up bauxite (actually a rock name, multiple minerals), the most important Aluminum ore.
Mining of minerals and ore-bodies -
* Airborne surveys - Broad information about the surface and subsurface geology over large areas can be quickly and easily obtained from the air.
* Mapping - Mapping to define the geology of an area is important in identifying regions prospective for minerals.
* Geochemical surveys - Surface Sampling - Laboratory analysis of samples collected from the surface can give indications of buried or nearby ore-bodies.
* Auger rigs are required where the samples needed are deeper in the soil profile.
* Geophysical surveys - Ground-magnetic surveys are used to better define aeromagnetic features, and to locate possible subsurface magnetic ore-bodies.
* Gravity surveys provide information on the density of rocks below the surface. This is used with other information to identify possible ore-bodies at depth.
* Electrical methods measure the conductivity of subsurface rocks which can identify the presence of metallic (conductive) ore-bodies.
* Reconnaissance drilling – Drilling which does not involve track clearing or on-site excavations is often used to test targets for signs of mineralisation. This will determine whether more detailed exploration is required.
The major stages of mineral exploration are (a) Area selection, (b) Target definition, (c) Resource evaluation, (d) Reserve definition, (e) Extraction.
Minerals exploitation - The process or business of extracting ore or minerals from the ground is called mineral exploitation or mining. It is the selective recovery of minerals and materials from the crust of earth. The term mining industry commonly includes such functions as geological exploration, mineral exploitation by drilling blasting, ore dressing, mineral separation, electrolytic reduction, and smelting and refining.
Mining is broadly divided into two basic methods: Opencast and Underground. Opencast mining is done on the surface, which involves extraction from a series of successive parallel trenches. Underground mining involves extraction from beneath the surface - from depths as great as some time 10,000 ft (3 km).
As discussed above, the activities of the mining industry begin with geological exploration of economic minerals or ore bodies. Geological exploration is very much complicated, expensive, and highly technical task. After suitable deposits have been found and their worth proved, development, or preparation for mining, is done for exploitation of deposit. Opencast mining involves stripping off overburden, before mineral deposit is approached. Removal of overburden is mostly done by drilling, blasting, lifting the blasted material by excavators on to dumpers and transporting the dumper to overburden disposal site called overburden dump.
The following list summarises the major environmental issues for mining in general:
(a) Destruction of habitat and biodiversity at the mine site
(b) Ecosystem/habitat/biodiversity protection in adjacent land
(c) Landscape/visual impact/loss of land-use
(d) Site stabilisation and rehabilitation
(e) Mine waste/tailings disposal
(f) Sudden failure of tailings facilities
(g) Abandoned equipment, solid waste, sewage
(h) Air emissions
(i) Dust
(j) Climate change
(k) Energy consumption
(l) Siltation and changes in river regimes
(m) Effluent discharges and acid drainage
(n) Groundwater alteration or contamination
(o) Hazardous wastes and chemical residues
(p) Hazardous chemicals handling, safety, workplace exposure
(q) Noise
(r) Deforestation
(s) Radiation
(t) Workplace health and safety
(u) The impact of metals toxicity on the marketing of metals
(v) Cultural and archaeological values
(w) Public health and urban settlement issues around mines
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