What are minerals?
Minerals are naturally occurring, inorganic solids with a definite chemical composition and a regular atomic structure. From this basis come all their physical properties.
Some of the most obvious physical properties of a mineral are hardness (resistance to scratching), tenacity (resistance to impact), specific gravity (relative density), cleavage (the tendency of some crystals to split along planes of weakest atomic bonding), magnetism, conductivity of electricity, reactibility to acids etc. These measurable physical properties, along with a number of optical ones such as lustre, refractive index and selective absorption of light rays, enable minerals to be positively identified by scientific instruments.
The table below is a universally accepted standard of the comparative hardness of minerals. It is called Mohs' scale, after Frederich Mohs, the mineralogist who devised it. The range is from the softest mineral (talc) to the hardest (diamond).
- Orthoclase feldspar
Minerals form crystals in one of seven crystal systems. Crystals can be very large, like some quartz crystals from Brazilian pegmatites. Others are too small to see without powerful magnification. Chalcedony, with its structure of interlocking fibrous quartz crystals in hydrous silica is termed microcrystalline.
A mineral can occur in different colours. This may be caused by the addition of a particular trace element or by structural irregularities that cause it to absorb and transmit particular wavelengths of light. Such slightly different versions of a mineral or gemstone are called varieties. For instance, the mineral beryl comes in a number of colours - colourless, pink, yellow, orange, red, blue and intense green, each with a different variety name. The intense green variety is called 'emerald', the blue, 'aquamarine' and the pink 'morganite'.
Some mineral crystals reveal variations at different stages of their growth by colour banding or zoning. Tourmalines and sapphires show this effect strongly, often displaying two or more colours in a cut stone.
What is the difference between minerals and rocks?
A mineral is a naturally-occurring inorganic (there are some exceptions to this) crystalline solid (though mercury is regarded as a mineral) with a specific chemical composition and a characteristic internal regular geometric arrangement of atoms, sometimes expressed as natural crystal faces.
A rock is an aggregate of one (such as quartzite) or more (such as granite) mineral particles formed through either crystallisation of molten magma (igneous rocks), settling of particles (sedimentary rocks), or reheating and pressure applied to pre-existing rocks (metamorphic rocks), with no set chemical composition or atomic structure.
How are minerals made?
Minerals grow in a wide variety of geological environments - salt lakes, deep oceans, volcanoes, and cooling deep masses of molten rock. They can also grow under the influence of heat and/or pressure, and from solutions and gases carrying concentrations of certain elements. Usually, minerals grow when solutions or gases rich in certain elements cool or evaporate, molten rock cools, or heat and/or pressure change (re-organise) pre-existing chemicals. The chemicals are supplied (via solutions and gases) to a growth centre, which begins as a more favourable site or focus for crystallisation, and a 'seed' crystal develops. The 'seed' becomes larger as more chemicals are supplied and growth continues. A rock is an aggregate of mineral grains produced by settling in air or water (sedimentary rocks, e.g. sandstone), by cooling of lava or deep molten masses (igneous rocks, e.g. basalt and granite) or by alteration of sedimentary and igneous rocks by heat and/or pressure (metamorphic rocks, e.g. marble and gneiss).
How do we identify minerals?
Some minerals are instantly recognisable because of their unique colour or shape, but usually some simple non-destructive tests can be done to prove the identity of the most common minerals present. These tests include those for the crystal system (based on symmetry), colour, lustre, transparency, cleavage, fracture, streak colour, refractive index, ultra-violet fluorescence, magnetism, radioactivity, specific gravity, melting (fusion) and simple chemical tests. Sometimes more complex tests are necessary, involving complex and expensive analytical instruments which use X-rays, such as the X-ray Diffractometer (analyses crystal structure) and Electron Microprobe or Energy Dispersive Spectrometer (analyses chemical contents).
What are minerals used for?
Minerals are used for many purposes, including:
- Agriculture - fertilisers (superphosphate, limestone, dolomite)
- Automotive industries - steel, high temperature ceramics (spark plugs, cylinder heads), sulphur (used in vulcanising of rubber), lead and antimony (batteries) etc.
- Building materials - cement (from calcite), bricks (from clay and shale), glass (from quartz sand), plaster (from gypsum)
- Chemical industries - manufacture of paints, paper, ceramics, glass
- Cooking (salt, baking powder)
- Currency - coinage (copper, bronze, nickel, silver, gold)
- Electronics, copper electrical wiring, computers, space industry - gold, silver, copper, silicon, uncommon metals (gallium, tantalum, tungsten), special glasses and ceramics
- Jewellery and decorative items (gemstones, gold, silver, platinum)
- Metallurgy and metal smelting - iron, steel, copper, bronze, brass, lead, zinc, gold, silver, aluminium, special metal alloys etc.
- Plumbing - copper, lead (in the past)
- Surgery, medicines, pharmaceuticals, cosmetics (titanium, tantalum and stainless steel implants), dietary supplements (zinc and iron tablets), talcum powder (from talc)