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HISTORY OF SCIENCE AND TECHNOLOGY
Rocks, Minerals and Ores
What is a rock?
Rocks are a mixture of minerals. There are three main types of rock.
Image adapted from Understanding Science No. 88 Second Edition
Igneous (from the Latin ignis, meaning fire) : These rocks were formed from a molten state and originated from magma, the semi-fluid material that exists just under the Earth's crust. Igneous rocks can be formed at the surface, for example lava from an erupting volcano, or in chambers under the surface, forming rocks such as granite.
Sedimentary: Rivers, lakes, oceans and glaciers all transport and deposit small pieces of weathered rock. Wind can also transport small pieces of weathered rock. Deposits of this kind, sand and mud for example, compressed over long periods of time, form sedimentary rocks of which shale is an example. Some sediments have organic origins. The inorganic shells and outer coverings of small invertebrates, such as snails and crustaceans, survived the death of the organism and collected on the sea bed. When these became compressed they formed limestone and chalk.
Metamorphic: Both igneous and sedimentary rocks can be metamorphosed (changed) if they are exposed to high temperatures and pressures. This can happen due to movements of the Earth's crust or pressure from magma pushing up from below. Such increases in pressure and temperature, over long periods of time, can change limestone into marble and shale into slate.
What is a mineral?
A mineral makes up part of a rock. It usually occurs as a solid in the form of crystals and had a definite chemical formula. Minerals are inorganic in origin. Some minerals are found in rocks as the native element, for example gold and silver, but most are found as compounds.
What is an ore?
An ore is any rock that contains enough of a given mineral to be commercially viable to extract.
Using cobalt ores as examples:
There are more than 100 minerals that contain cobalt compounds but only around 30 of these contain enough cobalt to classed as ores. The 4 main cobalt ores are
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