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Photo: Erik Jonsson, SGU

Minerals

Minerals are the building blocks of Earth and other terrestrial planets. Rocks are composed of minerals and rocks build up the crust of terrestrial planets. But what characterizes a mineral? How are minerals formed? How many minerals are there? Below you will find the answers to these questions.

Minerals are often chemical compounds, which means they are composed of several element joined together by chemical bonds. Common examples of this are salt(sodium chloride, NaCl), ordinary salt, and quartz (silica, SiO2). They are both minerals that occur naturally in rocks.

A mineral can also be composed of only one element. Examples of this are native gold (Au) and diamond (C). Gold is a noble metal, which means that it is highly resistive to reactions with other elements and therefore seldom build chemical compounds. Because of this it is resistant to oxidation and corrosion. In nature, gold often occurs in its pure, metallic form, sometimes together with silver or copper.

Crystal structures

Minerals show a well-arranged, geometrical inner atomic structure, called the crystal structure. This means that the elements are symmetrically arranged in 3 dimensions. Strong magnification microscopy or analytical techniques such as X-ray measurements are generally needed to observe the crystal structure. However, minerals that have grown freely, without other crystals hindering the development of their shape, often acquire an outside form that resembles its inner crystal structure. One example of this is quartz, which under the right conditions may grow to become large, elongate hexagonal crystals with plane crystal faces. The number of crystal faces, their form, and the angles separating them are all factors controlled by the inner crystal structure.

Some mineral-like compounds are sometimes classified as minerals although they lack a crystal structure, an example of which is the gemstone opal. Opal consists of mostly amorphous, but partially ordered, hydrated silica. It occurs in a range of colours including grey, brown, white, and others. However, the most precious opals exhibit a rainbow-like variety of colours.

Minerals are formed through geological processes

All minerals are formed through geological processes, examples of such are:

  • when new rocks crystallize from slow cooling of magma deep in the Earth’s crust, or magma reaches the surface to form volcanic rocks,
  • when sediments are compacted and lithified to sedimentary rocks under the burden of overlying sediments on the sea floor or in inland lakes,
  • when existing rocks are metamorphosed through increased pressure and/or temperature, but also
  • when hot water circulates in the Earth’s crust and precipitates minerals by hydrothermal processes.

By geological processes we mean processes that take place on Earth. These processes may, however, to some extent also be active on other planets.

Can minerals form from plants or animals, or even human activity?

Materials produced by plants or animals are often called biogenic materials, but since these chemical compounds are not crystalline, nor show order crystal structures, they cannot be classified as minerals. However, these materials may be subject to geological processes, which can transform them into minerals. An example is when dead plants or animals settle on the sea floor, gradually buried under overlying sediments and eventually lithify to sedimentary rocks. Another example is when remnants of animals and plants rich in carbon are exposed to different geological processes to form graphite and diamond, both of which are minerals composed of pure crystalline carbon.

Non-crystalline geological materials rich in carbon , such as peat, oil, pitch, lignite, coal, and others, are however not minerals. The popular jewellery material amber is a fossilized resin, and thus neither a mineral.

Some substances and compounds that result from human activity, for example slag from melting processes, were previously called minerals. But because these compounds are not naturally formed they are currently not classified as minerals. Thus, no anthropogenic material is classified as a mineral.

Last reviewed 2021-02-01