What are Minerals? (geology)

Minerals are Solid Chemical Compounds

A mineral is an inorganic solid that has occurred naturally through a geological process. Inorganic means it does not contain any living matter. 

It must have a specific chemical composition and an internal crystalline structure.

Having a specific chemical composition means it's composed of specific chemical elements arranged in a defined ratio.  This is a fundamental characteristic of a mineral. It ensures the mineral's crystal structure remains stable and consistent.

Any variation would result in a different mineral with different properties.

In the vast majority of cases a crystalline solid will be classed as a mineral.  I say "the vast majority of cases" because as with anything, there are always exceptions.

A crystal structure is an arrangement of atoms that comes together in a highly ordered, repeating symmetrical pattern.  The arrangement forms a crystal lattice.  The lattice forms a crystal.

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So crystals are composed of an orderly repeating arrangement of atoms.  If the atoms do not form in this way, the substance will not be crystalline.  If it's not crystalline, it's unlikely to be a mineral.

Naturally occurring, non-crystalline solids are known as mineraloids.  Examples include amber, which is fossilised tree resin, shungite, opal, pearl, moldavite which is a tektite, and obsidian which is natural volcanic glass.

The word 'amorphous' is also sometimes used, but it describes non-crystalline solids that are natural or man-made.  

Rocks and minerals are not the same.  Minerals are made up of crystals, rocks are made up of minerals.

Although close to 4000 minerals have been identified on Earth, only about 100 are common.  More than one hundred new minerals are discovered each year.

Most rocks are formed from a small number of the most common 'rock-forming' minerals.  Examples include quartz, olivine, calcite and pyroxenes.  Peridot is the gemstone variety of olivine, jadeite is a pyroxene.

Identifying some minerals can be straightforward but others can be more difficult.  Charoite can often be easily identified on sight alone but apatite can be far more complex.

The name apatite comes from the Greek for 'deceive' or 'to be misled'.

When identifying a mineral, colour is usually a good place to start. It can also be the most unreliable characteristic because many minerals occur in a range of colours.

Streak and lustre can also help with identification.  Streak describes the colour of a mineral in powdered form.  It never changes and can be different to the visible colour of the mineral.

Lustre describes the way light interacts with the surface of a stone.

Mohs scale of mineral hardness was created in 1812 as a way of testing the scratch resistance of one mineral against another.

Diamond which grades 10/10 is the hardest mineral on the scale.  Talc at number 1 is the softest.  Corundum which grades 9 can scratch quartz which grades 7.  Apatite which grades 5 can scratch fluorite which is 4.

Harder minerals can scratch those which are softer and those of the same hardness can scratch each other.  Hardness is not the same as toughness.

Quartz, feldspar and mica are three of the most common minerals on Earth.  Painite, taaffeite, and grandidierite are three of the rarest.

From a nutritional perspective the word 'mineral' has a slightly different meaning.

Minerals are required in small amounts to enable our bodies to function normally.  Different amounts are needed by different people depending on age, sex and overall health.

Article Pictures

Our photograph at the top of the page is amethyst. Photograph courtesy of Stan Celestian. The image is clickable and redirects to the original photo.

The second image is an example of atoms arranged in a crystalline and non-crystalline solid.