What are Minerals? (Geology)
How Minerals are Defined in Geology
A mineral is a naturally occurring, inorganic, crystalline solid.
The term 'naturally occurring' is important because ice produced in a freezer and lab-grown crystals, such as diamonds, are crystalline solids but are not natural. Sugar is also crystalline, but it's organic rather than inorganic. Therefore, these substances are not minerals.
Minerals must have a specific chemical composition and an internal crystalline structure. A specific chemical composition means it's composed of elements arranged in a defined ratio.
A crystal structure is an arrangement of atoms that comes together in a highly ordered, repeating, symmetrical pattern. This arrangement is known as a crystal lattice. If the atoms do not form in this way and are jumbled up, the substance will be amorphous.
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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 a volcanic glass.
Amorphous describes a non-crystalline solid that could be natural or man-made. An example of a man-made non-crystalline solid is glass.
How Minerals are Identified
Identifying some minerals can be straightforward, but others can be more difficult. Charoite can often be identified on sight alone, whereas 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, but 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 and doesn't change, but it can differ from the mineral's external colour.
Lustre, which describes the way light interacts with the surface of a stone, can also be useful.
The 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 out of 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 is 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.
There are more than 6000 known minerals on Earth. About 200 are common, and about 20 are very common. Many new minerals are discovered each year.
Most rocks are formed from a small number of the most common minerals. Examples include quartz, olivine, calcite, and pyroxenes. Peridot is the gemstone variety of olivine. Jadeite is a pyroxene.
Quartz, feldspar, and mica are three of Earth's most common minerals. 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. The amount each person needs differs 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 second image shows atoms arranged in a crystalline and non-crystalline solid.
Pop-up photos
Diamond: Courtesy of Géry Parent. Corundum: Courtesy of James St.John. Fluorite, charoite: Courtesy of Stan Celestian.
The mica and apatite are from our collection.
