What are Minerals? (Geology)

How Minerals are Defined in Geology

A mineral is a naturally occurring crystalline solid formed by geological processes. Minerals are almost always inorganic, although a small number of organic minerals are recognised. Two of the best known are whewellite and mellite.

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 crystalline, but it isn't formed by geological processes. Pearl is a biological composite of calcium carbonate, usually aragonite, together with organic material and water, so it isn't a single mineral. None of these are minerals.

What Makes a Mineral a Mineral?

Geologists look for five characteristics. Most minerals show all of them, but there are a few exceptions which are accepted.

It occurs naturally
Formed by geological processes
It's a solid under the natural conditions in which it occurs
It has a definite chemical composition
Its atoms are arranged in an ordered crystal structure.

Liquid water fails the third test, so it isn't a mineral, but natural ice passes all five and is. Coal fails the last two, so it's classed as a rock. Native mercury, which is liquid at ordinary temperatures, is an accepted exception.

Minerals normally have a specific chemical composition and an internal crystalline structure. Some, such as gold and diamond, consist of a single element. Where two or more elements are present, they're combined in a defined ratio.

A crystal structure is an arrangement of atoms that comes together in a highly ordered, repeating, symmetrical pattern. If the atoms lack this long-range order, the substance is amorphous.

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Naturally occurring, mineral-like, non-crystalline solids are known as mineraloids. Examples include amber, which is fossilised tree resin, opal, 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.

A graphic showing the difference between crystalline and amorphous atoms

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 is generally more consistent than its external colour, although it can differ.

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.

More than 6,000 minerals have been identified on Earth, but only a few dozen are common. A small number of these make up most of the rocks around us. Many new minerals are discovered each year.

Examples of the most common minerals include quartz, olivine, calcite, and pyroxenes. Peridot is the gemstone variety of olivine. Jadeite is a pyroxene.

Quartz, together with the feldspar and mica groups, makes up much of Earth's crust. Painite, taaffeite and grandidierite are rare minerals.

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.