Rocks are the solid stuff of the geosphere, and minerals are what rocks are made of. Granite, for example, is a mix of the minerals quartz, feldspar and mica. To count as a mineral, a substance must pass five tests.
- Naturally occurring: formed by nature, not made in a factory or lab.
- Inorganic: not made by living things.
- Solid at Earth's surface temperatures.
- Definite chemical makeup: a fixed recipe of elements, such as SiO₂ for quartz or NaCl for halite (rock salt).
- Orderly crystal structure: its atoms are lined up in a repeating pattern.
Key idea
A mineral is a naturally occurring, inorganic solid with a definite chemical makeup and an orderly crystal structure. A rock is usually a mixture of minerals.
Why it works
The crystal structure is why minerals have regular shapes. In halite, sodium and chloride ions stack in a cube pattern, so salt grains are tiny cubes and break into smaller cubes. The atoms in volcanic glass are jumbled because the lava froze too fast for them to line up, so glass breaks along smooth curves and is not a mineral.
Worked example: Ice and glass
Is the ice in a glacier a mineral? Is window glass?
- Glacier ice: natural, not made by living things, solid, always H₂O, and its molecules form a six-sided crystal pattern. It passes all five tests.
- Window glass is made in factories, and its atoms are not in an orderly pattern. It fails two tests.
Answer: Glacier ice is a mineral; window glass is not
Worked example: Coal
Coal is a natural solid dug from the ground. Why is it not a mineral?
- Coal formed from the remains of ancient plants, so it is organic.
- Its makeup varies from place to place, and it has no single crystal pattern.
Answer: It comes from living things and has no definite makeup or crystal structure
Common mistake
Calling any rock, gem or crystal a mineral. Granite is a rock (a mixture of minerals), coal and amber come from living things, and glass has no crystal structure. Meanwhile ice, which few people think of, counts. Check each of the five tests one at a time.