Minerals and crystal structure — Earth science, 14–17 years
Minerals are the ordered ingredients of most rocks. Their atoms, properties and formation conditions help explain why rocks look and behave differently.
What makes a mineral
A mineral is a naturally formed solid with a particular chemical composition and an ordered internal structure. That structure repeats like a three-dimensional pattern of tiny particles. Quartz, feldspar and calcite are minerals; a rock is usually a mixture of several minerals, not one single substance.
Why identify minerals
Geologists needed a reliable way to describe rocks instead of judging only by colour or name. Mineral composition gives clues about how a rock formed and how it may change. Hardness, cleavage, density, lustre and reactions with acid are useful tests because they reflect the mineral’s structure and chemistry.
Worked example: identifying calcite
A clear sample may be quartz or calcite. First, a steel nail scratches calcite more easily, so the sample is not very hard. Next, a drop of dilute hydrochloric acid fizzes: calcite reacts and releases carbon dioxide. Quartz does not fizz, so these two tests together support the identification of calcite.
Trap: colour is not enough
Colour feels like an obvious clue, so it is reasonable to identify a mineral by sight. But small amounts of impurities can change its colour, and different minerals can share the same colour. A better identification uses several properties, such as streak, hardness and density, rather than trusting one impressive feature.
Minerals in ordinary objects
Mineral properties guide practical choices. Quartz is used in glass and in some electronic timing devices; gypsum is used in plasterboard; halite provides table salt; and graphite’s layered structure lets it mark paper. Mining these materials also has environmental costs, so knowing what is needed matters.
Keep exploring
Other languages
Loading MyLeoNes™…