Minerals and Crystals

A mineral is a naturally occurring, inorganic solid with a definite chemical composition and an ordered internal atomic structure. That ordered arrangement of atoms is what gives many minerals their characteristic crystal forms, with flat faces meeting at constant angles. Mineralogy, a branch of geology, studies how minerals form, their composition and structure, and the physical properties used to tell them apart. Rocks, in turn, are aggregates of one or more minerals.

By far the most abundant minerals in the Earth's crust are the silicates, built around silicon and oxygen tetrahedra that link together in chains, sheets or three-dimensional frameworks. Common silicates include quartz, the feldspars, micas and the dark iron-magnesium minerals of many volcanic rocks. Non-silicate groups include carbonates such as calcite, oxides, sulfides and native elements. Crystals grow when atoms or ions arrange themselves in a repeating pattern as a melt cools, a solution evaporates, or hot fluids deposit material in cavities.

Mineralogists identify specimens using a set of diagnostic properties. Hardness is commonly assessed with the Mohs scale, which ranks minerals from soft talc to very hard diamond by which can scratch which. Other tests include colour, the colour of a powdered streak, lustre, cleavage and fracture, crystal habit, and density. Some minerals are prized as gemstones, the focus of gemmology, where qualities such as colour, clarity and the way light is refracted determine value. Collectors who gather mineral and rock specimens in the field are often called rockhounds.

Frequently asked questions

What is the difference between a mineral and a rock?

A mineral is a single naturally occurring substance with a fixed composition and ordered structure. A rock is a solid mass made up of one or more minerals grouped together.

What is the Mohs hardness scale?

It is a relative scale that ranks minerals by scratch resistance, running from talc as the softest to diamond as the hardest. A mineral can scratch any other that ranks lower than it.

Why are silicates so important?

Silicate minerals make up most of the Earth's crust and mantle. Built from silicon-oxygen tetrahedra, they include common minerals such as quartz, feldspar and mica.