Minerals

A piece of Peterhead granite is a mosaic of three minerals: pink feldspar, grey glassy quartz and black biotite mica.

Pick up a pebble of granite and look closely. It is not one material but a jumble of interlocking grains, each with its own colour and shine. The British Geological Survey describes a granite from Peterhead in Scotland: the pinkish grains are feldspar, the grey glassy ones are quartz and the black ones are biotite mica. Each of those is a mineral. The granite, the whole mixture, is a rock.

What counts as a mineral

The USGS defines a mineral as a naturally occurring inorganic element or compound with an orderly internal structure and a characteristic chemical composition, crystal form and physical properties. Each part of that sentence rules something out. Steel and window glass are made by people, so they are not minerals. Glass also lacks an orderly internal structure. Sugar has a fixed formula and forms crystals, but it is made by plants, so it is organic. Quartz, SiO₂, passes every test: it forms in nature, its atoms sit in a repeating pattern, and any sample of it has the same composition.

A rock, in the USGS wording, is an aggregate of one or more minerals. Common minerals include quartz, feldspar, mica, amphibole, olivine and calcite; common rocks include granite, basalt, limestone and sandstone. The same minerals can turn up in very different rocks. Sandstone can contain quartz, feldspar and mica just as granite does, but in sandstone they are worn grains cemented together, while in granite they are crystals that grew into one another as molten rock cooled. How the minerals are put together tells you how the rock formed.

Why are silicates like quartz and feldspar so common? Look at what the crust is made of. In Earth's crust and atmosphere taken together, oxygen makes up about 49% of the mass and silicon about 26% (OpenStax). Quartz, the simplest example, is nothing but silicon and oxygen, SiO₂.

Testing hardness

Geologists identify minerals by their properties: colour, shine, the shape they break into, and hardness. Hardness here means resistance to being scratched. A harder mineral scratches a softer one, never the other way round. In the early 1800s the German mineralogist Friedrich Mohs picked ten minerals and ranked them from 1 to 10. People had compared hardness long before; the National Park Service notes that Theophrastus mentioned it in his treatise On Stones around 300 BC.

To test a sample, try to scratch it with something of known hardness, then try the reverse. If a steel nail scratches the mineral but a copper coin does not, its hardness lies between 3 and 4. Test on a fresh, clean surface of the mineral, and try both directions when the two materials are close in hardness.

The numbers only give an order. They are not equal steps. The USGS bulletin on gemstones points out that diamond, at 10, is many times harder than corundum at 9; corundum sits next to it only because nothing natural comes closer. The same bulletin gives a practical rule: much ordinary dust is quartz, hardness 7, so a gemstone worn every day should be harder than quartz or it will slowly lose its polish.