Rocks and minerals

What is a mineral?

A mineral is a naturally occurring solid with a definite chemical composition and an orderly internal arrangement of atoms (a crystal structure). Quartz, for example, is always silicon dioxide (SiO₂), with its atoms arranged in the same repeating pattern, which is why quartz crystals have six sides.

A rock is a solid mixture of one or more minerals (or, occasionally, natural glass or organic material, as in obsidian and coal). Granite is a mixture of quartz, feldspar and mica; limestone is made almost entirely of one mineral, calcite. So rocks are made of minerals, and minerals are made of elements.

About 6000 different minerals are known, but a few dozen make up almost all rocks. The most common minerals in the crust are the feldspars (roughly half the crust), followed by quartz. Quartz is so hard and so resistant to chemical weathering that it survives when other minerals are broken down, which is why most beach sand is quartz.

Identifying minerals

Geologists identify minerals by testing their physical properties:

• Colour is the first thing you notice, but it is unreliable. Tiny amounts of impurities change it: pure quartz is colourless, but quartz can also be purple (amethyst), pink (rose quartz) or smoky grey.
• Lustre is the way the surface reflects light: metallic (like polished metal), glassy, pearly, silky, greasy or dull/earthy.
• Streak is the colour of the mineral's powder, seen by scraping it on an unglazed white tile (a streak plate). Streak is much more reliable than colour. Haematite can look silver-grey, black or red, but its streak is always red-brown.
• Hardness is how easily a mineral is scratched, measured on the Mohs scale (below).
• Cleavage and fracture. Minerals with cleavage break along flat, smooth planes where the bonds between atoms are weakest. Mica peels into thin sheets (one cleavage); halite and galena break into cubes (three cleavages at right angles). Minerals with no cleavage, such as quartz, fracture into uneven or curved surfaces.
• Crystal shape: cubes (halite, pyrite, galena), six-sided columns (quartz), flat sheets (mica).
• Density: galena and gold feel surprisingly heavy for their size.
• Special properties: magnetite is magnetic; calcite fizzes in dilute acid and a clear piece shows a double image; fluorite glows under ultraviolet light; talc feels soapy.

The Mohs hardness scale

In 1812 the German mineralogist Friedrich Mohs chose ten minerals and ranked them by which scratches which. A mineral scratches anything softer than itself and is scratched by anything harder. The scale only tells you the order, not how much harder: diamond (10) is about four times as hard as corundum (9).

A simple key for common minerals

Identifying rocks

Rocks are harder to identify than minerals because they are mixtures. Ask these questions, in this order:

1. Can you see crystals or grains? Interlocking crystals suggest an igneous or metamorphic rock. Rounded or broken grains stuck together suggest a sedimentary rock.
2. Are there layers or bands? Layers of different grain sizes and fossils suggest sedimentary rock. Wavy bands of lined-up minerals (or a sheen of mica) suggest metamorphic rock.
3. How big are the grains or crystals? This helps tell granite from rhyolite, or conglomerate from sandstone from shale.
4. Does it fizz with acid? If so, it contains calcite: limestone or marble.
5. Holes? Glassy? Gas holes mean pumice or scoria; glassy with curved fractures means obsidian.

Properties decide uses

We choose rocks and minerals for jobs because of their properties. Granite is used for benchtops because it is hard, resists scratches, heat and most chemicals, and takes a high polish. Slate is used for roof tiles because it splits into thin waterproof sheets. Marble is used for sculpture because it is soft enough to carve but strong. Quartz is used in watches and electronics because a quartz crystal vibrates at a very steady rate when electricity is applied; a typical quartz watch stays accurate to within about 15 seconds a month.