What is a mineral?
A mineral must meet five rules:
- It forms naturally.
- It’s a solid.
- It’s inorganic (not made by living things).
- It has a definite chemical makeup.
- Its atoms are arranged in a crystal structure.
Quartz, salt (halite), diamond and gold are minerals. Glass isn’t (it has no crystal structure), and coal isn’t (it comes from living things). There are more than 5000 known minerals, but only about 100 are common.
Minerals are the building blocks of rocks. Granite, for example, is made of quartz, feldspar and mica.
Identifying minerals
Geologists use several tests.
Colour
Colour is easy to see but can be misleading. Quartz can be clear, white, pink, purple (amethyst) or smoky grey because of tiny impurities.
Streak
Streak is the colour of a mineral’s powder. Rub it across the back of an unglazed white tile. Streak is more reliable than colour.
| Mineral | Colour | Streak |
|---|---|---|
| Hematite | Silver-grey or reddish | Red-brown |
| Pyrite ("fool’s gold") | Brassy yellow | Greenish-black |
| Gold | Gold | Gold-yellow |
That’s how prospectors could tell real gold from fool’s gold.
Lustre
Lustre describes how a surface reflects light.
| Lustre | Looks like | Example |
|---|---|---|
| Metallic | Shiny like metal | Pyrite, galena |
| Glassy (vitreous) | Shiny like glass | Quartz |
| Pearly | Like a pearl | Talc |
| Dull (earthy) | Not shiny | Kaolin clay |
Hardness and the Mohs scale
Hardness is how well a mineral resists scratching. In 1812, German geologist Friedrich Mohs created the Mohs scale of hardness from 1 (softest) to 10 (hardest). A harder mineral scratches a softer one.
| Hardness | Mineral | Simple test |
|---|---|---|
| 1 | Talc | Scratched by a fingernail |
| 2 | Gypsum | Scratched by a fingernail (about 2.5) |
| 3 | Calcite | Scratched by a copper coin (about 3.5) |
| 4 | Fluorite | Scratched by a steel nail |
| 5 | Apatite | Scratched by a steel nail (about 5.5) |
| 6 | Orthoclase feldspar | Scratches glass |
| 7 | Quartz | Scratches glass easily |
| 8 | Topaz | |
| 9 | Corundum (ruby, sapphire) | |
| 10 | Diamond | Scratches everything |
Diamond isn’t just a little harder than corundum; it’s about four times harder.
Cleavage and fracture
- Cleavage: splits along smooth, flat surfaces. Mica peels into thin sheets; halite breaks into cubes.
- Fracture: breaks unevenly or with curved surfaces. Quartz breaks with curved, shell-like surfaces.
Other properties
- Density: gold is very heavy for its size.
- Magnetism: magnetite attracts a magnet.
- Fizz test: calcite bubbles in weak acid like vinegar.
- Taste: halite tastes salty (but never taste minerals in a lab!).
- Fluorescence: some minerals glow under UV light.
Crystal shapes
Minerals grow in characteristic crystal shapes when they have space.
- Halite: cubes
- Quartz: six-sided columns with pointed tips
- Garnet: 12-sided balls
Why minerals matter
| Mineral | Uses |
|---|---|
| Quartz | Glass, watches, electronics |
| Halite | Table salt, road salt |
| Gypsum | Drywall, plaster |
| Hematite and magnetite | Iron ore for steel |
| Bauxite | Aluminum |
| Chalcopyrite | Copper |
| Graphite | Pencils, batteries |
| Lithium minerals | Rechargeable batteries |
| Potash | Fertilizer |
Your phone contains dozens of elements mined from minerals. Canada is a world leader in mining potash (Saskatchewan has the largest reserves in the world), uranium, nickel and diamonds (in the Northwest Territories).
Common mistakes
- Relying only on colour.
- Thinking all shiny yellow minerals are gold.
- Thinking rocks and minerals are the same.
Try it yourself
- Do a streak test on rocks you find, using the back of a bathroom tile.
- Test hardness: which of your rocks can be scratched by a fingernail, a coin or a nail?
- Grow salt crystals by dissolving salt in hot water and letting it evaporate on a plate.
Quick recap
- Minerals are natural, inorganic solids with a definite makeup and crystal structure.
- Streak is more reliable than colour.
- The Mohs scale ranks hardness from talc (1) to diamond (10).
- Lustre, cleavage, density and other tests help identify minerals.
- Minerals supply the materials for almost everything we make.