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Earth and Space · Chapter 2: Structure of the Earth
Minerals
What minerals are, how they are identified by colour, streak, lustre, hardness and cleavage, the Mohs scale, and minerals of Pakistan, with a virtual scratch test.
Key termRememberExampleWatch out
01Concept
A mineral is a naturally occurring, inorganic solid with a definite chemical composition and an orderly arrangement of atoms (a crystal structure).
Each part of the definition matters:
- Naturally occurring: formed by nature, not made in a factory. Factory-made diamonds are not minerals.
- Inorganic: not made by living things. Coal comes from plants, so it is not a mineral.
- Solid: water is not a mineral, but ice in a glacier is.
- Definite composition: quartz is always SiO₂; halite (rock salt) is always NaCl.
- Crystal structure: its atoms are arranged in a repeating pattern.
Identifying minerals
| Property | What it means | Example |
|---|---|---|
| Colour | What it looks like. Not reliable on its own, because small impurities change it. | Quartz can be clear, white, pink or purple |
| Streak | Colour of the powder left when rubbed on a rough white tile. More reliable than colour. | Haematite looks black or silver but its streak is red-brown |
| Lustre | How the surface reflects light: metallic, glassy, pearly, dull. | Pyrite has a metallic lustre |
| Hardness | How easily it is scratched, measured on the Mohs scale from 1 to 10. | Talc is 1, diamond is 10 |
| Cleavage / fracture | Whether it breaks along flat surfaces (cleavage) or unevenly (fracture). | Mica peels into thin sheets |
| Density | How heavy it is for its size. | Gold is very dense |
The Mohs hardness scale
In 1812, Friedrich Mohs arranged ten minerals so that each one can scratch the ones before it. To test an unknown mineral, try scratching it with things of known hardness.
Minerals of Pakistan
| Mineral | Where | Use |
|---|---|---|
| Halite (rock salt) | Khewra, one of the world's largest salt mines | Table salt, chemicals |
| Gypsum | Salt Range, Balochistan | Cement, plaster |
| Chromite | Muslim Bagh (Balochistan) | Stainless steel |
| Copper and gold ores | Saindak and Reko Diq (Chagai, Balochistan) | Wires, jewellery |
| Emerald, ruby, peridot | Swat (emerald), Hunza (ruby), Kohistan (peridot) | Gemstones |
02Virtual Lab
Try this topic for yourself in the virtual lab. It opens on its own page with steps, tasks and questions.
03Real World Examples
Pink salt
The pink colour of Himalayan salt from Khewra comes from tiny amounts of iron oxide. It is still the mineral halite.
Scratched phone screens
Sand is mostly quartz (hardness 7), which is harder than most phone-screen glass. That is why sand in a pocket can scratch a screen.
Cement factories
Gypsum is added to cement so it does not set too quickly.
Gems of the north
Rubies from Hunza and emeralds from Swat are sold around the world. Both are hard minerals that keep their shine.
04Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.Define a mineral.Show answer
Answer
A mineral is a naturally occurring, inorganic solid with a definite chemical composition and an orderly crystal structure.
Q2.Is coal a mineral? Give a reason.Show answer
Answer
No. Coal is formed from the remains of ancient plants, so it is organic, and it has no fixed chemical composition. Coal is a rock.
Q3.Why is streak more useful than colour for identifying a mineral?Show answer
Answer
Because the colour of a mineral can change with small impurities, but the colour of its powder (streak) usually stays the same.
Q4.A mineral is scratched by a steel nail but not by a copper wire. What is its hardness?Show answer
Answer
- Steel nail (5.5) scratches it, so hardness is less than 5.5.
- Copper (3.5) does not scratch it, so hardness is more than 3.5.
- Hardness is between 3.5 and 5.5, so it is about 4–5 (for example fluorite or apatite).
Long questions
Q5.Describe the properties used to identify minerals. Name three minerals found in Pakistan and their uses.Show answer
Answer
- Colour: easy to see but not reliable.
- Streak: colour of the powder on a white tile.
- Lustre: how it reflects light (metallic, glassy, dull).
- Hardness: Mohs scale, 1 (talc) to 10 (diamond).
- Cleavage or fracture: how it breaks.
- Density: how heavy it is for its size.
Pakistan: halite from Khewra (table salt), gypsum from the Salt Range (cement), chromite from Muslim Bagh (stainless steel).