Physical Characteristics of Minerals (Crystal Form, Cleavage, Fracture, Lustre, Colour, Streak, Hardness, Specific Gravity)
Geography › Geology – Minerals and Rocks · Topic 2.2
Minerals are identified by their physical properties. These are crystal form, cleavage (breaking along flat planes), fracture (irregular breaking), lustre (the way light is reflected), colour, streak (colour of the powder), hardness (resistance to scratching, measured on the Mohs scale of 1 to 10) and specific gravity (density compared with water). No single property is enough to identify a mineral, so geologists use several together, and colour is the least reliable because impurities change it.
5 min read · UPSC Prelims · Geography · Geology – Minerals and Rocks
At a Glance
- Hardness scale Mohs, 1 (talc) to 10 (diamond)
- Cleavage Splitting along flat planes
- Streak Colour of the powder
- Lustre Metallic or non-metallic
- Specific gravity Density relative to water
- Exam link UPSC Prelims, Geography: Geology – Minerals and Rocks
Where Does It Fit?
This topic follows the one on the elements of the crust and explains how minerals are recognised in the field and the laboratory. These properties are needed to understand the minerals and rocks described in the next topics. Previous topic: Minerals: Major Elements of the Earth’s Crust. Next topic: chromite, nickel, cobalt, tungsten Non-ferrous Copper, bauxite, lead, zinc, tin Precious Gold, silver, platinum Main iron…">Metallic Minerals (Precious, Ferrous, Non-Ferrous).
What Are the Main Physical Properties of Minerals?
| Property | Meaning | Examples |
|---|---|---|
| Crystal form | The external shape of a mineral grown freely. There are seven crystal systems: cubic, tetragonal, orthorhombic, hexagonal, trigonal, monoclinic and triclinic | Halite is cubic; quartz is hexagonal |
| Cleavage | Tendency to split along smooth, flat planes because of weak bonds | Mica has perfect cleavage in one direction; calcite splits into rhombs; halite into cubes |
| Fracture | Irregular breakage where there is no cleavage | Quartz breaks with curved, shell-like (conchoidal) surfaces |
| Lustre | The shine of the surface | Metallic in galena; glassy in quartz; pearly in talc; silky in asbestos; brilliant in diamond |
| Colour | Appearance in light; unreliable because of impurities | Quartz can be clear, pink, purple or black |
| Streak | Colour of the fine powder made on an unglazed tile | Hematite gives red-brown streak even when it looks silvery |
| Hardness | Resistance to scratching | Gypsum is soft; quartz is hard |
| Specific gravity | Weight compared with an equal volume of water | Quartz about 2.65; galena about 7.5; gold about 19 |
Other properties are transparency (clear, translucent or opaque), tenacity (brittle, flexible or malleable), magnetism (magnetite is magnetic) and reaction with acid (calcite fizzes in dilute hydrochloric acid).
What Is the Mohs Scale of Hardness?
The Mohs scale, devised by the German mineralogist Friedrich Mohs in 1812, ranks ten minerals from soft to hard. A mineral can scratch those below it on the scale and be scratched by those above it. The scale is relative, not equal in steps: diamond is far harder than corundum, although they are next to each other.
| Hardness | Mineral | Everyday test |
|---|---|---|
| 1 | Talc | Crumbles under 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 knife easily |
| 5 | Apatite | Scratched by a knife or glass (about 5.5) |
| 6 | Orthoclase feldspar | Scratches glass |
| 7 | Quartz | Scratches steel file |
| 8 | Topaz | Scratches quartz |
| 9 | Corundum | Ruby and sapphire |
| 10 | Diamond | Hardest natural substance |
Why Is Colour Not Enough and How Are Properties Used Together?
- Colour misleads: the same mineral may have many colours. Quartz is colourless as rock crystal, purple as amethyst, and yellow as citrine. Different minerals can share the same colour.
- Streak is better: the powder colour is steadier. Pyrite looks like gold, but gives a greenish-black streak, and real gold gives a golden streak. Pyrite is also hard and brittle, while gold is soft and can be hammered.
- Cleavage and fracture: these reflect the internal structure and help separate mica from other shiny flakes.
- Specific gravity: the ‘heft’ of a mineral is used in panning for gold, where the heavy grains settle out of the water.
As an analytical point, geologists therefore combine several tests, and use microscopes and X-ray methods when the field tests are not clear. The elements and groups behind these properties are in Minerals: Major Elements of the Earth’s Crust.
Why Do These Properties Matter in Practice?
Hardness decides the use of abrasives and gemstones: corundum and diamond are used in cutting and polishing. Cleavage makes mica useful as thin insulating sheets and graphite useful as a lubricant. Lustre and colour decide the value of gemstones. Specific gravity helps in separating heavy ore minerals from lighter waste rock, and magnetism in separating magnetite from other iron minerals. The uses of important minerals are described further in Metallic Minerals (Precious, Ferrous, Non-Ferrous) and Non-Metallic Minerals (Sulphur, Phosphates, Cement Minerals).
Why Is This Topic Important?
- Scale Mohs scale is asked
- Practice Basis for identifying minerals
- Use Decides industrial use
What Came Next?
The next topic describes metallic minerals, which are of great economic use. Next topic: Metallic Minerals (Precious, Ferrous, Non-Ferrous).
Key People and Terms
- Cleavage Splitting along smooth planes
- Fracture Irregular breaking
- Lustre Shine of the surface
- Streak Colour of the powder
- Mohs scale Ten-step hardness scale
- Specific gravity Density relative to water
Exam Corner
Points to Remember
- Mohs scale: talc 1, quartz 7, diamond 10.
- Streak is more reliable than colour.
- Mica has perfect cleavage in one direction.
- Pyrite (fool’s gold) has a greenish-black streak.
- Gypsum can be scratched by a fingernail.
Do Not Confuse With
- Cleavage and fracture Flat splitting versus irregular breaking
- Colour and streak Look of the lump versus colour of the powder
- Hardness and toughness Resistance to scratching versus resistance to breaking
Memory Hook
Talc, Gypsum, Calcite, Fluorite, Apatite, Orthoclase, Quartz, Topaz, Corundum, Diamond: one to ten.
Mains Angle
Questions on this topic are mostly factual. These points help in a short note.
- Method Use several properties together
- Use Industry and gems
- Limit Colour is unreliable
A Question You May Face
An original practice question, not a past paper question.
“Describe the physical properties by which minerals are identified, with the Mohs scale of hardness.”
How to Answer
- List Form, cleavage, lustre, streak.
- Hardness Mohs scale.
- Caution Colour misleads.
- Use Industrial examples.
GS Relevance
Frequently Asked Questions
What is the Mohs scale of hardness?
The Mohs scale ranks ten minerals by their ability to scratch one another, from talc as 1 to diamond as 10. The intermediate minerals are gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz and corundum. A mineral scratches those below it on the scale and is scratched by those above.
What is cleavage in a mineral?
Cleavage is the tendency of a mineral to split along smooth, flat planes because the bonds between its atoms are weaker in those directions. Mica splits into thin sheets in one direction, halite splits into cubes, and calcite splits into rhombus-shaped pieces.
What is the streak of a mineral?
Streak is the colour of the fine powder of a mineral, found by rubbing it on an unglazed porcelain tile. It is more reliable than the colour of the lump, because it is steady. Hematite, for example, leaves a red-brown streak, however it looks outside.
What is lustre?
Lustre is the way a mineral's surface reflects light. It is divided into metallic lustre, as in galena, and non-metallic types such as glassy in quartz, pearly in talc, silky in asbestos and brilliant or adamantine in diamond. It helps to separate similar-looking minerals.
Why is colour not a reliable test for minerals?
Colour is not reliable because small amounts of impurities can change it greatly, so one mineral can appear in many colours, as quartz does, and different minerals can look alike. Properties such as streak, hardness, cleavage and specific gravity are steadier guides.
PYQ Practice — Statement Analysis
1 Quartz has a hardness of 7 on the Mohs scale.
It scratches a steel file.
2 Diamond has a hardness of 5 on the Mohs scale.
It is 10, the hardest mineral.
3 Streak is the colour of the powdered mineral.
It is a steadier guide than colour.
4 Mica shows cleavage in one direction.
It splits into thin sheets.
Past Mains Questions on This Topic
This topic comes under Geology – Minerals and Rocks. UPSC Mains questions asked on the matching syllabus topic, year-wise with marks:
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