Physical properties of minerals
Topic: Minerals, Rocks and the Rock Cycle · NCERT: Beyond NCERT
Meaning
Physical properties of minerals are the features we can see or test to identify a mineral. They include crystal form (the outer shape, set by how its molecules are arranged inside), cleavage (breaking along flat planes) and fracture (uneven breaking where there is no such plane). They also include lustre (how the surface shines, such as metallic, silky or glossy), colour and streak (the colour of the mineral's powder). The rest are transparency, structure, hardness (measured on the Mohs scale) and specific gravity (its weight compared with an equal volume of water). Together these let geologists name a mineral without a laboratory.
Example
Hematite, an iron ore, can look black, grey or reddish. When rubbed on rough porcelain, it always leaves a red-brown streak. Mica splits into thin sheets (cleavage), but quartz breaks unevenly (fracture).
Don't confuse with
- Colour vs streak: colour can mislead, because small impurities change it. Streak stays the same, so it is more reliable.
- Cleavage vs fracture: cleavage gives smooth, flat breaks. Fracture gives rough, uneven breaks.