Physical properties of minerals

The physical properties of minerals depend on chemical composition and crystal structure, and are used to identify minerals in hand specimens.

1. Colour

Appearance in reflected light. Idiochromatic minerals have constant colour (malachite green); allochromatic minerals vary because of impurities (quartz can be white, pink, purple). Colour is not always reliable.

2. Streak

Colour of the powder of a mineral, seen on an unglazed porcelain plate. It is more constant than colour: hematite gives a red-brown streak even when the specimen looks black.

3. Lustre

Appearance of a fresh surface in reflected light.

• Metallic (galena, pyrite).
• Non-metallic: vitreous (quartz), pearly (talc), silky (asbestos), resinous, greasy, earthy (kaolin), adamantine (diamond).

4. Transparency

Transparent, translucent or opaque.

5. Hardness

Resistance to scratching, measured on the Mohs scale (1 to 10).

6. Cleavage

Tendency to split along smooth planes of weak bonding. Described as perfect (mica, one direction), good (feldspar, two directions), poor or absent. Calcite has three directions, not at right angles.

7. Fracture

Irregular breaking where there is no cleavage: conchoidal (quartz), uneven, splintery, hackly.

8. Tenacity

Behaviour under stress: brittle, malleable, sectile, flexible, elastic.

9. Specific gravity

Ratio of the mineral's density to that of water (quartz 2.65, galena 7.5).

10. Form / habit

Crystal shape: prismatic, tabular, fibrous, granular, massive.

11. Other properties

Magnetism (magnetite), taste (halite), effervescence with dilute HCl (calcite), feel (talc greasy), and fluorescence.