C. PHYSICAL PROPERTIES

Each mineral has a characteristic set of physical properties that depend on its elements and its type of bonding. These can be used to identify it.

1. Crystal form - growth form - Crystals sometimes have an external shape that is bounded by smooth plane faces. 29

2. Luster - The amount of light reflected form a mineral and how it is reflected. 29

3. Color - Those colors of the rainbow that are reflected (or transmitted); not always diagnostic. 37

4. Streak - color of a mineral powdered on a streak plate; more diagnostic than color.

5. Hardness - The resistance to scratching. The Mohs Hardness Scale lists 10 common minerals in order of increasing hardness. 30

Mineral

Hardness

Other

Mineral

Hardness

Other

Talc

1

  

Orthoclase

6

~6.5 steel file

Gypsum

2

~2.2 fingernail

Quartz

7

7.0 streak plate

Calcite

3

~3.2 penny

Topaz

8

  

Fluorite

4

  

Corundum

9

  

Apatite

5

~5.1 knife

~5.5 glass

Diamond

10

hardest substance

 6. Cleavage - breakage form - some minerals break along definite parallel smooth surfaces, which are planes of weak bonds. The number of planes and the angles between them are both important. 30,31

Examples:

mica has one perfect plane of cleavage.

halite has three planes of cleavage all at right angles to each other.

calcite has three planes of cleavage not at right angles to each other.

7. Specific gravity - comparison of the mass of a mineral to an equal volume of water (density). Most minerals are 205 to 3.0 times as heavy as water, galena is 7.5 times as heavy, gold is 19.3 times as heavy.

8. Magnetism - magnetite is magnetic.

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