Hardness and Tenacity of Minerals
Minerals vary quite widely in their hardness, and a determination of their degree of hardness is often an important aid to their identification. A series of minerals has been chosen as a scale by comparison with which the relative hardness of any mineral may be told. The scale consists of crystallized varieties of the following minerals, each species being harder than those preceding it in the scale.
- Talc
- Gypsum
- Calcite
- Fluorite
- Apatite
- Orthoclase
- Quartz
- Topaz
- Corundum
- Diamond
In order to determine the relative hardness of any mineral in terms of this scale, it is necessary to find which ones of these minerals it can and which it cannot scratch. In making the determination the following precautions should be observed: Sometimes when a mineral is softer than another, portions of the first will leave a mark on the second which may be mistaken for a scratch. It can be rubbed off, however, while a true scratch will be permanent. Some minerals are frequently altered on the surface to material which is much softer than the original mineral. A fresh surface of the specimen to be tested should therefore be used. Sometimes the physical structure of a mineral may prevent a correct determination of its hardness. For instance, if a mineral is pulverulent, granular or splintery in its structure, it may be broken down and apparently scratched by a mineral much softer than itself. It is always advisable when making the hardness test to confirm it by reversing the order of procedure.
The following materials may serve in addition to the above scale: The finger nail is a little over 2 in hardness, since it can scratch gypsum and not calcite. A cent is about 3 in hardness, since it can just scratch calcite. The steel of an ordinary pocketknife is just over 5, and ordinary window glass has a hardness of 5.5.
Crystals frequently show different degrees of hardness, depending upon the direction in which they are scratched. Ordinarily the difference is so small that it can be detected only by the use of delicate instruments.
Tenacity Of Mineral
The following terms are used to describe various kinds of tenacity in minerals:
- Brittle. When a mineral breaks or powders easily.
- Malleable. When a mineral can be hammered out into thin sheets.
- Sectile. When a mineral can be cut into thin shavings with a knife.
- Flexible. When a mineral bends but does not resume its original shape when the pressure is released.
- Elastic. When, after being bent, the mineral will resume its original position upon the release of the pressure.
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