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3.1.2.3. Mechanical properties of minerals

Mechanical properties of minerals

The mechanical properties of minerals depend on the behavior of the mineral against the application of forces on its surface. The main mechanical properties of minerals are:

Toughness.

Fracture.

Exfoliation.

Tenacity.

Hardness of a mineral

The hardness of a mineral is the resistance offered by its surface to being scratched. It depends on the cohesive forces of the mineral, which will be higher or lower depending on its structure.

Although there are several scales, the most used is the Mohs scale, which takes the hardnesses of ten minerals as a reference. These minerals are arranged in such a way that a mineral scratches the previous ones and is scratched by the later ones. For example, fluorite, with hardness 4, can scratch calcite, with hardness 3, and it can be scratched by apatite, with hardness 5. Similarly, it can be said that the hardness of a mineral is between 7 and 8 if it scratches quartz  and is scratched by topaz.

It should be noted that the hardness values ​​do not have a real arithmetic relationship between them, but are only a reference.

Hardness

Material

Chemical composition

1

Talc, (can be easily scratched with the nail)

Mg3Si4O10(OH)2

2

Gypsum, (it can be scratched with the nail with more difficulty)

CaSO4 2H2O

3

Calcite, (can be scratched with a copper coin)

CaCO3

4

Fluorite, (can be scratched with a knife)

CaF2

5

Apatite, (can be hardly scratched with a knife)

Ca5(PO4)3 (OH-, Cl-, F-)

6

Orthoclase feldspar, (can be scratched with a file)

KAlSi3O8

7

Quartz, (scratches glass)

SiO2

8

Topaz, (lines all of the above)

Al 2 SiO 4 (OH-, F-) 2

9

Corundum, (only scratched bydiamond)

Al2O3

10

Diamante, (the hardest mineral)

C


Complete the Mohs scale

Complete with the name of the corresponding mineral:

  • Hardness 1:
  • Hardness 2:
  • Hardness 3:
  • Hardness 4:
  • Hardness:
  • Hardness:
  • Hardness 7:
  • Hardness 8:
  • Hardness 9:
  • Hardness 10:

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Fracture of a mineral

The fracture of a mineral is the way in which it breaks along an uneven surface.

There are different types of fractures, such as conchoid (shell-shaped, typical of flint), fibrous, or splinter fractures.

Un hendedor o hendidor es un instrumento prehistórico que aprovecha la fractura concoidea del sílex para ser usado como un objeto cortante

Exfoliation of a mineral

The exfoliation is the property by which a mineral tends to be broken in some preferred directions, such as in films as micas , rhombohedral in calcite  or cubes in galena.

The form of exfoliation is closely related to the crystalline structure of the mineral.

If the minerals do not have a regular internal structure, it is called a fracture, breaking into irregular surfaces.

Tenacity of a mineral

The tenacity  is the resistance of a mineral to be broken, crushed or bent.

Caution: Do not cofuse toughness with harshness.

Depending on its toughness, a mineral can be:

  • Brittle: if it breaks easily. As a curiosity, thediamante, which is the hardest mineral, is one of the most brittle.
  • Malleable: if sheets can be obtained by pressing or hitting it. For example, gold is rolled by blows.
  • Sectile: if you can cut with a knife forming chips.
  • Ductile: if it can be stretched into a thread.
  • Flexible: if it can be bent but does not regain its shape after the effort ceases.
  • Elastic: if it can be bent but recovers its shape after stopping the effort. For example, micas .

Mechanical properties of minerals in other courses

The mechanical properties of minerals are also covered in other courses: