GeologyMinerology and Petrology

Studying Mohs Scale of Mineral Hardness and its use

In this article, we will study the Mohs scale of mineral hardness and how to use it to identify different minerals.

What are Minerals?

A mineral is a naturally occurring chemical compound, usually of crystalline form and abiogenic in origin (not produced by life processes). A mineral has one specific chemical composition.

There are over 5300 known minerals. The silicate minerals compose over 90% of the earth’s crust. Silicon and oxygen constitute over 75% of the earth’s crust. Minerals are distinguished by various chemical and physical properties. Differences in chemical composition and crystal structure distinguish the various species, which were determined by the mineral’s geological environment when formed. Changes in the temperature, pressure, or bulk composition of a rock mass cause changes in its minerals

Properties of minerals

Following are some physical properties of minerals that help in the identification of minerals:

  • Hardness
  • Color
  • Streak
  • Cleavage
  • Crystalline structure
  • Tenacity
  • Magnetism
  • Luster
  • Odor
  • Taste
  • Specific gravity

Hardness of minerals

When we scratch a mineral with some other mineral or any other surface the resistance offered by the mineral is known as its hardness.

The hardness of minerals is very useful in their field identification. Mohs scale of hardness is a useful tool for field geologists for the quick identification of minerals specimens.

Mohs scale of mineral hardness definition

The Mohs scale of mineral hardness is a qualitative ordinal scale characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. Created in 1812 by German geologist and mineralogist Friedrich Mohs, it is one of several definitions of hardness in materials science, some of which are more quantitative.

Mohs scale of mineral hardness
Mohs scale of hardness kit

Despite its lack of precision, the Mohs scale is highly relevant for field geologists, who use the scale to roughly identify minerals using scratch kits. The Mohs scale hardness of minerals can be commonly found in reference sheets. Reference materials may be expected to have a uniform Mohs hardness.

Mohs scale of mineral hardness chart

This chart shows the list of minerals in the Mohs scale that is assigned a value between 1 to 10 to show the hardness.

Mohs scale of mineral hardness
Mohs scale of hardness chart

Procedure-Mohs hardness test

  • Begin by locating a smooth, unscratched surface for testing.
  • With one hand, hold the specimen of unknown hardness firmly against a table top so that the surface to be tested is exposed and accessible. The table top supports the specimen and helps you hold it motionless for the test.
  • Hold one of the standard hardness specimens in the other hand and place a point of that specimen against the selected flat surface of the unknown specimen.
  • Firmly press the point of the standard specimen against the unknown specimen, and firmly drag the point of the standard specimen across the surface of the unknown specimen.
  • Examine the surface of the unknown specimen. With a finger, brush away any mineral fragments or powder that was produced. Did the test produce a scratch? Be careful not to confuse mineral powder or residue with a scratch. A scratch will be a distinct groove cut in the mineral surface, not a mark on the surface that wipes away.

Importance of Mohs hardness test for a mineral

The Mohs Hardness Test is almost exclusively used to determine the relative hardness of mineral specimens. This is done as part of a mineral identification procedure in the field, in a classroom, or in a laboratory when easily identified specimens are being examined or where more sophisticated tests are not available.

Check out this article to find out the identification of minerals using streak

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