Laboratory Manual for Introductory Geology 4e

3.4.5 Hardness
The hardness of a mineral is a measure of how easily it can scratch or be scratched
by other substances. A 19th-century mineralogist, Friedrich Mohs, created a mineral
hardness scale, using ten familiar minerals, that we still use today. He assigned a
hardness of 10 to the hardest mineral and a hardness of 1 to the softest (TABLE 3.1).
This scale is a relative one, meaning that a mineral can scratch those lower in the
scale but cannot scratch those that are higher. It is not an absolute scale, in which diamond
would be 10 times harder than talc and corundum would be 3 times harder
than calcite. The hardness of common materials, such as the testing materials listed
in FIGURE 3.3, can also be described using the Mohs hardness scale. To determine
the hardness of a mineral, see which of these materials it can scratch and which ones
can scratch it.
TABLE 3.1 Mohs hardness scale and its relationship to common testing
materials
Mohs hardness
Mineral
number (H)
Diamond 10
Corundum 9
Topaz 8
Mohs hardness of testing materials
Quartz 7
Orthoclase 6
Streak plate (6.5–7)
Apatite 5
Window glass; steel cut nail (5.5)
Common nail or pocket knife
Fluorite 4
(5.0-5.5)
Calcite 3 U.S. penny (3.0)
Gypsum 2 Fingernail (2.5)
Talc 1
HARD MODERATE SOFT
EXERCISE 3.4
Constructing and Using a Relative Hardness Scale
Name:
Course:
Section:
Date:
Your instructor will tell you which specimens from your mineral set to use for this exercise. Arrange them in order of
increasing hardness by seeing which can scratch the others and which are most easily scratched.
Softest
Hardest
Specimen no.:
Now use the testing materials listed in Table 3.1 to determine the Mohs hardness of minerals in your set.
Mohs hardness:
60 CHAPTER 3 MINERALS