Hardness

Minerals vary in hardness, depending on their composition and, consequently, their crystal structure and bonding. Minerals with a very regular structure and covalent bonds exhibit particularly high hardness (e.g., corundum). In geosciences, the Mohs scale of mineral hardness, developed in 1812 by Friedrich Mohs (1773–1839), is most commonly used to quantify the hardness of a mineral. Mohs discovered that harder materials scratch softer materials. Based on this observation, he arranged 10 minerals in an ordinal scale.

To determine the Mohs hardness of an unknown sample, you take a mineral with a known hardness (e.g., quartz) and test whether the sample can be scratched. If it can (as in the image), you know that the mineral must have a hardness of less than 7. If the sample cannot be scratched, it must be harder than 7. You then work your way up or down in increments until the sample can be scratched or can no longer be scratched. In the field, you can also use a fingernail, knife, or glass. However, due to the different types of glass and steel used, the accuracy naturally decreases.

Figure 2: The mineral shown here could be replaced by another mineral or
A tool can be clearly scratched and is therefore softer than the test material used.

When determining Mohs hardness, a few things need to be considered. It's usually necessary to wash the scratched area afterward and/or examine it under a magnifying glass to determine whether the sample itself scratched or if the test mineral simply left powder residue. It should also be understood that minerals of the same hardness generally do not scratch. Furthermore, many minerals exhibit a more or less pronounced anisotropy (direction dependence of their properties). This means that the hardness of a mineral often differs depending on the direction in which it is scratched. Kyanite, for example, has a hardness of 4.5–5 in the longitudinal direction and 6.5–7 in the transverse direction. As we can also see, the Mohs hardness of certain minerals can vary by 0.5–1.

To obtain more precise results in hardness testing, a sclerometer can be used. This instrument, from materials science, measures hardness using a weighted diamond tip. The weight at which the mineral is just barely scratched corresponds to the mineral's hardness.