Hardness of Minerals

the property whereby minerals resist penetration by other bodies. Hardness is an important diagnostic and typomorphic mineral feature and is a function of composition and texture, which in turn reflect in differing degrees the conditions surrounding the mineral’s formation. Mineral hardness increases with a decrease in interatomic distances in the crystal, an increase in the valence and coordination number of the constituent atoms, and a transition from the ionic type of chemical bond to the covalent type. The presence of hydroxyl groups or water molecules in the structure and the existence of gas and liquid inclusions in minerals reduce hardness to a marked extent. Hardness also depends on such factors as the amount and composition of isomorphous impurities, on defects in the structure, on the presence of microscopic inclusions and decay products of solid solutions, and on the degree of alteration of the mineral.

The hardness of minerals is a vector property, depending on direction even in crystals in the isometric system. (The classic example of anisotropy in mineral hardness is kyanite.) Mineral hardness is measured on a relative mineralogical scale known as Mohs’ scale. Most natural compounds have a hardness of 2–6, with anhydrous oxides and silicates being the hardest minerals. Microhardness is measured with sclerometers, and the data on microhardness are used in describing the genetic type of a deposit and the generations of minerals and ore types and in studying the history of individual minerals.