Exploring the relationship of scratch resistance, hardness, and other physical properties of minerals using Mohs scale minerals

The Mohs scale is enshrined in geoscience curricula as a simple and effective tool for identifyin minerals and understanding the influence of crystal structure and chemistry on physical properties; e.g., hardness. Measuring scratch resistance is different from measuring hardness, however, because scratching involves components of loading and shearing, whereas "absolute" hardness is measured by the response of a material to vertical loading (indentation). Although it is not practical for most undergraduate classes to do indentation hardness testing students can evaluate tabulated quantitative hardness data and compare these data with their own determination of relative scratch resistance. To help students better understand physical properties of minerals, and in particular the concept of mineral hardness, we present an example exercise based on recent systematic measurements of the hardness of Mohs scale minerals using indentation techniques. This exercise allows students to explore the diffierences in hardness among minerals of the Mohs scale, enhancing their understanding of the Mohs scale itself as well as the chemical and physical factors that influence mineral hardness. The exercise is most appropriate for Earth materials and mineralogy classes, but can be adapted !or students with different levels of expertise, including introductory physical science students.