THE RELATIONSHIP BETWEEN MOHS HARDNESS AND MATERIAL PROPERTIES OF COMMON MINERALS

The Mohs hardness scale ranks common minerals according to their resistance to scratching. Owing to its simplicity and usefulness for mineral identification, the Mohs scale is widely used to describe mineral ‘hardness’ and is enshrined in many undergraduate geoscience courses. Mohs hardness (MH), however, is not the same as hardness (H). Scratch resistance is a function of modulus (E), fracture toughness (T), and H, and these do not vary monotonically up the Mohs scale. To better understand how scratch resistance relates to material properties, we used microindentation and depth-sensing indentation (nanoindentation) to determine H, E, and T for the first nine Mohs minerals and other common minerals.

For the Mohs minerals, values for H vary from 0.14 GPa for talc to 21.2 GPa for corundum. H, E, and T generally increase with Mohs number, and between each Mohs mineral the values for each property increase by about a factor of two. There are exceptions to this trend: for example, fluorite (MH=4) and apatite (MH=5) have similar values for E and T, but H for apatite (H=5.5 GPa) is significantly greater than that of fluorite (H=1.9 GPa). Orthoclase (MH=6) has T=0.8 MPa mE0.5, similar to fluorite and apatite, but H is higher and E is lower. Both orthoclase and quartz have lower E values than fluorite and apatite. Quartz and topaz have similar H (12 GPa) and T (1.4-1.5 MPa mE0.5), but the modulus of topaz is over twice that of quartz. Also, the T of topaz is significantly lower than would be expected compared with the trend of the rest of the data.

Based on their scratch resistance, garnets have been assigned a Mohs value of 6.5 - 7.5, similar to quartz (MH=7). However, H of pyrope (16 GPa) and other garnet compositions is greater than H of quartz, and pyrope’s E (256 GPa) is more than twice that of quartz (E=118 GPa). Andalusite is typically assigned the same Mohs hardness as quartz and garnet, but has a lower H than either (10 GPa), whereas its T (1.8 MPa mE0.5) and E (232 GPa) are similar to those of pyrope.

The quantitative data obtained in this study illustrate how scratch resistance (MH) varies with H, E, and T. The difference between MH and H is not significant for introductory-level geoscience courses, but a distinction should be made in mineralogy and similar courses that discuss hardness as a material property.