A combined nanoindentation and numerical modeling investigation on the apparent mechanical properties of constituent minerals in soft rock

Although nanoindentation has been widely employed to measure the mesoscopic mechanical properties of rock-forming mineral particles, a critical gap exists in understanding how the mechanical response of a given mineral varies across different soft rocks. Here, nanoindentation tests were conducted to measure the mechanical properties, such as hardness, elastic modulus, and plastic energy ratio, of quartz and kaolinite minerals in both sandstone and mudstone. Mesoscopic observations show that quartz particles constitute the skeleton of the sandstone, and clay minerals, i.e., kaolinite, present as filling materials in the pores between the quartz skeleton; in mudstone, however, quartz particles are scattered in the matrix of clay matrix. The nanoindentation measured apparent values of hardness and elastic modulus of quartz are 8.24% and 11.75% higher, respectively, in sandstone than in mudstone. For kaolinite, these differences are even more pronounced, with hardness and elastic modulus values being 51.28% and 70.06% greater in sandstone. The plastic energy ratio of quartz is lower in sandstone, while that of kaolinite is higher. The surrounding constraint effect is numerically investigated to explain the variability in measured properties of the same constituent mineral across different soft rocks. The quartz is surrounded by mudstone clay matrix decreases its apparent hardness and elastic modulus and increases its plastic energy ratio compared to when it is in a quartz-rich sandstone framework. The kaolinite is confined by a rigid quartz skeleton in sandstone increases its measured properties compared to when it forms the unconfined, bulk clay matrix in mudstone. It is recommended that the ratio of indentation depth to mineral particle diameter be maintained below 0.1 to measure the absolute mechanical properties.