Comment on “Elastic modulus and hardness of muscovite and rectorite determined by nanoindentation” by G. Zhang, Z. Wei and R.E. Ferrell [Applied Clay Science 43 (2009) 271-281]

In their paper, Zhang et al. (2009) report on a series of nanoindentation experiments on muscovite and rectorite, two clay minerals with very different interlayer structures. Their work raises a fundamental question regarding the mechanical definition of a single clay mineral. Furthermore, if such a definition is possible, is nanoindentation capable of assessing the mechanical behavior of a single clay mineral? Comparison of Zhang et al.'s results with previously published nanoindentation results on shale, and with results of other methods of assessing the mechanical behavior of single clay minerals, provides an avenue to address these questions. In a final analysis, it may be impossible to strictly define the mechanical behavior of a single clay mineral without the additional context of specific applications.