Mineralogical application of nanoindentation testing

The study of elastic properties of rocks and minerals had a significant development because these data are crucial for the interpretation of the propagation of seismic waves on which all models of Earth's internal structure are based. In addition, these properties are essential for determining the equation of state of geological materials [1,2]. Other physical properties related to elasticity, such as hardness, fracture toughness, contact stiffness, creep resistance, can be used to understand the mechanisms of ductile deformation of rocks at the level of the mantle or deep crust, and of the brittle behaviour of rocks at the level of upper crust. Finally, since the elastic properties are closely related to the crystal-chemistry of minerals, they can be used to probe the structural deformations induced by phase transitions and/or ionic substitutions [3,4]. The measurement of the elastic properties of minerals is essentially based on three methods: 1) direct mechanical methods; 2) sound wave velocity measurements and 3) electromagnetic wave scattering methods (Brillouin, inelastic X-ray/neutron). All of these methods are now well established, mature and widely used however they have an inherent limitation: they do not allows the measurement of elastic properties of materials at micro to nano scale [5]. Nanoindentation testing (introduced by Oliver and Pharr in 1992 [6]) has been widely adopted in the last two decades for the surface mechanical characterization of bulk materials and coatings. The method involves the controlled penetration of a diamond pyramidal indenter into the material: by measuring the load and displacement during the loading and unloading parts of the test, hardness (i.e. resistance to plastic deformation) and elastic modulus can be calculated [6-7]. In this way, a very accurate characterisation of the elastic properties at material’s surface can be achieved, with a depth resolution and a lateral spatial resolution of the order of few nanometres. Three factors motivate the use of nanoindentation for the study of the mechanical properties of minerals. Firstly, in such tests the load and displacement of the indenter tip, are continuously monitored thus the method is ideal for probing local gradients and heterogeneities in samples. Second, no extensive sample preparation is required prior to mechanical testing. Third, most nanoindentation instruments provide experimental control that allows for a variety of different deformation modes. The data so far published are scarse and cover a limited number of minerals (some phyllosilicate, kyanite, K-feldspar, periclase, garnet, quartz and the first nine minerals of the Mohs scale) [8,9,10,11]. However, the results show the possibility of very interesting developments in the field of mineralogy, applied mineralogy and gemmology. [1] J. Schreuer, S. Huassühl, EMU Notes in Mineralogy 7 (2005) 95-116. [2] G.D. Price, Mineral Physics: Treatise On Geophysics. Elsevier (2009) 656 pp. [3] M.A. Carpenter, E.K.H. Salje, Eur. J. Mineral. 10 (1998) 693-812. [4] W. van Westrenen, J. Blundy, B. Wood, Am. Mineral. 84 (1999) 838-847. [5] R.J. Angel, J.M. Jackson, H.J. Reichmann, S. Speziale, Eur. J. Mineral. 21 (2009) 525-550. [6] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564-1583. [7] C.A. Schuh, Materials Today 9 (2006) 32-40. [8] M.E. Broz, R.F. Cook, D.L. Whitney, Am. Mineral. 91 (2006) 135-142. [9] D.L. Whitney, M.E. Broz, R.F. Cook, Am. Mineral. 92 (2007) 281-288. [10] A. Mikowski, P. Soares, F. Wypych, C.M. Lepienski, Am. Mineral. 93 (2008) 844-852. [11] G. Zhang, Z. Wei, R.E. Ferrell, S. Guggenheim, R.T. Cygan, J. Luo, Am. Mineral. 95 (2010) 863-869.