The effect of cation order on the elasticity of omphacite from atomistic calculations
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Omphacite, a clinopyroxene mineral with two distinct crystallographic sites, M1 and M2, and composition intermediate between diopside and jadeite, is abundant throughout the Earth’s upper mantle and is the dominant mineral in subducted oceanic crust. Unlike the end-members, omphacite exists in two distinct phases, a P2/n ordered phase at low temperature and a high-temperature C2/c disordered phase. The crystal structure and full elastic constants tensor of ordered P2/n omphacite have been calculated to 15 GPa using plane-wave density functional theory. Our results show that several of the elastic constants, notably C 11, C 12, and C 13 deviate from linear mixing between diopside and jadeite. The anisotropy of omphacite decreases with increasing pressure, and at 10 GPa, is lower than that of either diopside or jadeite. The effect of cation disorder is investigated through force-field calculations of the elastic constants of special quasi-random structures supercells with simulated disorder over the M2 sites only, and over both cation sites. These show that cation order influences the elasticity, with some components displaying particular sensitivity to order on a specific cation site. C 11, C 12, and C 66 are sensitive to disorder on M1, while C 22 is softened substantially by disorder on M2, but insensitive to disorder on M1. This shows that the elasticity of omphacite is sensitive to the degree of disorder, and hence the temperature. We expect these results to be relevant to other minerals with order–disorder phase transitions, implying that care must be taken when considering the effects of composition on seismic anisotropy.
KeywordsElasticity Omphacite Cation order Special quasi-random structures Density functional theory
AMW is supported by a fellowship from the Natural Environment Research Council (Grant No. NE/K008803/1). Calculations were performed on the Terrawulf cluster, a computational facility supported through the AuScope initiative. AuScope Ltd. is funded under the National Collaborative Research Infrastructure Strategy (NCRIS), an Australian Commonwealth Government Programme. Ian Jackson and two anonymous reviewers are thanked for their helpful comments.
- Boffa Ballaran T, Carpenter MA, Domeneghetti MC, Tazzoli V (1998) Structural mechanisms of solid solution and cation ordering in augite–jadeite pyroxenes: I. A macroscopic perspective. Am Mineral 83:419–433Google Scholar
- Hazen RM, Yang H (1999) Effects of cation substitution and order-disorder on P–V–T equations of state of cubic spinels. Am Mineral 84:1956–1960Google Scholar
- Herzberg C (1995) Phase equilibria of common rocks in the crust and mantle. In: Ahrens TJ (ed) AGU Ref. Shelf. American Geophysical Union, Washington, pp 166–177Google Scholar
- Levien L, Prewitt CT (1981) High-pressure structural study of diopside. Am Mineral 66:315–323Google Scholar
- Mainprice D, Bascou J, Cordier P, Tommasi A (2004) Crystal preferred orientations of garnet: comparison between numerical simulations and electron back-scattered diffraction (EBSD) measurements in naturally deformed eclogites. J Struct Geol 26:2089–2102. doi: 10.1016/j.jsg.2004.04.008 CrossRefGoogle Scholar
- Matsui M, Busing WR (1984) Calculation of the elastic constants and high-pressure properties of diopside, CaMgSi2O6. Am Mineral 69:1090–1095Google Scholar
- Perdew JP (1991) Unified theory of exchange and correlation beyond the local density approximation. In: Ziesche P, Eschrig H (eds) Electronic structure of solids '91. Akademie Verlag, Berlin, pp 11–20Google Scholar
- Vanderbilt D (1998) First-principles theory of structural phase transitions in cubic perovskites. J Korean Phys Soc 32:S103–S106Google Scholar
- Vinograd VL, Sluiter MHF, Winkler B, Putnis A, Gale JD (2004) Thermodynamics of mixing and ordering in silicates and oxides from static lattice energy and ab initio calculations. In: Warren M, Oganov A, Winkler B (eds) First-principles simulations: perspectives and challenges in mineral sciences (Deutsche Gesellschaft fur Kristallographie. Berichte aus Arbeitskreisen der DFK) 14, pp 143–151Google Scholar