Abstract
Abstract. Olivine, a ferromagnesian orthosilicate, is the most abundant mineral in Earth's upper mantle and is stable down to the olivine–wadsleyite phase transition, which defines the 410 km depth mantle transition zone. Olivine also occurs in crustal environments in metamorphic and hydrothermal rocks and is expected to be the major mineral constituent of the Martian and Venusian mantles. The olivine atomic structure is also used in materials science to manufacture lithium batteries. Like any other crystalline solid, including minerals, olivine never occurs with a perfect crystalline structure: defects in various dimensions are ubiquitous, from point, line, and planar defects to three-dimensional (3-D) inclusions. In this contribution, I review the current state of the art of defects in olivine and several implications for key processes occurring in Earth's mantle. Intrinsic and extrinsic point defects are detailed, exemplifying the astonishing diversity of atomic impurities in mantle-derived olivine. Linear defects, one of the key defect types responsible for ductile deformation in crystalline solids, are examined in light of recent progress in 3-D transmission electron microscopy, which has revealed an important diversity of dislocation slip systems. I summarize the principal characteristics of interface defects in olivine: the free surface, grain and interface boundaries, and internal planar defects. As the least-studied defects to date, interface defects represent an important challenge for future studies and are the main application of numerical simulation methods in materials science. I provide an overview of melt, fluid, and mineral inclusions, which are widely studied in volcanology and igneous petrology. Special attention is given to new crystalline defects that act as deformation agents: disclinations (rotational defects) and the potential occurrence of disconnections in olivine, both of which are expected to occur along or near grain boundaries. Finally, I detail outstanding questions and research directions that will further our understanding of the crystalline specificities and paradoxes of olivine and olivine-rich rocks and ultimately their implications for the dynamics of Earth's upper mantle.
Highlights
Minerals with perfect crystalline structures do not exist at temperatures above 0 K
One of the primary limitations is the paucity of accurate empirical interatomic potentials used in molecular dynamics calculations and pseudo-potentials and interatomic potentials used in ab initio calculations for silicates (e.g., Mott and Littleton, 1938; Catlow and Mackrodt, 1982; Sanders et al, 1984; Lewis and Catlow, 1985; Eastwood et al, 1980; Vanderbilt, 1990; Kresse and Hafner, 1994)
Ions that are theoretically too large or small for a given lattice site radius can still be incorporated, in smaller quantities. These rules ignore pre-existing intrinsic defects, i.e., defect interactions controlled by electroneutrality, and imply that minerals reached equilibrium with their host melt; they are not appropriate for mineral–mineral partitioning in melt-free systems, in which energy minimization occurs after thermodynamics changes to the system
Summary
Minerals with perfect crystalline structures do not exist at temperatures above 0 K. Other studies focus on physical processes and/or properties involving these defects in Earth minerals, for example ionic diffusion (e.g., Zhang and Cherniak, 2010), plastic deformation (e.g., Poirier, 1976, 1985; Karato and Wenk, 2002), electrical conductivity (e.g., Shankland, 1975; Yoshino et al, 2012), or metasomatic changes in the chemical composition of olivine (e.g., O’Reilly et al, 1997; Foley et al, 2013; O’Reilly and Griffin, 2013; Demouchy and Alard, 2021) These processes and properties notably involve hydrogen, by far the most popular atomic impurity in olivine (e.g., Beran and Zemann, 1969; Beran and Putnis, 1983; Kohlstedt et al, 1996; Mosenfelder et al, 2006; Férot and Bolfan-Casanova, 2012; Demouchy and Bolfan-Casanova, 2016; Blanchard et al, 2017; Demouchy and Alard, 2021). Recently observed defects and those expected to occur, not yet observed in olivine, are discussed: disclinations and disconnections, respectively
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