Stishovite and CaCl 2 -Type Silica

When compressing pure SiO₂ along a typical adiabatic geotherm, coesite would transform to stishovite at about 10 GPa and approximately 1800 K (Hemley et al., 1994; Akaogi et al., 1995; Zhang et al., 1996). Seifertite, a high-pressure SiO₂ polymorph with the α-PbO₂ crystal structure (El Goresy et al., 2008), becomes stable at pressures and temperatures in excess of 120 GPa and 2500 K (Murakami et al., 2003; Tsuchiya et al., 2004; Grocholski et al., 2013). In a rock of basaltic (MORB) composition, the volume fraction of stishovite increases from 0 to about 10 vol-% between 10 and 15 GPa at the expense of clinopyroxene (Irifune et al., 1986; Irifune and Ringwood, 1987). The breakdown of garnet gives rise to a second pulse of stishovite formation between 20 and 30 GPa that raises the volume fraction of stishovite to 20 vol-% (Irifune and Ringwood, 1993; Ono et al., 2001; Perrillat et al., 2006; see also Fig. 1.2c). At higher pressures, stishovite might react with the NAL phase to form bridgmanite. This reaction would decrease the volume fraction of stishovite to about 15 vol-% at 50 GPa where the NAL phase disappears leaving the volume fraction of stishovite unchanged up to about 100 GPa (Perrillat et al., 2006; Ricolleau et al., 2010).

Hirose et al. (2005) found the incorporation of alumina into seifertite to stabilize seifertite at around 110 GPa and 2500 K, i. e. at lower pressures than in the pure silica system.

Figure 1.4: Crystal structure of stishovite (P4₂/mnm, a) and CaCl₂-type SiO₂ (Pnnm, b) viewed along the c axis; drawn from own unpublished structural data. Si occupies the center of each octahedron formed by oxygen atoms on the corners. Note how the phase transition from stishovite to CaCl₂-type SiO₂shears the red square ina) into a rhomb inb).

Stishovite crystallizes in the rutile structure type with tetragonal symmetry and space groupP4₂/mnm(Stishov and Belov, 1962; Sinclair and Ringwood, 1978). In contrast to its adjacent low-pressure polymorph coesite, stishovite hosts silicon in octahedral coordination.

[SiO6]⁸⁻ octahedra share two opposing edges with neighboring octahedra to form chains along thecaxis. Neighboring chains are rotated by 90^◦and displaced along thecaxis with respect to each other to link via corner sharing. The alignment of edge-sharing chains along thecaxis results in a strong elastic and compressional anisotropy (Ross et al., 1990; Andrault et al., 2003; Jiang et al., 2009; see also Fig. 3.6 on page 61). Complete elastic stiffness tensors of stishovite have been determined by Weidner et al. (1982) at ambient conditions, by Brazhkin et al. (2005) up to a temperature of about 800 K at ambient pressure, and by Jiang et al. (2009) up to 12 GPa at ambient temperature. The results of numerous studies on the equation of state of stishovite have recently been compiled by Fischer et al. (2018).

When crystallized in basaltic rocks at high pressures, stishovite was found to contain up to several weight percent Al₂O₃ (Irifune and Ringwood, 1993; Kesson et al., 1994; Hirose et al., 1999). In addition to aluminum, Pawley et al. (1993) demonstrated the presence of structurally bonded hydroxyl groups in stishovite and proposed coupled substitutions with Si⁴⁺ being replaced by Al³⁺ charge balanced either by H⁺or by oxygen vacancies. These in-corporation mechanisms of aluminum and hydrogen were confirmed by later studies (Smyth et al., 1995; Bromiley et al., 2006; Frigo et al., 2018). The alumina content in stishovite was found to be sensitive to pressure and temperature (Ono, 1999; Liu et al., 2006) and potentially increases upon partial melting of a basaltic rock at pressures of the lower man-tle (Panero et al., 2003) raising the potential of stishovite to retain H₂O (Chung and Kagi, 2002; Panero et al., 2003). The coupled substitution of Si⁴⁺by Al³⁺and H⁺was also studied by first-principle calculations (Gibbs et al., 2004; Panero and Stixrude, 2004) suggesting a solubility of 0.3 wt-% H₂O in stishovite at 25 GPa and 1500 K (Panero and Stixrude, 2004).

A similar H₂O solubility was found experimentally by Litasov et al. (2007). Substantially

1.3 Mantle Minerals at Focus

higher water contents in excess of 1 wt-% H₂O were found in Al-free stishovite crystallized from silica glass or coesite at pressures of 10 GPa and temperatures below 820 K in the pres-ence of free H₂O (Spektor et al., 2011; Spektor et al., 2016). These very hydrous stishovites incorporate H₂O by a mechanism similar to the hydrogarnet substitution, i. e. 4 H⁺replace Si⁴⁺ (Ackermann et al., 1983; Spektor et al., 2011; Spektor et al., 2016).

In addition to the potential to transport or retain H₂O in Earth’s lower mantle, the stishovite crystal structure provides another geophysically interesting feature. With increas-ing pressure, theB_1goptical vibrational mode softens (Kingma et al., 1995) and couples with acoustic modes that involve shear motions in thea-bplane (Hemley et al., 2000; Carpenter et al., 2000; and references therein). The resulting shear instability gives rise to a second-order phase transition around 50 GPa at ambient temperature that involves a reduction in symmetry to P nnm and a structural distortion to a CaCl₂-type structure (Kingma et al., 1995; Karki et al., 1997b; Andrault et al., 1998). Figure 1.4 compares the crystal struc-tures of stishovite and CaCl₂-type SiO₂. On the atomic level, the chains of octahedra rotate around thecaxis by a few degrees with a concomitant change of interatomic distances (An-drault et al., 1998). As the phase transition from stishovite to CaCl₂-type SiO₂ involves a spontaneous shear strain in thea-bplane, stishovite becomes infinitely soft with respect to shear stress in this plane, i. e. (c₁₁−c₁₂)→0, as the phase transition is approached (Karki et al., 1997a; Carpenter et al., 2000). The elastic softening due to the ferroelastic phase transition is predicted to substantially reduce the sound wave velocities of polycrystalline silica aggregates as well as to increase the elastic anisotropy of silica single crystals (Karki et al., 1997a; Carpenter et al., 2000; Yang and Wu, 2014).

Direct measurements of sound wave velocities across the stishovite–CaCl₂-type SiO₂ phase transition, however, have either been performed on polycrystalline silica compressed under nonhydrostatic stress conditions (Asahara et al., 2013) or were limited to a single direction in a hydrous Al-bearing stishovite single crystal (Lakshtanov et al., 2007). Laksh-tanov et al. (2007) found that hydrous Al-bearing stishovite transforms to the CaCl₂-type phase at substantially lower pressures than pure SiO₂. A similar reduction of the transition pressure has been observed for Al-bearing stishovite (Bolfan-Casanova et al., 2009) and hy-drous Al-free stishovite (Nisr et al., 2017). Nonhydrostatic stresses were shown to reduce the transition pressure as well (Singh et al., 2012; Asahara et al., 2013). As an alternative to direct sound wave velocity measurements, the changes in elastic properties of stishovite and CaCl₂-type SiO₂ can be evaluated based on the evolution of unit cell parameters across the phase transition using Landau theory (Carpenter and Salje, 1998; Carpenter et al., 2000).

In chapter 7, we apply this approach to compression data of sintered polycrystalline silica across the stishovite–CaCl₂-type SiO₂ phase transition and show, by comparison to earlier studies on silica powder (Andrault et al., 2003), that sintered polycrystalline silica behaves differently. Our results emphasize how the strong elastic anisotropy of stishovite may affect the elastic response of a sintered polycrystalline aggregate that to some extend resembles a real rock. Combining the effects of chemical composition and temperature (Nomura et al., 2010; Yamazaki et al., 2014; Fischer et al., 2018) on the ferroelastic phase transition with an accurate model for the elastic properties of silica may turn silica-bearing rocks, such as subducted oceanic crust, into sensors for the thermal and compositional structure of Earth’s lower mantle.

Chapter 2

Experimental

This study focuses on the characterization of elastic properties of minerals at high pressures, and only those aspects of synthesis and characterization with a relevant contribution of the author will be included here. Moreover, to separate the results of synthesis experiments and chemical characterization from those of experiments aiming to determine the elastic properties at high pressures, I will include results of synthesis and characterization in this chapter, in particular where they aid to illustrate synthesis and characterization approaches.

A brief introduction to elastic properties of minerals at high pressures is included to provide the theoretical background for the analysis of experimental results.