The Effect of Fe‐Al Substitution on the Crystal Structure of MgSiO₃ Bridgmanite

Abstract

 Fe 3+ and Al mainly occupy the A and B sites respectively at 25 GPa when Fe 3+ = Al but disordering may be favored at higher pressures. Pressure favors charge-coupled substitution over oxygen vacancy formation and may suppress Fe metal precipitation.

Pressure Destabilizes Oxygen Vacancies in Bridgmanite

“…(2021). This is understandable because the molar volume of hematite (30.5 cm 3 /mol) is slightly larger than the FeFeO 3 component in bridgmanite (29.55 cm 3 /mol, Huang, Boffa‐Ballaran, McCammon, Miyajima, & Frost, 2021). Therefore, Fe 3+ may tend to be incorporated in bridgmanite by a MgSiO 3 ‐Fe 2 O 3 solid solution instead of forming hematite, consequently, the Fe 3+ solubility in bridgmanite is high.…”

“…On the other hand, in the case that bridgmanite coexists with MgAl 2 O 4 and MgO, the AlAlO 3 component in bridgmanite can be formed by, $$${\text{MgAl}}_{2}{\mathrm{O}}_{4}\enspace (\text{CF}\mbox{-}\text{phase})={\text{AlAlO}}_{3}\enspace (\text{bridgmanite})+\text{MgO}\enspace (\text{periclase})$$$ which has a small, but positive volume change ( ΔV = +0.5 cm 3 /mol at ambient conditions) using the molar volume of each component reported previously (Huang, Boffa‐Ballaran, McCammon, Miyajima, & Frost, 2021; Kojitani, Hisatomi, & Akaogi, 2007; Liu, Akaogi, & Katsura, 2019; Sueda et al., 2009). Thus, the small but positive volume changes of reactions (5) and (6) suggest that the AlAlO 3 content in bridgmanite should slightly decrease or be nearly constant with increasing pressure, which contradicts the tendency reported by Liu et al.…”

“…The suppression of Fe 3+ ‐linked oxygen vacancies with increasing pressure can be understood by the volume increase associated with MgFeO 2.5 formation. Partial molar volume of the MgFeO 2.5 component in bridgmanite is estimated to be 27.65 cm 3 /mol (Huang, Boffa‐Ballaran, McCammon, Miyajima, & Frost, 2021), whereas the molar volume of the MgFe 2 O 4 ‐phase is 41.4 cm 3 /mol (Ishii et al., 2020), and that of MgO is 11.25 cm 3 /mol at ambient conditions (Dorogokupets, 2010; Tange et al., 2012). Thus, the volume change in the reaction, $$${\text{MgFe}}_{2}{\mathrm{O}}_{4}({\text{MgFe}}_{2}{\mathrm{O}}_{4}\mbox{-}\text{phase})+\text{MgO}\enspace (\text{periclase})=2{\text{MgFeO}}_{2.5}(\text{bridgmanite})$$$ is about +2.7 cm 3 /mol at ambient conditions.…”

“…The negative pressure dependence of MgFeO 2.5 content is identical to that of the MgAlO 2.5 content in the MgO‐SiO 2 ‐Al 2 O 3 system (Liu, Ishii, & Katsu, 2017), which can also be understood by the positive volume change of the reaction: $$${\text{MgAl}}_{2}{\mathrm{O}}_{4}\enspace (\text{CF}\mbox{-}\text{phase})+\text{MgO}\enspace (\text{periclase})=2{\text{MgAlO}}_{2.5}\enspace (\text{bridgmanite})$$$ in which MgAlO 2.5 and MgAl 2 O 4 have molar volume of 26.6 and 36.5 cm 3 /mol, respectively (Huang, Boffa‐Ballaran, McCammon, Miyajima, & Frost, 2021; Kojitani, Hisatomi, & Akaogi, 2007; Liu, Akaogi, & Katsura, 2019; Sueda et al., 2009).…”

“…Bridgmanite is stabilized in the pressure range 23–125 GPa (e.g., Ishii et al., 2018; Murakami et al., 2004) and is the dominant mineral in Earth (e.g., Irifune & Ringwood, 1987a, 1987b). It can incorporate large amounts of trivalent elements such as Al 3+ and Fe 3+ in its crystal structure (e.g., Andrault et al., 1998; McCammon, 1997; Navrotsky, 1999; Navrotsky et al., 2003; Shim et al., 2017) by the formation of XXO 3 and MgXO 2.5 components (X is Fe 3+ or Al 3+ ) via charge‐coupled and oxygen‐vacancy mechanisms, respectively (e.g., Huang, Boffa‐Ballaran, McCammon, Miyajima, Dolejš & Frost, 2021; Huang, Boffa‐Ballaran, McCammon, Miyajima, & Frost, 2021; Lauterbach et al., 2000; Liu, Akaogi, & Katsura, 2019; Liu, Ishii, & Katsura, 2017; Liu, Boffa‐Ballaran, et al., 2019; Liu et al., 2020; Navrotsky, 1999; Navrotsky et al., 2003; Nishio‐Hamane et al., 2005, 2008; O'Neill & Jeanloiz, 1994). The different substitution mechanisms thus produce different types of defect species.…”