3.3 Xenon in the Earth’s lower mantle

Table 5.3 shows the representative chemical reactions involving xenon oxides and silicates in the Earth’s lower mantle at 100 GPa. Xe oxides are only stable above 83 GPa, i.e. at pressures corresponding to the lower mantle. Since in the Earth’s mantle metallic Fe should be present 167; 168, As the reference, Fe is in hcp and FeO is in the antiferromagnetic inverse NiAs structure 169; 170. All of the predicted xenon oxides are calculated to be very strong oxidants and they will oxidize Fe, producing iron oxide and free xenon (FeO + Xe). Therefore, Xe oxides cannot be present in the lower mantle, where free Fe should exist.

Table 5.3: Representative chemical reactions involving xenon oxides and silicates in Earth’s lower mantle at 100 GPa.

Reaction

   

$\Delta $H [eV]

$\Delta $V [$^3$]

FeO+Xe

$\rightarrow $

XeO+Fe

2.170

-1.35

FeO+Xe+1/2 O$_2$

$\rightarrow $

XeO$_2$+Fe

2.203

-3.09

Fe+2Xe+SiO$_2$

$\rightarrow $

FeSi+2XeO

8.540

-1.88

Fe+Xe+SiO$_2$

$\rightarrow $

FeSi+XeO$_2$

8.687

-2.03

4XeO + 2SiO$_2$

$\rightarrow $

2Xe$_2$SiO$_4$

0.910

-0.61

4XeO + 2MgSiO$_3$

$\rightarrow $

2Xe$_2$SiO$_4$ + 2MgO

1.490

0.27

4XeO + 2CaSiO$_3$

$\rightarrow $

2Xe$_2$SiO$_4$ + 2CaO

4.409

-0.33

Xe+SiO$_2$

$\rightarrow $

Si+XeO$_2$

12.205

0.33

2Xe+SiO$_2$

$\rightarrow $

Si+2XeO

12.057

0.48

Since xenon oxides are not stable in coexistence with metallic Fe, we investigated the formation of stable xenon silicates under pressure, focusing on XeSiO$_3$ and Xe$_2$SiO$_4$, which contain the least oxidized divalent xenon. All of the investigated compositions were unstable towards decomposition into XeO, XeO$_2$, SiO$_2$ and the elemental Xe; Xe$_2$SiO$_4$ (Fig. 5.5) proved to be one of the least unstable silicates, but still is unstable. In this structure, Xe atoms tend to terminate the silicate perovskite layers, suggesting that xenon could also be stored in perovskite/post-perovskite defects, or be incorporated in microscopic defects, such as stacking faults.

\includegraphics[scale=0.6]{chapter5/pdf/fig4.png}
Figure 5.5: Crystal structure of Cmc2$_1$ Xe$_2$SiO$_4$.