Compressibility and thermal expansion of hydrous ringwoodite with 2.5(3) wt% H2O

Ringwoodite (γ-Mg ₂ SiO ₄ ) is the stable polymorph of olivine in the transition zone between 525-660 km depth, and can incorporate weight percent amounts of H ₂ O as hydroxyl, with charge compensated mainly by Mg vacancies (Mg ²⁺ = 2H ⁺ ), but also possibly as (Si ⁴⁺ = 4H ⁺ and Mg ²⁺ + 2H ⁺ = Si ⁴⁺ ). We synthesized pure Mg ringwoodite containing 2.5(3) wt% H2O, measured by secondary ion mass spectrometry (SIMS), and determined its compressibility at 300 K by single-crystal and powder X-ray diffraction (XRD), as well as its thermal expansion behavior between 140 and 740 K at room pressure. A third-order Birch-Murnaghan equation of state (BM3 EOS) fits values of the isothermal bulk modulus K _T0 = 159(7) GPa and (dKT/dP) _{P = 0} = K' = 6.7(7) for single-crystal XRD; K _T0 = 161(4) GPa and K' = 5.4(6) for powder XRD, with KT0 = 160(2) GPa and K' = 6.2(3) for the combined data sets. At room pressure, hydrous ringwoodite breaks down by an irreversible unit-cell expansion above 586 K, which may be related to dehydration and changes in the disorder mechanisms. Single-crystal intensity data were collected at various temperatures up to 736 K, and show that the cell volume V(cell) has a mean thermal expansion coefficient α _V0 of 40(4) ×10 ^-6 /K (143-736 K), and 29(2) ×10 ^-6 /K (143-586 K before irreversible expansion). V(Mg) have α ₀ values of 41(3) ×10 ^-6 /K (143-736 K), and V(Si) has α ₀ values of 20(3) ×10 ^-6 /K (143-586 K) and 132(4) ×10 ^-6 K (586-736 K). Based on the experimental data and previous work from ²⁹ Si NMR, we propose that during the irreversible expansion, a small amount of H ⁺ cations in Mg sites transfer to Si sites without changing the cubic spinel structure of ringwoodite, and the substituted Si ⁴⁺ cations move to the normally vacant octahedral site at (1/2, 1/2, 0). Including new SIMS data on this and several Mg-ringwoodite samples from previous studies, we summarize volume-hydration data and show that the Mg ²⁺ = 2H ⁺ dominates up to about 2 wt% H ₂ O, where a discontinuity in the volume vs. H ₂ O content trend suggests that other hydration mechanisms become important at very high H ₂ O contents.