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

Abstract

Ringwoodite (γ-Mg₂SiO₄) is the stable polymorph of olivine in the transition zone between 525-660 km depth, and can incorporate weight percent amounts of H₂O as hydroxyl, with charge compensated mainly by Mg vacancies (Mg²⁺ = 2H⁺), but also possibly as (Si⁴⁺ = 4H⁺ and Mg²⁺ + 2H⁺ = Si⁴⁺). 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 K_T0 = 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 α₀ values of 41(3) ×10⁻⁶/K (143-736 K), and V(Si) has α₀ values of 20(3) ×10⁻⁶/K (143-586 K) and 132(4) ×10⁻⁶K (586-736 K). Based on the experimental data and previous work from ²⁹Si 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 Si⁴⁺ cations move to the normally vacant octahedral site at (½, ½, 0). Including new SIMS data on this and several Mg-ringwoodite samples from previous studies, we summarize volume-hydration data and show that the Mg²⁺ = 2H⁺ dominates up to about 2 wt% H₂O, where a discontinuity in the volume vs. H₂O content trend suggests that other hydration mechanisms become important at very high H₂O contents.