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 KT0 = 159(7) GPa and (dKT/dP)P=0 = K′ = 6.7(7) for single-crystal XRD; KT0 = 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 αV0 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.
© 2015 by Walter de Gruyter Berlin/Boston