DOE OSTI2016
Ringwoodite ({gamma}-Mg{sub 2}SiO{sub 4}) is the stable polymorph of olivine in the transition zone between 525-660 km depth, and can incorporate weight percent amounts of H{sub 2}O as hydroxyl, with charge compensated mainly by Mg vacancies (Mg{sup 2+} = 2H{sup +}), but also possibly as (Si{sup 4+} = 4H{sup +} and Mg{sup 2+} + 2H{sup +} = Si{sup 4+}). We synthesized pure Mg ringwoodite containing 2.5(3) wt% H{sub 2}O, 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{sub T0} = 159(7) GPa and (dK{sub T}/dP){sub P = 0} = K' = 6.7(7) for single-crystal XRD; K{sub T0} = 161(4) GPa and K' = 5.4(6) for powder XRD, with K{sub 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 {alpha}{sub V0} of 40(4) x 10{sup -6}/K (143-736 K), and 29(2) x 10{sup -6}/K (143-586 K before irreversible expansion). V(Mg) have {alpha}{sub 0} values of 41(3) x 10{sup -6}/K (143-736 K), and V(Si) has {alpha}{sub 0} values of 20(3) x 10{sup -6}/K (143-586 K) and 132(4) x 10{sup -6}K (586-736 K). Based on the experimental data and previous work from {sup 29}Si NMR, we propose that during the irreversible expansion, a small amount of H{sup +} cations in Mg sites transfer to Si sites without changing the cubic spinel structure of ringwoodite, and the substituted Si{sup 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{sup 2+} = 2H{sup +} dominates up to about 2 wt% H{sub 2}O, where a discontinuity in the volume vs. H{sub 2}O content trend suggests that other hydration mechanisms become important at very high H{sub 2}O contents.