| Olivine, wadsleyite and ringwoodite are the three phases of Mg2 SiO4, which dominate 60 ∼ 70 vol. % of mineralogy in the upper mantle and transition zone. Water may be transported to the transition zone or deeper in the forms of dense hydrous magnesium silicates, and also by hydrogen incorporated into nominally anhydrous silicate phases. Wadsleyite and ringwoodite in transition zone could contain up to ∼3 wt% H2 O, and these small amounts of H2O can have significant effects on the physical properties and seismological velocities. Single-crystal XRDs were conducted to refine unit-cell parameters and internal structures at temperatures and pressures to study effects of hydration on equation of state. Hydration significantly increases thermal expansion and compressibility. At Argonne National Laboratory, APS, we measured the compressibility of hydrous wadsleyite (2.8(5) wt% H2O) and hydrous ringwoodite (2.5(3) wt% H2O), and the 3rd-order B-M EOS fits the results: KT0 = 137(5) GPa, K&feet; = 4.6(3) for hydrous wadsleyite; KT0 = 160(2) GPa, K' = 6.2(3) for hydrous ringwoodite. For both wadsleyite and ringwoodite, one wt% increasing in water content would decrease the isothermal modulus (KT0) by about 12 GPa, while increase K'. Dehydration phenomena of hydrous wadsleyite were observed at about 650 K, and after dehydration, M2 -- O1 and M3 -- O1 bonds' lengths decreased by 3 % and 2.5 %, respectively. Irreversible expansion phenomena were observed at high temperatures for hydrous ringwoodite samples: a small mount of H+ cations in Mg sites appear to transfer to Si sites without changing the spinel structure of ringwoodite, and the substituted Si4+ cations move to the normally vacant octahedral site at (½ , ½, 0). SiO4 tetrahedral volume for hydrous ringwoodite increases significantly during irreversible expansion, but does not change for hydrous wadsleyite during dehydration. In addition, thermal expansion study of aragonite-group carbonate indicate that aragonite, strontianite and witherite have very similar &agr;0(V) values, whereas that of cerussite is significant larger. |
