Photon Factory Activity Report 2004 #22 Part B (2005)
Crystallography

10A/2001G037

Effect of hydrogen on the density and compressibility of hydrous ringwoodite
Yasuhiro KUDOH*1, Takahiro KURIBAYASHI1, Hiroki MIZOBATA1,Eiji OHTANI1, Satoshi SASAKI2 and Masahiko TANAKA**3
1

Institute of Mineralogy, Petrology, and Economic Geology, Faculty of Science,
Tohoku University, Sendai 980-8578, Japan
2Materials and Structures Laboratory, Tokyo Institute of Technology Nagatuda,
Yokohama 226-8503, Japan
3
Institute of Materials Structure Science, High Energy Accelerator Reasearch Organization,Oho, Tukuba, 305-0801, JapanWith a single crystal of 35x35x24 micron hydrous
ringwoodite Mg1.97SiH0.06O4, synthesized at conditions of1680 °C and 22 GPa, sets of X-ray intensities weremeasured up to 7.9 GPa with the modified Merrill-Bassett
type diamond anvil pressure cell [1], using synchrotron
radiation at the beam line BL-10A, Photon Factory, HighEnergy Accelerator Reasearch Organization, Tukuba,
Japan. The calculated isothermal bulk modulus using theunit cell volumes at ambient pressure, 3.2 GPa, 5.0 GPa,
6.2GPa and 7.9 GPa with the third-order BirchMurnaghan equation of state assuming K’=4 was
K0=184(6) GPa. The results of structural analyses up to7.9 GPa indicate that the compressibility of the unit cell isclose to the compressibility of the MO6 octahedron. The
mean Si-O distance stay almost constant up to 7.9 GPa.
This is considered to be due to the fact that the ratio of(M-O)/(T-O) in the silicate spinel is larger than the ideal
value of the closest packing of oxygen atoms. With
increasing pressure, the ratio of (M-O)/(T-O) decreases
and approaches to the ideal value. The Mg/Si ratio versusH2O content diagram for hydrous ringwoodite [2][3][4]indicate the existence of Si↔4H substitution inringwoodite together with Mg↔2H substitution. This isconsidered to be due to the fact that the substitution ofSi↔4H has less effect on the reduction of density than
the substitution of 2Mg↔4H because the molecularweight of SiO2 (=60) is smaller than the molecular weightof 2MgO (=80). The Si↔4H substitution is favorable athigh pressure. The partial disorder of Mg-Si found fromthe crystal structure analysis of hydrous ringwoodite, γMg1.89Si0.98H0.30O4 with 2.0 wt% H2O [5] may reduce thenumber of vacancies at the tetrahedral site. The bulkmodulus versus number of Mg atoms diagram (Fig. 1)
indicates that the bulk modulus of hydrous ringwoodite isinversely proportional to the number of Mg atoms.
Incorporation of hydrogen in the ringwoodite structure
has more effect on the bulk modulus than on the density(20% decrease of bulk modulus and 4 % decrease ofdensity with 2.8 wt%H2O [10]).
**Now at WEBRAM, National Institute for Materials
Science, Spring 8, Hyogo 679-5198, Japan

Fig. 1. Bulk modulus (K0) versus number of Mg atoms
for ringwoodite. 1; Weidner et al. (1984)[6]. 2; Hazen(1993)[7]. 3; Meng et al. (1994)[8]. 4; This study. 5;
Inoue et al. (1998)[9]. 6; Yusa et al. (2000)[10]References[1] Y. Kudoh, H. Takeda, Physica 139&140 B, 333(1986)[2] T. Inoue, H. Yurimoto, Y. Kudoh, Geophys. Res. Lett.,
22, 117 (1995)[3] E. Ohtani, H. Mizobata, Hyurimoto, Phys. Chem.
Minerals, 27, 533 (2000)[4] Y. Kudoh, Phys. Chem. Minerals, 28, 523 (2001)[5] Y. Kudoh, T. Kuribayashi, H. Mizobata, E. Ohtani,
Phys. Chem. Minerals, 27, 474 (2000)[6] D.J. weidner, H. Sawamoto, S. Ssasaki, M.
Kumazawa, J. Geophys. Res., 89, 7852 (1984)[7] R.M. Hazen, Science, 259, 206 (1993)[8] Y. Meng, Y. Fei, D.J. Weidner, G.D. Gwanmesia, J.
Hu, Phys. Chem. Minerals, 21, 407 (1994)[9] T. Inoue, D.J. Weidner, P.A. Northrup, J.B. Parise,Earth. Planet. Lett., 160, 107 (1998)[10] H. Yusa, T. Inoue, Y. Ohishi, Geophys. Res. Lett.,
27, 413 (2000)*ykudoh@mail.tains.tohoku.ac.jp

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