J. R. Smyth, C. M. Holl,
H. M. S. Laustsen
Department of Geological Sciences, University of Colorado,
Boulder, CO 80309, USA
F. Langenhorst, C. A. McCammon
Bayerisches Geoinstitut, Universität Bayreuth,
D-95440 Bayreuth, Germany
G. R. Rossman
Division of Geological and Planetary Sciences, California
Institute of Technology, Pasadena, CA 91125, USA
A. Kleppe
Department of Earth Sciences, University of Oxford, Parks Road,
Oxford, OX1 3PR, UK
T. Kawamoto
Institute for Geothermal Sciences, Kyoto University,
874-0903 Beppu, Japan
P. A. van Aken
Institut für Angewandte Geowissenschaften, Technische
Universität Darmstadt, D-64287 Darmstadt, Germany
Wadsleyite II is a variably
hydrous magnesium-iron silicate phase similar to spinelloid IV
and a potential host for H in the Transition Zone of the Earth's
mantle. Two separate samples of wadsleyite II synthesized at
17.5 GPa and 1400°C and at 18 GPa and 1350°C have
been characterized by electron microprobe, single-crystal X-ray
diffraction, visible, IR, Raman, and Mössbauer spectroscopies,
and transmission electron microscopy including electron
energy-loss spectroscopy. The two samples have the following
chemical formulae: Mg1.71Fe0.18Al0.01H0.33
Si0.96O4 and Mg1.60Fe0.22Al0.01
H0.44Si0.97O4. Mössbauer
spectroscopy and electron energy loss spectroscopy (EELS)
indicate that about half of the iron present is ferric.
Refinement of the structures shows them to be essentially the
same as spinelloid IV. Calculated X-ray powder diffraction
patterns show only subtle differences between wadsleyite and
wadsleyite II. The hydration mechanism appears to be protonation
of the non-silicate oxygen (O2) and possibly the oxygens
surrounding the partially vacant tetrahedral site Si2,
charge-balanced by cation vacancies in Si2, M5 and M6. The unit
cell volume of this phase and its synthesis conditions indicate
that it may be an intermediate phase occurring between the fields
of wadsleyite and ringwoodite, if sufficient trivalent cations
are available. The unit cell parameters have been refined at
pressures up to 10.6 GPa by single-crystal X-ray diffraction
in the diamond anvil cell. The refined bulk modulus for the
sample containing 2.8 wt% H2O is 145.6 ±
2.8 GPa with a K' of 6.1 ± 0.7. Similar to wadsleyite and
ringwoodite, hydration has a large effect on the bulk modulus.
The presence of this phase in the mantle could serve to obscure
the seismic expression of the phase boundary between wadsleyite
and ringwoodite near 525 km. The large apparent effect of
hydration on bulk modulus is consistent with hydration having a
larger effect on seismic velocities than temperature in the
Transition Zone.
