Wadsleyite [b-(Mg,Fe)₂SiO₄]

theoretical structure
Smyth (1994) [Mg₇(Si₄O₁₄[OH]₂)]

Structure Orthorhombic
Space Group : Pmmb (No. 51)
a=5.6921 Å, b=11.4600 Å, c=8.2530 Å
a=b=g=90.00
Z=2

Atomic Positional Parameters

H1  0.0000 0.2500 0.3459
H2  0.5000 0.7500 0.5915
Mg1 0.0000 0.0000 0.0000
Mg2 0.5000 0.5000 0.5000
Mg3 0.0000 0.2500 0.9701

Mg4 0.2500 0.1264 0.2500
O1  0.0000 0.2500 0.2187
O2  0.5000 0.7500 0.7187
O3  0.0000 0.2500 0.7168
O4  0.5000 0.7500 0.2168

O5  0.0000 0.9895 0.2556
O6  0.5000 0.4895 0.7556
O7  0.2698 0.1229 0.9929
O8  0.7698 0.6229 0.4929
Si1 0.0000 0.1201 0.6166

Si2 0.5000 0.6201 0.1166

References
H Horiuchi and H Sawamoto (1981) b-(Mg,Fe)₂SiO₄: Single crystal X-ray diffraction study. American Mineralogist, 66, 568-575

GD Price, A Putnis, SO Agrell and DGW Smith (1983) Wadsleyite, natural b-(Mg,Fe)₂SiO₄ from the Peace River meteorite. Canadian Mineralogist, 21, 29-35

JR Smyth (1987) b-Mg₂SiO₄: A potential host for water in the mantle? American Mineralogist, 72, 1051-1055

JR Smyth (1994) A crystallographic model for hydrous wadsleyite: An ocean in the Earth's Interior? American Mineralogist, 79, 1021-1024

Mineral Chemistry
Studies of phase transformations in the Mg₂SiO₄-Fe₂SiO₄ system were initiated to simulate behaviour of the deep earth. Olivine (a-Mg₂SiO₄) transforms to b-Mg₂SiO₄ and then to spinel-structured g-Mg₂SiO₄ with increasing pressure. Horiuchi and Sawamoto (1981) reported the crystal structure of synthetic b-Mg₂SiO₄, and a new mineral from the Peace River meteorite, wadsleyite, was described by Price et al. (1993). This mineral is natural b-(Mg,Fe)₂SiO₄ and was formed during an extraterrestrial shock event.

Wadsleyite is stable at pressures which are found in the upper part of the transition zone of the Earth's mantle, and reports of wadsleyite containing high concentrations of H led Smyth (1987) to propose this mineral as a potential storage site for water in the mantle. Si₂O₇ groups in wadsleyite result in oxygens not bound to silicon; these oxygens may be easily hydrated. Estimates show that if all wadsleyite was fully hydrated the volume of water contained in the mantle would be greater than that of Earth's oceans.

The CHIME figure shows a theoretical structure for wadsleyite proposed by Smyth (1994) based on the crystal structure reported by Horiuchi and Sawamoto (1981). Magnesiums are represented as green, silicons as ochre, oxygens as red, and hydrogens as white spheres. Note the Si₂O₇ groups, and the location of hydrogen in the structure.