Wadsleyite - Structure

Structure

Wadsleyite is a spinelloid, and the structure is based on a distorted cubic-closest packing of oxygen atoms as are the spinels. The a-axis and the b-axis is the half diagonal of the spinel unit. The magnesium and the silicon are completely ordered in the structure. There are three distinct octahedral sites, M1, M2, and M3, and a single tetrahedral site. Wadsleyite is a sorosilicate in which Si₂O₇ groups are present (Ashbrook, Berry, Farnanf, Le Polle, Pickard and Wimperise, 2006). There are four distinct oxygen atoms in the structure. O2 is a bridging oxygen shared between two tetrahedra, and O1 is a non-silicate oxygen (not bonded to Si). The potentially hydrated O1 atom lies at the center of four edge-sharing Mg2+ octahedra (Smyth, 1987, 1994). If this oxygen is hydrated (protonated), a Mg vacancy can occur at M3. A structure of β-Mg₂SiO₄ is shown in reference figure 3. If water incorporation exceeds about 1.5% the M3 vacancies can order in violation of space group Imma, reducing the symmetry to monoclinic I2/m with beta angle up to 90.4º.

Wadsleyite II is a separate spinelloid phase that might occur between the fields of wadsleyite and ringwoodite. It has both a single (SiO₄) and double (Si₂O₇) tetrahedral units. It is a hydrous magnesium-iron silicate with variable composition that occurs between the stability regions of wadsleyite and ringwoodite γ-Mg₂SiO₄.(Kleppe, 2006) One-fifth of the silicon atom is in isolated tetrahedral and four-fifths is in Si₂O₇ groups so that the structure can be thought of as a mixture of one-fifth spinel and four-fifths wadsleyite.(Horiuchi and Sawamoto, 1981). In the phase of wadsleyite II, there is considered to be possible host hydrogen in the transition zone of the Earth’s mantle. Since forsterite is thought to be about or little over 50% of the mantle, the transition region in the upper mantle could be an important water reservoir.(Kleppe, 2006) Wadsleyite is very water soluble and can accept up to 3 wt. % H₂O as hydroxyl at this site. Its water content is very significant in understanding the way Earth developed. Synthetic hydrous wadsleyite II is pictured in reference figure 4. Wadsleyite II in a variably hydrous magnesium-iron silicate phase. It is a potential host for hydrogen in the transition zone of the Earth's mantle. However, if the water composition of wadsleyite surpasses a 0.1–0.2 wt% amount, it could cause partial melting. As a result, an upwelling flow of water could affect the distribution of particular elements in the Earth.(Huang, Karato and Xu, 2005)