Wadsleyite - Composition

Composition

Wadsleyite is a polymorph of forsterite Mg₂SiO₄, an end-member of the solid-solution series of olivine(Horiuchi and Sawamoto, 1981). In the phase transformations of forsterite to fayalite Mg₂SiO₄-Fe₂SiO₄, this magnesium-rich olivine α-Mg₂SiO₄ changes to wadsleyite β-Mg₂SiO₄ under certain pressure and temperature conditions, and then with increasing pressure, it transforms to ringwoodite γ-Mg₂SiO₄ which is a spinel structure (Price, Putnis, Agrell and Smith, 1983). Wadsleyite is synthesized stably at 1000–1500 °C (1800–2700 °F) and 13 to 18 GPa of pressure between depths of 410–525 kilometres (250–326 mi). Figure 1 of the reference shows the high pressure phases of olivine polymorphs beginning with wadsleyite β-Mg₂SiO₄. Geologically speaking, it is a very fine-grained “reactive” forsterite that had been synthesized from hydrous starting materials.

In values of weight percent oxide, the pure magnesian variety of wadsleyite would be 44.5% SiO₂, 52.2% MgO, and 3.33% H₂O. The average microprobe analysis for wadsleyite yielded: MgO 38.21, SiO₂ 38.7, CaO 0.07, Cr₂O₃ 0.01, MnO 0.43, GeO 22.37, NiO 0.11 and ZnO 0.10.(Smyth, 1987) A recalculation of the number of cations on the basis of four oxygens will yield MgO 1.51, SiO₂ 1.03, CaO 0.0019, Cr₂O₃ 0.0002, MnO 0.0096, GeO 0.4032, NiO 0.0023 and ZnO 0.0019. An analysis of trace elements in wadsleyite suggests that there are a number of elements included in it. Results demonstrate traces of rubidium Rb, strontium Sr, barium Ba, titanium Ti, zirconium Zr, niobium Nb, hafnium Hf, tantalum Ta, thorium Th, and uranium U in wadsleyite relative to olivine.(Mibe, Nakai, Orihashi, and Fujii 2006) This information suggests that the concentrations of these elements could be larger than what has been supposed in the transition zone of Earth’s upper mantle. Moreover, these results help in understanding chemical differentiation and magmatism inside the Earth (Mibe, Nakai, Orihashi, and Fujii 2006).

Although nominally anhydrous, wadsleyite can incorporate more than 3 percent by weight H₂O, which means that it is capable of incorporating more water than Earth's oceans and may be a significant reservoir for H (or water) in the Earth's interior.