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Wadsleyite is a high-pressure phase of Mg2SiO4 and is polymorphous with the olivine phase forsterite. An orthorhombic mineral with the formula β-Mg2SiO4, it was first found in nature in the Peace River meteorite from Alberta, Canada. It is formed by a phase transformation from forsterite (α-Mg2SiO4) under increasing pressure and eventually transforms into spinel-structured ringwoodite (γ-Mg2SiO4) as pressure increases further. The structure can take up a limited amount of other bivalent cations instead of magnesium, but contrary to the α and γ structures, a β structure with the sum formula Fe2SiO4 is not thermodynamically stable. Its cell parameters are approximately a = 5.7 Å, b = 11.7 Å and c = 8.24 Å. Wadsleyite is found to be stable in the upper part of the transition zone of the Earth’s upper mantle between in depth. Because of oxygens not bound to silicon in the Si2O7 groups of wadsleyite, it leaves some oxygen atoms underbonded, and as a result, these oxygens are hydrated easily, allowing for high concentrations of hydrogen atoms in the mineral. Hydrous wadsleyite is considered a potential site for water storage in the Earth’s mantle due to the low electrostatic potential of the underbonded oxygen atoms. Although wadsleyite does not contain H in its chemical formula, it may contain more that 3 percent by weight H2O, and may coexist with a hydrous melt at transition zone pressure-temperature conditions. The solubility of water and the density of wadsleyite depend on the temperature and pressure in the Earth. Furthermore, the transformation resulting in wadsleyite is thought to occur also in the shock event when a meteorite impacts the Earth or another planet at very high velocity. Wadsleyite was first identified by Ringwood and Major in 1966 and was confirmed to be a stable phase by Akimoto and Sato in 1968. The phase was originally known as β-Mg2SiO4 or “beta-phase”. Wadsleyite was named for mineralogist Arthur David Wadsley (1918-1969). == Composition == In values of weight percent oxide, the pure magnesian variety of wadsleyite would be 42.7% SiO2 and 57.3% MgO by mass. 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 and suggest 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 et al. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「wadsleyite」の詳細全文を読む スポンサード リンク
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