Olivine–Modified Spinel–Spinel Transitions

OlivineSpinel of Mg-rich (Mg,Fe)2SiO4 composition is accepted as the most abundant mineral in the Earth’s upper mantleUpper mantle. High-pressure phase transitions of (Mg,Fe)2SiO4 olivine have been investigated to clarify the constitution and dynamics of the Earth’s mantle. Mg-rich olivine transforms into the modified spinelModified spinel-structured phase and next into the spinel-type phase as pressure increases. In this chapter, the high-pressure phase relations of olivine in the Mg2SiO4-Fe2SiO4 system are discussed; they were studied by means of high-pressure and high-temperature experiments and thermodynamic calculations based on calorimetric measurements. The historical aspects of these studies are also described. The modified spinel-type and spinel-type phases can incorporate waterWater of up to about 3 wt% as OH− in the structures. Therefore, the transition zoneTransition zone may be a major reservoir of waterWater in the Earth’s interior. The effects of water on the phase relations in the Mg2SiO4-Fe2SiO4 system are discussed. Modified spinelModified spinel-type phases belong to a group of spinelloidSpinelloid. The structures and phase relations of some representative spinelloid compounds are described in the final section of this chapter.