Seismic Mantle Discontinuities and Phase Transitions

When looking at seismic data, the waves travel through the mantle and there are certain points where the wave speed changes, on these jumps there is also reflection.

Polymorphs

The layers have the same composition, the exact same chemical formula, but the differ in structure.

Discontinuities

410-km Discontinuity

At a pressure of 14 GPa Olivine goes from its $\alpha$-phase to it's $\beta$-Phase called Wadsleyite. The looser structure goes to a "spinel-like" structure that is much denser.

Olivine is "dry", it cannot hold water, Wadsleyite can hold up to 3% water by weight in its crystal structure. As rocks rises above this boundary it loses its stored water.

520-km Discontinuity

Harder to see. Here Wadsleyite, the $\beta$-Phase turns into Ringwoodite, the $\gamma$-Phase. Rindwoodite is also a major sponge for water.

660-km Discontinuity

The sharpest and important boundary. It marks the end of the transition zone and the start of the lower Mantle. Ringwoodite breaks apart into two separate minerals Bridgmanite and Ferropericlase. This is a bigger change since in the Crust and Upper Mantle the silicon sits inside a Tetrahedron made of oxygen. Here the pressure is so high that it is forced into an Octahedron made of oxygen. This makes the Lower Mantle stiff and viscous. It acts as a barrier to convection.

The Core-Mantle Boundary (D'' Layer)

Gutenberg Discontinuity

Just above the core, around 2700 km, the final change occurs. Bridgmanite transforms into Post-Perovskite. This mineral is layered, which makes the wave travel at different speed through it depending on which direction they move near the core.

Question and Answers

Do seismic discontinuities in the mantle (like the 410 and 660) generally represent a change in Chemical Composition or a Phase Transition?::Phase Transition (Iso-chemical).

What is a "Polymorph" in mineralogy?::A mineral with the same chemical composition as another but a different crystal structure (packing arrangement).

The 410-km Discontinuity marks the transformation of Olivine into what mineral?::Wadsleyite ($\beta$-phase).

The 660-km Discontinuity marks the breakdown of Ringwoodite into which two minerals?::Bridgmanite (Silicate Perovskite) and Ferropericlase (Magnesiowüstite).

What is the major chemical/structural change regarding Siliconthat occurs at the 660-km boundary (Lower Mantle)?
?
Silicon changes coordination number from 4 (Tetrahedral) to 6 (Octahedral). It is now surrounded by 6 Oxygen atoms.

What transformation occurs in the D'' layer (just above the core)?:::Bridgmanite transforms into Post-Perovskite.