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The D'' layer is the 200-300-kilometer thick layer lying between the lower mantle of the Earth and the liquid iron outer Core. The layer is sometimes considered part of the lower mantle.

This is probably the most interesting layer of the Earth besides its surface1, even surpassing that of the layer it sits on top of.

At the actual boundary between the D'' layer and the outer core is the D'' mirror, which is actually the surface of the liquid iron. It reflects the type of seismic waves known as S waves and allows P waves to pass through (refracted).

The D'' layer is differentiated from the lower mantle in its density, and most importantly, its higher conductivity. This suggests that the layer is enriched with iron from the Core.

The last argument leads to all sorts of interesting speculations. The best one is that the silicate rocks of the lower mantle and the liquid iron of the outer core are violently reacting with each other chemically. It has been doing this since the Earth was formed, and will continue until all the iron in the outer core is used up.

The layer varies widely in thickness but of course it is at the bottom of the mantle's convection cells. I don't know if material is drawn up into them or not, but might that not explain the presence of metals in the crust1?

Source: A Scientific American article I once read, plus some Web-posted introductory Geology lectures (Indiana.edu and le.ac.uk) which refreshed my memory.
1Hey, I was a Geography major, not a geology major.

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