Transition Zone mineralogy

Composition and mineralogy of the Mantle. The Earth s Mantle consists of Upper and Lower regions separated by the Transition Zone at depths between about 350 km and 650 km. Several phase changes occur in the Transition Zone in which common ferromagnesian silicates of the Upper Mantle, all containing Fe2+ ions in distorted six-coordinated sites and tetrahedrally coordinated Si, transform to dense oxide structures with cations occupying regular octahedral... 

Of particular significance are the density increases which take place at the upper and lower boundaries of the mantle transition zone, at the 410 km and 660 km discontinuities. In the past it has been argued that the large density increase which takes place at 660 km depth reflects a change in the bulk composition of the mantle with depth. However, the present consensus is that the contrasts can be accommodated simply by phase changes in the mantle mineralogy. (This debate has huge consequences for whether or not the mantle is chemically layered, and is an important factor in the current debate about the nature of mantle convection). 

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