juvenile ultramafic association strongly predominates, and it is composed of ferropericlase, MgSi-,
CaSi- and CaTi-perovskites, stishovite, tetragonal almandine-pyrope phase (TAPP), and some others. The association
analogous to the upper-mantle eclogitic association, formed from subducting lithosphere, comprises:
majorite, CaSi-perovskite bearing compositional Eu anomalies, phase ‘Egg’ with a tetragonal
structure, and stishovite.
The carbonatitic association is represented by various carbonates, halides, and associated
minerals. Some mineral associations (wüstite+ periclase and native iron+iron carbides) are, possibly,
related to the D″ layer at the core/mantle boundary.
The mineralogical composition of the lower mantle is
now understood to be more complex than had been suggested in theoretic and experimental works.
The proportion
of ferropericlase in the lower mantle is higher than it was suggested before, and its composition is
more iron-rich (mg= 0.36–0.90) as compared to experimental and theoretical data.
Free silica (stishovite)
is always present in lower-mantle associations, and a separate aluminous phase (TAPP) has been identified
in several areas. These discrepancies suggest that the composition of the lower mantle differs to that of the
upper-mantle, and experiments based solely on ‘pyrolitic’ compositions are not, therefore, applicable to the
lower mantle.
These data indicate a probability of an alternative to the CI-chondrite model of the Earth's
formation, for example, an enstatite-chondrite model.