The lower mantle is primarily composed of bridgmanite, a silicate mineral with the chemical formula (Mg,Fe)SiO3. This phase is perovskite structured, meaning it has a cubic crystal structure with a cation (Mg or Fe) at the center of the unit cell, an oxygen anion at each corner, and a silicon cation at the center of each face.

However, the lower mantle is not homogenous. While bridgmanite is the dominant mineral, other phases exist in smaller proportions:

* Ferropericlase: This mineral is a mixture of magnesium oxide (MgO) and ferrous oxide (FeO), with a rock salt structure.

* Calcium silicate perovskite: This mineral is a calcium-rich silicate perovskite with the formula CaSiO3.

* Other phases: In addition to these, there are trace amounts of other minerals like oxides and even potentially some metallic iron.

It's important to note that the specific proportions and distribution of these phases within the lower mantle vary depending on depth and pressure. The extreme pressure and temperature conditions deep within the Earth cause mineral transformations, leading to the formation of these high-pressure phases.

The exact nature and behavior of these phases are still being researched, but they play a crucial role in understanding the Earth's structure, composition, and evolution.