It’s made of silicates and oxides. The most abundant elements there are oxygen, silicon, magnesium, iron and calcium.
The lower mantle is not to be confused with the core; it’s made of silicate and oxide rocks rather than metal. The lower mantle rock differs in two ways from what we see in the crust and upper mantle:
1) Elemental composition: rock in the mantle (both upper and lower) has more iron and magnesium, and less calcium and aluminum, than what we see in the crust. Calcium and aluminum tend to form lower-density silicates than magnesium and iron, so the former silicates tend to float up to the crust.
2) Mineral structure: this applies especially to silicates, which are “perovskite silicates” instead of ordinary silicates. In ordinary silicates we see silicon form four covalent bonds to oxygen, filling up the usual “octet” of valence electron states. Under the high pressure inside the lower mantle, however, the silicon atoms will form six bonds making ##SiO_6## octahedra (like sulfur does in the “expanded octet” molecule ##SF_6##).
A detailed discussion of Earth’s mantle is described here: