Understanding the lattice thermal conductivity at high temperatures is important for many applications. We characterize phonon quasiparticles numerically through a hybrid approach that combines first-principles molecular dynamics and lattice dynamics. We find no lower-bound limits on phonon mean free paths in the MgSiO3 perovskite. This contradicts the widely used minimal mean free path idea. The clear identification of phonon quasiparticles validates the use of a phonon gas model when the phonon mean free paths are shorter than the lattice constants of solids. Using the phonon quasiparticle properties, we have calculated the lattice thermal conductivity of the MgSiO 3 perovskite. The results are reasonable compared to recent experimental measurements.