Deformation and texture formation in (Mg, Fe)SiO₃ post perovskite (ppv) is a potential explanation for the strong seismic anisotropy that is found in the D″ layer of the Earth. However, different experimental approaches have resulted in different lattice preferred orientations (LPO) in deformed ppv that have led to ambiguity in the interpretation of deformation in the lowermost mantle. Here, we show that deformation of the analogue substance CaIrO₃ during a phase transformation from perovskite to ppv leads to a transformation texture that differs from the CaIrO₃ ppv deformation texture but resembles the results from ppv deformation experiments in diamond anvil cells. Assuming material spreading parallel to the core-mantle boundary, our results predict a widespread shear wave splitting with fast horizontal S-waves, which is compatible with seismic studies. Downwelling material that undergoes a phase transformation may develop a transformation texture that would locally result in vertically polarized fast S-waves.