High-pressure studies of (Mg0.9Fe0.1)(2)SiO4 olivine were performed at ambient temperature using X-ray diffraction, Raman spectroscopy, and M6ssbauer spectroscopy. At similar to 40 GPa, a change of compressibility associated with saturation of the anisotropic compression mechanism was detected. This change is interpreted to result from the appearance of Si2O7 dimer defects, as deduced from Raman spectroscopy; the appearance of such defects also accounts for the previously reported pressure-induced amorphization observed for this material upon additional compression. Furthermore, this behavior is followed by a spin crossover of Fe2+ that occurs over a wide pressure range, as revealed by Mossbauer spectroscopy.