Transient high pressures and temperatures generated during meteor or asteroid impacts induce mineral phase transformations that can mimic those occurring at depth within the silicate mantle of terrestrial planets. Olivine (alpha-(Mg,Fe)(2)SiO4), the primary constituent of the Earth's upper mantle and of chondritic meteorites, transforms to the high-pressure polymorphs wadsleyite and ringwoodite (beta-, gamma-(Mg,Fe)(2)SiO4) that are observed in shocked chondrites. The observed phase transitions place constraints on the shock P-T conditions attained and they lead to models that describe the impact event. We studied the olivines present within two newly catalogued Martian meteorites NWA 2737 and NWA 1950 using micro-Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) techniques. The shock conditions were not sufficient to cause melting or transformation of the olivines into wadsleyite or ringwoodite (Mg,Fe)(2)SiO4. The shocked olivines are dark-coloured in hand specimen and thin section due to the presence of FexNiy, metallic nanoparticles formed during the shock. Molecular Dynamics simulations (MD) are consistent with the observation that the shocked olivines give rise to a new orthosilicate polymorph (zeta-(Mg,Fe)(2)SiO4) that is formed metastably during the shock process and that is subsequently recovered to ambient conditions. The presence of the new (Mg,Fe)(2)SiO4 polymorph in shocked ultrabasic rocks including meteorites may have remained undetected due to its structural and spectroscopic similarities with olivine. The existence of the metastable a- phase transition also allows rationalising previously unexplained results of shock compression experiments on olivines. zeta-(Mg,Fe)(2)SiO4 is also formed at ambient pressure as a metastable intermediate during back-transfon nation from wadsleyite. (c) 2007 Elsevier B.V. All rights reserved.