Hydrous chain silicates (amphiboles) are central to understanding many geological processes in the Earth¿s Crust, Upper Mantle and Transition Zone, where they influence melting, rock rheology, seismicity and the distribution and availability of water. Consequently, the recognition of new mechanisms for the incorporation of stoichiometric amounts of structural H (as OH) in hydrous minerals in excess of their ¿normal¿ OH contents is of considerable relevance to quantifying the water budget of the upper mantle. This proposal seeks to determine the location of this excess stoichiometric OH in a well-characterised synthetic amphibole by difference-Fourier synthesis of neutron powder diffraction data collected at 2K and 298K on POLARIS. Success in this experiment will allow the interpretation of NMR and IR spectra, thereby identifying the signature of "excess-OH" in mantle amphiboles.