9526916 Smyth The proposed research is to investigate the crystal chemistry of hydroxyl substitution in a group of representative silicates. Several mineral structures will be refined by x-ray and/or neutron single crystal methods at various temperatures in order to test critical aspects of an electrostatic model for prediction of physical and chemical properties of hydroxyl-bearing silicates. H positions determined by x-ray and neutron diffraction will be compared to see if systematic differences are present that would allow use of x-ray data to estimate proton positions. Electrostatic site potentials will be computed for both x-ray and neutron h positions to see if systematic relations exist with IR data, relative thermal expansion of various nearest neighbor distances, 18O/18O and D/H fractionation and chemical weathering rates. These studies will hopefully improve the predictive capability for physical and chemical properties of hydroxyl-bearing silicates and may yield a constraint on the H flux returning into the mantle.
