OH substitution in garnets: X-ray and neutron diffraction, infrared, and geometric-modeling studies

Abstract

Tetrahedral-site occupations were determined for three titanian andradites (San Benito County, California) and a synthetic deuterated hibschite by using X-ray single-crystal and neutron powder data, respectively. Site refinements reveal the presence of tetrahedral vacancies (4-14%) in all three andradites. Infrared absorption spectra measured for the same material used in the X-ray analysis indicate structurally bound water (as OH^-) in amounts of 0.8 to 5. 7 wt% OH, which is in good agreement with water contents derived from the refinements. These data confirm that the tetrahedral site is not fully occupied and that charge balance can be achieved by the substitution (O_4H_4)^(4-) = (SiO_4)^(4-). The proton position could not be determined because of the small amount of H present. Diffraction maxima in the neutron profile for hibschite exhibited small, well-defined shoulders related to chemical inhomogeneity. The data were fit using multiphase Rietveld techniques assuming four phases with slightly different Si/D ratios. Structural parameters [ɑ= 12.0105(3) A; O(x,y,z) = 0.03561(14), 0.04653(12), 0.64957(12); d site= 0.767(6) Si] refined for the major phase [62(3)% mole fraction) were consistent with X-ray refinements of natural hydrogrossulars. The deuterium atom [0.0965(5), 0.0520(4), 0.6600(5)] was located (ΔF map) outside the tetrahedral volume near the position reported for the Si-free end-member. A significant improvement in R factor was obtained after refinement of a split-atom model to describe the oxygen positional disorder. The short O-D distance [0.744(6)] calculated for the ordered (average) structure can be interpreted within the context of this model. Distance-least-squares (DLS-76) calculations were used to simulate the effect of the (O_4H_4)^(4-) = (SiO_4)^(4-) substitution on the grossular structure. If the tetrahedral d-O distance, calculated from vacancy concentration, is weighted heavily in the geometric refinement, structural variations in the hydrogrossular series [Ca_3Al_2(SiO_4)_(3-)Ca_3Al_2(O-4H_4)_3] can be predicted. Application of DLS to other garnet structures suggests that mantle garnets (rich in pyrope component) may contain only very limited amounts of water.