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

Lager, George A. and Armbruster, Thomas and Rotella, Frank J. and Rossman, George R. (1989) OH substitution in garnets: X-ray and neutron diffraction, infrared, and geometric-modeling studies. American Mineralogist, 74 (7-8). pp. 840-851. ISSN 0003-004X. https://resolver.caltech.edu/CaltechAUTHORS:20130515-082732990

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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.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://ammin.geoscienceworld.org/content/74/7-8/840.citationPublisherUNSPECIFIED
ORCID:
AuthorORCID
Rossman, George R.0000-0002-4571-6884
Additional Information:© 1989 Mineralogical Society of America. Manuscript received August 8, 1988. Manuscript accepted March 7, 1989. G.A.L. acknowledges support of this research by the National Science Foundation (Experimental and Theoretical Geochemistry) through Grants EAR-8205605 and EAR-8719848. This work has benefited from the use of the Intense Pulsed Neutron Source at Argonne National Laboratory, which is funded by the U.S. Department of Energy, BES-Materials Science, under Contract W-31-109-ENG-38. Support from NSF Grant EAR-8618200 to G.R.R. is also acknowledged. R. Oberhiinsli provided valuable assistance with the electron-microprobe analyses.
Funders:
Funding AgencyGrant Number
NSFEAR-8205605
NSFEAR-8719848
Department of Energy (DOE)W-31-109-ENG-38
NSFEAR-8618200
Issue or Number:7-8
Record Number:CaltechAUTHORS:20130515-082732990
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20130515-082732990
Official Citation: George A. Lager, Thomas Armbruster, Frank J. Rotella, and George R. Rossman OH substitution in garnets; X-ray and neutron diffraction, infrared, and geometric-modeling studies American Mineralogist, August 1989, v. 74, p. 840-851
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:38515
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:15 May 2013 15:57
Last Modified:26 Nov 2019 11:15

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