### Background

The proliferation of stable isotope laboratories in recent years has led to a marked increase in the routine use of stable isotope analyses in geochemical studies. This trend is likely to continue because of the development of micro-analytical techniques (e.g. laser ablation, ion microprobe, femtomole-carrier gas methods, etc.) for stable isotope analysis and the potential of these techniques to reveal the detailed sequence of thermal and fluid histories preserved in the rock record (see McKibben et al. 1998). Equilibrium isotope fractionation factors and rates of isotopic exchange form the cornerstones for the interpretation of stable isotope data from natural systems. Indeed, determination of the distribution of the stable isotopes of C-O-H-S in gases, fluids, and minerals has become a standard and extremely powerful approach in elucidating the temperatures, material fluxes, rock and fluid origins, and time scales associated with ancient and active fluid-rock interaction processes in the Earth’s crust (see Valley et al. 1986). Although the thermodynamic principles underlying isotopic fractionation behavior were developed more than fifty years ago by Urey (1947), the calibration of fractionation factors has proven to be difficult. During the last four decades, a great deal of progress has been made in determining the equilibrium fractionations of 18O/16O, D/H, 13C/12C, and 34S/32S among fluid and mineral components at high temperatures (≥400°C), both from experimental exchange studies, theoretical calculations (using vibrational spectra), semi-empirical calculations (using bond-type considerations), and empirical calibrations based on natural assemblages (see references in O’Neil 1986; Kyser 1987; Criss 1991, 1999; Chacko et al., this volume). Of these, the experimental method is the most direct because it involves the least number of a priori assumptions.

Despite the utility of the equilibrium approach in quantifying the isotopic behavior in select water-rock …

Rates and Mechanisms of Isotopic Exchange

Plastic deformation of garnets: systematics and implications for the rheology of the mantle transition zone

Oxygen ion diffusion in single crystal SrTiO3

Oxygen self-diffusion studies using negative-ion secondary ion mass spectrometry (SIMS)

Atomistic simulation calculations have been used to study disorder and deviation from stoichiometry in synthetic garnets A3B5O12 as a function of A3+ (Y3+, Lu3+ to La3+) and B3+ (Al3+, Ga3+, Fe3+) ionic radius. Trends corresponding to the energy of several defect reactions have been obtained. Results compare favorably with the available, albeit limited, experimental data. Disorder in garnets is dominated by formation of antisite defects and the corresponding energy decreases with decreasing A cation size. Incorporation of excess Y2O3 or rare-earth oxide is energetically preferred with respect to incorporation of excess B2O3 (B = Al, Fe, Ga). Solution of excess binary oxide occurs by the formation of antisites rather than vacancies. Neither an excess of A2O3 or B2O3 seems to be the main source of oxygen nonstoichiometry in Al and Ga garnets. On the contrary, accommodation of oxygen nonstoichiometry in iron garnets is energetically more favored if accompanied by the reduction of Fe(III) to Fe(II). The energ...

Disorder and Nonstoichiometry in Synthetic Garnets A3B5O12 (A = Y, Lu−La, B = Al, Fe, Ga). A Simulation Study

Sintering and grain-growth rates were determined for yttrium iron garnet as a function of the yttrium/iron ratio. Rates decreased with an increase in this ratio; this behavior is interpreted as a result of the oxygen content variation through the garnet field. For constant sintering time, density and grain size, as well as microstructure-dependent properties, varied through the garnet field. Remanent magnetization and coercive force in particular depended on composition and sintering temperature. The rf field for nonlinear spin-wave excitation, hcrit, measured for dense samples with grain sizes from ∼4 to 30 μm, varied by a factor of four.

Microstructure Development in Yttrium Iron Garnet

The oxygen ion self-diffusion coefficient was determined for single-crystal and polycrystalline yttrium iron garnet (Y3Fe5O12). The rate of exchange between oxygen gas enriched with the stable isotope 18O and solid yttrium iron garnet was measured. Oxygen ion diffusion rates were found to be the same in single-crystal and 8μ polycrystalline Y3Fe5O12 between 1100° and 1400°C. This is in contrast to previous measurements of anion diffusion in several alkali halides and in AI2O3 where a strong dependence of diffusion rates on the presence of grain boundaries was found. Enhancing oxidation rates in dense, reduced yttrium iron garnet at low temperature by minimizing the final fired grain size in the sintering process does not appear to be possible on the basis of the results obtained in this investigation. The temperature dependence of the diffusion coefficient of oxygen measured at 100 torr can be represented by D= 0.40 exp (-65.4/RT) cm2 per second.

Oxygen Ion Diffusion in Single‐Crystal and PoIycrystaIIine Yttrium Iron Garnet

Two-phase composites of spinel ferrites and lithium aluminosilicates were prepared and investigated as microwave-absorbing materials. The magnetic and dielectric loss parameters which determine the heating characteristics were examined. Low-thermal-expansion composites were developed which were effective absorbers of microwave radiation. Some compositions investigated exhibited a temperature maximum limited by the Curie temperature of the ferrite in the composite.

Microwave‐Absorbing Ferrite‐Dielectric Composites

Polycrystalline garnets with the general composition Gd3-xYxFe5-yInuO12 were investigated. Ferrimagnetic resonance line widths at S- and X-bands and saturation induction are reported. The temperature dependence of the magnetization was determined on materials in which x= 0 to 3 and y = 0 to 1. Special emphasis is given to compositions having relatively stable magnetizations over the range 20° to 100°C.

Magnetic Properties of Gadolinium-Yttrium-Iron-Indium Garnets

The saturation inductions of polycrystalline garnet materials with the general composition Y3–x GdxFe5–y–z AlyInZO12 were determined. The compositions investigated included yttrium substitutions of 0 to 50%, aluminum substitutions of 0 to 10%, and indium substitutions of 0 to 10%. Temperature dependences of the saturation inductions are reported with special emphasis on materials having small temperature coefficients in the 20° to 100°C region.

E. A. Maguire

Papers

Microstructure Development in Yttrium Iron Garnet

Oxygen Ion Diffusion in Single‐Crystal and PoIycrystaIIine Yttrium Iron Garnet

Microwave‐Absorbing Ferrite‐Dielectric Composites

Magnetic Properties of Gadolinium-Yttrium-Iron-Indium Garnets

Magnetic Properties of Yttrium-Gadolinium-Iron-Aluminum-Indium Garnets