Dielectric constants of silicate garnets and the oxide additivity rule



E.I. Du Pont de Nemours and Co., Inc., Central Research and Development Department, Experimental Station
Wilmington, Delaware 19880-0356, U.S.A.



Division of Geological and Planetary Sciences, California Institute of Technology
Pasadena, California 91125, U.S.A.



The dielectric constants and dielectric loss values of a series of garnets were measured at 1 MHz using a two-terminal method and empirically determined edge corrections. The results are:

five intermediate pyrope-almandine samples, k' = 11.96 - 12.35;
spessartine, k' = 11.65;
two andradite samples, k' = 10.53 - 10.59;
three grossular samples, k' = 8.53 - 8.81.

The deviations of measured dielectric polarizabilities as determined using the Clausius Mosotti equation from those calculated using the sum of oxide polarizabilities according to aD(M2M'X4) = 2aD(MK) + aD(M'X2) is +5.0 to +6.5% for the pyrope-almandine samples, +1.9% for spessartine, -2.3% for the andradite samples, and -5.5 to -7.0% for grossular. These deviations from additivity are believed to result from garnet structural constraints leading to "rattling" Mg ions and "compressed" Ca ions.

American Mineralogist, Volume 77, pages 94-100, 1992