gems, garnet, garnet properties, pyrope-almandine, spessartine, andradite, grossular

The dielectric constants and dielectric loss values of a series of garnets were measured at 1 MHz using a two-terminal method with empirically determined edge corrections. The results yielded dielectric constants (ε’) of 11.96–12.35 for five intermediate pyrope-almandine samples; 11.65 for spessartine; 10.53–10.59 for two andradite samples; and 8.53–8.81 for three grossular samples. Deviations in measured dielectric polarizabilities, calculated using the Clausius-Mossotti equation and compared to theoretical values based on the sum of oxide polarizabilities α₀(MrM’X₀) = 2α₀(MX) + α₀(M’X₁), were +5.0% to +6.5% for the pyrope-almandine group, +1.9% for spessartine, −2.3% for andradite, and −5.5% to −7.0% for grossular. These deviations from ideal additivity are attributed to structural constraints within the garnet lattice, leading to “rattling” magnesium ions and “compressed” calcium ions.