Dielectric Constants of Tephroite, Fayalite and Olivine and the Oxide Additivity Rule

The dielectric constants and dissipation factors of synthetic tephroite (Mn_2SiO_4), fayalite (Fe_3SiO_4) and a forsteritic olivine (Mg_(1.80)Fe_(0.22)SiO_4) were measured at 1 MHz using a two-terminal method and empirically determined edge corrections. The results are: tephroite, κ′_a= 8.79 tan δ_a = 0.0006 κ′_b = 10.20 tan δ_b = 0.0006 κ′_c= 8.94 tan δ_c= 0.0008 fayalite, k′_a = 8.80 tan δ_a = 0.0004 k′_b= 8.92 tan δ_b = 0.0018 k′_c = 8.58 tan δ_c = 0.0010 olivine, k′_a = 7.16 tan δ_a = 0.0006 k′_b = 7.61 tan δ_b = 0.0008 k′_c = 7.03 tan δ_c = 0.0006 The low dielectric constant and loss of the fayalite indicate an exceptionally low Fe^(3+) content. An FeO polarizability of 4.18 Å^3, determined from α_D(FeO) = [α_D(Fe_2SiO_4)-α_D(SiO_2)]/2, is probably a more reliable value for stoichiometric FeO than could be obtained from Fe_xO where x = 0.90–0.95. The agreement between measured dielectric polarizabilities as determined from the Clausius-Mosotti equation and those calculated from the sum of oxide polarizabilities according to α_D(M_2M′X4) = 2α_D(MX) + α_D(M′X_2) is ∼+2.8% for tephroite and +0.2% for olivine. The deviation from additivity in tephroite is discussed