Dielectric Constants of Crystalline and Amorphous Spodumene, Anorthite and Diopside and the Oxide Additivity Rule

Abstract

The dielectric constants and dissipation factors of LiAlSi_2O_6, CaAl_2Si2O_8 and CaMgSi_2O_6 in both the crystalline (α-spodumene, anorthite, and diopside) and amorphous forms were determined at 1 MHz using a two-terminal method and empirically determined edge corrections. The results are: spodumene κ'_(11)=7.30 tan δ= 0.0007 κ′_(22)=8.463 tan δ= 0.0002 κ′_(33) =11.12 tan δ= 0.0007 anorthite κ′_a^*=5.47 tan δ= 0.0009 κ′_b^*=8.76 tan δ= 0.0010 κ′_c^*=7.19 tan δ= 0.0013 diopside κ′_(11)=9.69 tan δ= 0.0016 κ′_(22) = 7.31 tan δ= 0.0007 κ′_(33)=7.29 tan δ= 0.00019 LiAlSi_2O_6 κ′=8.07 tan δ= 0.047 amorphous CaAl_2Si_2O_8 κ′=7.50 tan δ= 0.0024 amorphous CaMgSi_2O_6 κ′=8.89 tan δ= 0.0021 amorphous The dielectric properties of a spodumene glass, progressively crystallized at different conditions, were also determined. As the crystallization temperature was increased from 720 to 920° C, κ′ increased from 6.22 to 6.44. The dissipation factor, tan δ, remained constant at 0.020. Similarly, as the crystallization time at 750° C increased from 0.5 hr to 6.0 hr, κ′ increased from 6.28 to 6.35. The deviations of the measured dielectric polarizabilities as determined from the Clausius-Mosotti equation from those calculated from the sum of oxide polarizabilities according to α_D(mineral, glass) = σ α D(oxides) are +7.4% for α-spodumene, +1.2% for diopside, and +28.0, +19.6 and +15.9% for amorphous spodumene, anorthitie and diopside compositions, respectively. Positive deviations in α-spodumene and anorthite are consistent with lower than normal apparent cation bond valence sums and are believed to be evidence for loosely bonded “rattling” Li and Ca ions. Diopside, with Ca and Mg ions having normal bond valence sums, exhibits no abnormal deviation from additivity. Larger positive deviations in amorphous SiO_2, LiAlSi_2O_6, CaAl_2Si_2O_8 and CaMgSi_2O_6 are postulated to arise from a combination of loosely bonded cations and disordered O= ions where the oxygen dielectric polarizability increased from its normal value of 2.0 Å^3 in well-behaved oxides to 2.2–3.0 Å_3 in the amorphous phases.