Phys Chem Minerals (1992) 19:148—156

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

Robert D. Shannon', James E. Dickinson 2, and George R. Rossman3
1 Central Research, Experimental Station 356/329, E.I. Du Pont de Nemours, Wilmington, DE 19880-0356, USA
2 Research, Development and Engineering Division, Corning Incorporated, Corning, NY 14831, USA ~' Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

Received October 2, 1991 / Accepted March 9, 1992

Abstract. The dielectric constants and dissipation factors of LiAlSi2O6, CaAl2Si2O8 and CaMgSi2O6 in both the crystalline (a-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 K'11 = 7.30 tan 6=0.0007
K22 = 8.463 tan 6=0.0002
K'33 =11.12 tan 6=0.0007
anorthite K'a* = 5.47 tan 6=0.0009
K'b* = 8.76 tan 6=0.0010
K'c* = 7.19 tan 6=0.0013
diopside K'11 = 9.69 tan 6=0.0016
K'22 = 7.31 tan 6=0.0007
K'33 = 7.29 tan 6=0.00019
LiAlSi2O6 K' — 8.07 tan 6=0.047


CaAl2Si2O8 K' — 7.50 tan 6 =0.0024


CaMgSi2O6 K' = 8.89 tan 6=0.0021


The dielectric properties of a spodumene glass, progres sively crystallized at different conditions, were also de termined. As the crystallization temperature was in creased from 720 to 9200 C, K' increased from 6.22 to 6.44. The dissipation factor, tan 6, remained constant at 0.020. Similarly, as the crystallization time at 750°ree; C increased from 0.5 hr to 6.0 hr, K' increased from 6.28 to 6.35. The deviations of the measured dielectric polari zabilities as determined from the Clausius-Mosotti equa tion from those calculated from the sum of oxide polari zabilities according to aD(mineral, glass) = Z aD(oxides) are + 7.4% for a-spodumene, +1.2% for diopside, and +28.0, + 19.6 and ± 15.9% for amorphous spodumene, anorthitie and diopside compositions, respectively. Posi tive deviations in a-spodumene and anorthite are consis tent with lower than normal apparent cation bond va lence 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 Si02, LiAlSi2O6, CaAl2Si2O8 and CaMgSi2O6 are postulated to arise from a combina tion of loosely bonded cations and disordered 0 = ions where the oxygen dielectric polarizability increased from its normal value of 2.0 Å3 in well-behaved oxides to 2.2—3.0 Å3 in the amorphous phases.