American Mineralogist, Volume 74, pages821-825, 1989

Electrical propertiesof opal M. Y. Xu, H. JuN, M. R. Norrs Department of Materials Scienceand Engineering,Leh4h University, Bethlehem,Pennsylvania 18015, U.S.A.

Ansrnacr The electrical conductivity (o) of one natural and two synthetic opals has been determined from ac complex impedance analysis. The value of o is lower and its activation enthalpy higher for the synthetic opals presumably becauseof a lower concentration of the Na* chargecarriers. In contrast to o, the dielectric constant G') ofthe synthetic opal is anisotropic and is dominated by the presenceof water. Its value is higher perpendicular to the columns than parallel to the columns. With increasingtemperature,e' decreasesfor the latter casebut increasesfor the former configuration becauseofthe extra interfacial polarization when the chargecarriers must crossthe boundary betweenthe columns.

IxrnonucrroN It is known that the physical properties ofnatural and synthetic materials can be significantly different. In the caseof quartz, for example,the activation energyfor elec-trical conductivity of the natural material, irrespective of its origin, is always found to be substantially lower than that of a syntheticquartz crystal, although structural techniques do not indicate any difference(Jain and Nowick, 1982a). The different electrical behavior of natural and synthetic quartz has been explained by the manner in which alkali ions associatewith other defectsin the respective crystals (Jain and Nowick, 1982a, 1982b); yet another possibility exists for structural differencesarising from different growth conditions but being too small to be detectedby availabletechniques.In view of this dilemma between structural and compositional factors, it is interesting to investigate a closely related but amorphous mineral, namely opal. A typical natural opal is made of regularly packed monosizesilica glassspheres(approximately0.25 pm in diameter) with the spacebetween the spheresalso filled with silica but of much higher water content (Nassau, 1980).Synthetic opal, which was first preparedby Gilson (Nassau, 1980, chapter 22) and sometimesknown as gilsonite, also consistsof a similar packing of silica spheres, but the interstitial spacein this case is uniformly filled with additional smaller-size zirconia spheres(Gauthier, 1986; Simonton et al., 1986).The commonly observed colors (or fire) in both kinds of opals are due to the diffraction-gratingeffectfrom regularly packedsilica spheres (Nassau, 1980). Becauseof the special structure of synthetic opal, it is a good model systemto study the overall dielectric behavior of a composite ceramic consisting of two components, one with a low dielectric constant and the other with a high one (silica and zirconia, respective1V). l $02.00 0003-o04x/89/0708-082

ExpnnrvrBI.{TAL DETATLS Samples One natural white opal from South Australia and two synthetic platelike opul, 1-urrufactured by Nakazumi Crystal Laloratory, Japan) were used in this study. The chemical analysisof tnir" samplesas determined by plasma_emissionipectroscopy is given in Table l. A visual ..fire" examination showed that each opal had the usual and was not isotropic but consistedofa columnar structure. In synthetic opals the columns were parallel to the sample thickness and could be distinguished by the diffracted color. An electron-microscopeexamination had shownearlierthatthedirectionofpackingofsilicaspheres is uniform within one column but is different between columns(Cowenet al.. l9g9). Simontonet al. (1986)have reported ihat tne silica spheresare arrangedon a hexagonal lattice when viewed along a column but on a square hfiice when viewed in a perpendicular direction. Electrical measurements For both synthetic opals (Sl and S2), electrical propertieswere first measuredwith the electric field being perpendicular to the columns (which were parallel to the electrodeplane). Becauseof the columnar structure of our opals, it was considered important to determine the relationship betweenthe electrical properties and the macroscopic structuralanisotropyshown by the colors. Accordingly, the same sample 52 was also studied with the electric field in the direction of the columns. The columns in the natural sample (N) were not as well aligned as in synthetic samples;nevertheless,on the averagethey were normal to the electric field. All opal sampleswere polished on a 600-grit SiC paper to obtain uniform thickness. Next, Au electrodeswere deposited on the top and bottom facesby vacuum evap-

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XU ET AL.: ELECTRICALPROPERTIES OF OPAL

TABLE1. Chemicalanalyses(wt%) of opal samples Element

Synthetic opal 1

Synthetic opal 2

Zr Na Mg AI K

2.4+ 0j2 0.086+ 0.004 0.068+ 0.003

1 . 6+ 0 . 2 0.079+ 0.008 0.009+ 0.001

Natural opal

0.28+ 0.028+ 0.32+ 0.049+

lCr

+so

lc C

0.01 0.001 g.g2 0.002

oration. Pt wires were used to connect the Au electrodes to the ac bridge leads using flexible Ag paint. The ac conductance(G) and capacitance(C) were measured in the frequency rangeof 20 Hz to 100 kHz with a General Radio 162l bridge assembly. At each temperature of measurement(+0.25 'C), the sample was equilibrated for at least 1.5 h in flowing, dried, high-purity He.

++7 C

voc ) U O z.F O l O 7 LJI

OO

N A T U R A LO P A L I

O

Rnsur,rs

10

10

10o 10s 10 (Hz) FREQUENCY

I

Both conductanceand capacitanceof the opal samples are strongly frequency dependent, typically as shown in LL Figures lA and 1B for the natural opal at various tem- U peratures. To separateelectrode effects, G(