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Electrical Conductivity of Diopside: Evidence for Oxygen Vacancies

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Electrical Conductivity of Diopside: Evidence for Oxygen Vacancies American Mineralogist, Volume 73, pages1235-1254, 1988 Electrical conductivity of diopside: Evidence for oxygen vacancies J. SrnpnBN Hunnxnn, DoNnn E. Vorcr* U.S. Geological Survey, Reston, Virginia 22092,V-5.4. Arsrn-rcr Impedance spectra for two natural single crystals of diopside were obtained at 800 to 1300'C and l-bar pressureover the frequencyrange 0.001 Hz to 100 kHz in a system closed to all components but oxygen. For the near<nd-member composition, approxi- mately NarCanuMgruFerAlrsirooOuoo,over the logfor(forin bars)range -9 to -14 at 1000 lC and - 8 to - 12 at 1200 "C, a range exceedingthat of most igneousand metamorphic processes,the conductivity (o, in units of siemens per meter) for three crystallographic orientations at l-bar pressureis given by (100):log o: 1.43- 10850/f - 0.18log.fo,, (010):log o : 0.88 - 10170/T- 0.l2lo1fo,, (001):Iog o : 1.58- 11080/Z- 0.15logfo,, where I is in kelvins. The activation energyis 2.1 eV. At both higher and lower/o, values, no/o, dependenceofconductivity was observed,indicating the presenceofdifferent con- duction mechanisms.At temperaturesless than 1000 oC, the activation energyis 1.3 eV, also suggestinga different conduction mechanism.Thus, at leastfour regimesare necessary to describethe conductivity of this diopside in T-fo, space.The approximately -l/(7 ! l) value of d(log o)/6(logfo) in the high-temperaturegeologic region suggestsa reaction by which oxygenvacancies control the conductivity. This relatively pure diopside is much less conducting than olivine (Mg,Fe)rSiO"or orthopyroxene (Mg,Fe)rSirO.. A seconddiopside with greaterFe content (NarCaruMgrrFe,rAlrSi,rrO.oo),but otherwise similar in composition to the near--end-memberdiopside, is more conducting,has a small- er activation energy(1.0 eV) over the range 1050 to 1225'C, and showsonly a weak negative fo, dependence: (100):log o: -0.89 - 4640/T - 0.03logfo,, (010):log o: -0.25 - 5270/T- 0.02logfo,, where I is in kelvins, suggestingthat oxygen vacanciesare presentbut are not the domi- nant defect in controlling the conductivity. It is probable that oxygenvacancies are also presentin pyroxenesin nature, even though they may not be the dominant point defects in natural assemblages.If, as is commonly assumed,oxygen ions are the largestions in the pyroxene lattice, oxygen vacanciescould have a more important influence on chemical diffusion rates, especiallythe Ca diftrsion rate, than on electrical conductivities. Therefore, bulk chemical difusion in pyroxenes would be favored by reducing conditions, not oxidizing conditions as is commonly as- sumed. INtnonucttow neededto model pyroxenesthat exist under different con- When Huebner et al. (1979) measured the electrical ditions. Huebner et al. went on to stressthat differences conductivity oforthopyroxene as a function oftempera- in the states of aggregation might bring about uncertain- single crys- ture, composition, orientation, andfr, the resulting mea- ties in applying laboratory measurementson Point surements were not sufficiently systematic to be inter- tals to real rocks.The 1982Chapman Conference on importance of preted in terms of the activation energiesand mechanisms Defects (Schock, 1985) emphasized the consideringpoint defectswhen trying to interpret electri- cal and mechanical properties measured in the labora- tory. Recently,we have encounteredsimilar uncertainties * Present address: Department of Geosciences,204 D{ike Building, PennsylvaniaState University, University Park, Pefin- when applying laboratory measurementsof chemical dif- sylvania 16802,U.S.A. fusion rates to natural assemblages.To addressthese un- 0003-o04x/88 / | | | 2-r 235$02.00 r236 HUEBNER AND VOIGT: ELECTRICAL CONDUCTIVITY OF DIOPSIDE certainties,we choseto return to electrical measurement Duba et al. (1976)subsequently measured orthopyrox- techniquesbecause they are a fast, yet accurate,method ene from Bamble, Norway; we identified talc in this sam- for deducingdefect structures(Tuller, 1985).We chose ple and determinedthe bulk compositionto be remark- initially to focus on relatively pure diopside for three rea- ably devoid of Al and Cr, NarCa,,Mn,Mg,urrFerrrTir- sons. (l) Augite is an important mineral in mafic rocks Alosi20roo6000.At 1000 "C and log.for: -9.5 U"rin bars), and the mantle, where it contributes to observedelectro- initial valueswere again anomalouslyconductive and un- magnetic sounding profiles and has undergonetransfor- stable, presumably becauseof dehydration of the talc. mations involving chemical diffusion. (2) Diopside, the Subsequentmeasurements were stable. At 1000 "C and principal end memberof most augites,is chemicallysim- logfo,of - I to -l6,thesloped(logo)/0(logf",)was-\f6. ple, thereby reducing the number of compositional vari- At s-kbar pressureand log ,fo, approximately - 12, the ables. (3) There is no unambiguous set of electrical con- results were complicated by (presumably) dehydration and ductivity measurementsfor this phase or for any other melting. The apparent activation eneryieswere 1.0 eV at pyroxene. In this first report we limit ourselvesto mea- <1250 oCand 1.8eV at >1250 'C. The conductivitywas surementson the diopside singlecrystals that will be used greater at 5 kbar than at I bar. to synthesizeaggregates for future studies. In future re- Dvorak and Schloessin(1973), in an investigationde- ports, we expect to discuss how changesin the state of signed to reveal conduction mechanisms,also measured aggregationand the cation ratios will change the mea- an orthopyroxene from Bamble, Norway. We have found sured conductivities, enabling us to compare data on sin- two compositions(Mg:Fe : 96:3 and 86:13) of ortho- gle crystals and polycrystalline aggregatesand to extract pyroxene from this locality, each with included talc (and bulk conductivity data from aggregatesprepared to de- one with magnetite).Unfortunately, Dvorak and Schloes- sired compositions. This long-rangeintention influenced sin failed to identify the composition of their pyroxene, our choice of measurementtechnique. worked at a difficult experimental range of pressure(24- 56 kbar) and low temperature,and neither measurednor defined the redox state (/or) of the orthopyroxene. Mea- Pnrvrous wonx sured values ofthe activation energyranged from 0.45 to Reports in the literature present an ambiguous "pic- 0.65 eV, distinctly lower than most other values.On the ture" of pyroxeneelectrical-conductivity values (data that basis of the variation with pressureand temperature,the can be used to quantify geophysicalmodels that use nat- activation energycould be resolved into two terms. The ural compositions, temperatures, pressures,and redox dominant energyterm was associatedwith increasingthe states).A review of previous measurementssuggests that number of chargecarriers; only 0.06-O.08eV was attrib- the conductivity measurementsthemselves are precise; uted to misration. rather, the ambiguity is probably causedby subtlechanges Will et al. (1979) measured the conductivity of syn- in composition or by differencesin the state of aggrega- thetic Fe-free orthoenstatite (MgrSirOu)aggregates at l0 tion within the sample cell. The following summary will kbar and 1600 Hz, using quartz to buffer the silica activ- illustrate some of the uncertainties in past efforts and ity or forsterite to buffer the MgO activity. No attempt thereby provide a rationale for the design of our experi- was made to distinguish the bulk component from any menls. grain-boundary component of the measuredconductivi- Much early work dealt with crystalsthat contained in- ty. On the assumption that the conductivity would be clusionsof a secondphase (commonly talc). For instance, independent of/or, they made no attempt to control or Duba et al. (1973) measuredthree orthopyroxene single measure/or.At 600 "C the measuredconductivity of en- 'C, crystals at 525 to 1009 controlled oxygen fugacity, statite in equilibrium with quartz was slightly greaterthan l-bar pressure,artd 1592 Hz. Two of the samplescon- for enstatite in equilibrium with forsterite. With increas- tained talc (A. Duba, pers. comm.), which should have ing temperature, the differencesin conductivity declined dehydratedin this temperaturerange and probably caused until, at 1000'C, the conductivitieswere equal. The mea- the anomalously large and unstable conductivity values sured activation energieswere l.l I eV for enstatitein observed when the samples were first heated. Gem-quality equilibrium with quartz and 1.25 eV for enstatitein equi- orthopyroxene[NarCarrMnrMg,rruFe, ruNi,Cr.TioAIroSi,rrr- librium with forsterite. The usefulnessof these conduc- Ouooo],the third sample, had no talc inclusions and be- tivity valuesmust be regardedas uncertain becauseof the haved stably. Although it had an intermediate value of uncertaintyin prevailingf,. Fel(Mg + Fe) and higher concentrationsof minor com- In a similar study,Voigt etal. (1979)searched for sys- ponents,the gem-quality orthopyroxenewas lessconduc- tematic variation between the conductivity of synthetic tive than the two otherpyroxenes,Na2Ca3Mn3Mgr873Fers0- orthopyroxene aggregatesand pressureor Fe/(Mg + Fe). CroTi,AlrSi,nrrOuoooand Ca,rMg,rorFerrsNirAlTSiree5O6ooo.Conductivity regularly increasedwith increasingFe/(Mg Measured activation energiesat constant values of /o, * Fe) over the completerange of solid solution.Over the were in the range 1.2-1.9 eV with a slight tendency for range Fel(Mg + Fe) : 0.0 to 0.2, an increasein pressure higher valuesat temperaturesgreater than 900 .C. At 1000 from l0 to 20 kbar decreasedthe
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