American Mineralogist, Volume61, pages248-259, 1976
The electricalresistivity of galena,pyrite, and chalcopyrite
Doneln F. PnlorrronreNn RnlpH T. Suurv Departmentof Geologyand Geophysics,Uniuersity of Utah Salt Lake Cily, Utah 84112
Abstract.
The sulfidesgalena, chalcopyrite, and pyrite are semiconductorswhose electrical resistivity and type are controlled by deviationsfrom stoichiometryand impurity content,and henceby their geochemicalenvironment. We measuredelectrical resistivity,type, and the impurity content (emissionspectrograph and microprobe) on small volumesof sample.Our results, together with those obtained from a comprehensiveliterature analysis, are usedto construct histogramsof the natural variability in carrier density and resistivity. Sulfur deficiency is the dominant defect in chalcopyrite and hence almost all natural samplesare n-type. lt appearsthat the copper/iron ratio is also important electrically,the copper-richsamples being the more resistive. lmportant donor deiectsin galena(z-type samples)are antimony and bismuth impurities, and sulfur vacancies;acceptor defects(p-type samples) include silver impurities and lead 'Mississippi vacancies.P-type samplesappear to be restrictedto Valley' and argentiferous deposits. In pyrite, electricallyactive impurities include cobalt, nickel, and copper as donors, and arsenicas an acceptor.Deviations from stoichiometry,in the same senseas galena,may be important. Pyritesfrom sedimentaryand epithermaldeposits are usuallyp-type if cupriferous sulfidesare not present.Samples from hypothermaldeposits are usuallyz-type if there are no arsenicminerals in the assemblaee.
Introduction pyrite, chalcopyrite,and galena;in particular to iden- The sulfidesgalena, chalcopyrite, and pyrite are tify the dominant donors and acceptors,and under- semiconductors.The semiconductivityis due to free stand the geologic factors which may control their charge carriers, for which three sources may be concentrations.Our experimental method consisted distinguished:(l) deviation from stoichiometry, (2) of measuringthe resistivityand thermoelectricvolt- traceelements in solid solution, and (3) thermal exci- age on a suite of specimensof diverse geological tation across the energy gap. The energy gaps of origin. The resistivityis proportional to the product galena,chalcopyrite, and pyrite are 0.4,0.6, and 0.9 of the carrier concentration and mobility, while the electron volts, respectively (for referencessee Shuey, thermoelectric voltage gives carrier type and some 1975,Chapters ll, 13, l6); therefore,the contribu- information about the carrier concentration.On the tion of the last source is negligible at room temper- basis of thesemeasurements, representative samples ature. The crystal defectswhich produce the carriers were selectedfor chemicalanalysis (spectrograph and in thesesulfides can be classifiedas either donors or microprobe) to identify the electricallyactive impu- acceptors,depending on whether they "donate" elec- rities. trons to the conduction band or "accept" electrons In recent years there have been numerous pub- from the valence band, leaving a hole. Unless the lished measurementsof resistivity,thermoelectricity concentrationsof donors and acceptorsare almost and impurity, particularly in pyrite (e.9.,Fischer and exactly equal, the carriers are of predominantly one Hiller, 1956,Favorov et al., 1972).Rarely have all type. The semiconductionis termed n-type or p-type three kinds of data beencollected on the samespeci- according to whether electrons or holes are domi- men. As we report our experimentalresults, we in- nant. dicatewhen a similar result has beenpreviously pub- The main objectiveof the researchreported in this lished. For our interpretations we draw upon all paper was to determinethe sourcesof free carriersin available data, our own and that already published. ELECTRICAL CONDUCTIVITY OF GALENA. PYRITE, AND CHALCOPYRITE 249
Electrical measurementprocedure Spurious thermoelectricvoltages on polished sur- The thermoelectric and resistivity.measurements faces of galena, due to the polishing process, have were made with a linear, four-needleprobe (Signa- beendocumented by Granville and Hogarth (1951). tone Co.). The steel needleswere spaced 0.63 mm Tauc (1953) suggestedthat thesewere due to electri- apart, with tip radius 2.54 p"m,and loaded to 80 gm cally charged mechanicaldamage in a surfacelayer. per needle.Sulfide hand specimenswere preparedfor We found no evidenceof such a surfacelayer effect, measurementby grinding and polishing a cut face possibly becauseour probe load was an order of down to a 6 pm diamond lap. The probe did not magnitude greater than theirs. mark the pyrite surfaces,but left pits of diameter20 to 70 pm and depth I to 30 trrmin galena and chal- 'point' Resistivity distributions copyrite. In addition to the contacts on the polishedsurface, we useda largearea contact fixed to The three sulfidesbeing consideredhave a variable the rough surfaceof the specimenby a silver impreg- resistivity, due to variations in composition. To in- nated silicone paste (Eccobond 59C from Emerson vestigatethe statistical distribution of resistivity in and Cuming Inc.) each case,we combined our data with all the pre- Current for the measurement(0. I to l0 mA) was viously publisheddata we could find. In this way we supplied by batteries,and the voltage was measured reduced the statistical fluctuations due to limited with a Hewlett Packard 419A. A.C. pickup was re- sample number, and also averagedover many more duced to about 4 pV by floating all circuitry and localities.The histograms(Figs. 1,2, and 3) include grounding the specimenthrough the large area con- only resistivity measurementson natural specimens tact. for which type was also determined and for which The reported resistivity measurements(Table I ) ancillary information indicatedthe measurementsre- wereobtained by passingcurrent through the outside ferred to a region of electrical and mineralogical two needle electrodes and measuring the voltage homogeneity.Thus we excludedresistivity measure- acrossthe inner two electrodes(Wenner array). Lin- ments on grains of mixed type and on polymineralic earity and reciprocity were routinely checked. To ores. monitor sample homogeneity, adjacent electrodes Figures I and 2 show that for both pyrite and were usedfor current and voltage(Dipole array), the galena p-type samples have a higher averageresis- probe was raised and lowered severaltimes, and the tivity than n-typesamples, although the distributions sample was displacedseveral times transverseto the overlap considerably. For pyrite (Fig. 2) the resis- array, by a distanceabout equal to the array spacing. tivity distributions seemsto be log-normal, while for We considered the resistivity to be homogeneous galena (Fig. l) they are curiously flat, i.e., have a when the resultsfrom all thesemeasurements varied negative kurtosis. The individual collections com- by lessthan a factor of 2. In many casesthe variation bined for Figures I and 2 do not differ significantly was lessthan a fourth of the allowed range. from the total resistivitydistribution for given min- Thermoelectricvoltage was measuredbetween one eral and type. However, individual collections do of the needleelectrodes and the large area baseelec- differ significantly in the relative proportions of n- trode. We usedthe convention that the thermopower type andp-type.No homogeneousp-typegalena sam- (Seebeckcoefficient) is positivewhen the gradientsof ples were present in our collection, but in Figure I voltage and temperature are in opposite directions, about one quarter of the galena samplesarc p-type. i.e., when the hot electrode is electrically negative. For chalcopyrite all measurementsmeeting the The point electrode was heated by passingcurrent given criteria were on t?-typesamples. However, p- through a fine wire wrapped around it. The valuesin type CuFeS, is known. Some of the syntheticspeci- Table I are for 40 mA current in 10 turns of 138wire. mensof Donovan and Reichenbaum(1958) were p- From the magnitude of the voltage,the temperature type, while Austin et al. (1956)and Olhoeft (personal differencewas of the order of 1'C. The valuesgiven communication, 1974) eachreport one naturalp-type are strictly relativeto the thermopower of steel( l0 to chalcopyrite.The individual collectionsused in Fig- 15 p,Y/"C), but this is small enough to be neglected. ure 3 do show significantlydifferent resistivitydistri- In a few cases only a sign is reported because the butions. More specifically,our values are distinctly samples were used for other experiments before the higher than thosereported by Parasnis(1956,p.270). quantitative thermoelectric measurements were The respectivemodes are 3 X l0-3 ohm-m and 4 X made. l0-o ohm-m. We attributed this to a differencein ore 250 D. F. PRIDMORE AND R. T. SHUEY
Tnnr-sl. Sampledescriptions and electrical data
Theroo- Reslst- electllc ivtty qu Deposlt voltage ole Locatlod type* (yV) meters* ComeDts
EAIENA
114,1 Edvards 3100 level D-7M Met. Var. Coarse to fine gralned talena, BaI@t, New York nl.nor pyrite, slllcates. ).L7,L 5767 Bench, Berkeley HvP var.- 3.4xI0-'1 }tediuo grained galena, Dinor ?it, Butte, Montana py!ite, 6lllcatesi varlable themo- rr7 ,2 Var. - 2.6xIO-2 electrlc voltage probably due to lncluded phasee. 118 Baxter Sprlngs, Kansas L.L.Z. -403 9.6x10-t Coarse trained galena. 1L9,1 7100-7500 level, llecla HVR -490 t.5xIO-2 Fine gralned galena, silicates. SEar Mlne, l{allace LL9,2 Coeur D'Alene HVR -480 ).)xru _ rt9,3 -488 1.7xI0-3 L2l Lark Mlne, Tlnclc, Utah HVR -620 I.7xI0-'z Coarse gralned galem, Dinor chalcopyrite, q@rtz. 123,1 Lark Mine, TinElc, Utah HVR 1.1xI0-' Coalse gralned talena. L23,2 -530 2.2xLO'2 123,3 7.6x10-t L24,L Butte, l'1onta@ HVP -290 6.2x10-s coar6e gralDed galena. 125,1 No, 29 uine, St. Joe, L,L.Z. 8.2x10-3 Coarse gralned galena, minor Viburnm, Miasouri dolomlEe; variable chemo- t25,2 9.7x10-l elecEric sign probably due to changee in chenistry; no indicatlons of separate phases. L27,L SrokenHllI, Australia Strat. -300 1.2x10-' coarse gralned galeoa and sPhaIe!1te. L28,1 3850 level,Lucky Friday HV -488 8.2xl0-a coarse Eo flne glained galena, Mlne, Coeur D'Alene mlnor chalcopyrlte, slllcaEes. L29,I 7700 level,Lucky lriday lw -493 1,5x10-3 Medlw grained galena, nloor Mine, Coeur D'A1ene Eetrahedrite, quattz.
PYRITE
3,1 BinghaE, UEah l{P +185 5.0x1trr Single crystal, striations. 4,I Dugway, Utah HV Var.- 5.3x10-'? Single crystal' slrlaElons; variabllity in thernoefectrlciEy due to separate phases. 5,I Cactus Mine, Newhouse, HV -+ Var. Singlecrystal,sErlaclons, zone Utah of concentric hmisphelIcal ho1es. 6,1 Leadville, Colorado HRV +745 3.7x10-' Coarse gralded pyrlte, ouartz crv6tals. 1,L Sullivan, Missouri MaC.H -89 3,9xl0-a Single rwinned crystal, s!riated. 8,1 Sullivan, Missouri Mag, H t.9xt0-s Coarse grained pyriEe,magnetlte, 8,2 -36 2.3x10-5 nlnor chalcopyrlte, sllicaEes; 8,3 3,9x10-' Eherooelectric voltage quite variable. 9,L Sullivan, Missouri Mag.H -36 3.1x10-5 Coarse grained pyrite. 10,1 6600r level South face, HP - + 3.1x10-' Medlum to coarse grained vuggy 10,2 Binghan, Urah - + 2,2xIO-' pyrite, s{Iicates; variable themo- electric 6ign probably due to inc]uded phases. 11,1 6600' Ievel SouEh face, tlP -+ 7.3x10-r Medium to coarse grained vuggy II,2 Bingham, Utah -+ 2.7 pyrite, silicates; variable Lherho t.L,3 2.9x10-' eleclrIc slgn probably due to lncluded phases, f2,L ceco, Ontario Strat. -98 4xIO-2 Medium grained granular pyrite, L2,2 -+ 1.5xI0-r silicates; variable thermoelecEric 6ign probabLy due Eo included phases. 13,1 Geco, Ontario Strat. + 3.Ixl0-'? Coarse gralned pyrite, ninor sphal- 13,2 fr1not _ 4.7xl0-'? erlEe, chalcopyrite, galena, silicates; variable thetnoelectrlc sign probably due to included phases, 14,2 4933 leve1, Berkeley PiE, HVP 2.1x10-' Coarse gralned pyrite, silicates; L4,3 Butte, Montana -6r 1.2xI0-a chalcocite stainlnc. 15,2 4933 level, Berkeley Pit, HVP -8t- 1.5xIO-r Mediun Eo coarse grained pyrite, 15,3 Butte, Monlana -84 2,1xI0-' silicates; chalcocite stalnlng. 16,1 4900 level, N. t,li, Series IwP -7 4 4.9xI0-' Medium grained pyrite, sillcates; L6,2 veins, Butte, Montana -76 3,2xI0-' chalcoclte sEainlng. L7,I Leadvllle, Colorado HRV +153 3.5x10-'? Single crysEal, sEllallons, fractured. L7,2 + 2. 8x10-'z IE,1 Phyllic Zone D1abase, HP -83 3.1x10-3 Medium gralned pyrlte, quartz. Ilayden, Arizona L9,2 2900 level, (Leonard Mlne) HVp -65 1.8xlo-3 Fractured coarse grained pyrite, 19,l Berkeley Pir, Butte, 6.5x10-' quartz; chalcocite stainlng, llonEam 21,1 BaI@r No. 2, 1900 sub MeE. -46 2.8x10-5 Coarse gralned pyrite, pyrrhoEiter level, BalmaE, Nev york sl11cates.
*Abb"euiations as foLLoDs: H=hydrothemal; p=porphyrg; R=repLacenent; v=Dein, var. =uqriable; = strat. rc|siue stratifom; L.L.z. = Linestone-Lead-zinc; Mag. = rcgimatic; aet, = highla netmorphbeed; Cont. = contact net@or?hic. ELECTRICAL CONDUCTIVITY OF GALENA, PYRITE, AND CHALCOPYRITE 251
TABLE l, continued
j Thermo- Resis- - : electlic tlvity A 3 Deposit voltage ohm- tyPe* (lV) meters* Coments
CHALCO?YRIT E
301,1 }lt. I6a, Australia Strat. -363 2.2x70-' Fine grained chalcopyrite, silicates. 302,1 Park Clty, Utah HVR -465 5.1x10-3 coalse grained chalcoPyrite, mlnor galena, siLicates. 303,L Tlmlns, Ontario StraE. -348 2.6x10-a Fine grained chalcopylite 303,2 -350 3.4x10-a 304,1 cale@ Mlne, wallace, HVR 2.9xto-r },ledim grained chalcopyriEe, minor 304,2 coeur D'Alene. 3.0x10-3 pyrlte and galena' 305,r Anerlcan Fork, Utah HVR -420 1.9x10-3 Coarse grained chalcopyrite. 305,2 -408 1.7x10-3 305,1 Prescott, Arizona cont. -334 9,9x10-a Coarse Slained chalcopyrite' Pyrite 306,2 -326 1.5xI0-3 quattz. 307,1 Jewel Stope, Jerone, HR -334 1.8x10-3 Iine grained chalcoPyrite with sub Arizom connected silicates' 308,r Matahanbre, Cuba Strat. -444 2. Ox10-l Fine gralned chalcopyrlte. 309,1 Bingh@, Utah HP -365 3.7xI0-' Coarse grained chalcopyrlre; chalcocite io9,2 3. 6x|0-a stainlng. 3t1,1 No, 29 mlne, St. Joe, L.L.Z. 1.9x10-3 Coarse to mediun grained chalcoPyrite' 311,2 Viburnu, Ilj.ssouri 2.3x10-3 vuggy quartz. 312,1 Geco, ontalio SErat. 2.3x10-' Medim gralned cha.Lcopyrite, pyrite. 3r2,2 1. 6x10-a 313,1 BuEEe, lvlontana HVP -365 I.6x10-' Med1utugrained chalcopytile, Pyrite. 3L3,2 -435 I.6xI0-2 314,1 Ore Zone Dlabase, HP -423 2.6{.0-3 Vein oI medium Srained chalcoPyrite Hayden, Arizona in diabase. 3I5,1 4500 level, Steward Mine, HVP 2.6x10-r Medlun gralned chalcopyrite, minol 315,2 3utte, Montana 2.6xI0-r pyrlre. 3r5,3 3.0x10- I
environment. Our samplesare mostly from the por- the variations illustrated in Figures 1,2, and 3 must phyry copper deposits of the western U.S., while be due to variations in mobility and carrier density. thoseof Parasnisare from Swedishiron deposits.The Where the deviationsfrom stoichiometryandlor pur- same correlation was noticad by Harvey (1928, p. ity are known, carrier density can be calculated. 799) who "observed that many low-resistancespeci- Then, knowing the resistivity,the mobility can be mens came from mines that contained pyrrhotite, calculated.The only samplesof ours for which this is while many high-resistancespecimens came from possible are pyrites fr5 and fi21(Table 3). Here we can mines that contained bornite." equatethe density of carriersto that of Co atoms and then calculatemobility usingthe measuredresistivity. Mobility and carrier density For #21 the resultis 5.5 cm2V-'sec-'.Near sitel8-2 Resistivityis proportional to the product of carrier densityand mobility (e.g.,Shuey, 1975, p.46) so that
r5 a lrJ J20 (L (n ul d'o a olo z U' TJ I ---'l d5 -- -- z - " ll too to3 tdz tdr loo J ll ro5 L_J RESISTIVITY, OHM-M o pyrite. Solid line is for n-type, -R _j -l FIc. 2. Resistivity distribution for ro" to" ro dashed line is for p-type, and samples of mixed or uncertaln type RESISTIVITI OHM-M are omitted. Basedon the data in Table I plus the following values: Frc. l. Resistivitydistribution for galena. Solid line is for n- 22-Smith (1942): 2-Telkes (1950); 6-Marinace (1954); 20- type, dashed line is for p-type. Samples of mixed type are omitted. Sasaki (1955); 20-Hill and Green (1962); l8-Kireev e.tal. (1969\; Based on the data in Table l, the referencesto Figure 4, the 8 l-Fukui et at. (1971'l:7-Ovchinnikov and Kirvoshein (1972); samplesof Telkes(1950) and the 35 samplesof Kireev et al. (1969\. 6-Horita (1973). 252 D. F. PRIDMORE AND R, T. SHUEY
ably narrow, from 8 X l0r8cm-3 (Teranishi,l96l ) to 4 X lOr'gcm-'(Donovan and Reichenbaum,1958). This small range, together with the low value of the upper mobility limit, explains the relatively narrow a distributionin Figure3. UJ J (L
Figure 4. We find no statisticallysignificant correla- l< 17 lorc lor/ tion of mobility with carrier density. lol9 For pyrite thereis no statisticaldifference in carrier CARRIERSPER CUBIC CENTIMETER density betweenn-type and p-type. This differencein Ftc. 4. Publishedcarrier densityin natural galena.Solid line is resistivity(Fig. 2) is due entirely to the differencein for n-type, dashed line is for p-type. The 63 values are from the mobility of electronsand holes. following sources: l-Brebrick and Scanlon (1954); l4-Putley (1955);5-Finlayson and Grieg (1956);2-lrie (1956);5-Allgaier Although there are relativelyfew published values and Scanlon(f 959); 2-Grieg (1960);l-Farag et al. (1965);33- of carrier densityin chalcopyrite,the rangeis remark- Kireev e, al (1969). ELECTRICAL CONDUCTIVITY OF GALENA, PYRITE, AND CHALCOPYRITE 253
Impurity 200
According to the principlesof semiconductorphys- e ics, impurity atoms in solid solution generally have the following effects: a substitution of an element from the right in the periodic table constitutes a o donor defect, and conversely a substitution of an o element from the left constitutes an acceptor defect. Substitution from the samecolumn, such as Se for S, a ! has no effect other than slightly reducing the mobil- o ity. A metal interstitial is a donor, and anion inter- o roo + stitials are of negligibleabundance. We did not con- ()E + 1+ sider impurities in chalcopyrite because of the t+ + evidence below that departure from stoichiometry + has a greater influenceon the carrier density. + + Galena + For five galena samples chosen to span the full range of resistivity, the galena and included phases were scannedfor Ag, Bi, Cu, and Sb (Table 2) with the microprobe (n.n.l. rux-su). ln only one speci- -? -l ro- ro- lo' men, fi129,were any of theseelements detected in the RESISTIVITY, OHM-M galena.We did not notice any tendencyfor Ag and Sb concentrationsto fluctuatesynchronously. This is FIc. 6. Correlation of resistivity and thermoelectric voltage in marks are for p-type, the others are for n-type. somewhat surprisingin view of the extensivesolubil- pyrite. The circled Data are from Table l. ity of AgSbS, in PbS (Wernick, 1960).In the speci- men with p-type regions (#125) no trace elements were detectedin either n-type or p-type regions. graph(with a gratingof 30,000lines per inch) and by microprobe. The microprobe-sensitivityca. Pyrite 300-400ppm for Co and Ni but much lessfor As- Ni, Nine pyrites of various types were analyzed for showedthat only pyrites18 and 2l containedCo, minor elementsby a 3-meterBaird emissionspectro- or As in solidsolution. Samples of 40 mg for spectro- graphic analyseswere securedby drilling, using a tungsten-carbidetipped bit, from the polishedsur- faceswhere the resistivitymeasurements had been 600 + made.These powders were ground and mixedwith a germaniumoxide buffer (one part sample,3 parts a .+ buffer,total mass16 mg) andfired with a D.C. arcof F t+ ++ was detectedin any of the J 12 amps. No tungsten o 400 + samples,so we assumethere was no contamination o from the bit. Semi-quantitativestandards (1, 10,100, E 1000,10000 ppm) were made by mixing spectro- I ++ graphicallypure hgmatite and flowersof sulfurin the = 200 correctproportions for pyriteand pipetting in known amountsof impurities.Such a standardis not ideal becausethe sulfur is likely to comeoff earlierthan for pyrite, therebycreating different matrix effects'The good agreementof spectrographicand probe results -R to- ro3 rol (for Co in samples18 and f2l) suggeststhe spectro- REStSTIVITY,OHM-M graphstandards were adequate for this study' (Table3) for severalsam- Ftc. 5. Correlation of resistivity and thermoelectric voltage in The spectrographresults galena. Data are from Table l. plesshowed Ag, Bi, andCu to bepresent in amounts 254 D. F. PRIDMORE AND R. T. SHUEY
Tnrln 2. Microproberesults for Ag, Bi, Cu,Sb in galena
Ave raqe uarraer--:;1.- Sanple- resistlviry Comenta SIqD - ohm-neters
IL7 3'Ox1o-'? ChalcopyrJ.te, pyrite, sphalerite grains. Ex- solved phases of AgCu nineral (Jalpaite?) and Ag nlneral (argentite?). L23 1.4x10-2 sphalellte and pyrlte grains. L24 6. 2x10-5 No obsenable separate phases. L25 -+ 9'OxlO-3 No detectable change in Bi, Ag, Sb, Cu across aras where type changes. Exsolved Ag Eln- eral (argentite? ) , L29 1'5x10-3 b(solved SbCu(variable)Ag phases. Probably a mober of tennantite - tetrahedrite series. Detectable Ag, Sb in lattice. AEount of Ag qulte varlable. above the sensitivitylimit of microprobe. Hence we with the statisticsreported by Favorov et al. (1972). returnedto the probe and scannedfor theseelements. Also three of them have higher As than (Co + Ni), In lro case were they detected in the pyrite. Instead the exceptionbeing the one sampleof thesenine (ll I ) they were found in small inclusionsand (for Ag and for which type could not be reliably determined. Cu) filling cracks. The sample of mixed type (15) was a singlecrystal In Table 3 the four samples(#3, #5, #ll, #t7) with round holes about 30 to 70 pm in diameter, lowest in Co are the four which are not definitelyn- arrangedin a zone about I mm wide and concentric type. Three of them had no detectableNi. This agrees with the edgesof the crystal. The crystal is n-type in
Tasr-E3. Pyriteimpurity analyses
o z Resist- o lty ilq Emlsslon spectrograph analyses Microprobe results for Co, Nl, As, ohm- PPl..t Bi, Ag, Cu and coments on pollshed A@ Eeters# N1 Ag Mn Bi As sbv Ti section of saple.
n-type caarier slgn -
4,L 5.3x10-' 1oO 90 >100 40 5 100 100 5-100 Minor unidentifled p-type exsolved phase 8,1 1.9x10-s r0,ooo 400 >>100 300 50 20 r000 5-1 Up to 10,000 ppn cobalt ln structure, vari-able. Ulnor cracks contain co- balt. SmJ-l high sllver phase ln pyrlte, L4,3 1.2xr0-4 roo 5 >100 10,000 - 10 300 5 t Intergranular bornlte, cove11lte, chalcoclte dlgenite. 18,1 3.lx1o-' 70 15 r00 50 80 I t Intergranular silicates. 21,I 2.8xr0-s looo 10 >I00 30 40 200 I 1 1000 ppn cobalt ln structure
p-lype carrier sign +
3,1 5, Oxlo-r 50 >>>r00 40 - 700 400 IO Exsolutlons of CuAgBi ulneral. Possibly a Edber of tennanlite- tetrahedrite series. L7,2 3.5xI0-3 10 >r00 500 - 40 r00 No other obsewable phases, Copper present in major fractures.
nixed type carrier sign + -
5,1 Var. 50 50 r00 100 100 - 200 10 30 Heulspherlcal hole nargins and s@e cracks high ln silver. One hole nargln high in copper.
questionable type
11,1 20 >>100 - JAveragel 3000 3000 - 70 Chalcopyrite, chalcoclte, bornlte 11,2 r.7 I | grains. Exsolved phase of BtCuAg Eineral in pyrite. * Linits _of detection leVnl, fgi spectrcgraphic malyses uere aa follous: Co - 5; Ni - 1; Ag - .1; Ctt _ 7; I4n _ S; . Bi,- 1.0; As - 100; Sb - 1; V - 1; ri - l-. ** Abb?eDintdon: uan. = oa?inhle. ELECTRICAL CONDUCTIVITY OF GALENA, PYRITE, AND CHALCOPYRITE 255 the vicinity of the cavities but p-type both at the ao surfaceand in the deepinterior ofthe crystal(Fig.7). 5 Inward from the p-type zone the thermoelectric volt- o age is very erratic, indicating near equality of donor o E -50 and acceptor concentrations.Radial zoning of this (J sort has beenpreviously noted by Smith (1942,p.7) = and by Fischerand Hiller (1956,p. 287).The micro- probe showed Ag (and in one caseCu) on the edges o.4 of the cavities,and also in somecracks. No impurities were detected in the pyrite, either in n-type or p-Iype o (J regions. One possibleinterpretation of our data for pyrite 15 is as follows: The crystal initially grew asp- ;e o.2 type, due to content of As (and possibly Mn). Then F the content of Ag and Cu increased,these acting as = donors to compensatethe As acceptors.Then a Ag- rich phase precipitatedin the growing pyrite, which o by this time was completely n-type.For a final stage Frc. 8. Thermoelectric voltage (above) and wt percent cobalt profile pyrite The microproberesults of growth the initial conditions were repeated.The (below) along the same in f8. for Co are the average of two traverses. Ag-rich phase was later eroded out leaving the cav- ities, and possibly at this time Ag was deposited in some cracks. cobalt concentration varied from over I to 0'06 wt Among the five n-type pyrites of Table 3, the two percent.Figure 8 shows how the thermoelectricvolt- outstandingly low in resistivity(#8, #21) are also the age variessynchronously with Co content. This data two rich enough in cobalt for the probe to prove its is in accord with theory (Tauc, 1962)rf we assumea presencein the pyrite structure. For sample fi21 the temperaturedifference of l'C and an effectiveelec- Co content was fairly uniform at 1000ppm : 0. I wt tron mass of 0.1 times the free-electronmass. This percent : 0.2 at percent. By contrast, in pyrite 18 latter value is also implied by the thermoelectricand the Hall effect data of Bither et al. (1968) on one sample of syntheticn-type FeS2. Comparison of data for pyrites fi4 and 114 illus- trates the importance of Cu as a donor. Pyrite I has Co + Ni ) As without any of the impurity concen- trations being very high. lt is accordinglyn-type with fairly high resistivity.In pyrite ff14the specanalyses P show (Co = Ni) ( As which would suggestp-type semiconduction. But this sample is evidently satu- N rated with Cu, basedon the extraordinary high Cu analysisand the presenceof various Cu-rich miner- als. The observedlarge r-type conductivity must be attributed to the donor effectof Cu.
+rOO Discussion The lower limit of detection with spectrographic analysisis about 1016cm-3 for most impurities, o though somewhathigher for As (about l0'8 cm-3). In galenaand pyrite this is nearthe lower limit of carrier -roo density (cf Fie. 4). Although the spectroscopehas sufficientsensitivity for most impurities,many sulfide specimenscontain microscopicto submicroscopicin- Ftc. 7. Two typical profilesof thermoelectricvoltage (in micro- clusions which will contaminate any microsampling volts) across pyrite sample 15. Arrows show the zone of round cavities. of the specimen.Sampling with the microprobe is 256 D. F. PRIDMORE AND R. T. SHUEY
much more reliable,but the lower limits of detection Pyrite are close to l01s cm-3. In galena or pyrite, carrier densitiesthis large are exceptional(cf Fig. a). It seemsthat at high temperatureand a broad The measurementsreported above suggest that, rangeof sulfurpressures, pure pyrite is n-type.Crys- with few exceptions,large spectrographicimpurity tals producednear 700'C by Bittner(1950) from a levelsindicate contamination by separatephases. The bromidemelt and by Bitheret al. (1968)using chlo- correlationsreported by other authors betweentype rine transportwere all n-type.We heateda portion of and impurities (Hiller and Smoczyk, 1953;Fischer our samplefi17 for a weekat 500'C in a closedvessel and Hiller, 1956,Favorov et al., 1972) may strictly (freevolume about 100times sample volume). This representcorrelations with microscopicto submicro- samplewas initially p-type,and our purposewas to scopic inclusions containing these impurities. Of createdonors by drivingoffsulfur. After heating,the course trace elementspresent in separatephases are samplewas not quenchedbut ratherallowed tq cool likely present in solid solution in the host as well. naturally.There was no type changebut resistivity Within theselimitations, our resultsfor pyrites from did increase,suggesting that donorshad been created North American depositsagree with previouslypub- to compensatethe acceptorsinitially present. Resis- lishedresults from depositsin Germany (Fischerand tivity measurementsranged from 4.7 to 4.9 X l0-'z Hiller, 1956)and Russia(Favorov et al., 1972).Spe- ohm-m beforeheating, and from 6.5 Lo 7.4 X l0-' cifigally the important donor impurities are Ni, Co, after heating.When we repeatedthe experimentat and Cu, while As is the most important acceptor 550oC,the sampleshattered with visiblesulfur loss. impurity. Low resistivity occurs in pyrite if a single We could not then measureresistivity, but the type impurity is presentin significantlygreater concentra- had not changed.Bittner (1950, p. l8a) reportedthat tion than other impurities. Roughly equal concentra- in somecases he was ableto changep-type to n-type tions of donor and acceptordefects gives a high re- by drivingoff sulfur,but he givesno details. It is not clearwhether there is a p-typeportion sistivity (e.g., pyri& rt4). of the pyrite field at high temperature.We could not producea changein type by annealingan n-type Nonstoichiometry sampleat 550'C with excesssulfur. However, Bittner A metal excess(sulfur deficiency)corresponds to a (1950,p. 183)reported he could changenatural n- donor defect (n-type semiconduction) and a metal type pyrite to p-typeby heatingin a closedevacuated deficiency(sulfur excess)corresponds to an acceptor vesselnear 700"C and removing the pyrrhotite defect (p-type semiconduction).This is true regard- formed. lessof whetherthe atomic mechanismin either caseis a vacancyor an interstitial. Thesedefects are always Chalcopyrite presentto someextent, becauseonly a finite energyis neededto createthem. Thus in practice a mineral is Metal in excessof stoichiometricCuFeSz is in- never stoichiometric. The maximum deviation in a dicatedas a donor defectbecause of persistentre- homogeneousphase will depend somewhat on tem- ports of an irreversibledecrease in resistivityupon perature of atomic equilibration. lt may however be heating(references in Shuey, 1975,p. 248). It is too small to resolveby chemicalanalysis, while at the plausibleto supposethat the characteristicrr-type same time large enough to account for the observed conductionis a consequenceof the characteristic electricalcharge carriers. This is the situation in all slightmetal excess (Barton, 1973). The naturalrange three of the minerals under consideration. of carrierdensity (see below) corresponds to a metal excesson theorder of 0.I percent.The ratio of iron to Gslena copperin chalcopyritemay also deviatefrom stoichio- metricCuFeSr. Comparison between our resistivity The electrical effects of deviations from stoi- measurementsand thosepreviously published sug- chiometry in galenahave beenwell studiedby Bloem geststhat iron substitutedfor copperwill have a and Kroeger (1956).Their resultsshow that the com- donor effect,while substitutingcopper for iron will position can deviatein either direction dependingon createacceptors. This hypothesisis not at all incon- temperatureand sulfur pressure.Lowering the tem- sistentwith the establisheddonor effectof Cu sub- perature at fixed sulfur pressurechanges the equilib- stitutedfor Fe in pyrite, becauseof the differencesin rium composition from n-type to p-type. bandstructure (Shuey, 1975, p.249,313). ELECTRICAL CONDUCTIVITY OF GALENA, PYRITE, AND CHALCOPYRITE 257
In an attempt to verify this prediction, we coated Pyrile chalcopyrite sample with copper paint and #304-2 Becausepyrite occursin almostany type of geo- heated it in an evacuated vessel at 150'C for 4 logic environment,no simpleanalysis can be ade- months. This low temperaturewas chosenbecause of quate for the factorsinfluencing type and densityof the published report of irreversible sulfur loss at carriers.We are awareof more than a dozenpub- 200'C (Donovan and Reichenbaum, 1958). Light licationson this subject,but the questionis far from repolishingafter heating removed lessthan I mm of resolved.It seemsto us that in many situationsthe material. The measuredresistivity was the same as relevantdefects are the donor impurities Co, Ni, Cu, previouslymeasured (Table l). No definite inference and the acceptorimpurity As. The studieson our can be made, but a likely explanation is that the limitedsuite certainly support this. It can no longer diffusion of copper was too slow for penetration to be supposed,as Smith (1947) in effectpresumed, that the depth probed in the resistivitymeasurement. carriertype and densityare uniquefunctions of for- mationtemperature. Yet it seemsthat two patterns originally recognizedby Smith have been sub- Geochemistryof charge carriers stantiatedby subsequentinvestigations. First, pyrite from sedimentaryand epithermaldeposits is p-type The main motivation for the measurementsand with remarkableconsistency, provided cupriferous statisticalanalyses reported in this paper was to un- sedimentsare excluded. For example,Favotovet al. the geologic factors controlling the density derstand (l972,Table2) showallp-type for a lead-zincdeposit of free charge carriers. We are led to certain con- at Blagadatskoie(1616 samples) and also for a de- clusions, which should be regarded as working hy- posit at Tsentral'noiewith colloform pyrite (2073 potheses,to be testedand revisedby future work. samples).At Gilman, Colorado, Lovering (1958) identifiedpyrite depositedbefore and after the main Galena mineralization.The laterpyrite, with interpretedfor- mation temperaturebelow 150'C, was all p-type. Our suite contained no p-type galenaand only one What is not establishedis whetherthe dominantac- mixed-typegalena (#125). Therefore, we analyzedthe ceptor is iron deficiencyor arsenicimpurity. (lt localities given in the literature for p-type galena. should be rememberedthat arseniccan contribute Besidesthe referencesto Figures I and 4 we consulted almostl0r8 cm-s carriers yet be undetectablein spec- Hiller and Smolczyk (1953) and Smolczyk (1954), trographicanalyses). who type (but not resistivity) on 57 ga- determined The secondpattern, first shown by Smith'sdata, is lena samplesof diverseorigin. We found without any that pyritefrom high-temperatureveins is n-type with definite exception that p-type galena is restrictedto remarkableconsistency, provided arsenopyrite is not two kinds of deposits,namely strataboundlimestone- in the assemblage.As an example,Hill and Green lead-zinc ("Mississippi Valley") deposits and also (1962)found only n-Iype(38 samples)in the miner- those deposits considerednotably argentiferous.ln alizedquartz-porphyry dikes of Mt. Bischoff,Tas- both casesn-type and mixed-type occur with the p- mania,where the pyrite wasformed with cassiterite. type, all with comparable abundance.The acceptor Karasevet al. (1972,Table 2) showonly n-type(784 defectsin galena are Pb vacanciesand Ag substitut- samples)from a tungstendeposit at Kholtosonskoe. ing for Pb. In the argentiferousdeposits it is natural They imply the probablecause is sulfur deficiency to assumethat Ag in solid solution is the dominant dueto high formationtemperature. acceptor.The stratabound limestone-lead-zincdepo- sits are "characteristicallyvery low in silver" (Stan- ton, 1972,p. 5a8). By elimination, Pb vacanciesare Chalcopyrite indicated. This also makes sensebecause limestone- lead-zincdeposits were formed at low temperature, The evidenceavailable to dategives no indication and the extensive experiments of Bloem (1956) that impuritieshave any importanteffect on the re- showedthat lowering temperature(at constantsullur sistivity and type of chalcopyrite.Natural p-type fugacity) resultsin lead-deficientequilibrium galena samplesare tare, apparentlybecause of the donor composition. But the observation of exsolvedsilver effectof excessmetal. Copper substituting for iron which compensatesome of suffide in galena fi125 (Table 2) is a reminder of may produceacceptors caution. the donors,decreasing the carrierconcentration and 2s8 D. F. PRIDMORE AND R. T. SHUEY
so increasingthe resistivity.However, this has not yet FrNr-lysoN,D. M., rNo D. Cnrsc (1956)Electrical measurements been demonstratedby direct experimentation. on natural galena at low temperature. Proc. Phys. Soc. 69R, 796-80r. FrscHrn, M., eNo J. E. HTLLER(1956) On the thermoelectriceffect Acknowledgments of pyrite. Neues Jahrb. Mineral.89, 281-301 (in German). Fnueu, A. J. (1959) Use of zone theory in problems of sulfide Thisresearch was supportedby NSF Grant GA 31571to S.H. mineralogy, part II; the resistivity of chalcopyrite. Am. Mineral. Ward. One of us (D.F.P.) wasalso assisted by graduatefellowships 44, l0t0-1019. from the American Smelting and Refining Company and from the Furut, T., T. Mrvenlr, eNo S. MryeHene (1971)Photoconductiv- Commonwealth Scientific and Industrial Research Organization ity of natural pyrite. Phys. Soc. Japan,3l, 1277. (A ustralia). "/. GnnNvrr-ln, J. W., lNo C. A. Hoclnrn (1951) A study of For assistancein collectionof specimenswe thank R. Miller and thermoelectric effects at the surfaces of transistor materials. D. (Anaconda Gustafson Company, Butte, Montana), J Emery Proc. Phys. Soc. 64B,488494. (Meremec Mining Company, Sullivan, Missouri), D. B. Dill, Jr. Gntec, D. (1960) Thermoelectricityand thermal conductivity in (St. Joe Minerals Corporation, Balmat, New York), H. E Myers the lead sullide group of semiconductors. Phys. Rev. 120, and R. G. Dunn, Jr. (St. Joe Minerals Corporation, Bonne Terre, 358-365. Missouri), S H. Huff (ASARCO, Wallace, Idaho), J. Simos Hlnvnv, R. D. (1928).Electrical conductivity and polished min- (Hecla Mining Company, Wallace,ldaho), J. D. Stevens and S. A. eraf surfaces.Econ. Geol. 23,778-803. Hoelscher (Kennecott Copper Corporation, Salt Lake City, Utah, Htll, P. A., lNo R. GnssN (1962)Thermoelectricity and resistivity and Ray Mines Division, Hayden, Arizona), R. Pembertonand R. of pyrite from Renison Bell and Mt. Bischoff, Tasmania. Econ. Weeks(Noranda Mines Limited, Ontario, Canada). Geol. 57, 579-586 The help of J. Haseltonin making the microprobemeasurements HTLLER,J. 8., lNo H. G. Srrrolcvzr (1953) Spectroanalyticin- and F. Jensen in sample preparation is gratefully acknowledged. vestigations on galena with consideration of photoelectric, We are indebtedto Kennecott Exploration Inc. for the useof their thermoelectric, and rectification effects. Z. Elektrochem. 57, spectroscope facilities, and particularly to S. Cone for help in 50-58 (in German). making thesemeasurements. HoRIrA, H. (1973) Some semiconductingproperties of natural pyrite Japan.J. Appl. Phys.12,617-618 Inrr, T. (1956) Magnetoresistance effect of lead sulfide semi- References conductors I. Measurementson natural specimensof lead sul- fide. J. Phys. Soc. Japan, ll, 840-846. ALlcrren, R. S.,lNp W. W. SclNlox (1958)Mobrlity of electrons Knusev, A. P.. V. I. KusNlrov. V. D. PlNrepv. R. S. SElrul- and holes in PbS, PbSe, and PbTe between room temperature LIN, V. S. Svcuucov, eNo V. A. 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propertiesof chalcopy- Surrs, F. C. (1947) The pyrite geothermometer. Econ Geol. 42, TennNIssl,T. ( I 96I ) M agneticand electric 5l 5-523. ite. J Phys Soc.Japan, 16' l88l-1887. AgSbS,-PbS' SMoLczYK,H. G. (1954)The influenceofsulfur concentrationand WenNrcr, J. H. (1960) Constitutionof the Mineral' 45' of antimony content on lead sulfide photoelements. Z. Elektro' AgBiS,-PbS,and AgBiSr-AgBiSe,systems. Am. chem. 58,263-270 (in German). 59I -598. conductivity SraNtoN, R. L. (1972) Ore Petrology. McGraw Hill, New York WINTENBERcER,M. (1957) Measurement of electrical (in Tnuc, J. (1953)An explanationof someanomalous thermoelectric of crystals,case of chalcopyrite.C. R. 244, l80l-1803 phenomena on the surface of transistor materials. Czech. J. French). Phys.3,259. - (1962) Photo and Thermoelectric Efects in Semiconductors. Pergamon Press,New York and Oxford. TELKES,M. (1950) Thermoelectric power and electrical resistivity Manuscript receiued, September 16, 1974; accepted of minerals.Am. Mineral.35. 536-555. for publication, September 26' 1975.