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The Electrical Resistivity of Galena, Pyrite, and Chalcopyrite

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The Electrical Resistivity of Galena, Pyrite, and Chalcopyrite 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
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