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Electron Diffraction Study of Phase Transformations in Copper Sulfides

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Electron Diffraction Study of Phase Transformations in Copper Sulfides American Mineralogist, Volume 62, pages 107-l 14, 1977 Electron diffraction study of phasetransformations in coppersulfides ANonswPurNIs Departmentof Mineralogyand Petrology,Uniuersity of Cambridge DowningPlace, Cambridge C82 3EW, England Abstract The order-disordertransformation behavior of chalcociteand djurleite,and digeniteand anilitehas beenstudied by direct observationsof the transformationsin a transmission electronmicroscope. Chalcocite and djurleiterepresent low-temperature superstructures of the high-temperaturedisordered chalcocite subcell, and there need be no compositional differencebetween them. The djurleite superstructureis the more stable form at room temperature.The intermediatestructure in digeniteforms through modulatedstructures ratherthan throughtwinning. A metastable6a digenitetype superstructure,isochemical with anilite, forms on heating.This 6a superstructureis metastablerelative to the formation of anilite. Introduction cannotbe directlymeasured in this way,the courseof a transformationcan be followeddynamically, and For compositionsnear CurS a numberof phasesare subtle changeswhich would be very difficult to ob- known to existat room temperature.These may be serveby X-ray methodscan clearly be seen. dividedinto two broadcategories based on thenature of the sulfur close-packingin the structure:(1) I: The transformationbehavior of chalcociteand chalcociteand djurleitewith structuresbased on hex- djurleite agonal close-packingof sulfur atoms: (2) digenite- like structuresand anilitewith sulfur atomsin ap- Chalcociteis generallyconsidered to have a com- proximatelycubic close-packing.The low-temper- positionvery closeto CuzS,whereas djurleite, origi- aturephase relations have been summarized by Mori- nallythought to beCur.rrS (Roseboom, 1966; Takeda moto and Gyobu (1971)and Barton (1973).The et at., 1967a)is now consideredto be a solidsolution transformationbehavior of someof thesephases and with a compositionrange from about Cut.*S to the relationshipsbetween them havebeen studied by Cu' nzS(Mathieu and Rickert,1972; Potter, 1974). directobservation of the transformationsin a trans- The phasesand cell dimensionsof chalcociteand missionelectron microscope. djurleite may be summarizedas follows. Between 104' and 435'C chalcociteis hexagonalwith an : Method 3.95,cn = 6.75A and spacegroup P6"/mmc. Above Naturally-occurringcrystals of the phaseswere 435"C it has the cubicclose-packed digenite struc- crushed,after chilling in liquid nitrogen to prevent ture.Below 104'C a pseudo-orthorhombiccell with a deformation.The finestgrains were collected from a : ll.92, b : 27.34, c : 13.44A, spacegroup Ab2m is suspensionin absolutealcohol and mountedon a formed (Buergerand Buerger, 1944).This can be standardcarbon-coated copper grid for observationsdescribedas a superstructureof the hexagonalform in an AEI EM6G transmissionelectron microscope with a : 3en,b : 4y3av,,c : 2cn.More recentlyit operatingat l00kV. Observationswere made firstly has been shown that low-chalcociteis monoclinic on untransformedgrains by usinga smallcondenser (Evans,l97l), but the pseudo-orthorhombiccell will apertureand a defocussedbeam to keepthe illumina- be usedhere for convenience. tion and the heatingeffect of the beam minimal. Djurleite is orthorhombic,diffraction aspect P*t?* : Transformationsin the grains,both on heatingand with celldimensions a :26.90, b 15'72,c: 13.57 cooling,can be observedby focussingand defocus- A. the diffractionaspect is alsocompatible with the singthe beamor by movingthe beamlaterally on and monoclinicspace group P2'ln. Takedaet al. (1967a) off the grain. Although the temperaturein the grain point out that thesecell dimensionsare closelyre- 107 PUTNIS,' PHASE TRANSFORMATIONS IN COPPER SULFIDES 4Dl Electron diffraction patternsshowed that twinning wascommon in bothphases, and to illustratethis and to identifythe phasepresent, grains were sought in an orientation such that the diffraction patternscon- tained the a* and c* axesof the hexagonalsubcell, i.e.,the beam was parallel to the [010]direction in the grain. Such reciprocal lattice sections are used ab throughoutthis investigation. Interpretationof electrondiffraction patterns. Both low-chalcocite and djurleite are oo2 oF simple super- structuresof the high-chalcocitehexagonal subcell, andthis relationship gives rise to threepossible orien- tationsof the low-temperatureorthorhombic forms o9o. .ry ogo . ry relative to the subcell.These three orientationsare relatedby 60' rotationsabout the hexagonalc axis. Fig. I a,b. The top direct lattice illustrations show two of the Figuresla and b showtwo of theseorientations for three possible orientations of the chalcocite superstructure(dashed line) relative to the high-chalcocitesubcell (full line) The arrow on the chalcocitesuperstructure together with a*c* te- the left is the direction of the electron beam which leads to the ciprocal lattice sectionsobtained in each case.The formation of the a*c* diffraction pattern below each diagram. In third orientationof the superstructureproduces a these diffraction patterns the large spots are the subcell reflections diffractionpattern as for Figure lb. Figures2a andb while the smaller spots are due to the superlatticeorientation show the similar above. situationfor the djurleitesuper- structurewith the two differenta*c* reciprocallattice lated to the hexagonalchalcocite subcell with 4 : sections. 4c6,b : 4en, c : 2y4a1,.Roseboom (1966) and The diffraction patterns from untransformed Potter(1974) reported that djurleitedecomposes to grainsin variousorientations are consistent with the hexagonalchalcocite and high-temperaturedigenite aboveinterpretation of differenttwin orientationsin above93oC, whereas Cook (1972)found that djur- both chalcociteand djurleite.The twinningin djur- leitetransformed to a tetragonalform above93oC. leite is the sameas that describedby Takeda et al. Thistetragonal form, firstdescribed by Djurle(1958) (1967b). has beenencountered for compositionsCu,.r.S (Ja- Transformationbehauior. Crystal grains which nosi,1964) and CurS(Skinner et al.,1964;Serebrya- showed only a single phase were selectedfor the naya, 1966)and has sulfur atomsin approximately heatingexperiments. Increasing the temperatureby cubicclose-packing. beam heatingcauses the spontaneousdisappearance As chalcociteand djurleite are often finely inter- of the superstructurespots in both chalcociteand grown (Evans, l97l), and transformationsfrom djurleite, leaving the high-chalcocitesubcell reflec- chalcociteto djurleite have been reported(Cook el tions(Fig. 3a).On slowcooling, spots appear at the al., 1970),the relationshipbetween these two phases reciprocallattice points (lz VzO), indicating the for- needs further investigation.Transmission electron microscopyprovides a methodof studyingsuch inter- growthsand observingtransformations in thesema- terials. Results Crystalsof chalcocitefrom Carn Brae,St. Just, a b. Penzance,and St.Ives in Cornwall,and Bristol Mine, Connecticut,were examinedin the electronmicro- @. scope.In all samplesobservations made on a large numberof untransformedgrains indicated that both chalcociteand djurleite werepresent and that djur- leitewas often the more abundantphase. Bulk com- &o z'o &o 2bo positionalanalysis of the startingmaterial was not Fig. 2 a,b The equivalentsituation to Fig. I for the djurleite made. superstructure. PT.]TNIS:PHASE TRANSFORMATIONSIN COPPERSULFIDES r09 t T increasinq --r \t- (b)The Fig.3. Thesequenceoftransformationsobservedinanc*c*diffractionpattern.(a)Thehigh-temperaturechalcocitesubcell orientations of the intermediate 21,6X lca superstructure. (c,d) Twinned orientations of the chalcocite superstructure. (e,f) Twinned cycle, with equal djurleite superstructure.The key to these diffraction patterns is in Figs. 1,2 The arrows show a cooling and reheating possibility of forming the chalcocite or djurleite ruperstructure and the reversible chalcocite to djurleite transformation' mationof a 2a X lc hexagonalsuperlattice (Fig. 3b). the regionfrom which diffractedbeams are collected This diffractedintensity appears continuously and is the samein eachcase. (The selectedarea aperture changesas a function of temperature,characteristic usedcollects diffracted intensity from a circulararea of a second-ordertransformation. On furthercooling approximately1000 A in diameter')Commonly the the superstructurereflections corresponding to either djurleitesuperstructure forms from the chalcocitesu- chalcociteor djurleiteappear, irrespective of the su- perstructureas the graincools and transformsback perstructureof the original untransformedgrain. to the chalcocitesuperstructure with slight heating. This transformationoccurs as a distinctdiscontin- This transformationbehavior is illustratedin Figure uity. Repeatingthe heatingand cooling cycleshows 3, which showsthe sequenceof a*c* diffractionpat- that the endproduct of the transformationsequence ternsseen in a singlegrain cycledthrough the trans- has an approximatelyequal likelihood of beingthe formations. chalcociteor the djurleite superstructure,although The transformationcycle could be repeatedmany il0 PUTNIS: PHASE TRANSFORMATIONS IN COPPER SULFIDES times, and has been observedin many grains with ens with decreasingtemperature, although different various orientations.In eachcase the result has been investigators disagree on the estimated deviations the same, with the chalcocite and djurleite super- from stoichiometry. Roseboom (1966) detected no structuresforming with approximately equal proba- compositional
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