Polytypism in Molybdenite (I): a Non-Equilibrium Impurity-Induced Phenomenon
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American Mineralogist, Volume 64, pages 75g-767, Ig79 Polytypism in molybdenite (I): a non-equilibrium impurity-induced phenomenon ReruBn J. J. Npwnpnny Department of Geologlt, Stanford lJniversity S t anford, Califu rnia 94 3 05 Abstrsct MolyMenite is known to occur in either of two structur€s:the @mmon 2H, polytype and the rare 3R polytype. Most previous work has suggestedthat 3R forms in low sulfur fugacity environments and is sulfur-deficient relative to 2H,. A thorough review of the literature on molybdenite, however, suggeststhat the occurrence of 3R molybdenite in nature is not r€- lated to sulfur fugacities or temperatures of fonnation, but rather to impurity content. X-ray diffraction analysis of 184 specimensof molybdenite indicates (l) there is a semi-quantitative relationship between the mean 3R abundance and the mean impurity cont€nt of molybde- nites from a given deposit and (2) the 3R abundance is hdependent ofthe sulfidation states of the associatedsulfides. Microprobe studies indicate that there 41e11e 5ignifisant major-ele- ment di-fferencesbetween 3R and 2H, molyMenite in the samplesstudied. A synthesisof the experimental stability studies of molybdenite polytypes suggeststhat 3R grows by a screw-dislocation mechanism and is unstable with respect to 2Hr, but does not readily convert due to kinetic barriers. Growth by screw dislocations in nature most fre- quently occurs due to internal strains generated by high impurity contents in the growing crystals. Rhenium, tin, titanium, bismuth, iron, and tungsten are the most common impurities found in 3R molybdenite, and their role in the mineralization prooesswith respect to forma- tion of 3R molybdenite is discussedfor several deposit types. Introduction gest that the two polytypes are stoichiometrically dis- similar (Clark, 1970),whereas others believethat the The structure of molybdenite (MoSr) is based on trace-element contents differ (e.9. Somina, l!ff;. stacking of planar close-packed S-Mo-S layers. Al- Still others postulate di-fferencesin pressuresof for- though such layers could be stacked in a variety of mation @adalov et al., l97l), temperaturesof forma- ways, only two stacking sequenceshave been ob- tion (Arutyunyan et al., 1966),61soolingrate during served in natural and synthetic molybdenites. Be- formation (Chukhrov et al.,1968). cause both mins14fuconsist of structurally identical Frondel and Wickman (1970)have shown that 3R MoS, layers and differ only in the length of c, they nolybdenite is present in a variety of ore deposits are polytypes. The most common polytype in nature and is not restricted to "exotic" low/(Sr) environ- is hexagonal(2Hr, spaoegroup P6r/mmc), consisting ments as suggestedby Clark (1970). However, of two layers per unit cell. The lesscommon natural Clark's (1970) conclusion that 3R is I to 5 weight polytype is rhombohedral (3R, space group R3m), pefcent sulfur-deficient relative to 2H, has been ac- consistingof three layersper unit cell (Fig. l). cepted by several authors (e.9. Hasan, l97l; Ctarg The origins and controls of polytypism are not well and Scott, 1974).If, indeed,3R molybdeniteis an in- understood,although most studiesof the SiC, ZnS, dicator of low sulfidation states, its rarity in nature and CdI, systemshave indicated that the phenome- suggeststhat molybdenite deposition occurs under non in these systemsis related to dislocation/growth relatively high 5ulfq1 fugacity conditions. This hy- processes and di-fferences in energies of different pothesis has signifigaat implications for molybdenite stacking sequences(e.9. Trigunayat, l97l). In the ore deposition,e.9., it suggeststransport by high sul- case of molybdenite much controversy exists con- fide mechanisms. cerning the origins of polytypism. Some authors sug- This investigationis basedon a study of 184 mo- 00n.3-004x/79 / U08-0758$02.00 NEWBERRY: POLYTYPISM IN MOLYBDENITE 759 is found in a variety of environments.lndeed, 2H, and 3R polytypes are cofllmonly mixed within a c single specimen,as shown by single-crystalexperi- ments (Drits and Chukhrov, l97l). $imilafly, gpln t sizeis not related to polytype abundance,contrary to the suggestionof Wildervanck and Jellinek (1964)'as 3R indicated by a review of the literature. -+ o Polytypeabundance and impurity content [roro]-Mo :. Severalstudies, starting with Somina (1966)' have Fig. l. Structuresof someMS2 polytypesprojected onto (1120), suggestedthat 3R molybdenites characteristically (1963). modified from Takeuchi and Nowacki contain high impurity contents. Rhenium is the most lybdenite-bearinghand specimensto determinepoly- common impurity element,ranging in concentration greater 5000 ppm. No type content,vein mineralogy,alteration, major- and from lessthan I ppm to than polytypism trace-elementcontent, and environment of deposi- detailed study of impurity contents and but the tion, in order to better understandthe causesof poly- in the same sampleshas been undertaken, that high rhenium typism in molybdenite. Part I is a study of what general trend of the literature is contents(e.9. causesoriginal formation of the 3R polytype; it leads contentscan be correlatedwith high 3R trace elements, up to a hypothesisexplaining the relation between Ayres, 1974).High contentsof other correlatablewith trace-elementand 3R content. Part II examinesthe such as Ti, Bi, W, and Fe are also (Table l). In con- detailsof a particular system(Re/3R in the porphyry the presenceof the 3R polyype pure (Chukhrov et environment) and discussesthe geochemicalcircum- trast, 2H, polytypes are relatively stancesunder which 3R is preservedand/or de- al.,1968\. by post-depositionalalteration. stroyed Synthesisstudies (l) Reviewof past work Published experirrental data show that both polytypes can be synthesized (Bell and Herfert, Polytypismand sulfidationstates lg57), (2) 3R convertsto 2H, when heated for long 500oC (Wilder- Little information is available in the literature on periods at temperaturesin excessof (3) necessaryfor the major-elementcompositions of 2H' and 3R mo- vank and Jellinek, 1964), the time as temper- lybdenite. Available analyses,however, show that no complete conversionincreases drastically to 2H, faster systematiccorrelation existsbetween sulfur contents ature falls (Clark, 1970),(4) 3R converts pres€noeof sulfur gas and 3R abundances.Moreover, these data show no and more completely in the (5) 3R convertsto 2H, indication that the 2H' polytype is I to 5 percent (Zelikman et al., 1969),and richer in sulfur than the 3R polytype. Both polytypes only by recrystallization(Zeliknan et al.,1969). by reaction have been reported with compositionsranging from Experimental synthesisof molybdenite vapor or liquid 39.9VoS (Seebach,1926) to 4O.4VoS (Zelikman et al., of molybdenum metal with sulfur 2H' poly- 196l) (Fig. 2). Detailed tabulations of these and generallyresults in a mixture of the 3R and experiment other molybdenite data may be found in Newberry types (Dukhovskoi et al., 1976).In one to the (1e78). two MoS, layerswere formed: a2Hrlayer next + 2Hr layer next If natural polytype occurrences are a function of molybdenum metal and a mixed 3R (Zelikman These results are sulfidation stateof the environment,as has been sug- to the sulfur et al., 1976)- gestedby severalstudies, then there should be some correlation between 3R/2H' occurrencesand the sul- - o6 fidation statesof the associatedminerals. Literature o b9. data suggest,however, that there is no such correla- e7 EE tion. 3R molybdenitehas beenfound associatedwith z 40'4 sulfidation states ranging from high-native sulfur 39 I 400 402 (Znamenskiy, 1969)-{hroggh intermediate--chal- wt. 7" S copyrite + bornite + pyrite (Hasan, l97l)-to low- Fig. 2. Histogramof molybdenitechemical analyses taken from bismuth + pyrrhotite (Petruk, 1964).similatly,2}' the literature. NEIIIBERRY: POLYTYPISM IN MOLYBDENITE inconsistent with the theory that 3R forms in com- Table l. Impurity contents of some 3R molyMenites from the paratively sulfur-poor environments.Rather, as Ze- litcrature likman et al. (1976)point out, there are differencesin Location Impurity the growth mechanismsof molybdenite formed at the Reference* metal- and sulfur-interfaces. BinDental, Svltz. 0.7"1w 1 Kurdle Island, USSR ca, 500 ppn Fe 2 Similarly, molybdenite East 'Iit formed by reaction be- Siberia, USSR 27" 3O0 ppn Nb, V 3 portugal tween MoO, and sulfur is 2H, whereasin the pres- Panasqueira, 150 ppn Fe; "sore" W 4 ence Ahenia, USSR 1.882 Re 5 of an alkali-carbonate flux 3R forms. Again, syDtheElc o.L6'/.'t7 6 there are differences in growth mechanisms between Con I'line, Yellowknife, Can. ca. 500 ppn Fe 1 the two experiments, aReferences: rather than differencesin sulfur (t) Graeser (1964)j (Z) Zwenskiu eL aZ. (lg?0). activities. Elwell and Neale (1971) (3) soniw (1s66); (4) cL@k (.1g6s); ts) rad,rmja-oul"nuol"nuayon have shown that (1s63); (6) BeLl qd Herfeft (1gsz); (?) Bolle (is6s) fluxes generally cause crystal growth by a screw-dis- location mechanism. Finally, the addition of an impurity (in particular Five molybdenite-bearing samples representing Re) in synthetic molybdenite affectsthe 3R to 2H, different deposit types were trimmed to 0.5-cm conversionreaction. 3R molybdenlls colhining 0.05 pieces, mounted in epoxy, polished, and carbon- to 37oRe convertsmore slowly than pure 3R molyb- coated.Qualitative wavelengthscans of the five sam- denite, and 3R containing more than 3VoPiehas not ples on an ARr-Eux-sM microprobe indicated that been observedto convert at all, even at temperatures only Mo, S, and Re