Cu-Fe-S Phases in Lunar Rocks

American Mineralogist, Volume 58, pages 952-954, 1973

Cu-Fe-SPhases in LunarRocks

LawnnNcn A. Tevronl

Departmentol Geosciences,Purdue Uniuersity, Lalaye t te, I ndiana47 907

KBNNrrn L. Wnrrnus

Departmentof Applied Earth Sciences,Stanlord Uniaersity, Stanford, Calilornia 943 0 5

Abstract

The sulfide minerals reported to be present in lunar rocks include troilite, mackinawite, the discredited mineral "chalcopyrrhotite", sphalerite, chalcopyrite, and cubanite. Apollo 12 sample l2D2l,l34 contains these last two minerals, and the first chemical analyses of the Cu-Fe-S phases are presented. These minerals occur along cracks and grain boundaries within troilite and are probably exsolution products formed at low temperatures (i.e, 100"-300'C) from a cupriferous troilite.

Introduction Cu-Fe-S Minerals The opaqueminerals, although constituting only a "Chalcopyrrhotite", a discreditedmineral (Yund minor amount of a given sample,have proven useful and Kullerud, 1966; Cabri, 7967) , hasbeen reported as indicators of the genesisand cooling histories of from Apollo 12, 14, and 15 rocks (El Goresyet al, the lunar rocks (e.9., Reid, 1971; Taylor and Mc- l97la,b; 1972a,b). It wasdescribed as a yellowish, Callister, 19721,Taylor et al, I973b). The most chalcopyrite-coloredphase which appearsisotropic abundant opaque mineral is ilmenite, and many of and is alwaysfound within troilite. It is probablethat the oxide phasesin lunar rocks are unique (e.6'., this phase is either talnakhite, a mineral originally armalcolite as per Anderson et al, l97A). In addi- describedby Cabri (1967) andhaving a composition tion to native Fe metal, native Cu, and schreibersite, of CusFesSro(Cabri and Harris, 1971) near chal- the remaining opaqueminerals consistof sulfides. copyrite or cubaniteII, an isotropic form of cubanite The most abundantsulfide mineral in lunar rocks (Genkin et al, 1966). Theseare the only two iso- is troilite. It commonly occurs with native Fe in a tropic minerals in this portion of the Cu-Fe-S sys- eutectic texture and probably results from crystal- tem. Chalcopyrite was qualitatively identified as lization of an immiscible sulfide liquid (Skinner, occurring on the outside of native Cu rims around 1970). Other sulfideswhich have been reportedin- FeNi metal grainsin Apollo 15 sample 15475 (El clude mackinawite, (Fe,Ni)r.,S (Simpson and Goresy et al, 1972b). In light of the phaserelations Bowie, 1970; El Goresyet ql, l97Ia; Taylor et al, in the Cu-Fe-Ssystem (Barton and Skinner, 1967) l97l); the discredited mineral "chalcopyrrhotite" at low temperatures,the phase is more probably (El Goresyet al,l97la, b); chalcopyrite(El Goresy cubanite.The grain size was too small for positive et d, 1972b); and sphalerite(El Goresyet al, 1973; identification.None of theseCu-bearing phases had Taylor et aI, 1973a). However, becauseof the small been completely verified by quantitative chemical grain size of most of these minerals, troilite and analysesprior to this study.A preliminarydiscussion sphaleriteare the only sulfide minerals which have of this investigationwas presentedby Taylot et aI beenchemically analyzed prior to this study. (r973c). Within the Sample Library at the Lyndon B. 1 Present Address: Department of Geology, University of JohnsonSpace Center (formerly the Manned Space- Tennessee,Knoxville, Tennessee379 16. craft Center), Houston,Texas, an Apollo 12 sample 952 MINERALOGICAL NOTES 953

(12021,134; a porphyritic basalt) was found to con- Tesrs 1. Electron Microprobe Analyses of Chalcopyrite and Cubanite in Apollo 12 Sample tain yellow Cu-Fe-S phases up to 10 /am across in 12021,r34 association with troilite (Fig. 1). The phenocrysts, to 2 cm in length, consist of pyroxenes up with cpy (4)* CuFes, cub (3)* CuFerS, pigeonite cores mantled by augite-to-ferroaugite rims pyroxene Cu 33.6 34.6 22.8 23.4 set in a variolitic groundmass of and Fe 30.0 30.4 40,4 4L.2 plagioclase. The opaques consist of ilmenite, native Co 0.85 0.87 c ?q t 35.0 35.7 35.4 iron, troilite, titanian chromite, chromian ulviispinel, and the Cu-bearing phases. These latter are chal- Total 99.7 99.8 copyrite and cubanite (Figure 1). This is thus the Nwnber of analgses, first reported occurrence ol cubanite in lunqr rocks

and the first confirmatkm ol the presence ol chalcopyrite. The chalcopyrite and cubanite appear to have similar genesis in that both occur along cracks and grain boundaries within troilite. Both show the ap- propriate anisotropism for the respective phases and, therefore, neither is the supposedly isotropic "chalcopyrrhotite." The microprobe analyses of these minerals, the first Cu-Fe-S mineral analyses reported from lunar rocks. are shown in Table 1. No other elements were detected in quantities >0.05 wt percent. It is noteworthy that this is the first report of the presence of cobalt in Cu,Fe sulfides. It would appear from the analyses that cobalt substitutes for iron in these structures. X-ray scans of an assemblage

cLrBANlTfr1n 120?1"1'll,

Frc. 1. Photomicrographsof (A) Chalcopyrite and (B) Fro. 2. X-ray scans of chalcopyrite within troilite in Cubanite in Apollo 12 sample nA21,134. Gray is troilite; Apollo 12 sample 12021,134.The same specimin as shown light gray to white is Cu,Fe sulfide. in Figure 1A. 954 MINERALOGICAL NOTES further verify the nature of the occunence of these AND_ (l97lb) The geochemistryof Cu,Fe sulfides (Figure 2). the opaque minerals in Apollo 14 crystalline rocks. Earth Planet. Sci. Lett. 13. 121-129. Chalcopyrite is not stable with troilite, FeS, but L. A. TAyLoR, er.roP. R*rpou*, (1972a) Fra Mauro is stable with pyrrhotite, Fe1-,S, (Yund below 334'C crystalline rocks: Mineralogy, geochemistry and sub- and Kullerud, 1966). It is possible that the Fe-S solidus reduction of the opaque minerals. Proc. Third phase immediately adjacent to the chalcopyrite is Lunar Sci. Conl., Geochim. Cosmochim. Acta, Suppl 3, pyrrhotite; however, if present it is less than 10 pm Vol. 1,333-349. AND - (1912b) in width. The troilite beyond this distance is stoichio- Apollo 15 opaque minerals: Geochemistry, mineralogy and subsolidus re- metric FeS, and the Fe sulfide within this 10 pm duction (abstr. ). Meteoritics, 8, 32. distance could not be accurately determined due to P. Reuooun, M. Plvrdpvri, O. MeonNnecr, O. the interference effect of the chalcopyrite and the Mrirrrn, eNo W. GrNrNen (1973) Zinc, lead, chlorine. diffuse contact with the Fe sulfide. and FeOOH-bearing assemblagesin the Apollo 16 sample 66O95: Origin by impact of a comet It is possible that the troilite contained small or a carbonaceous chondrite? Earth Planet. Sci. Lett. 18, 4ll4l9. amounts of Cu at higher temperatures and that GENxrN, A. D., A. A. FrrruoNovl, T. N. SHepruN, S. V. exsolution of this Cu occurred at lower temperatures Sonoteve.,eNo N. V. TnoNpve (1966) On cubic cubanite as a result of slow subsoliduscooling. Subsolidusre- and cubic chalcopyrite. GeoI. Radnykh Mestorozhdenii, equilibration of sulfidesto temperaturesbelow 300'C 8,41-54. Rsro, J. B., Jn. (1971) is exemplified by the presence of mackinawite in Apollo 12 spinels as petrogenetic indicators.Earth Planet. Sci.Lett.10. 351-356. many lunar troilites, a phase only stable below SrursoN, P. R., eNo S. H. U. Bowe (1970) Quantita- =150'C (Z6ka et al, 1973). tive optical and electron-probe studies of the opaque Taylor et al (1973b) recently investigated the phases.Science, 167, 619-621. cooling rates of rock l2O2I versus 12052. Based on SxrNNrn, B. J. (1970) High crystallization temperatures indicated for igneous the Zr partitioning between coexisting ilmenite, they rocks from Tranquillity Base. Proc. Apollo lI Lunor Sci. Conf ., Geochim. Cosmochim. Acta, concluded that rock l2A2I has undergone consider- Suppl l, Vol. l,891-897. able subsolidus re-equilibration as a result of slow TAyLoR, L. A., G. Kurlenuo, rNo W. B. BnveN (1971) subsolidus cooling. These data support the slow Opaque mineralogy and textural features of Apollo 12 cooling (i.e., re-equilibration) hypothesis for the samples and a comparison with Apollo 11 rocks. Proc. Second genesisof the Cu-Fe-S minerals. Lunar Sci. Conf., Geochim. Cosmochim. Acta Suppl. 2, Vol. 1, 855-871. Acknowledgments eNb R. H. McCerusren (1972) An experimental investigation of the significance of zirconium partitioning We are grateful for the constructivecriticisms and sug- in lunar ilmenite and ulvospinel. Earth Ptanet. Sci. Lett. gestionsof Drs. L. J. Cabri,J. R. Craig,and W. Via. We 17.105-109. would like to thank the Departmentof Applied Earth Sci- H. K. Meo, lup P. M. BeLr (1973a) "Rust" in encesat StanfordUniversity for the useof their microprobe. Apollo 16 rocks. Proc. Fourth Lunar Sci. Conf., Geo- This studywas supported by NASA Grant NGL 15-005-175 chim. Cosmochim. Acta, Suppl. 4, VoI. l, (in press). andNSF GrantGA-31988 to L.A.T. R. H. McCerLrsrER, eNo O. Senor (1973b) Cool- ing histories lunar rocks References of based on opaque mineral geothermometerc. Proc. Fourth Lunar Sci. Conf., Geo' ANoensoN,A.T., et al (1970\ Armalcolite:a new mineral chim. Cosmochim. Acta, Suppl. 4, Vol. l, (in press). from Apollo 11 samples.Proc. Apollo 11 Lunar Sci. K. L. Wrrrreus, eNo O. Senor (1973c) Mineralogy Conf., Geochim.Cosmochim. Acta, Suppl. l, Vol. t, and geochemistry of opaque and non-opaque phases in 55-63. selectedApollo 17 samples.Trans. Amer. Geophys. Union, BentoN,P. B., Jn.,eNo B. I. SrrNNrn(1967) Sulfide mineral 54, (in press). stabilities.In Geochemistryof HydrothermalOre De- YuNo, R. A., eNr G. Kurrrnut (1966) Thermal stability posits,Ed. H. L. Barnes.Holt, Rinehartand Winston, of assemblagesin the Cu-Fe-S system. I. Petrolo'g, 7, Inc.,New York, pp. 236-333. 454-488. Cennr,L. I. (1967) A new copper-ironsulfide. Econ. GeoI. Z6s.t, H., L. A. Teyron, er.roS. TexeNo (1973) Composi- 62,910-925. tional variations in natural mackinawites and the results of heating exp€riments. l. Sci. Hiroshima Unio., Ser. 6, on talnakhite.Econ. Geol. 56, 673-67 5. l-25. Er Gonnsv,A., P. RrruooHR,AND L. A. Tlyron (l97la) The opaqueminerals in the lunar rocks from Oceanus Procellarum.Proc. SecondLunar Sci. Conf., Geochim. Manuscript receiued, Iune 25, 1973; accepted CosmochimActa, Suppl. 2, Vol. 1,219-215. for publication, Iuly 2, 1973.