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Arsenian Sphalerite from Mina Ai,Caran, Pampa Larga

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Arsenian Sphalerite from Mina Ai,Caran, Pampa Larga MINERALOGICAL NOTES AMERICAN MINERALOGIST, \IOL. 55, SEPTEMBER.OCTOBER, 1970 ARSENIANSPHALERITE FROM MINA AI,CARAN, PAMPALARGA, COPIAPO, CHILE Arew H. Cr-anr, Department oJ GeologicalSciences, Queen's Uniaersity, Kingston, Ontario, Canado. ABSTRACT Bright pink sphalerite from the arsenic-rich Alacr6n deposit, with a:5.4110 +0.0003 A, is shown by electron-probe microanalysis to contain only traces of iron but 17 +0.3 weight percent As INrnooucrroN The literature containsnumerous reports of appreciablesolid solution of arsenicin sphalerite(e.g. Schroll, 1953; Fleischer,1955), but the an- alytical evidenceremains largeiy unconvincing.Moss (1955;and in Hey, 1955,Frondel, 1967)proposed that the mineral voltzite has the general composition (Zn,As)S or Zn(As,S), but Frondel (1967) subsequently found no evidenceto support this suggestion.Most recent electron- probe microanalysesof sphaleritedo not record even traces of arsentc. Ilowever, Herzenberg(1932,1933) found 0.64 weight percentarsenic by chemical analysisof a raspberry-redsphalerite-like mineral from Llall- agua, Bolivia, which he named gumuc'ionite.Later workers (e.g.Hey, 1955)have assumedthat this material representsa mixture of sphalerite and realgarbut no further work appearsto have been carried out on the original or similar specimens.The presentnote describesthe occurrence of an apparently similar, and demonstrably arsenic-bearingsphalerite. Spner-Bnrrr rN rHE Ar,acnAN DBposrr Sphalerite is a minor constituent of the mineralogically complex, polymetallic Alacr6n vein deposit (Parker, Salas and P6rez, 1963),in the Pampa Larga mining district of northern Chile (Lat. 27"36'5.;Long. 70'11'W.). It occurs, however, in at least three parageneticcontexts: (1) in associationwith early arsenopyrite, pyrite, stibnite, smithite, aAsS (Clark, 1970a)and arsenolamprite(Clark, 1970b), among other minerals; (2) with late-stagegreigite, pyrite, native arsenic,realgar, and orpiment;and (3) rarely, as free-standingcrystals in vugs, in association with realgar and orpiment. Sphaleritegenerations I and 2 have a brownish-blackappearance in hand-specimen,and have been found by X-ray fluorescenceand elec- tron-probemicroanalysis to contain,respectively, 8-9 mole percent FeS, and 7.5*0.5 percent FeS (when in contact with greigite and pyrite) to MINI'RALOGICAL NOTES 1795 4.2+0.5 percentFeS (in associationrvith pyrite alone).The third gener- ation, however, has a bright reddish-pink color, and is shown by spectro- graphic anal,vsisto contain only traces (0.0X percent) of iron, in addi- tion to unusually small amounts of cadmium and manganese(both 0.00X percent). PtNr Spn.Ar.ul{rfL This iate-stagesphalerite exhibits minute, translucent tetrahedrons, overgrowing calcite rhombs and rosettes. Thin crusts and occasional crystalsof realgarand crustsof orpiment occurin contact with the sphal- erite, and the three sulfidesappear to have crystallizedsimultaneoulsl'. Native rhombohedral arsenic,pyrite, and greigite probably formed at the same stage in the vein filling, but have not been observedto occur in contact with the pink sphalerite,which apparently crystallizedin an arsenic-rich,but essentiallyiron-free micro-environment.The lustrous crystalsare extremelybrittle, and crumble at the touch of a needle.They showno color zoningin hand-specimenor in polishedsection, where they are characterizedby intense,roseate internal reflections,and are readily distinguishedfrom the iron-bearingsphaierites in the AlacrSn ores. X-ray powder patterns of this mineral confirm its identification as a cubic sphalerite,with less than 10 percc'nthexagonal packing (Smith, 1955;Shortand Steward,1959). The unit-celledges of two crystalswere measuredin a 114.49mm-diameter X-ra1' camera,using the Straumanis film mounting technique and Cu-Ka radiation, and by extrapolationof back reflectionlines to 20:180' by the Nelsonand- Riley (1945)function. Concordantvalues of 5.4110and 5.4111+0.0003A wereobtained, which are significantly greater than that found for pure BZnS by Kullerud (1953;5.4094+0.0001A, in Skinnerand Barton, 1960)and Skinnerand Barton(5.4093+0.0002 A; 1e60). ANar,yrrcer, D,\ra The unusual optical properties of this sphalerite prompted an an- alytical study. Qualitative spectrographicand X-ray fluorescenceanal- ysis of clean, hand-picked crystals revealed that, although containing only tracesof iron, the mineral is apparently rich in arsenic()1 weight percent). The possibility that the arsenicmight be present as discrete inclusions of realgar or oripment was investigated by electron-probe microanalysis of several intergrown crystals. Arsenic was confirmed as a constituent of the sphalerite,and no inclusionsof arsenicsulfides were detected.Wavelength scanning across each of the cr1'stalsshows that the arsenic is homogeneouslydistributed. An arsenic content of 1.7+0.3 weight percent was determined,using the associatedrealgar (assumed 1796 MINER,4LOGICAL NOTES to be stoichiometricAsS) as a standard.In in vacuoheating experiments' the arseniansphalertite broke down to a honey-yellow sphalerite and probable realgarin four hours at 200oC,but persistedwithout a change in color or in cell edgeat 150"Cfor 32 hours.Annealing in the presenceof magnesiummetal for 42 hours at 150'C resulted in no increasein cell edge (c/. Skinner and Barton, 1960), so the arsenian sphalerite is appar- ently free from oxygen. The responseof this phaseto heating suggests that it containsno organiccomponent (c/. Frondel, 1967). DrscussroN On the basis of the observationssummarized here, it is tentatively suggestedthat an arsenian sphalerite solid solution, probably of the form Zn(As,S), is a stable phasein the s-vstemAs-Zn-S at low temper- atures (below approximately 200'C). The earlier-formed,iron bearing sphalerite coexisting with arsenopyrite,pyrite, and native arsenic in this deposit contains no arsenic (i.e. 10.0t percent), suggestingthat either the solubility of arsenic in sphalerite decreaseswith increasing temperature in the system As-Fe-Zn-S or, more probably, that such substitution is prevented by even minor amounts of iron. The Mina Alacr6n arsenian sphalerite in some respectsresembles the "gumucionite" of Herzenberg(1932, 1933). However, it forms euhedral crystals in contrast to the botyroidal, fibrous habit of the material from Llallagua, which exhibited violent decrepitation and expulsion of water at Iow temperatures.Herzenberg considered the red colorationof "gumu- cionite" to be due to arsenicsulfide, but the presentstudy suggeststhat this striking feature may be an inherent proporty of arseniansphalerite. AcrNo'r,vlnocrurrvts Field'lvork in Chile was carried out in cooperation with the Instituto de Investigaciones Geologicas de Chile, and financed by the Instituto and the National Research Council of Canada. The late Merle Crozier kindly assisted with the electron microprobe analysis. Rrnrpowcrs Cr.enr, A. H., aNo R. H. Srr.r.rror (1970)Atpha arsenicsulfide, from Mina Alacr6n, Pampa Larga, Chile.,4 mer.Mineral.55, 1338-13,14. --- (1970) Arsenolamprite confirmed from the Copiap6 area, notthern Chile. Mineral. Mag 37,732-733. l'r.rtscuon, Mtcrrant (1955) Minor elements in some sulfide minerals. Eeon. GeoL 50th Annit:. Vol,.. Part IL 970-1024. Ftoxou,, Cr.rrronl (1967) Voltzite. Amer. M ineral,. 52, 617-634. Hrv, M. H (1955) Clrcmicali.nder oJ Mi.neral,s.2nd. ed.., London, p. 12. Hnnzrulrtc, R. (1932) Gumucionita, una nueva variedad arsenical de blenda. Ra,r. Mineral, Geol.Mmeral,., Soc.Argent. Mineral. GeoI.4,65-66. -- (1933) Gumucionit, eine neue arsenhaltige Varietat der Schalenbiende Zenlralbl. M ineral,.,A, 1933, 77 -78. MINERAI.OGICAL NOTES 1797 Kur.tenuo, G. (1953) The FeS-ZnS system: a geological thermometer. Norsk Geol. Tiilssk., 32,6r-141. Moss,A A. (1955)The constitution of voltzite[Abstr.]. Amer. Mineral.40, ?78. NntsoN, J. 8., enn D. P. Rrr.nv (1945) An experimental investigation of extrapolation methods in the derivation of accurate unit cell dimensions of crystals. Proc. Pltys. Soc. 57, t6j-177. Parxln, R. L., R. S.uas, eNo G. Pr6nez (1963) Geologia de los distritos mineros Checo de Cobre, Pampa Larga y Cabeza d eY aca B ol I n.st I nr est. Geol., Chil,e, 14, 46 pp. Scrnor,r,, E. (1953) Uber Unterschiede im Spurengehalt bei Wurtziten, Schalenblenden und Zincblenden. Si,tzungsber.Oesterr. Ahad. Wiss., Math. Naturtuiss. K1,., Abt. I 5,305-332. Srrotr, M. A.. ,lNl E. G. Srowanl (1959) Measurement of disorder in zinc and cadmium sulphides. Amer. M ineral. 44, 189-793. SrrNrnn, B. J., aNo P. B. BlnroN, Jn. (1960) The substitution of oxygen for sulfur in wurtzite and sphaletite.Amer. Mineral,.45, 612=625. Surrn, F. G. (1955) Structure of zinc sulphide minerals. Amer. Mineral.40, 658-675. THE AMERICAN MINERALOGIST, VOL. ss, SEPTEMBER-OCTOBER, 1970 A NEW TECHNIQUE IN THERMOLUMINESCENCE PHOTOGRAPHY SrBpnBN L. Coruccr, Department of Physics, Fresno Slate College, Fresno, C alifornia 937 26l Arsrnacr A simple and sensitivemethod has been developedto produce photographsof ther- moluminescencein minerals. Placing an X-irradiated mineral in direct contact lvith a photographic emulsion forms imageswith a sensitivity comparableto photomultiplier tubes and with resolutionof particles 10 pm in size.Some geological and paleontological applicationsare also discussed. INrnonucrroN Photography, in the usual sense, of thermoluminescing minerals is complicated by the typically low Iight intensity involved. Although the film may be exposed over the entire time a sample is being heated, physi- cal limitations allow only a small fraction of the Iight produced to enter the camera. The successof investigations (for example Roach, 1968, p. 591; and Fremlin, 1968, p. 407), fiilizing the photograph of strongly thermoluminescent materials, indicates the desirability of a method allowing its extension to low intensity cases. The method described be- low, although restricted to a smaller temperature range, is quite simple and provides sensitivities comparable to that of a photomultiplier tube. r Present address:Physics Department, University of Notre Dame, Notre Dame, Indiana 46556..
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