Canadlan Mineraloght Vol. 14, pp. 36a486 (1976)
PHASERELATIONIS INVOLVING ARSENOPYRITE IN THESYSTEM Fe-As-SAND THEIRAPPLICATION'
ULRICH KRETSCHMAR2 eNp S. D. SCOTT Departmentof Geology, University of Toronto, Toronto, Ontario, CanadaMsS lAl
ABSTRACT tions and the composition of arsenopyrite from metamorphosed ore deposits, they suggest that tlte Subsolidus recrystallization experiments rn the effect of confining pressure is much smaller than system Fe-As-S have been carried out using dry, ind:icated by Clark (1960c) and that arsenopyrite is eutectic, halide fluxes to speed reaction mtes. At unlikely to be a useful geobarometer. 300'C the composition range of arsenopyrite ex- tends from less than 3O at. Vo As in equilibrium with pyrite and arsenic to approximately 33.5 at.% Sotrluetnr, As in equilibrium with pyrrhotite and loellingite. At 700"C arsenopyrite contains approximately 38.5 at. 7o As irrespective of the assemblage in which Des exp6riences de recristallisation sub-solidus it is synthesized. The As-content of alsenopyrite dans le systdme Fe-As-S ont 6t€ effectu6es b I'aide may be obtained from its 4s, and the equation: de flux d'halog6nures secs et eutectiques afin d'aug- At, Vo arsenic : 866.67 dBFl38I.l2 menter la vitesse des taux de r6action. A une tem- Microprobe analyses show that natural arseno- p6rature de 300'C, la gamme de compositions pyrite crystals coexisting with pyrrhotite, pyrite or d'ars6nopyrite s'6tend de moins de 30Vo (atomique) jusqu'i pyritefpyrrhotite are commonly zoned with a Si- As en dquilibre avec la pyrite et l'arsenic, rich center and an As-rich rim. The zoning in arse- environ 33.5 at. Vo As en 6quilibre pour la pyrrho- nopyrite from As-rich environments is in the op- tine et la loellingite. A 700'C, l'ars6nopyrite con- posite sense. At preseDt, the most reasonable ex- tient presque 38.5Vo d'atomes d'As, ceci sans d6- planation is that the zoning reflects local disequi- pendre de I'assemblage dans lequel elle est synth6- librium during growth. However, the overall com- tis6e. Le contenu en As de I'ars6nopyrite est li6 positional limits of natural arsenopyrites are de- ir 4sr par l'6quation: pendent on the coexisting Fe-As-S minerals and Vo atomiqtue d'arsenic : 866.67 d3rl39l.lz these limits correspond very well with composi- tional limits determined experimentally. Because of Des analyses i la microsonde 6lectronique d6- its refractory nature, arsenopyrite does not readily montrent que les cristaux naturels d'ars6nopyrite en re
INTnopucnoN substantial improvements in experimental and analytical techniques and Barton's (1969) sul- Arsenopyrite, FeAs1.g"S1.r", is the most fidation study has provided an important frame- refractory of the common sulfides which, work within which to reconsider anenopyrite together with its wide range in As/S ratio, geochemistry. The general phase relations in- potentially geochemical makes it a useftrl tool volving arsenopyrite (Fig. 1) are in agreement for deciphering conditions of its formation. In with the results of Clark (1960a) and Barton possible recognition of the usefulness of such a (.1969),but tlere are significant differences from goethermometer-geobarometer, Clark (1960a) Clark's results in the composition of arseno- determined detailed phase relationships in the pyrite from various assemblages,and his effects condensed Fe-As-S system between 400" and of confining pressure could not be reproduced. 750'C and also investigated the effect of pres- The second part of our study consists of a de- sure to 2.07 kbar. The results of his study have tailed examination of the compositional variation been applied widely for estimating temperatures of arsenopyrite from a large number of occur- pressures and of formation or metamorphism of rences in order to test whether this composi- ore deposits. tional variation is consistent with the experi- Figure I depicts our present knowledge of mental results. phase relationships in the Fe-As-S $ystem over Symbols and abbreviations are given in Ta- the temperature range of interest to geologists. ble 1. Standard deviation of the mean of dr"r is Clark's geothermometer is based on the varia- expressedin the same unit as the last decimal(s) tion of d of. the (131) X-ray reflection of syn- quoted in the mean (e.g. dxt : L.6341!3 thetic arsenopyrite as a function of assemblage means 1.6341:t0.0003). (Fig. l) and temperature at which it is syn- thesized. The geobarometer stems from Clark's (1960b,c) observation that confining pressure ExpnntlteNrer. PRocEDUREs apparently produces a more $rich arsenopyrite for each assemblage and temperature, and Arsenopyrite was synthesized in different raises the invariant point involving arsenopyrite bulk compositions surrounding its stability field *pyritelpyrrhotitefliquidfvapor (491oC in (Fig. 1) by combination of the elements Fe, As Fig. l) by l.8"C/kbar. 'I. In applying Clark's data it is usually assumed TABLE SYMBOLSAIID ABEREYIATIOHS that naturally-occurring arsenopyrite lies at its As". at. g aisenic in arsenopyrite, determlned - by electron S- or As-saturated compositional limit. How- mlcrcprcbe ever, Barton (1969) cautioned that this is not I-sr. average of at. % As ln ars€nopyr.lte fron mlcmprcbe necessarily so. Proper application of arseno- " analyses, reported as mean! one standard devlatlon pyrite as a geochemicaltool requires an estimate of the mean of sulfur activity, a(Sz), during equilibration in As* at. t arsenlc ln arsenopyrlte, calculated from lts nature. Barton (1969) made an important con- dr3r (eguatlon l) tribution to our further understanding and use aest standard enor of estlmate -l4nr;F;F/{-z) of the ""r1 Fe-As-S system by measuring, with the lv ntnnberof lndlvidual deteminatlons (electron probe pyrrhotite-indicator method (Iouknin & Barton analyses or d nFasurenents of arsenopyrite) 1.964),the a(Sz)-dependenceof sulfidation reac- KL KCI - LICI flux tions involving arsenopyrite(Fig. 2). The essence - of the arsenopyrite geothermometer-geobaro- NL NHqCI LlCl flux meter is to determine the composition of arseno- asp arsenopyrlte pyrite as a function of temperature and pressure lo loelllnglte in each of the assemblagessurrounding arseno- ofp orpiment pyrite in Figure L or, in other words, to deter- mine how the composition of arsenopyritevaries po pynhotite along each segment of the buffer curves in Fig- py pyrlte ure 2. rl realgar
Our study is an attempt to present an inter- L sulfuf-arsenlc liguld whosecomposition ls reported nally consistent thermochemical model of ar- as Lc, wherec - wt. % S senopyrite formation and falls into two parts. V vapor First, it is an experimental reinvestigation and FAS designatlon for experlnents conducted in the systen extension to temperatures below 400oC of Fe-As-S phase relations involving arsenopyrite in the a(i),at actlvlty of component, system Fe-As-S. In recent years there have been 366 TTIE CANADIAN MINERALOGIST and S (99.9997o pure) or of previously syntle- heated in a thermal gradient to remove surface sized binary compounds FeaAs, FeAs, FeS, oxidation by distillation and, following Yund's FeSz and S-As glasses.The identity and homo- (1962) suggestion, the compositions of S-As geneity of starting materials were checked op- glasseswere checkedby measuring their specific tically and by X-ray diffraction. Arsenic \ras gravity.
688 - 491"C
702- 6gg"c
Fe2As FeAs lii
Fe2As FeAs tii
< 281"C
49t- 363"C
FeAs 16 As
Fe (Fe2As) FeAs lii As Frc. 1. Isotbermal, polybaric sections througb fhe condensedFe-As.S system.Isotherms are drawn for the temperature midway in the ranges indicated and at 275'C (after Barton 1969). Arseaopyrite composition is shown schematically as stoichiometrio FeAsS. Important invariant points involving ar- senopyriteare asp:lo+po+L at iO2*3'C (Clark 1950a);aspfAs:lo*L at 68813'C (Clark 1960a); asp{py:po-fL at 49ltl2'C (Clark 1960a);pffAs:aspftr- at 363'+50"C @arton 1969). PHASE RELATIONS INVOLVING ARSENOPYRITE 367
Two types of charges were prepared. In a -+-5oC or better. Periodically, the charges were few high-temperature runs we followed standard quenched in cold water and carefully reground procedures for evacuated silica tube experiments to facilitate reaction. At the termination of an as outlined by Kullerud (L971) and Scott experiment, charges were quenched and then (L974a). In most experiments, particularly at elamined optically, by X-ray diffrastion and low temperatures where sluggish reaction rates by electron microprobe. are a severeproblem, we added eutectic mixtures In using fluxes, it is essential that salt com- of binary halides which were molten at the run ponents do not enter the FeAsS structure and temperafures and served to flux the reactions. that the salt system is thermodynamically inert The halide mixtures which we used (with mole relative to the sulfide system. Although it is ratio and temperature range) were KCI-LiCI difficult to demonstrate the former satisfacto- (KL, 42:58, >360"C) and NII+Cl-LiCl (NL, rily, the following observations are an indication 5O:5O, 270-350'C). Procedures for preparing that the fluxes play no part in the reaction and and handling these fluxes are given by Moh & that their role is strictly that of a catalyst. Taylor (1971) and by Scott (L974a). In both types of experiments,the silica tubes (1) Experimental results at low temperature were heated in furnaces controlled within where fluxes wers used are consistent with
t03/T(K-l)
stt\
/jo, po* +L
(\l sat, ctl o
io' q _16 L 500 700 800 Temperoture(oC) Frc, 2. Activity of S2-temperature projection of buffered assemblages involving arsenopyrite in the Fe- As-S system (after Barton 1969). Pyrite-pyrrhotite (Scott & Barnes 1971), Sr-S" (Braune et al. l95t) and Fe-FeS (Robie & Waldbaum 1968) are shown for reference. Arsenopyrite stability field is shaded. 368 TT{E CANADIAN MINERALOGIST those at high temperatures determined with in a silica tube, open at the top, and sealed and without salt fluxes. inside a gold tube. A pin-hole in the gold al- (2) Qualitative electron microprobe analyses lowed accessof the argon pressure medium to for Cl in selected high-temperature arseno- the charge. pyrite produced in halide fluxes showed no detectable chlorine (i.e. less than about 0.1 wt. Vo A). ANer-Ysss (3) The atomic properties of K, Li, NIL and Cl are not similar to those of Fe, As or S X-ray diffraction so they are not expected to substitute into As Morimoto & Clark (1961) noted, changes the FeAsS lattice. in the position of the (131) peak of arsenopyrite Both Klemm (1965) and Delarue (1960) are a sensitive indication of differences in ar- found discoloration of the fluxes accompanied senopyrite composition. The (131) peak has reaction between the flux and sulfide system. sufficiently high intensity, is well removed from If the flux was clear upon termination of the most interfering peaks and, since it occurs at annealing period, we assumedthat no reaction relatively high 20, may be precisely measured. had taken place between the flux and the Arsenopyrite (131) spacingswere determined charge and that the salt system had remained using FeKa radiation, a single crystal mono- thermodynamically inert relative to the Fe-AsS chromator (with graphite crystal) as detector system. and reagent-grade CaF: which had been an- as an internal Attempts to synthesize FeAsS by hydrother- nealed for 24 hours at 700'C (311) CaF* was calculated mal recrystallization (Scolt t975) n NH4CI, standard. The line of of NHJ and HCI solutions met with failure. As to be at t.6470 A from its unit cell edge early experiments showed that neither gold nor 5.46264* (Swanson & Tatge 1953). Arseno- pyrite (131) peaks were scanned at least six platinum could be used as containers because of times at y2o zL/rrl.in and a chart speed of 60 reaction with arsenic (see also Clark, 1960a), cm/hr. 1as sample, a glass slide smear mount, silica tubes were used. These proved very dif- was rotated 180o about an axis normal to the ficult to seal especially for high-percent fill (high slide between scanning cycles. Enough CaF, was pressure) and many runs failed because the seal added to make the intensity of the CaFz (311) dissolved during the annealing period. Regard- peak roughly equal to (131) of arsenopyrite. less of experimental conditions used, arseno- Atl (131) measurements are reported as the pyrite dissolved nonstoichiometrically, forming mean and one standard deviation of the mean. large euhedral crystals of pyrite, pyrrhotite or The precision of at. 7o arcenic obtained from arsenic at the cool end of the tube. the (131) spacings, using a linear equation re- A few experiments were made at 5 kbar con- lating the two, is at least -+0,45 at. Vo As (one fining pressure in KL flux using the apparatus standard deviation) as discussedbelow. described by Scott (1973). Because of reaction Morimoto & Clark (1961) derived a linear between As and gold, the charge was contained equation relating d$t &nd arsenopyrite composi- tion but according to Barton (1969) the equa- TABLE2. dt31 PEAKP0SITIoN V5' ARSENoPYRITECoilPoSITIoN tion is misprinted. It is also misprinted in Bar- ton's paper and should read dnt - 1.6006-r sampleNo. dj3l(A) re L, (at. t) n a<*, _est 0.00098 at. % As. Inasmuch as this equation is Asp 2 1.6303i5 3l .5510.4 3l .82 0.08 based on only a few analyses,it was desirable Asp 56 1.6292!6 30.99*0.68 30.86 0.04 to refine it. Only natural unzoned arsenopyrites Asp 57 1.633213 34.28 0,01 with less than 1 wt. 7o conbrned Co, Ni and Sb Asp 58 1.630014 3l .75s0.23 31.55 0.07 and a few synthetic arsenopyriteswere used to Asp 130 1.635312 36.0410.45 36.15 0.03 derive our X-ray determinative curve in Fig- Asp 135 1.6312x2 33.2!0.45 ?, qE 0.10 ure 3. An equation for this line is: Asp I37d 1.634012 34.50d0,45 35.02 0,15 Asp 138* 1.6352*2 35.98!0.8 36.06 0.02 As* : 866.67dB,. - 1381.12 (1) Asp 200 1.630415 t4 31,77xO.45 24 3l .90 0.04 of Asp276 l.63llc4 d 32.9310.6 32.49 0.13 and has an estimated one standard deviation '15 '10 FAS41 1.637tr5 37.7t1.2 37.71 0.00 10.45 in As*. FAS46C 1.6330 33.7411.21 d 34.15 0.l2 FAS460 1.633319 34.43*0.93 34.41 0.01 FAS 46E 35.35c0.79 I 34.89 0.13 *This tcalculated fiz.la *dt3l value has been revised to 5.4638*0.00044 from equation (l). from Kelly & (Robie al. Swanson& Tatge value (1970); el 1967) but the Turneaure E', frcm at least l0 gralns in gmin munts. must be used with our calibration curve (Fig. 3). PIIASE RELATIONS INVOLVING ARSBNOPYRITE 369 The (131) values were obtained in the man- experimental reaction products in amoun8 ner described above and at, Vo arsenic is from gr6abr than about toVo)ft becomesimpossible electron microprobe analyses. Individual data io determine the d-value of the aneiopyrite points are presented in Table 2. The line was (131) line because of interference from ifote to pass-tu.q$_Asp a1d ut t.$3zl.and toeltingite (031) and-}zt) {tlY".Aeehand- fOO i:tri FAs 41 dlta points for which we have the lines at t.64ot ^a-t.eiu[itJp""ti*1. rmt greatest number of determinations. An un- -compositionproblem was overcome by prepiring the bulk weighted least-squares (dot{as! Jg9 in Fig. f) of the charge"v6ry'close-to arseno- with a correlation coefficient of 0.991 has t4e pyriti ro that other phasds weri present in minor equation: As* = 832.08 d& - 1324.'70.It is imounts relative to arsenopyriti. nearly coincident with the line for equation (1). For a given drsr a rnuch higher As value is ob- Electron microprobe tained from equation I than from Morimoto & Precise and accurate determination of the Clark's line (dashed in Fig. 3). Bquation 1 was composition of arsenopyrite is an integral part used throughout our shrdy. of this studv and considerable effort-was ex- If either pyrite or loellingite are present in pended in selection of a suitable standard for 38 7 7,2 ,,'a.." '-o a)34 U'' bq 33 .O E o32 + AspzOO // ?l /a---"./ $.* r.6280 t.6290 t.63oo l.63to r.632O t.633O 1.6340 t.6350 t.6360 t.6370 t.6380 d1,.,yin AngstrSms Frc. 3. Relationship between d61 X-ray peak position and arsenopyrite composition. Error bars represent one standard deviation of the mean for arsenic and d1s1determinations (see Table 2). The solid line representing equation (1) was drawn to include FAS 41 and Asp 200. A least-squares line based on the same data has the equation As* : 832.08 d$F1324.70 and is shown by-dot-dash symbols. Arsenic values obtained using this equation are not significantly different fron those obtained using equation (1) shown in the Figure. Equation (1) was used throughout this study. The curve from Morimoto & Clark (1961) has the equation 4$:1.6006+0.00091 at. Vo As as corrected (but mis- printed) in Barton (1969). 370 TIIE CANADIAN MINERALOGIST microprobe analysis. Several chemically anal- correction of raw data are very similar to those yzed samples that Morimoto & Clark (1961) described by Rucklidge et aI. (1971) who used described were available for study. Arsenopyrites the same electron microprobe (ARL-EI\DQ and were also provided by R. V. Kirkham (Kirkham associatedfacilities. Data were corrected using 1969). These chemically analyzed arsenopyrites EMPADR VII (Rucklidge & Gasparrini 1969). were checked for homogeneity and Co, Ni and Analytical conditions used throughquXthis stud] Sb. Inhomogeneous arsenopyritesor those con- are as follows: accelerating voltage - 15 kV; taining more than L wt. % combined Co, Ni specimencurrent on Asp 57 - 0.03 microamps; and Sb were rejected. Each of the lsmaining beam current - 0.50 microamps; spectral lines suitable specimenswas then used in turn as a - FeKa, Asltr, SKo, CoKc, NiKa and SbLa. standard to analyze the rest. Since the arseno- Eight 10 sec. counts were collected on Asp 57 pyrites had been chosen on the basis of superior and on each spot on the unknown. The number chemical analyses,that sample which gave the of spots analyzed per unknown varied widely closest correspondencebetween published chem- because it was not always possible to find the ical and microprobe analyses for the largest same number of suitable locations. number of specimenswas chosen as the micro- Considerable difficulty was encountered dur- probe standard. The wet chemical analysis for ing electron probe analysisof reaction products. presented the sample so chosen, Asp 57, is in Their fine-grained and intimately intergrown Table 3. All electron microprobe analyses of nature commonly precluded probing. Many re- natural and synthetic arsenopyrite reported in action products were porous (multigrain ag- this study were obtained using the same ap- gregates mixed with S-As liquiQ and resulted proximately polished 1 mm square area on a in low totals. A few zonal arrangements of section of Asp 57 as standard. reaction products persisted despite regrinding. Asp 200 is a natural, homogeneous, S-rich Only analyses from well-formed grains with arsenopyrite without detectable Co or Ni (see well-defined outlines, and which satisfied the below, however) from the Lucie pit of the Helen following rigorous criteria, were used for de- siderite mine near Wawa. Ontario. A 1 mm termining arsenopyrite phase relations: sqllare spot on a single crystal was analyzed ele- as an unknown at the beginning and end of each (1) The homogeneity index for all three set of analysesthroughout this study. Its com- ments, Fe, As and S must be less than 5. program in- position, based on the first 24 such independent The EMPADR VII computer analyseson different days, is shown in Table 4. dicates a homogeneity index for each analy- sis. This index is defined by Boyd et al, procedure, The analytical data collection and (1969) as the ratio of the observed standard deviation to the standard deviation predicted from counting statistics. Few analyses had TABLE3. UETcHEl'lIcAL AMLYSIS 0F AsD57 to be rejected becausethe H.I. was greater ut. % At. % than 5. In most casesit was (3 and usually <1, but in one or two instances it was ne- Fe 33.85 33.17 cessaryto accept a value of up to 5 for As. As 46.94 34.28 R I 9.08 32.55 (2) The analysis must sum to between 98.0 and Co 0.0042 t!|l 0.001I 101.0 wt. Vo. It is possibleto obtain accept' BI 0.01 able microprobe analyses summing to less sb 0.04 Cu 0.015 than 100.0 'ttt. 7o rf the reaction products porous. Ifowevero totals greater than Total 99.94 I 00.00 are 101.0 must be due to significant etlors. Ana]yslsby R.v. Kirkhani sanple65-144 of Klrkham(1969). (3) An "analysis" must consist of at least two setsof 10 sec. counts. With very fine-grained TABLE4. Tl{ECol,lPoSlTIol{ 0F Asp 200 BASED.0N24 INDEPENDET{T reaction products and a very fine electron ELECTRONPROBE AMLYSES' beam, some difficulty was occasionally en- Elemnt Average At. g one std. Dev. Range countered in keeping the beam on the sam- llt. , (rt. C) (wt. C) ple. Fe 34.51 33.10 0.20 33.8-35.3 (4) The final analysis for the elements Fe, As As 44.U 31.77 0.45 43.7-46.r in the secondary standard Asp 200 s and S 2l .03 35.13 0.31 19.8-21.6 must lie within two standard deviations of Total 99.98 the composition accepted for Asp 200 Cfa- ble 4). rA llnlted ntder of chips fncn Asp 57 and Asp 200, that can be used as pmbe standards, ar€ avallable fr.omS.D. Scott. (5) If arsenopyrite contains iron in stoichio- PTIASE RBLATIONS INVOLVING ARSBNOPYRITE 37r metric amounts (see discussion below), the E: C) o q) osp+ po+L 3 soo Iq, o- E (l) '*{ F osp+lii +Po tr osp+py+L 'o q [1 * c/t I I I /i / @ v/ / lii # / iu FeAs1.1Ss.9FeAs1.15Sg.g5 3233r343536 37 Aiomic % Arsenicin Arsenopyrite Fra. 4. Pseudobinary ?-X section along the pyrite-loellingite join showing arsenopyrite comlosition as a function of temperature and bulk composition in which it was synthesized. The composition of ar' senopyrite represented by solid symbols was determined by converting drr. measurements to at. /o As using equation (1). The precision of measurements for drsr is 10.00054 which corresponds to about 0.45-at.'% As.'Open syirbols denote arsenopyrite compositions measured by microprobe; the precision of these measurements is about '+0.45 at. Vo As, Nl assemblages include vapor. Data from Clark (1960a) are shown for comparison and were converted from his d131fleosurerDents to at. % As using equation (l). PIIASE RBLATIONS INVOLVING ARSBNOPYRITE 373 there must be a break in slope such that the paths and by the single point at 480'C. high-temperature metastable extension of the (3) Although experiments were inconclusive, asp+py+L+V surve lies outside of ftre geometrical restrictions require that tle stability field of arsenopyrite. asp+po+pyl-V equilibrium line must lie (2) The asp*lo*As*V equilibrium is well beffieen the asp+py+Ar+V and asp*As located at 550"C by bracketing runs of simi- *lo*V lines. lar bulk composition but different reaction (4) An unsuccessful attempt was made to de- TASLE5. DATAUSED IN CONSIRUCTINGIHE ARSENOPYRITEY-X SECTION(FI6I'IRE 4) FAS Te!p. Flux Duratlon Startlnq Bulk E-1 n *l No. ("C) (drvs) Materlais f;l.r;jilon 3l Assemblagel: asprpo+L+V '19 9l 6U asp 200,po '1.6377t11 F"q+.dtzs. osza.o 8 38,22 a4 ocu 38 PJ'1o F"ls.zAt42.ss2z,z 1.636815 5 37.45 72 KL 90 FeZAs,LSO F"37.0to40.052g.o 1.6364s3 ll 37.10 45E 600 KL 14 FeZAs,L5O F"gz./tqq. zszs.o 1.6355t5 7 36.32 46E 600 XL 21 FezAs,L5O F 3z.:At4q.7s2s.o 35.410.7 1.6350i7 I 35.89 900 550 KL t02 po, L2g F"q+.dtzs. oSrg.o 34.7r0.5 2 460 KL 56 FeZAs,L5o F"32,sAt44.75zg. o 34.4!0.9 1.6333i9 14 34.41 46C 550 KL 21 FeZAs,LsO F"sz.t't4q. zszr.o 33.7!1.2 a 1.632114 8 33.41 Assenblage: py+asp+L+V 46H 470 KL 26 FeaAs.,LSO F"32.gAt44.7sz3. o 3l.811.2 9 468 450 KL Fe2As,LSO F"32.:A'4e.7523.0 33.011.3 l2 1.6329!2 t2 '172 46A 400 KL Fe2As,L5O F"32.:At44.7s23.0 I Assenb'lage : py+porasplv 90c 480 KL 187 po,LA3 F"aa.dtzs. osss. o 34.0 90E 450 KL 68 po,LZ3 F"44. oAt2g.0S33.o 32.5r'1.8 904 400 .l.6314i12 KL 68 po,LZg Fuq+.dtz:. osrs.o 32.78 Assenbl age: pyraspri\s+V3 'l0lB 264 KL 102 Fe2As'L50 F.3O.OAt45.OSZS.O 29.030.3 974 358 KL 102 po,L23 F"31.74.3e.05:O.g 30.0i0.5 J 978 358 KL 179 po,L23 F"3.1.7A.3A.0530.3 31.8r].4 8 988 358 KL 102 FeAs,S Fust;dtqz.eszs.s 28,9 t Asselblage: As+asFlorv 96A 684 ?d FeAs,L4o F"2a./.Sg.SStz.g 38.0 voL 550 KL 96 FeAs'L4O F"Zg.7Ar58.5Sl2.g 35.2r'1.1 86 s50 KL 25 Asp200,lo,As rl: l: I 36.010.9 96D 480 KL 174 FeAs,L o Fu28.Z&Sg.SStZ.g 34,7r],0 Assmblage: k+asprL+V 4l 1s,123 F"lr.oAtz4.gStr.s 37.7t1.2 I .637115 lc 37.71 Asserblage: asFll orL+V lo,L23 F"tA.dtZq.ZSIO.+ 38.3r1.0 Assenblage: asprpollorv 71 680 ru 35 Fe2As,150 F":Z.dtAA.OStg.o 38.4g0.45 ot 650 KT Asp200 F"l4.SAtq4.AS2l.O 37;5r0.87 9 73A AEA KL 20 PJ'lo F"3Z.5AtSZ.BS]4.7 37.'1r0.65 8 7'tc 480 (L 159 FerAsiL5O F"37.04.44.oS]9.0 36.1i0.51 8 i!00 KL 130 FeAs,LsO F"3Z.OA.SS.7Sll.l 34.910.69 t2 Ean 320 l{L t30 FeAs,LSO F"gz. ? oArS5,75tl.7 33.610.t7 53C 300 It_ tra toqs,Lso F"3Z.6At5S.ZStt.Z 33.5 I rAssedlEgsat equlllbrlur for bulk coqosltion andtemperaturc. elhe mcertalnly associstsdrllh this yalue (onestandard devlatlon of the nean) ls at least 10.45at. Z As (see tert). 3Arsenlc can be.dlstlngulshed fm.mS-4s l.lquld only wlth dlfflcutty and experimnts to detemlne the asplp}lAs+L+Vlnyarlilt, uilch ms prcdlcted frcm thenEdynarlc data by Barton (1969). were unsuccissful. see further dlscusslin'ln text. 374 TTIE CANADIAN IVIINERAL@IST termine the asp+py+As+L+V inva,riant @arton 1969), a few imprecise and widely calculated to be at 363:L50oC by Barton scattered electron probe analyses and the (1969). After 102 days at 358oC, no reaction requirement that tlere be a break in slope had taken place between pyrite and arsenic. between contiguous curves. For different reaction paths and reaction times up to I79 days, pyrite and arseno- Agreement with the results of Clark (1960a) pyrite formed but arsenic was not present. is good at high temperatures, but below about Therefore, the position of the asp+py+As 550'C arsenopyrite in all assemblagesis more fV equilibrium line is based on acceptance S-rich than he indicated and the difference is of the invariant temperature of 363:L50'C not accounted for by our respective X-ray de- Frc. 5. Schematic isothermal sections showing se- quenceof tie lines surrounding arsenopyrite.The letters A, B, C . , . . depict the traces of arseno- pyrite compositionsin tle arsenopyritefield CIa- ble 6) for the isothermal buffer assemblagesof Figure 1. PIIASE RELATIONS INVOLVI\IG ARSBNOPYRXTE 375 terminative curves. Aboie 500'C in the as- sUghtly less than the maximum amount of ar- semblage asp+po+L+V there is a maximum senic possible (about 38.7 at. Vo As or FeAsr.ro 0.9 at. 7o dtfference between the two sets of So.enat 702"C), results. Since uncertainty in arsenic determina- The univariant reaction describing the upper tions is at least O.45 at. Vo (one standard de- stability timit of crystalline arsenic is represented viation), the results are compatible. As Clark by the line labelled As-L on Figures 2 and 4. (1960a) stated, failure of pyrite to nucleate in A portion of rhis line lying within the arseno- his 450 and 400oC rurn may render his drgr pyrite stability field originates at 688oC and measurements questionable. ClarKs work shows meets the asp*py*As+L+V invariant at that the single remaining point, at 500oC, is 363'C. Although formation and decomposition 1.5 at. 7o more As-rich than was found in our of arsenopyrite in this assemblage(asp*As*L study. There is no ready explanation other than *V) cannot be described as a simple sulfida- the failure of ClarKs arsenopyrite to attain tion reaction, it is compositions along this line equilibrium even after 86 days. For the assem- that are represented by point E. At all tempera- blage asptlo+As+V there is again quite good tures, this line is close to the S-rich limit of the agreement above 500oC between the two sets arsenopyrite stability field and one would there- of data. Below 500'C the discrepanciesmay be fore expect arsenopyrite to have a composition explained by postulating interference from loel- very similar to that in the assemblageaspfpog lingite during measurement of the arsenopyrite L*V (point C) and asp*py*L*V (point D. (13l)-spacing as is discussedbelow. If, as an approximation, one assumesregular Isothermal sections Schematic isothermal sections showing the TABLE6. ARSENICCON'IENT OF ARSENOPYRITEIN ATOMICg AS A FIJNCTIONOF TTilPERATUREAND ASSEMBLAGE configuration of tie-lies surrounding the arseno- pyrite field are depicted in Figure 5 and may Phase Assenblaget be derived directly from the T-X section of Tery. po+lo lo+L porl As+lo Asq- pJ+L porpy pylAs Figure 4. Table 6 lists arsenopyrite composition ('c) A B CD EF GH as a function of phase assemblage at various 702 38.7 38.7 8.7 temperatures. 688 38.6 38.4 38.3 38.4 38.4 The letters A, B, C . . . depict 650 38.r 37.2 37.7 37.5 the trace of arsenopyrlls ssmpositions in the ouu 37.5 35.9 36.8 36.2 550 36.8 34.6 35.9 34.9 arsenopyrite field for the various isothermal 491 36.0 33.0 35.0 33.4 33.0 33.0 buffer assemblages. 450 ?66 - 34.0 32.4 32.1 32.4 400 34.9 - 33.1 3't.2 31.0 3l .6 At 690'C, arsenopyrite has essentially con- 363 34.4 stant composition (about 38.4 at. Vo As or 300 33.6 FeAsr.*So.er)regardless of the phase assemblage *All asseublageslnclude asg+V. Lette6 correspondto in which it is synthesized and contains only very sinllarly labelled polnts ln Fig. 5. TABLE7. ATTB,IPTST0 REVERSEARSEI{OPYRITE C0|IP0SITI0N Pr.oduct FN Temp. Flux Duratlq Startlnq Burk ArsenopyrlteComposltlon llo. ("C) {davs) Materlals tfll5srtron (F, or Asr) 88 550 ru JU FAs4ll, po l:12 4sr.38.0dl31.'1. 6374i4 6 '165 89 480 K. FAS41, po, py l:l:1 I-1.38.3x1.0 4 asp200 nucleus' 7A 480 Kr. 9I reszz3 F sr.#no.oszr.o 8a"35'zst'o 12 728 450 Kr 104 FAS72 F gz.#eo.oseg.o q"36'4sl '7 4 998 600 KL 4 asp 2004,As I:l Ft"3l ' 7*o'g 92 680 30 asp 200 F"g4.5k44.4s2r. o Fz'36'9ro'l 2 86 550 KL 25 asp 200, Iil:l I-ta'se'ooo's 4 A3, lo 91 584 l9 asp200, po I rl Asr'38.2dt3l"1.6377rll 8 l@B 686 3l asp 200 F"go.gto+r.45zt 3 crystal . o 4'3?'slo's 1t2C 680 KL t9 asp 200 F"34.5kca.4szr I-s2"32.&o.e 4 crystal .o l. For aBenopyrite FAS41, d13I.1.637135,t!14: &rE37.7. 2. Indlcates apprurlnate ratlo of startlng naterla'ls. 3. For aBenopJrite FAS72, d131"1.6364i3,,bl'l: Asr.37.]. {. For asp 200 Ft 31.8i0.4, t ?4; r.I4. E31.1.630415, 376 TTIE CANADIAN MINBRALOGIST solution behavior, arsenopyrite composition examination of the reaction products to assure should be proportional to a(Sz) at a given tem- textural equilibrium, changes in the X.raj pat- perature. At 400'C the difference in log a(Sia) tern as a function of time, and by quantitative units between the assemblagesasp+po+L+V electron microprobe analysis of reaction prod- and As*L*V is 1.0, which is the maximum ucts. difference in a(Sz) between these two equilibria. Experiments designed to reverse arsenopyrite- A difference of 1.0 log a(Se)units corresponds forming reactions were only partly successful to 0.8 at. lo As n arsenopyrite. Because the (Table 7). In experiments FAS 88 and 89n the precision of As determinations by microprobe high-temperature (680'C) from -r-0.45 arsenopyrite is at. Vo As, the difference rn arseno- experiment FAS 41 (assemblage asp+As+L pyrite composition among the assemblages ap did not change composition even after represented by points C, E and F is experi- 166 days at 480oC and the Asp 200 nucleus in mentally indeterminate at all temperatures. An FAS 89 did not change weigbt. Similarly, ar- experiment at 515oC to check this prediction senopyrite in experiments 724 and B did not failed due to incomplete equilibration. change composition (r statistic at the 95To At 650'C the arsenopyrite field lies entirely level of confidence). between the compositions FeAsr.rSo.e and S-rich arsenopyrite seemsto react more readi- FeAsr.rsSo.es.At 55O'C the configuration of tie ly. After 4 days, using KL flux (FAS 99B), lines is essentially the same as at 650oC but Asp 200 did not change composition at 600oC; the arsenopyrite solid-solution field now lies however, at 680oC for 30 days (FAS 92), its between the limits FeAsS and FeAsr.rSo.* composition changed to 36.9 at. % As. Since With the establishment of the py-asp tie line only two suitable grains could be found for elec- at 491"C, points F and G attain the same com- tron microprobe analysis, this value is not plot- position. At 400oC, the entire stability field of ted on Figure 4, although it is in good agree- arsenopyrite has shifted to more S-rich compo- ment with FAS 91. In the assemblageasp*As* sitions and except for arsenopyrite from the lo*V at 550'C, Asp 200 changed composition assemblageaspfpoflofV (point A), arseno- to 36.0 at. % As after 25 days at 550"C (FAS pyrite is more S-rich than 33.3 at. 7o As in all 86) in agreementwith the accepted equfibrium equilibrium assemblages. value of 35.9 at. % As, Experiments demon- The isothermal section for 300oC is a distant strating that S-rich arsenopyrite becomes more extrapolation from higher temperature. The As-rich by exsolution of pyrrhotite and a S-As greatest uncertainty is associatedwith the posi- liquid (in agreementvrith relations in Fig. 5) are tion of point H. presented by Kretschmar (1973). Attempts to bracket equilibria and vary re- Reaction rates and atta,inrnent action paths were made by using S-rich and of equilibrium As-rich starting materials Clable 5). The differ- ence in composition of arsenopyrite in experi- The greatest single experimental problem in ments 46D and 90D at 550oC in the assem- this study of the subsolidus phase relations of blage aspfpo*L*V is statistically indeter- arsenopyrite was the extremely slow reaction minate at the 95/o level of confidence. In the rate. Neither Clark (1960a) nor Barton (1969) assemblageasp*Asflo*V at 550oC, arseno- were satisfied that equilibrium had been at- pyrite synthesizedfrom Asp 200 appears to be tained much below 500oC and Clark attaches more As-rich (36.0-F0.9 at. 7o As, FAS 96C) an uncertainty of. !1,2"C to the asp+po+py than the arsenopyrite syntlesized in the same +L+V equilibrium at 49IoC because sluggish assemblage using FeAs as starting material reactions prohibit bracketing in runs lasting (35.2-rl.l at. 7o As, FAS 86). However, the several tens of days. measurements are imprecise and are not statis- In the present study, the following precau- tically different at the 957o level of confidence. tions were taken to maximize the probability Where loellingite is one of the starting ma- that equilibrium arsenopyrite compositions were terials, it is significantly more difficult to attain attained: (1) starting materials were finely equilibrium, as run 73A demonstrates. Alson ground (<300 mesh); (2) fluxes were used; (3) after 30 days at 515oC, arsenopyrite synthe- charges were reground about every 4 weeks; sized from py*lo in KL flux contained 37.2 and (4) where possible, S-rich and As-rich start- at. % As (dr", = 1.6365:L34, N=12) which is ihg materials were used in order to vary tle far removed from expected values. In general, path of the reaction and attempt to approach there is a tendency for reaction rates to de- equilibrium from different directions. crease the more As-rich a phase on the Fe-As The extent of reaction was judged by optical join is used as starting material. (i.e. formation PIIASB RELATIONS NVOLVING ARSENOPYRITE 377 TABLE8. ETECIROIIIiIICROPROBE AMLYSES OF SYNTI{ESIZED ARSENOPYRTIES FAs. t Tedp. Duratlon Assenblaget- Starilng Fl ux llo. w ('C) (dcys) tihtertais 4l 33.2x0.29 37.711.21 29.2!0.67 27 680 lO3 As,L lo,L23 71 32.9210,29 38.38*0.45 Z8.gse0.7s 6 680 33 lo,Fo r"adir56 93 32.?j.0,2? 37.5!0.87 Z9.9rl.0l 9 650 4 lo,po asi ZOO-- KL 53E 33.1h0.15 34.8610.69 3z.lil*0.04 12 400 130 lo,po FeAstlso XL l0lB 32.93*0.45 28.96i0.35 [email protected] 3 358 102 py,As rerls,iio KL 978 n.3{}'0.27 -- 31.77r'l.64 36.0511.57 8 359 179 py,As mlq, KL 46D 32,73t0.21 34.43i0.93 33.0310.94 6 550 po,L 58 rerniiqo-- KL 900 n.4&0.29 s.7Lt0.s4 32.se0.74 z 5s0 toz po,L noitr, KL 468 V..32r0,fi 33.0411.26 34.6411,05 12 450 67 py,L rerniir-rs KL 45fr 32.68!0.43 34.41!0.96 32.90i0.78 3 5OO 28 py,L re"es,Lro KL 45Btt 3l.44io.ll 35.1211.39 33.1710.92 4 450 28 py,L re4r,Lii KL 45Att 3l,73!0.44 33.96s0.75 34.2910.86 5 4OO 28 py,L r"rlr,rri KL 464 31.5910.22 35.2610.86 33.2210.06 I ioo t7Z py,L rerls,Lri KL tAll lnclude 8sFlV, tfThes€ runs dld not reach equlllbrium. of arsenopyrite is significantly slower if FeAss sistently gave 33.3 at. Vo Fe. rather than FezAs is used). The apparent Fe-deficiency is an additional Our observations on equilibrium agree q/itb source of uncertainty which affects the T-X those of Clark, who found that arsenopyrite, section (Fig. 4), since this is drawn on the as- onse synthesized, will not change composition sumFtion that arsenopyrite is stoichiometric. even after 304 days at 550oC (Clark 1960a). It For example, if an arsenopyrite has a l.O at. Vo is also clear from our results tlat usine fluxes Fe-deficiency, it will plot at 0.5 at. Vo more and grinding starting materials very tiiety in- S-rich composition on the 7.ef,^,,section after creases reaction rates significantly and permits recalculation onto the FeSrFeAsz join. reversals of runs. Theoretical considerations dictate that, since the arsenopyrite stability field has a finite thick- Stoichiometry of arsenopyrtte ness (Fig. 5), arsenopyrite must be slightly non- A compilation of selected wet-chemical anal- stoichiometric. However, the Fe-deficiency is yses of natural arsenopyrite made by Klemm less than 1, at. Vo and, in agreement with Mori- (1965) indicates that the mineral may be Fe- moto & Clark (1961), arsenopyrite is probably deficient. Natural arsenopyrites examinbd in our stoichiometric to :t0.7 at. Voo the precisiou of study exhibit a similar trend. Fe determination from the interlaboratorv com- Table 8 shows electron microprobe analyses parison of arsenopyrite analyses. of arselopyrite from various phase assembliges synthesized at different temperatures along ihe Acrrvrry or FsAsS nr AnseNopynrrn S-rich and As-rich limit of the arsenopyrite In his derivation of univariant stability field. Included in ttis table are ana- curves for sulfidation reactions in the Fe-As-S lyses of arsenopyrite from runs that clearly had system, Barton (1969) noticed slight discrepancies for not reached equilibrium (e.g.45A, B and C), as alternative calculation paths well as those that do represent equilibrium and which he attri- buted to but significanf were used to plot Figure 4. None of tLese latter "small, apparently variations in experiments produced arsenopyrite with less activities of components within phases". Because the S/As ratio varies than 32.2 at. Vo Fe. Neverthel&i, the mean Fe- con- siderably in arsenopyrite, a(FeAsS) gontent for arsenopyrite in all assemblages is is expected to vary also. less than 33.3 at, Vo and there are verv few individual arsenopyrite analyses of greater than In is convenient to define tfie mole fraction 33.3 at. Vo Fe. Micro-inclusions of S-As liquid of FeAsS in arsenopyrite in terms of the com- in synthetic arsenopyrite could explain sohe, ponents FeAsS and FeSr in order to takd maxi- but not all, of the apparent nonstoichiometry in mum advantage of tiermodynamic data for the Table 8. However, for the assemblages not in- Fe-S system. Defrned in this way, the mole 91"{i"e S-As liquid, the Fedeficienry is not ftaction of FeAsS in arsenopyrite, lmt, is 1.0 likely a function of the analytical meihod be- for 33.3 atomic ft As, is positive for values less cause repeated microprobe analysis of loellingite than 33.3 at. % As and is greater than unity for and pyrite, which are known to 6e sbichiomeiric values greater than 33.3 at. % As, in which case @osenthal 1972; Kullerud & Yoder 1959), con- lffi, is negative. 378 THE CANADIAN MINERALOGIST TABLE9. CALCULATEDACTMTY 0F FeAsSIt{ ARSENoPYRITEAT 500"C corrected for the improved AGoy for pydte giv- en by Scott & Barnes (1971). Values of a(FeS) Assem- oto:t lonor" or", or.s, n?i!, and a(Sig)within the arsenopyrite stability field blage 4iirt "illrs are from Toulmin & Barton (1964). asprpot 33.3 1.000 -4.5 0.51 0.725 0 I .000 L+V 33.6 1.008 -5.0 0.56 0.4/A -0.008 0.999 Results in Table 9 show that at 500"C the aspll+V 33.7 L011 -5.2 0,58 0.368-0.0I1 0.998 activity of FeAsS in stoichiometric arsenopyrite 33.9 |.017 -5.3 0.59 0.334-0.017 0.998 34.4 1.032 -5.9 0.62 0.176-0.032 0.991 is, by definition, 1.0 in the most S-rich as- 34.7 1.041 -6.2 0,65 0.t31 -0.041 0.987 semblage, aspgpoal.lV, and only slightly -6.7 0.0779.-0.0560.977 asp+As+'| 35.2 1.056 0.69 lower (0.949) in the most As-rich assemblage' eV 35.4 I .062 -6.9 0.71 0.0637-0.062 0,972 35.8 1.074 -7.4 0,74 0.0373-0.074 0.956 aspfpollofV. This small departure from '1.086 asplpo+ 36.2 -7.6 0,76.0.0304 -0.086 0.949 unit activity of FeAsS is sufficient to sause the lsv discrepancies noted by Barton. Such small varia- tion in a(FeAsS) may be important for precise thermochemical calculations (see Barton 1969), Astivities of FeAsSwere calculatedfrom the but they are insignificant for interpretation of Gibbs-Duhem equation: natural arsenopyrite compositions and we have : (2) not evaluated them further. 4 nrdP, 0.. . 2 where dpr - 2.303 RT d Log a* Eprpcr oF CoNFININc PnsssuRE oN For the binary system FeAsS-Fe$: Ans gNopYRrrB Co*rposrnoN rlll* a logaffi"u + Nil".,a loga|[, : o. (3) Clark (1960c) found that arsenopyrite syn- thesized at 1 or 2 kbar confining pressure is and more S-rich than arsenopyrite synthestzed at r , .aap the same temperature under the vapor pressure - - r:aaD (4) d log o""oo N;;, /N;;;" d log aror. . . of the system,regardless of the assemblage.The crystallographic data of Morimoto & Clark Equation (4) was integrated for the range of (1961) shows tlat S-rich arsenopyrite has a arsenopyrite compositions across the stability lower molar volume than As-rich arsenopyrite field at 500oC. For a given ratio of Nffiz/ so is, indeed, the favored composition at high lff&*, log a(FeAsS) is given by the area under pressure. Howevero relative molar volumes do the curve in a plot of N/N vs. - log a(FeSz). not give the magnitude of the pressure effect The a(FeSz) in equation (4) was calculated from (1960c) the relationship We have attempted to check Clark's results by synthesizing arsenopyrite at 500"C 2FeS * Sz : 2FeSs. (5) under 5 1621 gqnfining pressure in the assem- blage aspfpofl- using KL flux. The vapor for which from the charges in these experiments was in a(FeSz): a(FeS). a(Sr'' K4...... (6) contact with the walls of the pressure vessel so the results Crable 10) must be treated with The equilibrium constant, K, was calculated caution. In summary, our experiments indicate from the free energy of reaction for equation that high-temperature arsenopyrite used as (5) using the data of Barton & Skinner (1967), starting material became only slightly more TABLEIO. ATIEI{PTSTO SYII'IIIESIZEARSEI{OPYRIIE AT 5 Kb AI{D sOOOCT FASl{o. Strrting ltaterlals Duration Products (dcys) FAS4l asP,po,Py ssPrPo,PJ -pyrlte asspeaE to be bmklng dm -lnltial asp co4osltlon (FAs 4l): Asi' 37.7*1.2 -prcduct asp d131 .1.635@4, t I0; Air - 35.9G!0.4 po,L?o asPrPo,PY -Droduct asD drrr' l.63it(EI3, X.l l, Asi . 35.Gl.I -irmduct py-mhotlte co@sitlon, fp.5 . 0.934 -lnltlal pyrrhotlte coqosltlon, tF;a " 0.9579 po,L20 99 asp,po,[ -F,: - 33.3*0.2, F5. llot enough left to l-ray. Thls vllue my be as-uch as 0.6 at. t As hlgh dE to malytlcal dlfflcultles. FAS4l,asp,po 99 asp,po, -flur dlssppeared, s€vere vapor loss, po gltr on spaclng rcd. FeAs, Fe2As (dlsequll lbrim asseEbl age) yaFr fmn charges ln contact rlth and Esy hrE reacted rlth mlls of the Pmssult vessol. PHASB RELATIONS INVOL\IING ARSENOPYRXIB 379 S-rich (5P3), but h clearly not in equilibrium the foregoing is that confining pressure does with pyfpo or pofl- at any pressure.Arseno- cause arsenopyrite to become more $rich, but pyrite grown from pyrrhotite and a sulfur-ar- the magmtude of the change is not known and senic liquid (5P4) was much less $rich than may be much smaller than indicated by Clark expected from Clark's results. In a longer ex- (1960c). periment (5P5), arsenopyrite was up to 0.6 aL Vo more S-rich than 33.3 at. Vo As (the equili- brium As-content at 500oC in the assemblage Tlm CorvrposrrroN oF NATURAL AnspxopvnrrB asp+po+L+V, from Fig. 4) but far less S-rich than expected from ClarKs data In trying to To what extent are the phase relations ap- explain this discrepancy between our results plicable to natural occunences? A noteworthy and Clark's it must be kept in mind that (1) preliminary observation, which suggests that Clark carried out his high-pressure experiments arsenopyrite does attain "equilibrium" composi- in gold tubes whereas our experiments were in tion in nature, is in the listing of arsenopyrite- glass-lined gold tubes and (2) he was unable to bearing assemblagesby Ramdohr (1969) which determine tle upper stability limit of arseno- shows only 5 of 98 arsenopyrite occurrencesto pyrite at high pressure QOZ"C at <1 bar) be- be "non-equilibrium" (i.e. the assemblagesof cause of reaction between gold and arsenic ore minerals belonging to the Fe-As-S system (Clark 1960c). We sealed arsenopyrite with Kt are not compatible at any temperature). flux in both Pt and Au tubes. After only 2 days Two recent investigations have dealt with at 500'C, the containers had melted almost natural arsenopyrite. Klemm (1965) compiled completely besausethe flux accelerated the re- what in his estimation were superior analyses action between Au and As. Clark states that determined by wet-chemieal methods. He found there is very low solubility of Au in arseno- that arsenopyrite composition varies approx- pyrite since drsr value of arsenopyrite, reacted imately between the limits FeAsr.rSo.eand for 19 days 600'C at with Au in silica tubes, FeAso.rSr.r,and more than two thirds were sul- was unchanged. There is, however, no assurance fur-rich. The maximum deviation of Fe in ar- that this is true pressures. at higher Moreovern senopyrite was approximately 1 at. 7o. Rosn..r any reaction between Au and As will lower the (1969) analyzed arsenopyrite from 25 localities activity of As [raise a(Sz)] and, in response,ar- by electron microprobe. He found no optical senopyrite should become more S-rich. There- or compositional zonation and no elemen8 fore, the apparent pressure effest of on arseno- other than Fe, As, S, Co and Ni in concentra- pyrite composition observed by Clark for tle tions exceeding L wt. Vo. The majority of nat- assemblage possibly asp+po+L may represent ural arsenopy:rites contained less than 1 vtt. Vo As-Au interaction. Co and/or Ni and the maximum deviation of For the assemblageasp*lo*As, there is a Fe from FeAsS was approximately 1 wt. %. similar discrepanry which may be explained by The compositional limits he found \trere Feo.ee analytical interference of loellingite, T\e Q2l) Asr.ouSo.saand FeAso.asSt.tt. X-ray line of loellingite is at 1.63444 (PDF Card No. 1.1-699),approximately in the middle We have examined arsenopyrite-bearing ores from of the commonly observed range of axseno- a wide variety of localities. Each was pyrite 4:r values. If this line is mistaken for classified into assemblages on the basis of Fe-As-S coexisting polished (131) of arsenopyrite, one obtains 35.4 at. Vo minerals in the same section. preliminary As from equation (1). Four out of five of After microprobe analyses, 54 arsenopyrites ,*t. com- Clark's 2 kbar data points lie within O,2 at, % with less than L Vo bined (with of 35.4, which suggeststhat it may have been Co, Ni and Sb a few exceptions) were the loellingite du that he measured and not accepted for detailed study. On averagen arsenopyrite (131). However, Clark (written three separate spots on three different arseno- pyrite grains communication 1976) was well aware of this were analyzed in each of these polished given possibility and attempted to minimize it. sections. Preference was to those grains which were enclosed in or were touching In trying to deduce from all available data other Fe-As-5 minerals. Each such spot is plot- what effect confining plessure will have on ted as a single analysis. Analyses from several anenopyrite compositions, it should be noted assemblagesare shown in Figure 6. The lines in that natural arsenopyrites 1i66 highly meta- Figure 6 indicate the permissible compositional morphosed ore deposits are not as $rich as they range of arsenopyrite in each assemblage @ig. should be if confining pressure has a drastic 4) and are drawn at a constant Fe content of effect. We will return to this point later. 33.3 at. %. With a few exceptions, the corn- A permissible, albeit tenuous, conclusion from positions of natural arsenopyrites, includlng TIIE CANADIAN MINERALOGIST -o o=9 ll o i>- oQ .Eti t.eaF itrEgnsii €fi9 .E€ "rP6e in €ge.:iFtr EEcstr $ ""-'-x I a5 tB 'i Ft H.i, -f:: _8e Egs+dX -c'& -9 F,ra .E16A Fs;fg i i{ 5'E IBEEi 3€Eo El F\l cs tr lc,6 llE EFIf E 9EgE.H oo 'ig€sfi €€s;€ B'6 Eer*'E'E .,ad sEXs E.H 9 R.^ H-H 6.9 a Eg€E: r€! t* anAo3{s gE€ o--oL <+ I ET f. \.|b E;E I1 Figure 6b,c,d shows that many arseno- (4) Both center and rim compositions of zoned pyrites coexisting with pyrite and,/or pyrrhotite arsenopyrite crystals lie vdfhin the permis- apparently have a slight iron deficiencn similar sible equilibrium range for each assemblage. to those grown s;mthetically. The existence of such marked zoning as Zoning in arsenopyrtte shown in Table 11 reflests changing conditions during formation of arsenopyrite and attests Many, if not all, natural arsenopyrites are to the strong resistance of arsenopyrite to in- compositionally zoned. The zoning is almost ternal homogenization. If arsenopyrite started impossible to detect optically without etching to form at high temperature and continued to and optically zoned arsenopyrites are not neces- grow as temperature dropped, the outer zones sarily compositionally zoned. In many case should be progressively more S-rich than the optical inhomogeneities are due to twinning center if buffering occurred during its growth which is almost universal in natural arsen(F (Fig. ). This is indeed what we see in As-rich pyrite (Morimoto & Clark 1961). Because it assemblages.Ilowever, the zoning is in the is a large-scalefeature, the zoning is not readily opposite sense in S-rich assemblages,so a dif- detectable by back-scatter elechon or X-ray ferent explanation must be sought. scan photos on the microprobe. If the effect of confining pressure is to make The zoning appears to be concentric; the arsenopyrite more S-rich, the observed sequence compositional change is stepped and irregular of zonesfor S-rich Out not As-rich) assemblages with no reversals. The difference in composi- reflects growth during gradual, nearly isother- tion between center and rim of some single mal release of pressure attending retrograde arsenopyrite crystals is shown in Table 11. A metamorphism. This may happen in some in- limitation of the data is that we are dealing stances but it is a highly unlikely explanation with point analyses only; nevertheless, the fol- for the diagenetic low-temperature arsenopyrite lowing generalizations apply regarding the zon- from Ovens, N.S. (Asp 43 in Table 11). Re- rng, and exceptions are explained by changes versals in slopes of the univariant curves below in assemblagewith time: the temperatures at which we investigated them (l) In S-rich assemblages (asp-po, asp-po-py could explain some of the zonal patterns, but not and asp-py), centers of arsenopyrite crystals those from high-temperature ores. are S-rich relative to the rims. The conclusion that we are forced to accept Q) Zonng in arsenopyrite from As-rich as- at present is that most of the zoning is a non- semblagesis much less common which, in equilibrium feature reflecting the kinetics of part, is a reflection of the rarity of such arsenopyrite growth and local fluctuations in the arsenopyrites.Nevertheless, zoning does ap- a(Sr)/a(Asz)ratio. Generally, howevet, the com- pear to be the reverse of that shown by ar- positional range of zoned arsenopyrite lies with- senopyrite from the S-rish assemblages,i.e. in the equilibrium range for each assemblage, with the centers As-rich relative to the rims. $o we suggest that zoning reflects only minor (3) The lower the indicated formation tem- and transitory conditions of non-equilibrium petature, the greater is the extent of zoning during growth. in arsenopyrite from the S-rich assemblages. CoNcl-usroNs: TnB AnssNopYRrrg TAELE1.I. I'IICROPROBEAMLYSES OF ZONINGlN NATURALARSENoPYRITE GsoillrnMoMETER AsP Fe-As-S Atomlc U As Locallty llo. Assemblage-len-terR-im- The excellent correspondence between the ?* asp only 1i7 2tt liloretonrs Harbour, Newfoundland 298 asp only 33.9 33.2 Cobalt, ontario composition of natural arsenopyrite and our asp only 32.8 3t.3 Cobalt,ontarlo experimentally-determined that, 33,3 32.3 limits suggest ti asp only tqa ttt Cschlova,Barrat, Hungary despite the zoning, arsenopyrite compositions l3 asp only ,oq t14 Trepca, Yugoslavla 42 Porasp' 30.5 32.3 uFuzlg, Hungary frequently indicate initial formation tempera- 27 PJrasP 30.0 3l.5 MUhlbach.Austrla tures. We ate not convinced of the magnitude 16 PYrasP 30.0 3l.0 Stall Lakemlne, lilanJtoba 2438 PY'asP 30.5 3l.0 Stall Lakemlne, lilanitoba of the effect of confining pressure as measuted 43 py,asp 29.7 31.1 ovens, NovaScotla 69 PJrasP 30.5 32,2 NocheBuena, l,lexlco by Clark (1960c). If, as we suspect, tle pres- 548 porpy,asp 36.7 33.4 Madelelnemine, Quebec sure effect is negligible, arsenopyrite may in- 228 po,py,asp 3t.4 30.4 Fox Lake mlne, lilanltoba 2N po,py,asp 29.8 31.5 Fox Lake mine, tilanltoba deed be used as a sliding-scalegeothennometer. 231 PorPJ'asP 30.8 3l:5 Fox Lake mlne, filanltoba 30.6 30.0 Fox Lake mlne. filanltoba As such, and because of its refractory nafute, 246 PorP$asP 30.0 32.3 osborneLake mlne, llanltoba arsenopyrite should be a good indicator of tem- 247 Po.P$asP 32.6 33.4 osborne Lake mlne, filanltoba 270 po,py,atp ??n ?l I lilattabi mlne, ontario perature and sulfur activity during ore forma- po,py,asp 280 33.8 35.3 Honestakemine, South Dakota tion but, becauseit is usually a minor phase in PTIASE RELATIONS INVOLVING ARSENOPYRITB 383 ore deposits, it is unlikely to buffer the ore- arsenopyrite occurs in veins of the Mount Pleas- forming environment. The stability field of ant tin deposit (Petruk 1964). There are no Fe-S arsenopyrite effectively straddles the range of minerals but tle univariant a(Sg)-? relationship temperatures'and sulfur activities sgtlining the may be est:mated from the Bi-Bir$ assemblage "main-line" ore-forming environment of Barton which lies within the stability field of &rsotro- (1970) and Holland (1965), so this geother- pynte. mometer should find widesprea{- application. - It is with such applications in mind that we It is essential that the sulfur activity be have constructed Fifure Z, which is a log a(SL)- known for proper use of the arseaopyrite-geo- temperature diagra"d oo whi"h arsenopyite iso- thermometer. In most c,uiesother fq:4"-S phases plet|s are contolured n at Vo As. In uiing fig- will provide this information and Figure 4 can ures 4 and Z for geothermometry the fo[owing be used directly, lqt the presence of these precautions shouldbe taken: phases is not essential. Any a(S:)-dependent as- semblage coexisting with arsenopyrite can be (1) Arsenopyrite should be chosen with care usedo at least theoretically. For example, the from equilibrium, a(Sz)-buffered assemblages. assemblagebismuth*bismuthinitefsphalerite* Equilibrium is difficult, if not impossible, to tooo/T(K-r) t.4 r.5 -a 684"C wsarztsl'l -3 6"* -5 osp+16+L -6 -7 N-q a, t! o 'Qo o -9 -15 L 3@ Temperoiure("C) Flc. 7. Activity of $u-temperatureprojwtion of tlo stability field of arsenopyrite, contoured in atomic Vo arsenfc from the data in Figure 4. All assemblagesinclude vapor. 384 TTIE CANADIAN MINERALOGIST prove in ores and is usually decided on para- metamorphosed sulfide ores and yet we were genetic and textural grounds (Bafton i al, not able to bracket it closely. Finally, we have 1963). In vein deposits, arsenopyrite can be not been able to resolve the cause of composi presumed to be in equilibrium with other simul- tional zoning of arsenopyrite. One of its implica- taneously deposited minel{s, surface equili- tions is ttrat temperature estimates based on b:ium among the minerals being readily- achieved careful electron microprobe analyses will appear through the fluid phase. However, once de- to be lsss precise because of the latge range of posited, tlre refractory arsenopylite is unlikely As-contents. However, if the zoning tums out to 10 changg its composition in response to chang- be an equilibrium phenomenon, such est:mates ing conditions in the vein Rather, it is expected may eventually be more precise because the to be epitaxially overgrown by younger arseno- zoning will then be an additional factor for pyrites of different composition as was found, evaluating the thermal history of arsenopyrite- for example, by Clark (1965) in the yltijirvi bearing rocks. copper-tungsten deposit where S-rich low-tem- In conclusion, despite the several remaining perature arsenopyrite rims As-rich high-tem- uncertainties, we believe that arsenopyrite is a perature arsenopyrite. viable geothermometer.Nevertheless, ore-form- Equilibrium is easier to reconcile in meta- ing environments can be effectively estimated morphosed ores by the fact that the coexisting only if all avaaTabledata are used. The informa- minerals have annealed together for a long tion that arsenopyrite provides must not be ab- pgriod of time, although compositional zoning stracted but must be used in conjunction with which persists in some arsenopyrite from highly silicate petrology and, above all, the geology metamorphosed ores may preclude complete of the deposits. equilibration. Experience gained in using spha- lerite, a less refractory mineral than arsino- pyrite,. as a geobarometer in metamorphosed ACKNowl.EDGEMENTs ores (Scott 1973, 1976) emphasizes that the This research is a portion arsenopyrite must touch the coexisting phases of Kretschmar,s Ph.D. dissertation (Kretschmar 1973) which constituting the buffered assemblageand pref- was started at McGill University under the dires- erably in triple junctions @eWitt & Essene 1.974). tion of L. A. Clark and was completed at the University of Toronto. Dr. Clark's (2) The combined minor-element content of the helpful ad- vice in the early stages and his review arsenopyrite should be <1 wt. Vo. Commonly, of our manuscript are gratefully acknowledged. Co and Ni substitute for Fe. We are uncertain The advice and criticism of G. M. Anderson. p. of their effect on As,/S ratio in arsenopyritebut, B. Barton, fr., L. J. Cabri, A. J. Naldrett more imFortantly, if arsenopyrite compositions and J. C. Rucklidge throughout the remainder are determined by X-ray diffraction their effect of the investigation were valuable and most is to increase 4s, (Morimoto & Clark 1961; appre- Gammon 1966). ciated. (3) The drgr value should be determined with We would like to thank the following persons annealed CaFz as a standard and using a= for providing samples: H. L, Bostock,-R. A. 5.46264 to be consistentwith our determiiative Both, R. W. Boyle, D. Card, L. A. Clark, p. curve in Figure 3. Girard, W. C. Kelly, R. V. Kirkham" c. H. (4) If arsenopyrite is analyzed by microprobe, lCtzler, J. T. Nash, H. Ohmqle, V. S. Papezik, an prsenoqynte standard of proven homogeneity W. E. Roscoe, N. Russell, R. J. Shegelski,J. F. and carefully determined composition ihoula Touborg, B. F. Watson and F. J. Wicls. be used. Arsenic determined using FeAs, or As For microprobe as standards will be high because the As is analyses of arsenopyrite we extend bonded differently than in arsenopyrite. our thanls to: D. C. Harris, W. H. Maclean, J. T. Nash and D. R. Owens. The There are still several important unresolved microprobe facility at the University of Toronto problems in the determination and application is,maintained by J. C. Rucklidge. His expert of phase relations of the Fe-As-S systim. Chief advice and encouragement aided us immensely among these is to determine the effect of pres- in overcoming some serious problems pertaining sure on arsenopyrite composition. Second, our to analyses of arsenopyrite, experimental results.below 450"C in the S-rich Diagrams portio!_ of tle system are not entirely satisfac- were drafted by W, M. Jurgeneit. tory The asp+py+po+V assemblageis partic- This research was funded through National ularly problematical in this respect. tf is a Research Council of Canada grants A-1111 to most useful assemblagefor geothermometry of L. A. Clark and A-7069 to S. D. Scott. PIIASB RELATIONS INVOLVING ARSBNOPYRITB 385 RBFERBNCFS KRBTScSMAR,U. (1973): PhaseRelations Involving Arsenopyrite ln the System Fe-As-S and Theh BAerON,P. 8., rn. (1969): Thermochemigal $tudy Application, Ph.D. thesis, Univ. Toronto. of the system Fe-As.S. Geochim. Cosmochtm. Kt[lmuo, G. (1971): Experimental technique* in Ada 88,841-857. dry sulfide research./z ResearchTecbniques for (1970): Sulfide petrology, Mlneral. Soc. Iligh Pressure and High Temperature (G. C. Amer. Spec.Paper 8,187-198. Ulmer, ed.), Springer Yerlag, 289-3L5. -, Bntre, P. M. & TouLl&Y, P., m & Yoonn, II. S. (1959): Pyrito stability (1963): Equilfbrium in ore deposits,Mtneral. Sor' relations in the Fe-S system- Econ. Geol. il, Amer, Spec.Poper 1,171-185. 533-572. & Smrrvn, B. I. (1967): Sulfiatemineral Mor, G. II. & Tevr-on, L. A.'(197t)z Laboratory gtabilities. Iz Geochemistryof Hy&othermal Orc xecbniques in experimental qtlfide Ftrology. Deposits (II. L. Barnes, ed.), Holt, trlin€haat & Neues lahrb. Mineral. Monatsh,, 450459. Wrnston,N. Y., 236-333. MoRrMoro, N. & Clenr, L. A. (1961): Arseno- Bovp, F. R., FnrcEn, L. W. & CrrAyBs,F. (1969): pyrite crystal-chemical relations. Amer. Mineral. Computer reduction of electron probe data. 46, t448-t469. CarnegieInst. Wash,Yearb. 67, 2I0-2t5. PETRnK,W. (1964): Mineralogy of the Mount BneuNg H., Perrn, S. & NBvrr:wc, Y. (1951): Die Pleasant tin deposit in New Brunswick. Mizes Dissoziation des Schwefeldampfes.Z. Naturf, Branch Tech. BuIl. TB 56, Dept. Energy Mines 62, 32-37. Res.,Ottawa, 35 p. CLARK,A. H. (1965): The compositionand condi- Prpru, A. C. (1967): Bau Mining District, West tions of formation of arsenopyriteand loellingite Sarawak,East Malaysia, Part II: Krokong. Geol. in the Yliijiirvi copper-tungstendeposit, Sout!- Surv., Borneo Region, Malaysia, Bull. 7, Pt. fr, west Finland BulL Comm. Geol, Finlande 217, 97 p. 56 p. L. A. (1960a): CLaRK, The Fe-As-Ssystem: Phase Intergrowths. Pergamon Press, London. relationsand applications.Econ. Geol.56, Pt. I: 1345-1381,Ft. tr: 1631-1652. RosIE, R. A., BrrmE, P. M. & Bnar,osr.rv; I( M. (1967)'. data, (1960b): Arsenopyrite As:S ratio as a Selected X-ray crystallographio possible geobarometer.Bull. Geol. Soe. Amer. molar volumes and deusities of minerals and 71, 1844 (Abstr.). related substances.U.S. Geol. Surv. Bull. l.LLB. (1960c): The Fe-As.S system.Variations & Wer-rsaur4,D. R. (1968): Thermody- of arsenopyrite composition as a function of T namic properties of minerals and related sub- and.P. CarnegieInst. Wash. Yearb. 59, ln-L30. stancesat 298.15'K (25.0'C) and one atmosphere (1.013bars) pressureand at higher tomperatures. DEwrrr, D. B. & EssrNr, E, l. (1974): Sphalerite U.S. Geol. Surv. Bull. L259,256 p, geobarometryapplied to Grenville marbles. GeoL Soc.Amer. ProgramAbstr. 6,709-7L0. RoscoB, W. E. (1971): Geology of the Caribou Bathurst, New Brunswick Can. t. Earth Druanue, G. (1960): deposit, Propri6t6s chimiques dans Sci.8, LL25-tt36. l'eutectique LiCl-KCl fondu. II. Soufre, sulfures, sulfites, sulfates. Bull, Soc. Chem, France L960, RosnNrner-, G. (L972): The preparation and prop 906-910. erties of FeAss and FeSbs./. Solid State Chem, GAMMoN,J. B. (1966): Someobservations otr min- 5, L36, erals in the system CoAsS-FeAsS.Norsk Geol. RosNeR, B. (1969): Mlkrosonden-Untersuchungen Tids. 46, 405-468. an Natuerlichen Mlneralien der Reihen Arsen- Hor.r.ar.ro,H. D. (1965): Some applications of ther- kies-Kobaltglanz-Gersdorttit und Loellingit-Saf- mochemical data to problems of ore deposits.U. florit-Rammelsbergit sowie der Gruppe der Skut- Mineral assemblagesand tho composition of ore- terudite. Dr.-Ing. thesis, Fakultaet fuer Bergbau forming fluids. Econ. Geol. 60, 1101-1166. und Huettenwesen det Meinisch-Westfaelischeu TechnischenHochschule, Aachen. Knulv, W. C. & TURNBAURB,F. S. (1970): Miner- alogy, parageuesisand geothermometry of tle RucKJDcE, I. C. & Gesumnu, E. L. (1969): Spec- tin and tungsten deposits of tle Eastern Andes, ifications of a computer progranme for pro- Bolivia. Econ. Geol. 65, 609-680. cessing electron microprobe analytical data. EMPADR Dept. Geol. Uoiv. Toronto. KnmAM, R. V. (1969): A tuIineralogicaland Geo- VII. chemical Study of the Zonal Distribation of Qres -, Gespennnqr,E., Sr"flTu,J. V. & KNowr-es, in the Hudson Bay Range, Brtrtish Columbia, C. R. (1971): X-ray emission miooanalysis of Ph.D. thesis, Univ. Wisconsin. rock-forming minera-ls.VIII. Amphiboles. Can, Kr-rrtlt, D. D. (1965): Synthesenund Analysen in l. Earth Sci. 8, 1171-1183. den Dreiecksdiagrammen FeAs$CoAs$.NiAsS Scorr, S. D. (1973): Experimental calibration of und Fe$-CoS:-NiSz.Naaes lahrb. Mineral. Abh. tle sphalerite geobarometer. Econ. Geol. 68, 103, 205-255. 466474. 386 TIIB CANADIAN MINERALOGIST (1974a): Experimental methods in sulfide SwANsoN,H. E. & TATcE, E. (1953): Standard synthesis..In Short Course Notes - Suffide Mitr- X-ray diffraction powder patterns. U.S. Nat. Bur. eralogy @. H. Ribbe, ed.),Mineral. Soc,Amer. Standards Clrc. 539, L, 69:70. 1, Sl-S38 Toull"flN, P., m & BenroN, P. 8., rn. (1964): A (1975): Hydrothermal sylthesis of re- thermodynamic study of pyrite and pynhotite. fractory sulfide minerals, Fortschr. Idlneral. 52, Geochirn. Cocrnochl,m.Acta. 28, 64t-67t, 185-195. YUND, R. A. (1962): The system Ni-As-S: Phase (1976): Application of tle sphalerite geo. relations and mineralogical significanc*. Amer. barometer to regionally metamorphosedterains. .1.Sci. 260,761-782. Amer. Mineral. 61 (in press). & BenNrs, H. L, (L97L): Sphalerite geo- thermometryand geobarometry.Econ. GeoI.66, Manuscript receivedFebruary 1976, emendedAprtl 653-669. r976.