sphalerite-wurtzite equilibria and stoichiometry 72_scott-barnes

Oeochiml~etCarmochlmicoAc~,l979, Vol. 86.pp.1276 to12@6. P~~@IIOUP~~XJL PrInted tnNortbmImlmd

Bphalerite-m equilibria and StoichiometrS

S. D. SOOTT* and H. L. BAZWES Ore Deposits Remaroh Se&on, The Pennsylvania State University, University Park,

Pa. 16802

(&c&xl 0 August 1971; acmpted in revimiform 21 April 1972)

A-t--Evidence from natural occurrences and from published syntheses indicates that the sphelerite_wurtzite inversion is not an invariant reaction at 1020%, 1 atm but ia a univariant function offs, and temperature. Single orystals of zino sulfide, grown in aqueous NaOH eolu- tions which were eeleeted to control fs,, have shown that a univeriant boundary existe between sphalerite and wurtzite near 600 atm from 405 f 4% to 617 rf: 2% over a corresponding oalculated fe, range of 1O-8’5 to le’ atm. Direct dete rmination of fe, for coexisting sphalerite end wurtzite, made by passing H, + H,S mixtures over ZnS powder, gave an fe, of 10-S atm at SBO”C, between 1O-6’5 and lad” atm at 800°C, and between 1O-6’6 and 1p*s atm at 700%.

The fe,-dependenoe of this phase change demonstrates that wurtzite is sulfur-defioient relative to sphalerite. Nonstoichiometry in zinc sulfide is also indicated by its color and by published luminescence studies, electrical measurements, and chemical analyses. Electrical measurements show the defects to be zinc vaoancies in sphalerite and sulfur vacauciee in wurtzite. The oombined range of nonstoichiometry in ‘ZnS’ is on the order of 0-B at per oent.

The hypothesis that wurtzite might be formed met&ably at low temperatures due to oxygen substitution for sulfur ie untenable. No oxygen was detected by cell-edge measurements of sphalerite or wurtzite from the hydrothermal experiments. In the gas-mixing experiments in which wurtzite waa produced from sphalerite below BOO%, oxygen was not present.

Wurtzite is thermodynamically stable at lower fe, than sphalerite. Above about 260%, this stability field liea well outside of the normal suliidation state encountered in ore-forming environments; however, at lower temperatures wurtzite may be deposited in highly reduoing environments. Beoause it is stable throughout the geologioally most important pH range within 2 or 3 uniti of neutrality, highly aoid solutions are not neceasaxy for its precipitation 88 advocated by k&EN et al. (1914).

INTRODUCTION

SPHALERITE and wurtzite are commonly regarded ss the low-temperature oubic and high temperature hexagonal polymorphs, respectively, of stoichiometrio ZnS, with aninversiontempersturene~r 102O'Catl rttm (ALLEN and CRENSHAW, 1912). Con- sequently, wurtzite which formed at very much lower temperatures in Mississippi Valley-type deposits and elsewhere is generally thought to be met&able (e.g. D~~~etaZ.,1962; BARTON andToa~rx, 1966; RAMDOHR,~~~~). Thereareressons to doubt these assumptions, however.

First, metastability is unlikely. The rapid equilibration found in hydrothermal solubility measurements, and textursl evidence of simultaneous deposition only of equilibrium assemblages in ores, both show that exposed crystal surfaces of sulfides must be in equilibrium, or nearly so, with the ambient solution both during deposition and solution reactions within minutes above about 100°C. Implicitly, natural wurtzite must be stable under conditions far below 102O’C.

Second, the inversion temper&ure is open to question. Attempts by some workers to reproduce the 1020°C inversion have been unsuccessful and, instead, both sphalerite and wurtzite have been synthesized over a wide range of temperatures. If these experiments were nesr equilibrium, then the inversion temperature is incorrect.

* Now at the Department of Geology, University of Toronto, Toronto 6, Canada. 1276

1276 S. D. SCOTT and H. L. BARNES

Third, sphalerite and wurtzite differ in composition, that is, the stoichiometry of ‘ZnS’ is not fixed at a Zn/S ratio of unity. This com~sitio~ variation requires that the phase change in general cannot be i~com~sitional* nor, therefore, invariant but must be a function of temperature and sulfur fugacity (is,). We have tested this hypothesis by examining phase changes in ‘ZnS’ in relation to its growth conditions in carefully controlled laboratory experiments. The primary problem in this system is that of obtaining equilibration at extremely slow rates of solid state diffusion ; however, our methods are designed to c~cumvent solid-solid kinetics by taking advantage of the rapid reactions of mineral surfaces and hydrothermal solutions.

This paper describes principles of nonstoichiometry which are applicable to many au&de minerals. An extremely sensitive test of nonstoilohiometry, which is adopted here, utilizes analysis of thef,%-temperature dependenoe of phase changes in sulfides. The basic concepts were outlined by BAMES and SCOTT (1966) and preliminary data on ‘ZnS’ were given by SCOTT and BAMES (1966, 1967). Similar variation in composition are the subject of our current research on ‘HgS’, (POTTER and BARNES, 1971) FeSz’, cMo& and ‘CdS’.

The terms ~lyrno~~rn and pol~~ism are ambiguous as applied to ‘Z&3’ and require clarification. &Iineralogists prefer to call ‘wurtzite’ those polytypes of ‘ZnS whose net symmetry is rhombohedral (nR) or hexagonal (nH) and reserve, as a polymorph, the name sphalerite’ for the oubic (SC) modiiioation (SMITH, 1955). Inasmuch se more than 100 dii%rent stacking sequenoes have been observed for zixm atide, this terminology is arbitrary. From a structural viewpoint there appears to be nothing special about three-layer pe~~city of sphalerite relative to the staek- ing sequence of any other polytype. Thus, there is no meaningful difference for ‘ZnS’ between the terms polymorph and polytype, as pointed out by SMITH (1955). Nevertheless, such a d&tin&ion between ‘ZnS’ phases is convenient beoause of the widespread occurrence of sphalerite in nature and the ease with w&h it oan be distinguished from the wurtzite polytypes by gross opt&al properties and morphology. We will use this ~r~olo~ for our present purposes to discuss general ~latio~hi~ without regard for the particular wurtzite polytypes which coexist with sphalerite. We will be presenting data on the systematio occurrence of the Berent polytypes elsewhere.

Deviation in stoichiometric proportions of a binary compound can be accom- plished by omission of one component or excess of the other. We summarize data below which shows that vacancies are the predominant s~i~~orne~ic defeats in ‘Z&Y. !lhmfore, we will discuss the composition of ‘ZnS’ iu terms of deficiency, rather than excess, of Zn or S.

EVIDENCE FOR NO~STO~CHIOMETRY IN ZINC SULFIDE

As early as 1934, Buerger, after con&&ng that pyrite and marcasite had slightly different Fe/S ratios, stated that a similar relationship held for sphalerite-“. . . the

* An exception would be at the thermal and compositional limits to the two-phase loop separating sphelerite and wurtzite on a T-X projection (ALBERS, 1967).

Sphalerite-wurtzite equilidria end stoichiometry 1277

high-sulfur form of ZnS . . .“, and wurtzite--“. . . the low-sulfur form of ZnS . . .” (BUERGER, 1934, p. 61). However, he did not publish the details of this study.

Many published data showing that ‘ZnS’ deviates from equal proportions of zinc and sulfur are summarized by NICKEL (1966). For example, electrical (BABA, 1963; MOREHEAD, 1963) and luminescence studies (KRUGER and Vmx, 1964; UCHIDA, 1964) have demonstrated that sulfur vacancies are produced in ‘ZnS’ which has been heated in a vacuum or zinc vapor, and zinc vacancies when heated in sulfur vapor. Similarly, Sm4nrM OVA and MOROZOVA ( 1966) have examined phase changes in thin flms consisting of a sphalerite and wurtzite mixture which were annealed first in zinc and then in sulfur vapor at 94O’C and approximately l-3 atm vapor pressure. After heating in zinc vapor, the proportion of wurtzite was greatly increased, in some cases to 100 per cent. Annealing in sulfur vapor produced the reverse effect ; the hexagonal phase changed to sphalerite. Optical absorption studies confirmed that this hexagonal phase contained a deficiency of S and that this deficiency was removed in the cubio phase. Shalimova and Morozova concluded that (p. 471) << . . . disruption of the stoichiometric composition of zinc sulfide has an effect on its crystal structure. When the crystals grow under high zinc pressure and zinc pene- trates the lattice, the hexagonal form results. When the composition . . . approaches stoichiometric composition, the lattice passes over into the cubic form . . .” In a later paper, MOROZOVA et al. (1969) reached a similar conclusion from a study of point defects in ‘ZnS’ powders. Similar results are shown by BANSAW et al. (1968) in their Fig. 5 summarizing the zinc and sulfur pressure effects on the kinetics of inversion.

Color may be another sensitive indicator of nonstoichiometry. We found that honey-yellow Joplin sphalerite, when heated for several weeks in aqueous sulfide solutions below 200°C and from 1 to 28 atm H,S pressure, became dark brown to black, depending on the duration of the experiment. The few tenths per cent Fe in the sphalerite remained constant and therefore was not responsible for the darkening. From an examination of many analyzed natural sphalerites, TOGARI (1961) concluded that color (chromaticity) was controlled by excess of sulfur over total metals and not just by iron. Iron was found only to enhance the brilliance (intensity) of the color. Similarly, ROEDDER and DWORNIK (1968) could not correlate color of natural sphalerite from Pine Point with iron content. Therefore, it is not necessarily true that dark colored sphalerite is riah in iron (PLATONOV and MARFUNIN, 1968). Rather it may indicate that the sphalerite is metal-deficient and formed under highly sulfidizing conditions.

Small differences in composition between sphalerite and wurtzite are found in careful chemical analyses of ‘ZnS’. NIUKEL (1965) noted that most analyses of natural sphalerite indicate a sulfur to metal ratio greater than one. Especially precise analyses by PANEU TZ and KINK (1966) of synthetic sphalerite and wurtzite (both pure and Fe doped) give S/(Fe + Zn) mole ratios of l-001, l-002, l-003 and l-004 for sphalerite and 0.996 and O-998 for wurtzite. These data indicate that sphalerite is Zn-deficient and wurtzite is S-deficient ; the argument that chemical analyses are not sufficiently accurate to distinguish such small compositional differences (KULLERUD and YODER, 1969) is belied by the consistency of these results,

1278 S. D. SrroTT and H. L. Banxs

Prsb~iieh&& inversion tempemtwrea

ALLEN and CRENSEMW (1912) first showed the sph&rite-wurtzite inversion to be reversible but very sluggish in the solid state. Using natural sphslerite containing 0.15 wt per cent Fe, they obtained an inversion temperature of 1020 f 5W as aa invariant point at one ~trn~phe~ of H,S. Recently, sevexd other ‘inversion’ temper&ures rangiug &om 600” to above 1240°C have been reported. KREMH~CLLER (1955) found that in the presence of chlorides, sphalerite converted to wurtzite at 97&W, but in the absence of chlorides there was no transform&ion even at 1175%. Similsrly, ADD-O and AVEN (1960) observed the appearance of the cubic phase when melt-grown wurtzite cxystsls were annealed in H,S at i150°C, 130” above the commonly accepted inversion tempera&ire. BUCK and ST~OCK (1965) synthesized a 3-liayer (3R) phs,se by vapor growth between 600 and 1020°C which they considered to be a third rn~~tion of zinc sulfide in~~~&te between sphalerite and wurtzite. SAMWON (1962) grew mixed hexagonal and cubic zinc sulfide cry&& over a wide range of temperatures from <800-106O’C by reacting gaseous ZnCl, with H,S. SAMELSON (1966) and S~LSON and BROPEY ( 1962) found only a cubic structure above 1240°C in non-equilibrium vapor tr&usport experiments conducted in an H,S atmosphere.

Most of our data on phase changes iu ‘ZnS’ were obtained from hypothermal recryst&llization experiments in which single crystals of sphalerite and wurtzite were grown in N&OH solutions over a. range of temperatures and pressures. This method is superior to dry synthesis because rapid equilibration occurs at temperatures as low as 3OO”C, suitable single crystals caa be obtained for X-ray studies, and precise (although imperfectly calibrated) control of& in the environment of erystallizstion can be s&ieved by fixing the NaOH con~nt~tion of the aqueous solution.

In side-beg alkaline s~utio~, fs, varies ~ont~uously over several orders of magnitude as a function of hydroxide ion activity (or pH), fo, and temperature. This relation is shown in Fig. 1. The general shape of the diagram, that is, the slopes of all field boundaries, rem&ins independent of temperature and sulfur conoentration and indicates the chemical relations of oonsequence here (for example, compm Figs. 2,7 and 8 in BARNEY and KULLERUD, 1961). Figure 1 shows that at constant fo, in the alkaline region, j’s, is & function of PH. Therefore, by bu%ring fo, on or nertr the P--SO,*- boundary by the reaction

S”-(aq) + 20,(g) rti; SW-(W (1)

fs, oan be oontrolbd over a range of about 20 orders of magnitude by v&ryinEr the hydroxide ion aotivity :

2S*-(aq) + 2H,O + O,(g) ;ti: S,(g) + 4OH-(aq) (2)

Hydroxide ion activities were varied principally by &snging the temperature and, less importantly, by &wing the con~n~~ion of N&OH. I3eWls of the calcu- lation of aoH- are given by SCOTT (1963). Within the ~rn~at~ range of interest, the ionization cons&& of N&H dews with incm&ng tempersture at 500 bar total pressure from lOaa at 450°C to lo-* at 600°C.

Sphalerite-wurtzite equilibria and stoiohiometry

ij s.

50 --

0 2 4 6 8 10 12 P”

Fig. 1. Distribution of p& minant aqueous ions and moleoules 8t 260’ and total aotivity of all sulfur-containing species, i.e. ZS, of 0.36. Fugaoity of 5, is aontoured

in atm. Calculated from data of HELGESON (1969).

Runs were made in welded gold capsules contained within temperature-gradient pressure vessels as desoribed by Baa~l~s (1971). Gradients, usually 12’C over the length of the oapsules, were maintained to f 1 and mean run temperatures from f2 to f6% between 300 and 7OO’C at 2ooo-9OOO psi. Runs lasted l-13 days but were usually 4-6 days. Run products were examined mioroseopioally and by X-ray diSka&ion. Single crystals up to 3 mm in diameter were obtained from runs in which nutrient material had dissolved at the hotter end of the tubes and precipitated at the cooler end. However, in many runs the nutrient did not transport but merely nxrtalyslized in eitu.

Zinc at@& starting material was obtained from three souroes. All three produoed the same results.

(i) JopZh qhdehte. Mottled, honey yellow orystals from Joplin, Missouri were casefully oleaued and crushed to about 100 mesh. An emission spectroscopic analysis gave 5070 ppm Cd, 1750 ppm Fe, 210 ppm Cu, 6O ppm Ga and cl0 ppm Pb.

(ii) S#xt&e u&c&t+#e. Fine powdered huniuescent grade, type S-10 zino sulfide from Sylvania Ektrio Produets Inc. was used as a high purity source of sphalerite. Speotrochemical analysis provided by the mauufauturer showed a total of <60 ppm in metallic impurities.

(iii) MOB wurtz&. Zinc sulfide powder from Matheson, Coleman and Bell gave a wurtzite X-ray diffmotion pattern with a small contribution by sphalerite and was used as wurtzite starting material. Its cell edge of 0 = 33224 A showed it to be oxygen-free (see below). The rn~~~t~r’s analysis showed as principal impurity 600 ppm obloride.

For eaoh NaOH solution, we found a univariant curve, as a fin&ion of total pressure and temperature, separating sphalerite (at lower temperature) from wurtzite

(at higher temperature). Figures 2 and 3 illustrate this relationship for 6.2 and 16 m

NaOH, respeotively. Similar diqyanx~ were obtained for 10 m and 104 m N&OH. These data show that the temperature where the two mirmrals coexist decremes as the N&OH concentration increases. For each solution the boundary has been reversed et least once; however, it was fixed best by single bracketing runs which produced sphalerite at the cooler end and wurtzite at the hotter end of one capsule.

-9

-I SPHALERITE -I

-4 *

--I

1 ‘u-0 -4

WURTZITE

C

I -I,

h , , , , , ,*, ,

350 400 450 500 550 TEMPERATURE, *C

--a _r]O’a-

Fig. 2. Sphalerita and wurtzite stabilities in 0.2 m N&OH. Rectqles indic&e the temperature and preasum range of ayatrd growth in one or more runs. Solid rectanglea indAte growth of q&&rite and open rectangles of wurtzite. Arrows

indicate the dire&on from which equillafium was approached.

SPHALERITE

I

WURTZITE

,=n, I I I t 1 1 400 450 500 550 600

TEMPERATURE, *C

Fig. 3. Sphalerite and wurtzite stabilities in 16 m NaOH. Symbols are aa in Fig. 2. Double arrows represent points that have been reversed.

1280

Sphale~~~~i~ equilibria and stoichiometry 1281

Figure 4 shows the observed sphah&e-wnrtzite relationships near 600 atm for six NaOH solutions. Well defined wnrtzite and sphalerite stability fields are separated by a univariant ourve which passes through 466 f 4’ in 15 m NaOH, 478 f 2” in 10.8 m NaOH, 485 f 5’ in 10 m NaOH, and 517 f 2” in 6.2 m NaOH. Wurtzite was not found in runs using 4 m NaOH up to 680% nor in 2 m NaOH as high as

650°C. Figure 4 also shows the contours of hydroxide ion activities which have been calculated from estimated ionization constants for concentrated NaOH solutions at

16

t4- .

8-

6-’

n

60 10

Fig. 4. SphJerite and wurtzite stabilities as ~b function of NaOE mokdity and temperature neaz BOO atm. The length of the rectangles represents the temperature range over which a ‘2X3’ phase wae grown in a single run or series of runs, The width of the rectangles has no significanae. Closed rectangles represent the grotih of spbalerite and open rectangles of wurtzite. The contours are negative

logarithms of hydroxide ion aotivities.

high temperatures. Due to the large uncertainties involved in calculating these ionic equilibria at elevated temperatures, the hydroxide ion activities are only order-of-magnitude approximations.

Figure 4 clearly shows that as the hydroxide ionaeti~ty ~c~ases~ the ~rn~rat~e of the sphalerite-wurtzite equilibrium decreases. In view of the control of fs, by hydroxide ion activity, the observed variation in the sphalerite-wurtzite inversion temperature is seen to be a function of fsp. Tbat is, as the hydroxide ion activity increases, fs, decreases, resulting in a lower temperature for the sphalerite-wurtzite equilibrium as shown by reaction (2).

Inasmuch as concentrated NaOH solutions were used, the effect offs, on the inversion ~rn~rat~e is ~on~tional upon derno~~t~g that OH- has not entered the wurtzite structure allowing wurtzite to grow met&stably at the temperatures of

7

1282 S. D. SCJO!IY and H. L. BAJWES

these experiments. If OH- has an appreciable solubility in wurtzite relative to sphalerite, the wurtzite stability field might be extended to lower temperatures at the expense of the aphalerite field. This effect of impurities on mineral stabilities has been discussed in detail by BARTON and S~IWNER (1967, p. 309). However, an infrared spectroscopic analysis of wurtzite grown in 6.2 m NaUH showed no hydroxide present at the minimum deteotion level of (O-1 mole per cent. Therefore, hydroxide ions do not participate directly in the reactions between the ‘ZnS’ phases, but serve only to controlfs, in the aqueous environment.

Sulfur fugacity could not be measured directly in the NaOH hydrothermal experiments. However, it was estimated for the sphalerite-wurtzite equilibrium by calculations of the aqueous equilibria, which showed the experiments to be near the stability field of ZnO. The calculations were oonfirmed by the growth of ZnO crystals as an additional phase in runs of a week or more duration above 600°C in the wurtzite stability field and above 600°C in the sphalerite field. The appearance of ZnO is the result of slow diffusion of H, through the gold tubes at high temperature as discussed later under ‘ZnO in Zinc Sulfide’. The stoichiometrically required relations among the stability fields of sphalerite, wurtzite and ZnO at the temperatures of the hydrothermal experiments are shown schematically as a function of fo, and hydroxide

HYDROXIDE ION ACTIVITY -

Fig. 6. Schematic relationships among the stability fielde of sphalerite, wurtzite, and ZnO near 600%. The solid light line is the sulfide-sulfate boundary. The dashed lines are contours of sulfur fugacity. The slopes of the boundaries are fixed

by stoichiometric relations but their actual positions are only estimated.

ion activity in Fig. 5. This figure is derived in the following manner. The oaloulated equilibrium constant for the reaction

ZnS + l/20, * zno + l/ZS, (3)

gives values of log (fs,lfo,) ; fo, can be estimated by

H,S(g) + 3/20,(g) % SO,(g) + H,O(g) (4)

beoause HBS and SO, are the dominant sulfur-oontaining gases here in the reduced and oxidized regions, respectively. At the temperatures of our experiments the fugacity of SO, is approximately equal to that of H,S for conditions on the Ss- -SOpa- boundary (H~ALD el aJ., 1963). Thus, forj’n*s = fsor,for can be oaloulated

Sphalerite-wurtzite equilibria and stoi&iometry 1283

which, in turn, gives an estimate of fs, for the sphalerite-wurtzite equilibrium (Table 1 and Fig. 6). Details of thejo, andfs, calculations are given by SCOTT (1968).

GAS-MIXING EXPERIXENTS

At high temperatures where equilibrium is possible in anhydrous experiments, the hydrothermsl results were tested by passing H,, H,S and their mixtures at

Table 1. Calculeted oxygen and sulfur fugacities (oorreoted to 1 atm) for the sphalerite-wurtzite equilibrium in the NeOH hydrothermal

experiments

NeOH Temperature molality (“C)

6.2 617 f 2 10 486 f 5 10.8 478 f 2 15 465 f 4

-1ogf0, --log&* Wm) (atm)

18.8 8.7 f 1 19.7 9.2 & 1 20.0 9.3 f 1 20.4 9.5 f 1

* Although the absolute uncertainties overlap all values, the systematio differenoes between values are signifioant.

1 atm over powdered zinc sulfide to locs,te the univariant curve as a function of fs, (&OTT and BARNES, 1967). At any given temperature, far is fixed by the PHI/ Pn,s ratio through the equilibrium:

H&3 rs H, + l/2& (6) Expsriinsnkrl method

Runs were made in a horizontal, gas-tight finnaoe whose temperature was controlled to f3’C. Powdered zino sulfide ohargea (MCB wurtzite or Sylvsnia sphalerite) were placed in a mullite boat and slid into the hot spot under a brisk nitrogen flow to prevent oxidation of the charge. At the end of a run, the charge wss quenohed in about 1 min by pulling it back on to the oooling jsoket under nitrogen flow.

The Hs/HsS ratios were oarefully controlled with a gas mixer similar to that described by DABKEN and QVBRY (1946). H&S mixing ratios of IO/l, 25/l, 60/l, and 100/l were used as well aa both pure H&3 and pure He.

Result8

In these anhydrous experiments, under pure H,, sphalerite was converted to wurtzite at 700, 800 and 900°C. The meohsnism involved vapor transport of zinc sulfide so that aciculsr wurtzite crystals grew immediately outside of the furnece hot spot. Similarly, wurtzite was converted to sphalerite under 1 atm of H&l between 600 and 845°C ; at 600°C about half of the wurtzite was converted to sphalerite in 4 days. To test the possibility that the latter reaction simply represented thermal equilibration of metastable wurtzite, identical wurtzite was heated in an evacuated silica, tube at the same temper&ire for the same length of time and showed no change to sphalerite. Consequently, the high fsl from the thermal dissociation of H,S was essential in producing sphalerite from wurtzite, and these experiments effectively reversed the polymorphic reaction.

By varying fs, using H, + H,S mixtures, the sphalerite-wurtzite univariant equilibrium was followed from 646 to 900°C. The results are plotted as triangles in Fig. 6. Although the curve was reversed at several temperatures, a narrow bracket

1284 S. D. SWTT and H. L. BARNES

I I I I I

*’ I

5-

6-

7- E z _

z 4 6-

1 -

s-

IO-

Al- - Y

SPHALERITE / A- /

'A

9' A

/y

4 /

/

1' A

/

A

WURTZITE

LI I I I I I I

400 500 600 700 800 900

TEMPERATURE,%

Fig. 6. The univariant sphalerite-wurtzite boundary as a function of sulfur fugwity and temperature at 1 atm. Triangles represent gas mixing experiments. Circles are calculated points from the NsOH hydrothermal experiments (Table 1). Closed symbols represent sphaltwite and open symbols, wurtzite. Estimated precision in the determination offer are shown for the hydrothermal experiments.

Uncertainty infs, in the gas mixing experiments is within each point.

was not obtrtinable owing to the sluggishness of the reactions, particularly that forming wurtzite. Because complete reactions were not observed in any of the runz, the stable phase at a particular fs, end temperature was determined from the direction in which the reaction was proceeding. For example, within the sphalerite stability field, wurtzite gradually reacted with S,(g) to produce sphalerite while sphalerite starting material remained unchanged. For 50 per cent reaction in a 10/l ratio of HJH,S, 3 days were required at 726°C and 6 kys at 645°C.

There is considerable asymmetry of reaction rates about the univariant boundary. The conversion of wurtzite to sphalerite proceeds much faster than the reverse reaction. For example, at 700°C the sphslerite-wurtzite boundary lies at fs, = lo--*' atm (Fig. 6.). In the sphalerite stability field at fs, = 1O-6'6 atm and 700°C, about 50 per cent of wurtzite starting material was converted to sphalerite in 3 days. At the same temperature in the wurtzite field no conversion of sphalerite to wurtzite was observed after 5 days at fs, = lo-"* atm and there was only a small amount of reaction after 3 days at fs, = 1O4'6 atm. Clearly, the activation energy for the production of wurtzite from sphaleritc is conside~bly greater than that required for the reverse reaction. In other words, the activation energy for the production of S- vacsncies is considerably greater than that required for Zn-vacancies as previously shown by MOREHEAD (1963).

Sphalerite-wurtzite equilibria and stoichiometry 1286

The position of the sphalerite-wurtzite boundary in Fig. 6 is located by a oombina- tion of the braokets obtained from the gas-mixing and hy~the~al experiments and natural occurrences of wurtzite at lower temperatures as discussed later.

DISCUSSION

The hydrothermal and gas-mixing experiments have shown that the sphalerite- wurtzite transformation is a univariant function of sulphur fugacity and temperature at boast pressure. Because fs, is a control variable in the tr~fo~ation, a change in stoichiometry must also occur; that is, the Zn/S ratio in ‘ZnS’ is not fixed at unity but must vary withfs, and temperature. Wurtzite, which is stable at lower fs,‘s than is sphalerite under isothermal conditions, must be S-deficient relative to sphalerite. The apparently ‘anomalous inversion temperatures and stability ranges cited earlier are consistent when viewed in terms of the univariant nature of the ~hale~~~rtzi~ eq~b~um. They simply reflect the serene in the experimental environments leading to the formation of Zn-deficient sphalerite at high fs, or S-deficient wurtzite at low fs,.

Several lines of evidence demonstrate that the products of our experiments are equilibrium phases. The results are clearly reproducible and were obtained by two ~de~ndent methods, hypothermal ~c~ta~ation and gas-mixing. In the hydrothermal experiments, a close bracket was obtained for the phase boundaries and results were independent of the reaction path followed (i.e. both wurtzite and sphalerite starting materials gave identical results). The ease of reversibility of the phase boundaries is well illustrated by the common occurrence on the hydrothermally synthesized wurtzite crystals of line overgrowths of sphalerite produced during quenc~g of the runs and detected as faint reflections in X-ray precession photo- graphs. Such overgrowths are expected because quenching into the sphalerite stability field is unavoidable as is shown by Fig. 4, but these surface growths were easily removed by soaking the crystals for a few minutes in an HCI solution, These overgrowths are further evidence that our stability boundary represents equilibrium because one would expect that if suurtzite were growing metastably it wouldepitaxially persist, instead of sphaleri~ being ab~ptly deposi~d under quench conditions most favorable to metastability.

It has been suggested (P. B. Barton, Jr., personal communication) that the configuration of our hydrothermal recrystallization experiments may have induced metastable growth of wurtzite. The difference in free energies between wurtzite and sphalerite is small enough (a few tens of calories) that the 12’ temperature gradient may have resulted in s~cient su~r~t~tion to precipitate wurtzite metastably at the cooler end of the gold tube in the manner of the Ostwald step rule. Even under the best of conditions it is diflicult to prove whether a reaction represents the most stable (i.e. lowest energy) or a metastable (i.e. higher energy) state. Both. represent equilibrium and can be tested for reproducibility and reversibility. However, there is ~o~iderable evidence that our hy~otherm~ experiments produced the stable equilibrium and not su~rsat~ation --induced metastability because isothermal experiments (i.e. those in which there was no transport but only in situ recrystallization) gave the same results as those conducted in a temperature gradient, In other words, the absence of a temperature gradient precluded the supersaturation necessary

1286 S. D. SCOTT and H. L. BNKNES

for a m&a&able equilibrium to be established. As well, the H&J/& gas-mixing experiments which produced wurtzite at low fs, were also isothermal experiments.

The well known kinetic barrier of the very slow difFusion rates in crystalline ZnS have been circumvented by our experimental methods. On the other hand, this problem may invalidate the results of experiments, such as those of SKINNER and BARTON (1960), in which sphalerite heated in the presence of excess Zn at 860°C did not convert to wurtzite. Apparently, solid state diffusion of Zn and/or S was too slow for a stable eq~b~~ to be attained. We have often found it difbault to produce a homogeneous zinc sulfide by heating liquid zinc and sulfur in an evacuated silica tube. A thin skin of ZnS forms on the liquid zinc which, because of the very slow diffusion of Zn and S through it, prohibits further reaution.

From a kinetic viewpoint, it is highly improbable that the deposition of wurtzite could take place at temperatures as low as 466% (Fig. 3) if 1029°C truly represented a sphaleri~~~~ invariant point because rates of such reactions are ex~~nt~~y dependent on temperature. The consistent temperature dependencies found in Figs. 2, 3 and 4 are completely inconsistent with such rate requirements for metastable deposition but do satisfy an equilibrium interpretation. However, even if subsequent studies were to show that suoh profound m&a&ability did ocuur in our experiments,it doesnot a&& thevalidity of our conelusions concerning nonstoiohiom- etry in ‘ZnS’ nor the generality of the ~a~rn~~ and geological examples that follow.

ZnO in zinc su.JpA&k

SKINNER and BARTOX (1960) found that by heating a mixture of sphalerite and zinc oxide (which has the wurtzite sauce) in an evacuated silica tube at lOOO”C, a small amount of wurtzite was produced. They concluded that a Zn(S, 0) solid solution promoted the growth of wurtzite below its ‘inversion temprature’ near 1020°C.

The proximity of the sphalerite-wurtzite boundary to the stability field of ZnO in our hydrothermal experiments (Fig. 6) necessitated an ~ve~tion of the possible influence of ZnO on the ZnS eq~b~um. In the NaOH e~~~~, ZnO appeared as an additional phase at the highest temperature only in runs in excess of a weak’s duration. Slow diffusion of Hz out of the gold tubas was responsible for the rise iu fo, and subsequent precipitation of ZnO. Evidence for this rise info, is found in the appearance of large NazSOo crystals above 66O’C in runs with 16 m NaOH and ranging to above 699°C in those with 2 m NaOH. The crystals indicate a large increase in the S012- ~n~~t~tion requiring oxidation via a reaction such as:

Z&(s) + BNaOH(aq) + 3HsO (b) s ZnO(s) + %,SO,(aq) + 3IMg) (6)

The oxidation of SB- to SO,s- requires 2 mole of Oa per mole of S2- oxidized. Water is the only potential source of oxygen oapable of providing the quantity required for the oxidation. The reaction shows that fH, inoreaaes rapidly as Sa- is oxidized by water. If the hydrogen had been retained in the system (which is experimentally improbable in unbuffered stainless steel bombs), the reaction would have stopped before much SO,“- was produced. However, because large amount of Na,SO, were

Sph~e~~-~~~ equilibria and ~i~ometry 1287

observed in some experiments, it is obvious that hydrogen must have leaked from the gold capsules at the higher temperatures.

There are several lines of evidence that ZnO sold-~lution was not a control variable in the sphalerite-wurtzite equilibrium observed in the NaOH experiments. Dire& analysis of two wurtzite and two sphalerite samples for ZnO-oontent was made by measuring their a cell edges as described by SIUIVNE~ and BARTON (1960). For peels, a = 3-8230 f O.OOlO~a~dforp~ sphalerite, a = 5.4093 + 0*0002~ (RO~IE et a#?., 1966). The presence of oxygen decreases a of both phases. The (300) reflection of wurtzite and the (331) reflection of sphalerite were measured against the (422) and (420) reflections, respectively, of NaCl as an internal standard. All measurements were made with smear mounts on carefully aligned Pioker or Norelco diffractom- eters using Ni-filtered 0.1 radiation. The cell edge of NaCl (Fisher reagent grade) was taken as 66403A (Sxrxx~a et rd. 1959). No correction was made for systematic errors; random errors were minimize d by oscillating three or more times over each pair of peaks at 1/4O 28 per min. The results, in Table 2, agree with cell edges for pure materials indicating that no detectable oxygen ( c O-2 mole per cent ZnO) had substituted for sulfur even when ZnO appeared as a separate phase (sample 107).

Table 2. Cell edge measurements of ~~othe~~y synthesized wurtzite and sphalerite

lKolnlity NaOH

Temp.

(“C)

Additional PhsseS

d&u+

(A)

wurtsita 68-l 6.2 613 f 4 Sph&Wite 1*1038 jc O&Xi2 3.8230 Ifi O-0007 wurtsiti 262 I6 613 f 6 NW% I.1036 & 0*0002 3-8230 f MOO7 Sphalerite 267 6-2 013 f 6 NM% l-2411 & O*OOOl 6.4087 f 0*0004 spimhite 107 4 677 f 4 zno 1.2409 If 0~0002 6.4001 f 0.0008

* wurt%ita (300). sph&rit..3 (331).

+ CeJaulstad from d-spminng. $r sphalerits, a(o) = (A* + Is’ + l*)W~(d).

For wurtzite, ~(a) = T (3

u(d).

Additional evidence that ZnO was not essential to the precipitation of wurtzite in the NaOH elements is indicated by the ~latio~~~ among& fo, and temperature along the S*--SO f- boundary. Buffering by the aqueous equilibria at any given temperature fixesf,, (Fig. 5) and yet either sphalerite or wurtzite can be stable, depending on pH (Fig. 4). Furthermore, where ZnO did form, often sphslerite was found showing that even when ZnO-saturated, sphalerite remained stable, These results again demonstrate that fo, and, hence, ZnO-content was not a controlling variable for the sphalerite-wurtzite equilibrium.

Finally, the gas-mixing experiments, in which wurtzite formed at 700°C and above in an oxygen-free atmosphere, present conclusive evidence that oxygen is not an essential component in low-temperature wurtzite.

In summary, the effect observed by SI~ICI~XE~ and BARTON (1960) of a Zn (S, 0) solution favoring the wurtzite stru&ure is expected at high temperatures. As ZnO and wurtzite are both hexagonally close-packed structures, ZnO is likely to be more soluble in wurtzite than in sphalerite at the same temperature so there would be an apparent lowering of the inversion temperature (BARTON and SKIKNEI~, 1967). The ma~tude of this lowering marmot be calculated without the~~~amic data on the Zn-S-0 ternary. However, our data show it to be insignificant compared to the

1288 S. D. Scorn and H. L. BARNES

effects offs, up to QOO’C. The maximum solubility of ZnO in ZnS ranges from only 1 mole per cent at 12OO’C (~K.I&JER and DIKHOFF, 1952) to 0.8 mole per cent at 860°C (SKINNER and B-TON, 1960) and must be even less at lower temperatures, It is doubtful that such a small amount of ZnO solid-solution can have a very large effect on the stability of wurtzite in low-temperature environments. On the other hand, the results of the present and other research have demonstrated that defects such as vacancies (no~~iehiomet~~ have a profound effect on the subsolidus phase relations of ‘Z&Y.

ExtrapoWon to lower temperature

The ~v~ia~t nature of the sphale~~~~zi~ phase change, as a fur&ion of fs, and temperature at constant pressure, has demonstrated that wurtzite is a S-deficient phase stable over a wide range of geologioally common temperatures. Therefore, natural occurrences of wurtzite indioate depositional environments which were lower in fs, than those which would have produced sphalerite at the same temperature. Extrapolation of the experimental sphalerite-wurtzite equilibrium from between 466 and 890°C to lower temperatures of geological interest can be used to put an upper limit on fs, during deposition of wurtzite. In addition, the intersection of this boundary with other univariant reactions in a me~~ogeneti~ grid (BARTON, 1970) would provide useful limits for both fs, and temperature for equilibrium assemblages.

We have implied above that wurtzite does not have a geologically important lower temperature stability limit. Evidence is found in the occurrence in sediments of perfectly formed single crystals of wurtzite several millimeters in diameter. Such crystals commonly grew in open cavities under low-temperature conditions (SEAMAN and HAMILTON, 1960; ?&USSGHL and MILLER, 1963). As noted previously, other evidence is found in very rapid reaction rates between sulfide minerals and aqueous solutions (BARNIS and CZAMANSKE, 1967) so that, unless quenched from solution, the precipitating phase is normally the stable one. Similarly, ore textures indicate that there is generally a rapid, close approach to equ~b~um between the crystal surface and the aqueous phase (BARTON et al., 1963).

In Fig. 7, the experimentally determined sphalerite-wurtzite curve has been extrapolated to lower temperatures as a linear function of log fs, and reciprocal ~mperat~e. The troilite-iron (Tout and BATON, 1964) and the p~~p~ho- tite (extrapolated from SCOTT and BARNES, 1971) univariant curves are shown for reference. Except for the point at 89O*C, the gas-mixing experiments gave a wide bracket and the hydrothermal experiments, besides having a large uncertainty in&,, cover only a small ~mperature range with the result that there is a large ~~~a~ty in the extrapolation. The experimental data lie atfs, ‘s and temperatures well within the pyrrhotite stability field as shown in Fig. 7. However, in nature at low temperatures, wurtzite of low FeS-content is normally found with pyrite (or marcasite) rather than pyrrhotite so the actual sphale~te-~zi~ boundary must, with decreasing temperature, cross the pyrite-pyrrhotite solvus and pass into the pyrite stability field. The low angle of intersection of the boundary with the pyrite- pyrrhotite solvus as well as the upper limit offs, imposed by the gas-mixing data confine the extrapolation of the ~h~e~t~~zi~ equ~ib~um within fairly narrow


TEMPERATURE, ‘C

200 300 400 500 600 600

6-

I I, I I I I I I , , ,, , ,

2.4 2.2 2.0 I.8 1.6 I.4 I.2 I.0

1000 OK-1 -8 T

Fig. 7. Extrapolation of the sphalexite-wurtzite equilibrium as a function of log&, and temperature. Data points and error bars show the bracket obtained from the gee-mixing end NaOH hydrothermal experiments. The haohured area ia the

‘main line’ ore-forming environment of BUTON (1970).

limits. The influence of iron content on the sphalerite-wurtzite curve in Fig. 7 has been neglected in this extrapolation.

Effect of irrvpurities

Iron, cadmium and manganese, the major impurities in ‘ZnS’ (~EISCHER, 1955),

are all more soluble in wurtzite than sphalerite and, therefore, should stabilize wurtzite relative to sphalerite under a given set of conditions (KULLERUD, 1953;

BARTON and TOULMXN, 1966; KR~~UER, 1940). The sphalerite-wurtzite boundary would be displaced by solid solution of these components towards higher fs,‘s, increasing the temperature range over which wurtzite is stable in the pyrite field. The magnitudes of the effects of impurities have not been determined and will have to await a quantitative thermodynamic study on free energies of sphalerite and wurtzite in multicomponent systems as a function offs, and temperature. However, this effect is unlikely to be of significance for low temperatures where wurtzites occur due to the limited Fe-contents possible, particularly in the presence of pyrite (SCOTT and BARNES, 1971). In the hydrothermal experiments with NaOH, 0.26 wt per cent Cd and 0.06 wt per cent Fe in crystals, grown using Joplin sphalerite as nutrient, had no measureable effect on the position of the univariant boundary.

Natural 0cczGrrences of wwtxite

That wurtzite is relatively rare in nature compared to sphalerite is not surprising in view of its stability field (Fig. 7). At high temperatures wurtzite is stable under


prohibitively low fsl’s compared to the normal sulfidation stafe of ore-forming environments (BARTON 1970) as outlined by hachures in Fig. 7. Wurtzite in ore deposits is formed at relatively low temperatures where it can coexist with sulfur-rich pyrrhotite, marcasite or pyrite at fs*‘s just slightly lower than the ‘main line’ environment of BARTON (1970).

Figure 8 is a log fo, -pH diagram illustra&ing the stabilities of Zn species in an aqueous system at 26°C and 1 atm. These stability fields are qualitat5vely similar at

Fig. 8. Distribution of equeoua ions and eolide (heavy lines) in the Zn-S-0 system at xs = lad, ZZn = N-6, and 26%. Regior~ of predm of eulfilr- containing equeous speaies (light solid lines) are from BARNES and KTJLLERUD

(1961). Sulfur fug&ties (light dashed lines) sre contoured in atm.

high temperafures for the same XS (total of the activities of all aqueous sulfur- containing species). Raising ZS will expand the fields of sphalerite and wurtzite at the expense of other zinc species. The field of stability of wurtzite found at low fo,‘s below the sphalerite field, wraps around sphalerite at high pH and haa a narrow stability range in the SOf- field at higher to,. Within the S04!‘g- field the maximum fo, at which ‘ZnS’ (both sphalerite and wurtzite) is stable is limited at geologic&y important pH’s (neutral f 2 or 3 pH units; BARNES, 1966) by the Zns+ and ZnO fields, a.nd at higher pH by the zincate ion. The wurtzite stability field is very small at these higher fo,‘s within the geologically important pH range even at very high BZn. It is improbable thatfs, and pH would be adequately buffered to maintain the solution within this stability range because of its small size. Therefore, the presence of large amounts of wurtzite, and the absence of oxidized phases such as ZnO. indicates low fo, during wurtzite deposition.

Suoh a low temperature, low fs, and reducing environment for wurtzite deposition is iliu&r&.ed by the occurrences desoribed by SEIABMN and HAMILTON (1960) of small single crystals of wurtzite in siderite concretions within a black carbonaceous shale. The presence of siderite indicates reducing ConditGons in solutions of high dissolved carbonate content and low dissolved sulfur (CARRELS and CHRIST, 1965, p. 225).

The pH is restricted to within 2 or 3 units of neutral for any reasonable activities of dissolved ferrous iron, carbonate and sulfur (GBRRELS and CHEWT, 1965, Fig. 7.2 I), and where wurtzite is stable, fo, is very low ( 10-8*5 to 1O-77’5 atm).

Sphaleri&wurtzite equilibria and stoiobiometry 1291

Much of the sphderite from Mississippi Valley-type Bb-Zn deposits is concentrically banded and has a colloform, bladed habit which suggests that it was originally deposited as wurtzite and later inverted to the cubic form (B&r8 et aE., 1937 ; BE-E,

1939). Ind88d, in the Zig Zag deposit near Joplin, MO. (M&night and Fischer, 1970), wurtzite had persisted intergrown with sphalerite and homogeneous single crystals of wurtzit8 grew on botryoidal stalactitic sphalerit8 (EVANS and MCKNIGZIT, 1959).

These ooouz~8nm of wnrtzite are oompatible with the lowf,,‘s calculat8d by Barton (in Barton and Skinner, 1967) for a typical Mississippi-Valley deposit.

Amm et al. (1914) found that heating solutions of zinc sulfate in sulfurlo acid in the presence of H,S produced wurtzit8, sphalerite or a mixture depending on the temperature and concentration of H,SO4. Above 350% only sphalerite formed and below 250% they obtained an ‘amorphous’ material. Between these temperatures the amount of wurtzite incr8ased with inoreasing H6S0, ~on~nt~tion.

Thes8 experiments have been widely cited in the literature as evidence that the precipitation of wurtzite in nature requires acid solutions. HOW8V8r, COREY (1953), using similar experimental methods as Allen and coworkers found only whalers at 250°C over a range of H&SO, concentrations. The disagreement between Corey’s results and the earlier results casts doubt on the experimental basis for the association of wurtzite with acid solutions. There are several lines of evidence which indica;te

that the pH of hydrothermal solutions is within 2 or 3 units of neutrality (i.e. weakly acidic to moderately alkaline) for temperatures to at least 250% (B-s, 1966). Wurtzito has a large field of stability within this pH range (Fig. 8) so it is not necessary to postulate exotic solutions for its deposition.

A possible explanation of why Allen and coworkers experiments produced wurtzit8 is given in Fig. 9. This figure, drawn for the approximate conditions of their

40 - SPHALERITE

H2S HS- s2-

60 , , I I t 0 2 4 6 8 IO I2

PH

Fig. 9. Equilibrium relations among zinc minerals (heavy lines) as a function of log&, and pH at 260% and XS = 0.36. The light lines show the fields of pre- domiuence of sulfur-bearing aqueous species ea in Fig. 1. The spheJerite-wurtzite boundary is from Fig. 7. Other boundties - calculated from HELG~SON (1969) and ROBE and WALDBA~ (1968). The dashed line ikstratea the probable

reaction p&h followed by ALLEN et al.‘8 (1914) experiments.


experiments (ZS = 0.13 to 0.57 and T = 260" to 35O“C), shows a small field of wurtzite at highf,, in the HSO,- field. Initially, their experiments were at highf,, within the ZnSO, field. Release of H,S, by the decomposition of sodium thiosulfate or ammonium thiocyanate that was added to the runs, causedjo* to decrease gradually Thus a traverse, indicated by the arrow in Fig. 9, was made with falling fo,, starting in the ZnSO, field, passing through the stability field of wurtzite, then of sphalerite, and eventually reaching the field of elemental sulfur. Evidence that such a traverse info, was indeed followed in their experiments is found in their examination of the products of the runs in which “. . . wurtzite . . . very commonly coated over the sphalerite . . .” (p. 423) and “. . . the universal occurrence of sulfur . . .” (p. 413) are described. The reason that they did not produce wurtzite above 360°C is probably because at these temperatures ZnO truncates the wurtzite field at higher pH’s as indicated schematically in Fig. 5.

CoNCLUsIoNs

Our hydrothermal and gas-mixing experiments, supported by published information, have demonstrated that the sphalerite-wurtzite equilibrium is a univariant function offs, and temperature over a wide temperature range. Published inversion temperatures for ‘ZnS’ ranging from 600°C to above 124O’C from a variety of experimental environments are not anomalous but are merely indicative of the fsl-dependence of the phase change. Natural occurrences of wurtzite are consistent with our experimental results and indicate that either mineral may be stable to low temperatures.

The f8, -dependence of the inversion requires wurtzite to be sulfur-deficient relative to sphalerite at a given temperature consistent with the relationship:

wurtzite spilalerite

ZnS,_, + l/2 ( 2 - 1 + &)%" (&) Z%*S

Thus the stoichiometry of ‘ZnS’ which, on the basis of PANKRATZ and KING’S (1965) analyses, may vary within approximately 0.9 at per cent S is all-important to the stability of each phase. The observations of SEULIMOVA and MOROZOVA (1965) indicate that wurtzite compositions lie predominantly to the S-deficient side of stoichiometric ZnS while sphalerite can be stoichiometric or lie on the Zn-deficient side. The conductivity data of BABA (1963) and MOREHEAD (1963) show the predominant defect in sphalerite to be zinc vacancies rather than interstitial sulfur and in wurtzite to be sulfur vacancies rather than interstitial zinc.

Nonstoichiometry is by no means unique to ‘ZnS’ but is a phenomenon exhibited to greater or lesser degree by all compounds and is amply documented in publications in solid state physics and chemistry. Likewise, variations in ‘polymorphic’ inversion temperatures with composition are not only expected but are demanded by thermodynamics (ALBERS, 1967). In the case of ‘ZnS’ this variation extends over a very large temperature range. Preliminary experimental results indicate similar behavior for ‘HgS’ (cinnabar and metecinnabar; POTTER and BARNES, 1971) and ‘MO& (2H and 3R molybdenite ; CLARK, 1970). The thermodynamic relations of such systems are exhaustively considered by KR~ER (1964).


Previously, we (BARXES and SCOTT, IBM) have discussed no~~i~ornet~ among sulfides and have pointed out that there are many mineral pairs for which small differences in the metal(s) to sulfur ratios are known to exist. We suggested that inversion temperatures for these mineral pairs be re-examined to determine the effects of varying sulfur fugaoity. Knowledge of the&, and temperature dependence of ooexisting sulfide minerals should provide useful information on hydrothermal environments during one deposition.

The classical defbritions of polymorphism and polytypism require the phase change to be ~~rn~sitio~l snd hence isob~~~y invariant. Phase changes between sphalerite and wurtzite or any other nons~ic~omet~c mineral pairs in the general case fit neither of these criteria so, within the rigid definition of the terms, these phases are not polymorphs. In view of the universal usage of the term polmorph or polytyps for such phases and the fact that the compositional differences between the closely-related defect solids are quite small, we suggest that the de6nitions of these terms be broadened to include phases with a maximum difference in composition iu the order of 1 or 2 at per cent.

&kkww&?&~ This study is a portion of a Ph.D. dissertation by S. D. S. which was supported by the Advanced Research Projects Agency through the Materials Researoh Laboratory of the Pennsylvania State University and by National Soience Foundation grauts GA-477 and GA-1689 Many suggestions aud criticisms from colleagues J. R. FISEEE, J. L. HMS, JR., G. R. BELZ N. G. LIVERY and 5. B. ROEEEE~EE have been included here. Espeoially valmble have been extended discussions with devil’s advocates, P. B. BAEToE, JR. and P. TOULMIN III, of the U.S. Geological Survey; in addition, the manuscript required oonsiderable modification after Dr. BAETON’S expert dissection which we greatly appreciated.

REFERENCES

AEDAEXAEO A. and AVEN M. (1960) Some properties of zinc sulfide crystals grown from the melt. J. Appl. Phya. M, 36-39.

A~EEEs W. (1967) Physical chemistry of defeats. In Phyaia ad Ohem&try of II-VI C%nnputi (editors M. AVEN and J. 5. PEEEEE), pp. 165-222. Interscienoe.

ALLEN E. T. and CEEESYL&W J. L. (1912) The sulfides of zinc, cadmium and mercury; their crystalline forms and genetio conditions. Amer. J. Sci., 8ep. 4,84, 341-396.

ii.IXJZN E. T., &tENSJxAW J. L. and M~nwrE H. E. (1914) Effect of temperature and acidity in the formation of marcasite (Fess) and wurtzite (ZnS); a ~ont~bution to the genesis of unstable forms. Amer. J. h-G&, ser. 4, 88, 393-431.

BA.BA H. (1963) Some effects of zinc atmospheres on zinc sulfide. J. Etectrochem. Sot. 110, 74-81.

BAMAUI T., SECCO E. A., SE~VA~TAVA 0. K., and MAXMN R. R. (1968) Kinetics of hexagonal- cubic phase transformation of zinc sulfide in vocou), in zinc vapor, and in sulfur vapor. Can. J. Chum. 46,2881-2886.

BmEEs H. L. (1966) Enviromental limitations to mechanisms of ore trausport. In Sympokm, &&ems of Podmagnetic Ore Depx&iora, 8, pp. 316-326. Geol. Survey Czechoslovakia.

BmEEs H. L. (1971) ~nv~~tio~ in hydrothermal sulfide systems. In Bsasarck ~~~~~ for H&h PFe&we and B&h Tern~~ye (editor G. C. Urar~~), pp. 317-366. Springer-Verlag,

BAEEEs H. L. and C ~AMAES~E G. K. (1967) Solubilities and transport of ore minerals. In Gkuc?bemz%y of Hy&oth.evmaE Ore Depo&a (editor H. L. B.&EXE(I), pp. 334-381. Holt, Rinehart and Winston.

BAEEEs H. L. and K~~ERUD G. (1961) Equilibria in sulfur-containing aqueous solutions, iu the system Fe-S-O, and their correlation duriug ore deposition. EcM). Ueol. 56, 648-688.

BAEEEs H. L. and SCOTT S. D. (1966) Stoichiometry of sulfide minerals (abst.). Ecora. Geol. 61, 1297.

1294 S. D. Scan and H. L. BARr?ES

B~!CON P. B., JB. (1970) Sulfide petrology. M&er& Sot. Amer. Spee. Paper 8, 187-198. B-TON P. B., JR., BK~K~K~ P. M. and TOUIXXN P. III (1963) Equilibrium iu ore deposits.

MinsraZ. Sot. Ame?. Speo. Paper 1, 171-186. BARTON P. B., JR., and S~TNKIC~ B. J. (1967) Sulfide mineral stabilities. Iu &c&em&&s-y of

Hater Ope Bepoe% (editor H. L. B-a) pp. 238-333. Holt, Rinehart and Winston. RA.KroN P. B., Ja., and Tom P. III (1986) Phase relatious involving sphalerite in the

Fe-&l-S system. &kc?&. UseI. i& 815-849. I%EKr%K C. H., JR. (1939) Wisconsiu-Illiuois-Iowa lead-zinc district, In Co~&&&+m-r to a

.Knowledge of tke Lead and Zinc Depoeits of the Miaeiesippi Vdey Resjon (editor E. S. BAFfIN) pp. 114-118. Geol. Sot. America Speoial Paper no. 24.

BEKKK C. H., JR., So- E. R. and B kNKIELD A. F. (1937) The Wiscousiu lead-zinc district. Pmhmmary paper. .&en. i.7eoZ. 88,783~809.

Buog D. Cl. and STX&OUK L. W. (1966) Trimorphism in zinc sulfide. Amer. MineraL 40,192-200. BmoKK I& J. (1934) The pyrite-marces ite relation. Amer. BC&w+d ZS, 37-81. C&A.BK A. H. ~lQ70) Com~sitio~ d.ifYerences between hezagonal aud rhorn~h~ molyb-

de&e. Netsss &.h&. hfke&. Mmotek. lW@, 33-38. CoKKx A. S. (1953) Nate on sphalerite and wurtzite. Amer. Mi?wd. 88,141-143. DAXEN L. S. and GXJRKY R. W. (1945) The system iron-oKygeu. I. The w&kite field and

related equilibria. Amer. Chem. Sot. J. 67, 13981412. DE?EZ W. A., Howrz R. A. aud Zosearar~ J. (1962) Rock-Bbrming Minerals: Vol. 5, Non-Silicatee,

371 pp. Freeman. EVANS H. T. and MCXXIUKT E, T. (1969) New wurtzite polytypes from Jophu, Missouri. Aw.

i%.Z~nepoZ. 44,1210-1218. Ikrsoms~ M. (1955) I&or cIemeuts iu some sultide minerals. In Ecoa. Ueo2. ~~~~t~ An&v.

VoZ. (editor A. M. ~A%%lMAN), Part 2, pp. 970-1024. Gv R. M. and CKK~~T C. L. (1966) Sol&&e, Mine&, and Epui&ria, 460 pp. Harper and

Row. Hkusstkr~ S. and MEL- G. (1963) Neue ZnSPolytypen (QR, 12R uud 21R) iu mesozoischen

Sedimeuten NW-Deutsohlauds. Beitr. Mineral. Petrog. 9,28-39. HEAX,D E. F., NAUOKTON J. J. aud BARNES I. L. (1963) The chemistry of volcanic gases. 2. Use

of equilibrium caloulatious in the interpretation of volcauic gas samples. J. Geophye. Rec. 68, 645-567.

JACKSON H. C. (1909) Thermodynamics of hydrothermal systems at elevated temperatures and pressures. Ame+, J. Sei. 887,?2%804.

m A. (1965) Zinc sulfide single crystals for phosphor research. &&x&z T~h~~~t 8, 11-16.

Kr&orca F. A. (1940) Solid solutions in the ternary system, ZuS-CdS-MnS. Zeit. K&t. 108, 132-13s.

K~iiorc~ F. A. (1964) The Chemniatrg of Imperfect Cryetds, 1039 pp. North-Holland. K~tiaxm F. A. and Ih~~ornr~ J. A. M. (1952) The function of oxygen in zinc suhlde phosphors.

J. E~ectroehem. Sot. eS, 144-163. KR&~EK F. A. and V~mr H. J. (1954) The origin of the fluoresceuce in self-activated ZnS, CdS

and ZnO. J. Chsm. Pkge. Ida, 250-252. Xurx,rmxm G. (1963) The FeS-ZnS system, a geological thermometer. NOP8k. &02. !I%&&. a,

61-147. Kmxrrm~~ G. aud Youag II. S. (1959) Pyrite stability relations in the Fe-S system. Eccn.

Geol. 64, 533-672. MoIkxaar E. T. aud FISUKK~ R. P. (1970) Geology and ore deposits of the Picher field, Olda-

homa and Kansas. U.S.GLS. Prof. Paper 588. MOIUDCEAD F. F. (1963) A Dember effect study of shifts in the stoichiometry of ZnS. J. EZectro-

Oh&f& f&G. 110, 286-288. MO~XOVA N. H., VE~ELKOVA M. M, BOTNEV A. F. =d Sm OVA K. V. (1969) Ir&ueuce of

point defects on the crystalline structure of z& sulfide powders. Sou. P~e.-~~a~. 14, 74-78 (English trauslatioz).

NXXE+L E. H. (1966) A review of the properties of zinc sulphide. Dept. Miw Tech. Swrveys, Ottawa, Inf. Circ. 170.

Sphalerite-wurtzite equilibria and stoicbiometry 1296

Pl~~gaarz L. B, and KING E. G. (1966) High-temperature heat contents and entropies of two xina sulfides and four solid solutions of zinc and iron sulfides. U.S. Burera~ of Mkea, Rep. frrue~t. 6708.

%ATONOV A. N. and MARBWNX N A. 5. (1968) Optic& absorption spectra of sphak&es. @eochem. Im?ernat. 6, 245-260 (English translation),

Poxrxn R. W. II and BONES H. L. (1971) Merourio aulflde stoichiometry and phase relations (abst.). &of. Sot. Amer., Abetrack with Prv, 8, 674.

Raas~tonn P. (1966) The Ore M&WC& and Their Intergrowth (third edition, English trauslation), p. 668. Pergamon.

ROBS R. A., Bsrrrtxx P. M., TOULXIN M. 5, and EDWABD~ J. L. (1986) X-ray c~~o~ap~c data, densities and molar volumes of minerals. In Handbook of Phy8icaZ Con&ants, Reti& Ed. (editor 5. P. CLILBK, JR.), p. 34. Geol. Sot. America Memoir 97.

ROBIE R. A. and WALDBAUDX D. R. (1968) Thermodynamic properties of minerals and related substances at 298.16’K (26%) and one atmosphere (1.013 bars) pressure and at higher temperatures. U.S. Ueol. r3u+vef( B&Z, 1269.

ROEDDER E. and Dwonxz E. J. (1968) Sphalerite color banding: lack of correlation between color and iron content, Pine Point, Northwest Territories, Canada. Amer. MineraZ. Ss, 1623- 1629.

S-ON H. (1961) Vapor phase growth and properties of zinc sulfide single crystals. J. AppZ. Phya. @@,309-317.

SAMXLEION H. (1962) Growth of cubic ZnS single orystals by a chemical transport process. J. AppZ. Phya. 8@,177Q-1783.

S-ON H. and BROPEY V. A. (1961) The synthesis and ~~s~o~phy of structurally pure cnzbic and hexagonal single crystals of ZnS. J. .EZe&ochem. Sot. loS,l60-164.

Soon S. D. (1968) Stoichiometry and phe changee in zinc sulfide. Ph.D. dissertation, The Pennsylvania State University.

SCOTT S. D. and BARNES H. L. (1966) Sphalerite-wurtzite stability relations (abst.). Amer. Geophys. Union Trans. 1, 200.

SOOTT 8. D. and BARXES H. L. (1967) Nonstoichiometric phase chsnges in spbalerite and wurtxite (abst.). Cam. i%f&eraZ. B, pt. 2, 306.

Scan S. D. and BARNES H. L. (1971) Sphalerite geothermometry and geobarometry. &on. UeoZ. 66, 663-669.

SE- D. M. and HA~XILTON II. (1960) Occurrence of polymorphous wurtzite in western Pennsylvania and eastern Ohio. Amer. Mineral. @t&43-60.

S-OVA K. V. and MOROZOVA N. K. (1966) Effect of excess zinc on the crystal structure of ZnS. Sov. Phys.-Cryabawog. @, 471-472 (English translation).

S~MN~ER B. J. and B~BTON P. B, JR. (1960) The ~bstitution of oxygen for sulfur in wurtxite and sphalerite. Amer. Mineral. &5, 612-626.

SW B. J., BARTON P. B. JR. and K VLLERWD G. (1969) Effect of FeS on the unit cell edge of sphalerite. A revision. Ewn. UeoZ. W, 1040-1046.

SMITE F. G. (1966) Structure of zinc sulfide minerals. Amer. MineraL 40, 666-676. TOU~I K. (1961) Mineralogical studies on Japauese sphalerite. J. Fat. Sci. Hokkaido Univ.

8er. 4, GeoZ. and M&be&. 10, 703-733. TO- P. III and B-~N P. B. Jn. (1964) A th~~~~iu study of pyrite and pyrrhotite.

&o&m. Gwmochim. Acta $$@, 641-671. UCHIDA I. (1964) Fluorescence of pure zino sulfide in correlation with deviation from stoichi-

ometry. J. Phye. Sot. Jawn I@, 670-674.

sphalerite-wurtzite equilibria and stoichiometry 72_scott-barnes

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