Phase Relations Among Tellurides, Sulfides and Oxides I 88_Afifi-Essene

Economic Geology Vol. 83, 1988, pp. 377-394

Phase Relations among Tellurides, Sulfides, and Oxides: I. Thermochemical Data and Calculated Equilibria*

ABDULKADER M. AFIFI, WILLIAM C. KELLY, AND ERIC J. ESSENE

Department of Geological Sciences, University of Michigan, 1006 C. C. Little Building, Ann Arbor, Michigan 48109

Abstract

Thermodynamic data for binary tellurides are compiled and combined with available data for sulfides and oxides to calculate phase relations among tellurides, sulfides, and oxides at low temperatures (100ø-300øC). Limits on the natural range in fxe•, yS•, and fo• values during telluride deposition and/or reequilibration are estimated from natural assemblages relative to reactions, such as 2Fe•O4 + 3S2 q- 3Te2 -- 3FeS2 q- 3FeTe2 q- 402 and 2HgS q- Te2 = 2HgTe q- S2. Calculated equilibria show that tellurides of Mo, Sn, Mn, and Zn are unstable with respect to corresponding sulfides and/or oxides in natural systems and that some tellurides, such as As2Te•, are likely to occur in nature, although they have not yet been found as minerals. The composition of electrum in equilibrium with hessitc is useful as a sliding-scale indicator of the fugacity of Te2 at relatively low values offT• ß The calculated phase relations explain particular associations or antipathies among ore minerals and may be used to predict other minerals which should be present at equilibrium.

Introduction

TELLURIDE minerals are known from a wide variety of mineral deposits in veins, massive sulfides, skarns, greisens, and magmatic segregates. While volumetrically minor, tellurides are important car- riers of Au, Ag, and platinum-group elements. De- spite this, little is known about the geochemical controls of telluride mineralization in general, and the systematic association of tellurium with specific metal suites in particular.

Kelly and Goddard (1969) observed several systematic relations among vein minerals in the telluride deposits of Boulder County, Colorado. They noted, for example, that galena displayed antipathetic relations with both calaverite (AuTe2) and native tellurium, whereas altaitc (PbTe) was antipathetic to native gold. In attempting to explain these observations, Kelly and Essene (1982) applied thermodynamic data to evaluate the relative stabilities of native elements, binary sulfides, and tellurides. Since tellurides in general are invariably associated with sulfides, oxides, and/or native elements, thermodynamic data are essential to quantify the com- petition of sulfur, oxygen, and tellurium to form compounds with metals and to estimate the activities of the mobile components S2(g), O•(g), and Te•(g) during ore deposition.

Since the last compilation of thermodynamic data for tellurides by Mills (1974a), a significant volume of new thermochemical and thermophysical data for

* Contribution 437 from the Mineralogical Laboratory, Univer- sity of Michigan.

tellurium compounds has been published. In this paper (Part I), available thermodynamic data for geologically significant tellurides are compiled and used to construct fTe•-fs•-fo•-T diagrams. In the following article in this issue of Economic Geology (Part II), this information is used to interpret the conditions of telluride formation in ore deposits.

Thermodynamic Data The stability of metal tellurides with respect to

their corresponding native elements, sulfides, and oxides is, among other factors, a function in part of the activity of tellurium in the system. For tellurides, Te• ideal gas is adopted as the most appro- priate reference state, because Teag ) is the most abundant gaseous species over geologically significant ranges in temperature (Fig. 1). This is consistent with the common usage of the diatomic molecules S2(•) and O•(•) as indicators of sulfidation and oxidation states.

Thermodynamic data for pure elements, sulfides, and oxides were taken from the compilations of Kelley (1960), Barin and Kniicke (1973), Mills (1974a), Barin et al. (1977), and Bobie et al. (1978). We have further attempted to include more recent measurements of thermophysical and thermochemical properties, such as those for chalcopyrite and bornitc by Bobie et al. (1985) and for arsenic and antimony sulfides by Johnson et al. (1980, 1981).

The free energies of formation (/XGf) of geologically significant tellurium compounds were calculated relative to a standard state of ideal Te•(g) as functions of temperature. Input for these calculations typically consists of heat capacity (Cv) or heat

/H o H o • content • T- •9S/ measurements, which are corn-

0361-0128/88/789/377- ! 952.50 3 7 7

378 AFIFI, KELLY, AND ESSENE

lo

12

..J

t

4oo

i i i • [

Te 2 fgl _

t I N I I I

600 800 1000

!' I*KI

FIG. 1. Partial pressures of gaseous tellurium molecules as a function of temperature at Ptotal = 10 -8 bars.

bined with emf, equilibrium, or calorimetric measurements in order to calculate the enthalpy of formation (AH•9s) of the compound of interest. Where heat capacity data were unavailable, estimates of standard molar entropy (S•9s) and heat content were made following the methods outlined by Mills (1974a). Calculations were aided by the programs CP and REACT written for this purpose. Copies of these programs, the thermodynamic database, and a database of published telluride analyses may be obtained on request to the senior author. Internal consistency within binary metal-tellurium systems was checked against known phase relations. No thermodynamic data are available for ternary metal-tellurium compounds, such as petzite (AuAg3Te2). Consequently, the stabilities of such compounds cannot be presently estimated with suf- ficient accuracy to be reliable.

Appendix I includes a tabulation of the free energies of formation (AG•') of relevant tellurium compounds as functions of temperature. Appendix II gives a tabulation of the standard molar enthalpies

A o of formation (Hf,2•s), the standard molar entropies (S•s), heat capacity equations, and sources of thermodynamic data.

Table 1 is a summary of melting points for tellurides which may be of use in geothermometry. The relatively low melting points of some common telluride assemblages, such as calaverite + altaitc + tellurium (388øC), or minerals, such as sylvanitc (354øC), help constrain the temperatures of deposition and subsequent metamorphism of specific telluride districts. The data compiled in this paper supplement previous compilations by Vaughan and Craig (1978) and Barton and Skinner (1979).

The selection of thermodynamic data in conjunction with description of phase relations and mineralogy for geologically significant telluride systems are described in the following section.

Tellurium and Te-H20

Tellurium is stable as a solid to its melting point at 450øC at i bar. Native tellurium is typically found as a primary mineral in telluride deposits, implying that itc2 had reached saturation and that tellurium, unlike sulfur, is relatively insoluble (D'yachkova and Khodakovskiy, 1968).

Thermodynamic data for the gaseous species Te2(g), H•Te(g), TeO(g), and TeO2(g) were calculated from the compilation of Mills (1974a). Thermody- namic data for aqueous species of the acids H2Te, H2TeO3, and H6TeO6 were estimated between 25 ø and 300øC by D'yachkova and Khodakovskiy (1968) and were revised by Ahmad et al. (1987). These estimates, however, are subject to uncertainties that can only be resolved through acquisition of experimental data.

Unlike native tellurium, which generally occurs as a primary (hypogene) mineral in telluride deposits, tellurite (orthorhombic TeO2) and paratellurite (tetragonal TeO2) are restricted to zones of near-surface (supergene) oxidation. Switzer and Swanson (1960) determined that tellurite inverts to paratellurite above 600 ø to 650øC, but the transition is extremely sluggish as evidenced by the inability to synthesize tellurite in the laboratory.

Ag-Te

Phase relations in this system are critical to understanding equilibria in the system Au-Ag-Te. The phase diagram (Kracek et al., 1966) contains the compounds Ag2Te (hessitc), Ag2_xTe (x -- 0.08- 0.12)--the 3' phase, and Ags-xTe3 (stuetzite). Hes- sitc is stable in three structural modifications from

room temperature to its melting point at 960øC. Stuetzite is stable in at least two structural modifica-

tions to its final melting point at 420øC. The 3' phase is stable over the temperature range 120 ø to 460øC, but it decomposes rapidly on cooling below 120øC into a mixture of hessitc and stuetzite, which accounts for its apparent absence from natural telluride ores. The intimate intergrowths of hessitc and sylvanitc (AgAuTe4) that are commonly found in telluride ores were attributed by Cabri (1965) and Kelly and Goddard (1969) to the unquenchability of the Au-rich 3' phase in natural systems.

The rare mineral empressitc (AgTe), which could not be synthesized in this system (Markham, 1960; Cabri, 1965; Kracek et al., 1966), was observed by Honea (1964) to decompose to stuetzite and tellurium on heating above 210øC. This low thermal stability could explain failures to synthesize empressitc, which appears to be restricted to low-temperature, gold-poor systems.

Since the cryogenic heat capacity measurements of Walsh et al. (1962) on Ag•.ssTe were almost cer-

TELLURIDE PHASE RELATIONS: I. DATA 379

tainly measurements on a mixture of hessitc and stuetzite (decomposition products of the 3' phase), the entropies of all silver tellurides are subject to uncertainty. The data of Mills (1972) and Mills and Richardson (1973) were adopted for both hessitc and stuetzite since they are consistent with the emf measurements of Zaleska (1974). More recent thermodynamic data for hessitc by Castanet and Laffite (1974), Vecher et al. (1974), and Blachnik and Gunia (1978) differ from the previous measurements but cannot be checked for internal consistency with other silver tellurides. The absence of heat content measurements for the 3' phase, the small energy difference from hessitc, and the de- pendence of AG• on composition (Zaleska, 1974) do not allow a reliable estimate of its thermodynamic functions. The combined measurements of Kiukkola and Wagner (1957), Zaleska (1974), and Kracek et al. (1966), however, allow an estimate of the field of 3' stability in fTe2_T space over the range 25 ø to 300øC.

Au-Te

The only stable compound in this system is calaverite (AuTe2), which melts at 469øC. Thermody- namic data for calaverite were obtained from the compilation of Mills (1974a).

The rare mineral montbrayite (Au2Te3) has defied synthesis in the systems Au-Te and Au-Ag-Te (Markham, 1960; Cabri, 1965). Experimental work by Bachechi (1972) has demonstrated that mont- brayitc from the type locality (Au•.89Ago.o3Pbo.o4- Sbo.o7Te2.88Bio.•) is stable in the system Au-Sb-Te to its melting point at 410 ø _ 5øC and may be stabilized by small amounts of Sb.

Bi-Te-(S)

Tellurobismuth (Bi•Te•) is the tellurium-rich end member of a homologous series of bismuth tellurides which include hedleyite ('--BivTe•; Zav'yalov et al., 1976), unnamed Bi•Te (Gamyanin et al., 1982; Goncharov et al., 1984), and tsumoite (for- merly wehrlite, '•BiTe; Shimazaki and Ozawa, 1978, Gamyanin et al., 1982). These four minerals have successfully been synthesized in the system Bi-Te and are stable from below 150øC to their

melting points, which range from 312øC for hedleyite to 588øC for tellurobismuth (Table 1). Sztro- kay and Nagy (1982) and Garuti and Rinaldi (1986) have reported analyses of a natural telluride corresponding to '•Bi•Te•, but the identity of this compound remains unknown.

The system Bi-Te-S contains several minerals derived by regular alternation of sulfur, bismuth, and tellurium layers within a common substructure. These include joseite-A ('•Bi4TeS2) and joseite-B ('--Bi4Te2S; Thompson, 1949), tetradymite (Bi•4-

Te•Ss; Pauling, 1975), ingodite (Bi•TeS; Ren, 1986), and sulphotsumoite (Bi3Te•S; Zav'yalov and Begizov, 1982; Ren, 1986). Selected analyses of naturally occurring compounds in the system Bi-Te-S reported in Thompson (1949), Spiridonov et al. (1974), Zav'yalov et al. (1976), Lipovetskiy et al. (1977), Zav'yalov and Begizov (1977), Shima- zaki and Ozawa (1978), Gamyanin et al. (198'2), Zav'yalov and Begizov (1983), Goncharov et al. (1984), and Ren (1986) are shown in Figure 2. It should be noted that these minerals are indistin- guishable by optical means and that the majority are nonstoichiometric.

The AG• of tellurobismuth was calculated from the selected data of Mills (1974a). Data on AHf or Cp are not available for the remaining bismuth tellurides and the proliferation of ternary homologues, and ignorance of phase relations in the system Bi-Te-S renders quantitative estimates of their thermochemical properties of little practical utilit)•.

Consideration of the Te/Bi ratios of minerals in the Bi-Te system, and reactions such as:

4BiTe + Te•/g / = 2Bi•Te3 , (1) tsumoite tellurobismuth

shows that, with increasing fTez, the following minerals are successively stabilized: bismuth -• hedleyite -• Bi2Te -• tsumoite -• tellurobismuth. Like- wise, with increasing fsdfT•, sulfotellurides of bismuth are stabilized with respect to pure bismuth tellurides as shown for the reaction:

2BizTe3 + Sang /: 2BizTezS + Te•g I. (2) tellurobismuth tetradymite

The phase diagram for this system (Klepp and Komarek, 1973) contains compounds of two phases: a (CoTel+•; x = 0.1-0.8) and 'y (CoTe2+•; x = 0.0-0.2). The 3' phase has been reported as the mineral mattagamite (Thorpe and Harris, 1973; Spiridonov et al., 1974). Natural occurrences of mattagamite invariably contain significant solid solution of FeTe• (frohbergite).

Cu-Te

The phase diagram for this system (Blachnik et al., 1983) is complicated, particularly at lower temperatures, by the presence of compounds contain- ing both Cu + and Cu +• in defect structures. The three principal compounds are the minerals weissite (Cu•_•Te), rickardite (Cu4-•Te•), and vulcanitc (CuTe). Blachnik et al. (1983), among others, have reported several additional compounds intermediate in composition between weissite and rickardite whose thermal stabilities, structures, and existence are uncertain.


TABLE 1. Invariant Points in Telluride Systems of Possible Use in Geothermometry

System Low-temperature assemblage High-temperature Temperature

assemblage (øC) Reœerence

Te

Ag-Te

As-Te

Au-Te

Bi-Te

Co-Te

Cu-Te

Fe-Te

Hg-Te

Ni-Te

Pb~Te

Pt-Te

Sb-Te

Crystal Melt 450

Hessite + stuetzite Low Y 120 _ 15

AgaTe AgsTe3 Agl.9Te Empressite Stuetzite + tellurium

AgTe AgsTe3 210

As•Te3 Melt 382

Calaverite + tellurium Melt 416 AuTe•

Calaverite + gold Melt 447 AuTe•

Calaverite Melt 464 __+ 3 AuTe•

Bismuth + hedleyite Melt 266 Bil4Te6

Hedleyite Unnamed: Bi•Te + melt 312 Bil4Te6

Unnamed: Bi•Te Tsumoite + melt 420 BiTe

Tsumoite Tellurobismuth + melt 540 BiTe Bi•Tea

Tellurobismut h Melt 588 Bi2Te3

Tellurobismuth + tellurium Melt 413 Bi•Tea

Mattagamite CoTel.7 + melt 764 CoTe•

Vulcanite + tellurium Melt 340 CuTe

Vulcanite Rickardite + melt 367 CuTe Cu4-xTea

Rickardite Cu3_xTe• + melt 647 Cu4-xTe•

Weissite Melt 1,125 Cu•_•Te

Frohbergite FeTel.8 + melt 649 FeTe•

Coloradoite Melt 670 _ 20

HgTe

Melonite Melt 900 NiTe•

Altaite + tellurium Melt •405 PbTe

Altaite Melt 924 PbTe

PtTe + platinum Melt 860

PtTe Moncheite + melt 920 PtTe•

Moncheit e Melt 1,125 PtTe•

Te!!urantimony + tellurium Melt 424 Sb•Te•

Tellurantimony Melt 617 - 2 Sb•Te•

Kracek et al. (1966)

Honea (1964)

Pankratz et al. (1984)

Barton and Skinner (1979)



Elliott (1965)

Elliott (1965)

Elliott (1965)

Elliott (1965)

Shunk (1969)

Elliott (1965)

Klepp and Komarek (1973)

B!achnik et al. (1983)

Blachnik et al. (1983)



Ipser et al. (1974)


Klepp and Komarek (1972)



Shunk (1969)

Shunk (1969)

Shunk (1969)

Elliott (1965)

Shunk (1969)


T•,BLE 1. (Cont.)

System Low-temperature assemblage High-temperature Temperature

assemblage (øC) Reference

Ag-Au-Te

Ag-Cu-Te

Au-Bi-Te

Au-Pb-Te

Au-Sb-Te-Bi?

Bi-Te-S

Pb-S-Te

Pd-Bi-Te

Petzite + hessite

Ag3AuTe2 Ag2Te Electrum + calaverite + petzite (AuAg) AuTe• Ag3AuTe2

Tellurium + stuetzite + sylvanite AgsTe3 AuAgTe2

Sylvanite AuAgTe• Krennerite

(Au,Ag)Te• Hessite + weissite

AgaTe Cu•Te

BisTea + maldonite + bismuth Au2Bi

Tellurobismuth + calaverite + gold Bi2Tea AuTe•

Tellurobismuth + calaverite + tellurium

Bi2Te3 AuTe•

Calaverite + altaite + tellurium

AuTe• PbTe Calaverite + altaite + gold

AuTe• PbTe Calaverite + altaite

AuTe• PbTe Gold + altaite

PbTe

Montbrayite

B-tetradymite Bi•Te2+xS•_x

Galena + altaite

Kotulskite + merenskyite PdTe PdTe•

Kotulskite PdTe

Merenskyite PdTe•

Michenerite solid solution •PdBiTe

X •50 Cabri (1965) Agl l-xAu•+xTe6 Melt 304 _ 10 Cabri (1965)

Melt 330 Cabri (1965)

Krennerite + melt 354 _ 5 Cabri (1965) (Au,Ag)Te• Calaverite + melt 382 _+ 5 Cabri (1965)

AuTe2

Extensive solid solution 125 Legendre et al. (1983)

Melt 235 Gather and Blachnik (1974)



Melt 388 Legendre and Souleau (1972)




Melt 410 _ 5 Bachechi (1972)

-Tetradymite + melt 581 _ 3 Glatz (1967) Bi•TeL59S•.4•

Complete solid solution 805 Barton and Skinner (1979)

Complete solid solution 575 to 710 Hoffman and Maclean (1976)

Melt 720 Hoffman and Maclean (1976)

Melt 740 Hoffman and Maclean (1976)

Kotulskite + melt 489 to 501 Hoffman and Maclean (1976) PdTe

The copper-rich end member, weissite, is stable in five structural modifications from room temperature to its melting point at 1,125øC. Rickardite from the type locality has the structural formula Cu4-xTe•. (x = 1.0-1.2; Forman and Peacock, 1949), but it is frequently described either as Cu4Te3 or Cu?Tes. Rickardite undergoes at least one transition and melts incongruently to a phase with the composition Cu3-xTe2 at 647øC. Vulcanite melts incongruently to rickardite at 367øC and forms a eutectic with native tellurium at 340øC.

The complexities in the phase relations in this system render the interpretation of thermodynamic measurements by different workers difficult, partly

due to uncertainties regarding the composition of the phases whose properties were measured. Nev- ertheless, it is desirable to obtain thermodynamic data for the three principal copper tellurides. One approach is to estimate the heat capacity of rickardite using an approximation such as:

Cp(Cu4Te3) = IS"s(Cu4Te3)I-Cp(CuTe), (3) [ Ss(CuTe) J

or by assuming that Cp (g/atom) -1 is constant in the system Cu-Te. This may be combined with the heat content measurements of Blachnik and Gunia (1978) for Cu2Te and those of Mills and Richardson

382 AFIF1, KELLY, AND ESSENE

Bi

Hd

Ts

S Te

FIG. 2. Compositions of natural compounds in the system Bi-Te-S. Minor amounts of Sb and Pb grouped with Bi, and minor Se grouped with Te. Abbreviations: Bs -- bismuthinite, Hd -- hedleyite, In -- ingodite, J-A = joseitc-A, J-B -- joseitc-B, St = sulfotsumoite, Tb = tellurobismuth, Td -- tetradymite, Ts -- tsumoite (-- wehrlite), and unnamed -- Bi2Te.

(1973) for CuTe in conjunction with emf measurements for all three compounds by Abbasov et al. (1976) in order to calculate AGf •r these compounds at high temperatures. This approach is valid only up to 270øC due to a transition in rickardite. Alternatively, the emf data of Abbasov et al. (1976) over the range 25 ø to 155øC may be combined with the emf data of Terpilowski and Fuglewicz (1978) over the range 260 ø to 350øC to estimate the fields of stability of the three copper tellurides in fTe2-T space. The two approaches are in disagreement, but the former is preferred until further experimental work resolves this discrepancy.

The phase diagram for this system at temperatures below 519øC (Ipser et al., 1974) contains the compounds of two phases,/• (FeTe0.9; mp -- 844øC) and • (FeTe•.; mp -- 649øC). The only iron telluride known as a mineral is frohbergite (FeTe2), which occurs in a variety of telluride deposits (Rucklidge, 1969; Ramdohr and Ububasa, 1973; Berzon and Merkur'eva, 1976; Berbleac and David, 1982; Afifi et al., 1984). Although frohbergite from most occurrences is generally pure FeTe•., analyses from some deposits (Thorpe and Harris, 1973; Spiri- donov et al., 1974) indicate that it forms a continu- ous solid solution with mattagamite (CoTe•).

The heat content measurements for FeTeo.9 and FeTe• of Mikler et al. (1974) appear to be in error,

since both compounds apparently dissociated during measurement. Thermodynamic functions for these two compounds were therefore calculated by using Mills' (1974a) extrapolation of the heat capacity data of Westrum et al. (1959) in conjunction with the emf measurements of Geiderikh et al.

(1961). This approach is in reasonable agreement with high-temperature calorimetric data by Vladi- mirova et al. (1982).

Ni-Te

The phase diagram for this system at temperatures below 690øC (Klepp and Komarek, 1972) contains the compounds of three phases, /• (Ni3-xTe•.), 3' (Ni4Te3+x; x = 0.08-0.4); and • (NiTe•_x; x -- 0-0.9). The i5 phase corresponds to the mineral melonitc which generally has the composition NiTe2 (Spiridonov et al., 1974; Ebner, 1978; Shimada et al., 1981). Melonitc from high- temperature (magmatic sulfide) deposits, however, may contain significant substitution of Pd and/or Pt for Ni, and Bi for Te (Garuti and Rinaldi, 1986). Yushko-Zakharova (1964) reported imgreite (NiTe), which may be the natural equivalent of the 3' phase.

Thermodynamic functions for the two i5 end members (NiTe•.• and NiTe•) were calculated from the heat capacity data of Mills (1974b) and the emf measurements of Geiderikh et al. (1980). Data on the heat of formation of the /• and 3' phases are currently not available.

Sb-Te

The phase diagram for this system (Voronin and Degtyarev, 1981) contains at least three compounds: /•(SbTe•_x; x = 0.41-0.78), 3'(SbTex; x = 0.69-1.17), and Sb•Te•. The latter has been found as the mineral tellurantimony (Thorpe and Harris, 1973). The apparent rarity of this mineral is probably due to accommodation of Sb in other tellurides, notably tellurobismuth (Spiridonov et al., 1974) and nagyagite (Stumpfi, 1973). Thermody- namic functions for tellurantimony were calculated from the selected data of Mills (1974a). No thermodynamic data are available for the remaining Sb-Te compounds.

Other binary systems

As•Te• (top = 382øC), HgTe (top = 670øC), PbTe (mp = 924øC), SnTe (mp = 806øC), MnTe, MnTe•, and ZnTe (mp = 1,297øC) are the only known compounds in the corresponding binary systems. From this group, only coloradoitc (HgTe) and altaitc (PbTe) have been reported as minerals, whereas the remaining compounds are replaced in nature by their sulfide and oxide counterparts. With the ex- ception of MnTe2, for which AH•' is not known,


thermodynamic data for these compounds are in- cluded in Appendix I.

Phase Relations in Multicomponent Systems

Aside from minerals in the system Au-Ag-Te, ternary telluride minerals are rare and mostly escaped detection prior to the advent of the electron microprobe. Although no thermodynamic data are available for ternary tellurides, it is important to consider their stability with respect to their binary ana- logues. As a first approximation, the free energies of formation of ternary tellurides are probably not very different from the sums of the equivalent mix- tures of binary tellurides, by analogy with ternary sulfides and sulfosalts (Craig and Barton, 1973). For example, the free energy changes for the reactions:

Ag2Te + Bi2Tea -- 2AgBiTe• (4) hessite tellurobisrnuth volynskite

and

PbTe + Bi•Tea = PbBi•Te4 (5) altaite tellurobisrnuth rucklidgeite

are probably small, as suggested by various combi- nations of these five minerals in assemblages where other tellurides appear to be in equilibrium (Afifi et al., 1988).

Au-Ag-Te

Experimental work on this system by Markham (1960) and Legendre et al. (1980) has suffered from their inability to synthesize known minerals, and in

some cases, their failure to detect unquenchable high-temperature phases.

Our understanding of phase relations in this system is based on the superior experimental work of Cabri (1965). In addition to minerals stable along the binary Au-Te and Ag-Te joins, this system contains the minerals electrum (Au, Ag), krennerite ((Au, Ag)Te•), sylvanite (AuAgTe4), and petzite (AuAgaTe2) and the naturally unquenchable phases 14 (tg11-xtt11+xWe6) and •, ((Ag, Au)•-xTe; x = 0.09- 0.12; Fig. 3A).

Compilation of reliable analyses of gold-silver tellurides confirms the unquenchability of both 14 and •, in natural ores (Fig. 3B). In addition to these relatively common minerals, Spiridonov and Chvi- leva (1986) have confirmed the existence of the rare mineral muthmannite (AuAgTe•) in supergene ores.

The decompositions of 14 and •, have important consequences to the interpretation of natural telluride assemblages. The minerals hessite, sylvanite, petzite, and/or stuetzite can be either part of an original equilibrium assemblage or a product of the decomposition of 14 and/or •, (Fig. 3A and B). Cool- ing of the divariant assemblage •,-petzite, for example, will produce the assemblage petzite(I)-petzite(II)-hessiteosylvanite at room temperature (Fig. 3B). It is therefore important to distinguish primary (hypogene) sylvanite, whose thermal stability below 354øC (Table 1) is a useful thermometer, from sylvanite produced by dissociation of •,. The common

a Te

A • 280-120'c / / ] • after Cabr, 11965,

Ag

FIG. 3. A. Phase relations in the system Au-Ag-Te between 120 ø and 280øC (Cabri, 1965). The calverite-sylvanite tie line becomes unstable above •280øC due to formation of krennerite (Au, Ag)Te2. B. Compositions and inferred natural tie lines in the system Au-Ag-Te based on 133 microprobe analyses by Cabri and Rucklidge (1968), Stumpfl (1970), Spiridonov et al. (1974), Soeda and Watanabe (1981), Porter and Ripley (1985), and Afifi et al. (1988). Analyses with •0.03 wt percent other elements were excluded. A tie line exists between sylvanite and calaverite when krennerite is absent.


intergrowths of hessitc and/or stuetzite with petzite and/or sylvanitc in ores are usually a product of the decomposition of x and/or • at temperatures below • 120øC. A second consequence of the decomposition of x and/or • is that invariant assemblages that initially contained either of these phases will appear to violate the phase rule after cooling to room temperature.

The recognition of assemblages derived by decomposition of x and/or ? is critical to proper appli- cation of phase equilibria in this system. Kelly and Goddard (1969) described textural relations that are useful for this purpose, and from our experi- ence, are applicable to a wide variety of telluride ores.

Phase relations in the system Au-Ag-Te do not allow reliable estimates of AG•' for any of the ternary compounds, which should instead be obtained by methods such as emf measurements. For example, the method by which Ahmad et al. (1987) estimated AG? of petzite is not valid because they ig- nored the presence of the x phase.

Other systems

Experimental studies of the systems Au-Pb-Te (Legendre and Souleau, 1972) and Zn-Bi-Te (Maru- gin et al., 1984) did not detect any ternary compounds. Although ternary compounds are known in the systems Ag-Sb-Te (Werniek et al., 1958), Cu-Ni-Te (Singh and Bhan, 1982), Cu-Te-S (De Medieis and Giasson, 1971), and Sb-Ag-Te (Marin et al., 1985), none of these compounds have yet been reported as minerals.

The following ternary tellurides have been reported from natural occurrences: volynskite (Ag- BiTes; Shimada et al., 1981), unnamed AgaBiTe2 (Patterson and Watkinson, 1984), kostovite (Au- CuTe4; Table 2), henryitc ('•Cua.77Aga.0•Te4; Criddle et al., 1983), and several Pb-Bi-Te compounds, including rueklidgeite ((Pb,Bi)aTe4; e.g., Lipovetskiy et al., 1977). Other tellurium-rich minerals include nagyagite (PbsAu(Te,Sb)4Ss-8) and goldfielditc (telluriah tetrahedrite). Experimental data on the stability of these compounds are essentially lacking, and the majority are less common than corresponding binary telluride-sulfide assemblages.

Telluride Stability Diagrams

Method of calculation The thermochemical data compiled in Appendix I

are referenced to a state of ideal Tes(g) and other pure elements in their stable form at 1 bar and the temperature of interest. Unlike the choice of Ss(g) as a reference state for sulfur, the choice of ideal Tescg/ as a reference state for tellurium does not provide a computational advantage but is consistent with con-

vention. Switching from a reference state of pure elements to a reference state of ideal Tescg• may be accomplished by adding AGr for the reaction:

Tes•g• = 2Te•c,•. (6)

For example, consider the formation of hessitc by reaction of pure silver with either pure tellurium or pure

4Ag + 2Te½c) = 2AgsTe (7) and

4Ag + Tescg• = 2AgsTe. (8)

The free energy change for reaction (8) is equal to the sum of the free energy change for reactions (6) and (7).

For the general reaction:

m vii 4- • vii = 0, (9) 1

where n and m equal the number of solid phases and gaseous phases, respectively, and vi equals the reaction coefficient for the ith phase in the reaction, the equilibrium constant (K), the free energy for the reaction (AG•,0, the activities of solid phases and the fugacities of gaseous species (fi) are related by the expression:

log K - --AC•,r _ • Pi log "i 4- • Pi log.)•. (10) 2.303RT i i

For pure solid phases (ai = 1), log K becomes a function of the free energy of the reaction and the fugacities of the gaseous species. For the silver-hessite example (8), log K -- --1ogfTe• may be calculated at different temperatures from the free energy data in Appendix I.

Equilibria between sulfides and tellurides may be represented as a function of• andfTe• by reactions such as:

FeSs+Tes= FeTes +Ss. (11) pyrite frohbergite

The equilibrium constant for this reaction may be written as:

K: (fs2)(fTe2) -1/ ,frohbergite\/ ,pyrite\-I •,"FeTe• ] •Ul•eS • / (12)

which, for pure pyrite and frohbergite, is a function of./•,fTe•, temperature, and pressure. Likewise, the oxidation of frohbergite may be represented by the reaction:

3FeTes + 2Os = Fe304 4- 3Tes, (13)

which, for pure frohbergite and magnetite, is a function offT•, fo•, temperature, and pressure.


25 100 T[øCl 200 300 400 500

0 -20

-30

103. TiKi -•

FIG. 4. Fugacity of Te2-temperature diagra•n showing stable co•npounds in the syste•n Ag-Te, and calculated isopleths of electrum in equilibrimn with hessitc.

The effect of confining pressure on these reactions can be evaluated by the expression:

O In K --AV(s) 0• - RT (14)

Since AV(s) in these reactions is small (pyrite-froh- bergitc and magnetite-frohbergite, respectively), the effect of pressure on the equilibrium constant is negligible for moderate (up to 1 kb) increases in pressure. The only reaction which is significantly influenced by this range of pressure is the condensation of tellurium (Fig. 4), for which a 1-kbar increase in pressure results in a 10 ø'5 increase in fTe•.

The fugacity of Tee(e) in a hydrothermal fluid is related to the total concentration of tellurium in the

fluid, and the partitioning of tellurium among aqueous species by:

fTe2 _•_• ( O/H2Te/2 o fO2 \ aH•O / K•6

O•H20

in which K•6 is the equilibrium constant for the reaction:

Tee(g) + 2H20(/) = 2HeTe(,q) + 02(0. (16)

XH•Te is the mole fraction of HeTe(,,), mzTe is the sum of the molalities of all aqueous species of tellurium, and 3'H•Te is the activity coefficient for HeTe(,,). The speciation of tellurium in hydrothermal fluids is not adequately known, and thermody-

namic data for some Te-O-H species are unreliable, which prevents quantitative evaluation of reaction (15). Furthermore, the possible presence of metal- tellurium complexes requires additional thermodynamic data. However, the existence of both reduced (e.g., HTe-, Te• e) and oxidized (e.g., HTeOj, We(OH)I) species indicates that XH2Te is also a function of both fo2 and pH. Thus in a hydrothermal fluid, fTe• is a function of several independent variables, including temperature, fro, pH, salinity, and the total concentration of tellurium in the fluid. Al-

though variations in fTee, rs2, and fo• may be re- vealed by mineral assemblages, reasons for varia- tion infTe• cannot be reliably ascertained at present.

The effect ofJ•, fTe•, fo•, and temperature on sulfide-telluride and oxide-telluride equilibria may be evaluated by construction OffTe•-T diagrams and isothermal fTe•-fs• and fTe•-fo• diagrams.

f Te•temperature diagrams The condensation of solid or liquid tellurium,

represented by reaction (6), sets an upper limit on fTe• in natural systems and is shown as a function of /Te• and 1/T in Figure 4.

By analogy with the electrum tarnish method (Barton and Toulmin, 1964) the reaction between silver dissolved in electrum and hessitc,

4Ag + Tee = 2AgeTe, (17) electrum hessitc

was suggested by Barton and Skinner (1979) as a potentially useful sliding-scale indicator of fTe2 in


experiments. This reaction can further be applied to natural electrum-hessite assemblages for the same purpose. Assuming unit activity of Ag2Te in hessire and using the regular solution model of White et al. (1957) for Au-Ag alloys, the fugacity ofTe2tg ) in the assemblage hessite-electrum may be expressed as:

1 } { AGq(Ag•Te) log fTe• -- 4.576T - 18.302T log XAg + 4(1 - XAg)•[5,650

- 1,600(1 -X•g)- 1.375T]}. (18)

The derivation of this expression is essentially iden- tical to that shown by Barton and Toulmin (1964) for the reaction argentite-electrum. An expression for the free energy of formation ofhessite is listed in Appendix I and may be substituted directly into reaction (18).

The effects of gold substitution in hessire may be taken into account by assuming that solid solution of gold in AgaTe is similar to that in Ages and by apply- ing the corrections of Barton and Toulmin (1964) to fTe• values calculated with reaction (18). These corrections only affect isopleths of Au-rich electrum in equilibrium with hessite at relatively high temperatures (Fig. 4).

Although natural hessire is generally nearly stoichiometric, the applicability of reaction (18) is lim- ited by the restriction that hessite and electrum are the only Au-Ag tellurides present in an assemblage.

The reason for this restriction is that hessire inter-

grown with petzite is ultimately a product of decomposition of the unquenchable x phase (Cabri, 1965) in which the reduced activity of Ag2Te cannot be estimated.

The applicability of reaction (18) can be defined for a range of electrum compositions. The experimental data of Cabri (1965) show that the limits of electrum composition for the assemblage electrum- hessire are Ag]oo-Au•6Ag74 at 356øC and Ag]oo- Au•lAg7,at 290øC. This indicates that the restriction imposed by the x phase does not apply to assemblages formed at low temperatures (<300øC) with the electrum composition not exceeding •21 mole percent gold.

Potential applications exist for using electrum- hessite assemblages as sliding-scale indicators offx• in ore deposits. For example, Soeda and Watanabe (1981) reported the composition of electrum (X•g -- 0.31-0.39) coexisting with hessire in the Takeno mine, in the Green Tuff region, Japan, and noted that petzite and argentire are rare or absent. Using their fluid inclusion filling temperatures and reaction (18) indicates a range in logfx• from -13.8 to -14.8 at 210øC, and from -11.7 to -12.6 at 270øC.

Univariant equilibria for geologically pertinent telluride reactions are shown as a function of the

difference from fugacity of Te• at saturation and temperature (Fig. 5). Equilibria among the three copper tellurides are extrapolated above 270øC

-5

i-

o

-10

-15

25

0

oo.

co• e• • %o

100 T øC 200 400 I

CuTe

As2T'

3.0 2.5 103. TiKi_ 1 2.0 1.5

FIG. 5. Diagram showing various reactions as a function of the difference in the fugacity of Te• from saturation and temperature.

TELL URIDE PHASE RELATIONS: I. DATA 387

due to limitations on the applicability of reaction (3). The reactions pyrrhotite + Te2 = pyrite + frohbergite and gold + Te2 -- calaverite may be used to estimate or limit fTe2 for these assemblages. The remaining reactions in Figure 5 illustrate the relative propensities of various metals to form tellurides, but they have no applicability to natural systems because the fugacities of S2 and O2 must also be considered. The reaction magnetite + Te2 • hematite + frohbergite is unstable above room temperature, indicating that hematite cannot be in equilibrium with frohbergite in geologic systems.

f Te2-f s2 diagrams Holland (1959, 1965) showed that sulfide and

oxide assemblages place broad limits on the ranges of•2 and fo• during ore deposition. Tellurides are generally associated with pyrite, and only rarely with pyrrhotite, which indicates fs• conditions generally above the pyrrhotite-pyrite buffer. Further- more, most telluride deposits contain magnetite or hematite either as ore minerals or in associated al-

teration assemblages, which places fo2 conditions near the magnetite-hematite buffer. The lower limits of fs• and fo• are typically buffered by reactions between iron sulfides or oxides and iron in

silicate minerals. Such reactions are commonly referenced to the buffers fayalite + O2 = quartz + magnetite and fayalite + S2 -- quartz + pyrrhotite + magnetite. The upper limits for • and fo2 are defined respectively by the condensation of sulfur

and by the fugacity of O2 in the atmosphere, although these conditions are generally not attained during telluride deposition.

Available geothermometric data indicate that most tellurides in veins were deposited at temperatures less than 350øC. Experimental data (Cabri, 1965; Vladimirova et al., 1982) and textural evi- dence (Kelly and Goddard, 1969) further indicate that some telluride systems, such as Au-Ag-Te and Fe-Te, may undergo extensive reequilibration during cooling. In order to bracket these variations, we have constructed isothermal fTe•-fs• diagrams at temperatures between 100 ø and 300øC. These diagrams are useful in estimating the prevailing fTe• and • during telluride deposition and/or reequilibration from observed mineral assemblages. Con- versely, these diagrams are also useful in predicting stable sulfide-telluride assemblages given partial information about mineralogy or the chemical constitution of the system.

Several reactions depicted in Figures 6, 7, and 8 are metastable with respect to phases for which thermodynamic data are not currently available. The Bi-Bi2Te3 and Bi2Te3-Bi2Sa univariant reactions, for example, are roetastable with respect to reactions involving the remaining bismuth tellurides and bismuth sulfotellurides. Consequently, the divariant fields shown for native bismuth, tellurobismuth, and bismuthinite represent maximum fields of stability for these minerals. Likewise, the fields of stability for hessite, stuetzite, and calaver-

-20 -15 Log fS 2 -10 -5 FIc. 6. The stabilities of some sulfides and tellurides as a function of the fugacities of S2 and Te2 at

300øC. The shaded region includes the normal range in fTe• and.• for hydrothermal ore deposits. Additional reactions are shown in Figure 7.


ite in the binary Ag-Te and Au-Te systems will be reduced when thermodynamic data for the ternary Au-Ag tellurides (x, •, petzite, sylvanite, and krennerite) become available. The fields of stability for tellurantimony and stibnite will be reduced if minerals in the Fe-Sb-S system (Barton, 1971) are considered. Nevertheless, the depiction of these metastable reactions is useful since they provide some restrictions on the stability fields of known or possible ternary compounds.

Figures 6, 7, and 8 explain the rarity or absence of many tellurides from natural assemblages. The compounds MnTe, ZnTe, FeTe0.9, CoTel+x, and NiTe2_x are unstable with respect to sulfides over the normal range in,•. In the system Fe-S-Te, frohbergite (FeTe2) is restricted to relatively high values offTe• and,•, which explains its rare occurrence. The assemblage frohbergite-pyrite is restricted to a narrow range in fTe2 and,• at temperatures below 272øC (Fig. 5).

In the system Cu-Fe-S-Te, reactions between copper sulfides (chalcocite, digenite, and covellite) and copper tellurides (weissite, rickardite, vulcanite) are metastable with respect to reactions involving chalcopyrite and bornite (Fig. 6). Copper tellurides are unstable with respect to chalcopyrite over the normal range in S•, which accounts for their rarity in hypogene assemblages. Since the ternary Cu-SoTe compounds reported by DeMedicis and Giasson (1971) have not been reported from nature and since these compounds would occupy an intermediate position between copper tellurides and chalcopyrite in ,•-fv•2 space, the presence of

copper tellurides in hypogene assemblages becomes even less likely.

In nature, the upper limit offv• is defined by the condensation of native tellurium. The absence of

argentite and native silver (or silver-rich electrum) from the majority of telluride deposits may be used to define the lower limit of fv•. These limits are combined with the normal ranges infs• to define the general conditions of telluride deposition (shaded regions in Figs. 6, 7 and 8).

The majority of known telluride minerals (including altaite, calaverite, coloradoite, frohbergite, hessite, melonite, stuetzite, tellurobismuth, mattagamite, and tellurantimony) may coexist with specific sulfide suites. For example, the assemblage vaesite (NiS•)-altaite requires the presence of both galena and melonite. Specific telluride-sulfide-oxide assemblages may define narrow limits for the fugacities of the gaseous species during ore deposition. For example, the common assemblage galena-altaite-gold-pyrite defines fs• and fv• conditions on the galena-altaite reaction between the pyrrhotite- pyrite and Au calaverite reactions. Many minerals, such as galena and frohbergite, cannot be deposited simultaneously as an equilibrium pair, because they require different ranges in fTe• for their stability. The stability of the compound As•Tes with respect to loellingite, arsenopyrite, arsenic, realgar, and or- piment indicates that arsenic telluride is stable over the normal range in fs• and fv• (Fig. 7). Further- more, calculations show that arsenic telluride is stable with respect to arsenolite (As•Os) at fo• conditions near the magnetite-hematite reaction. Be-

-lO

-20 -15 Logfs 2 -10 -5

FIG. 7. Additional sulfide-telluride reactions at 300øC supplementing those shown in Figure 6.

TELLURIDE PHASE RELATIONS: 1. DATA 389

-18

-30 -20 -10

LOg/S 2

FIG. 8. The stability of some sulfides and tellurides as a function of the fugacities of S2 and Te2 at 100øC. The dashed line corresponds to the reaction magnetite + S= = pyrite + hematite. The dotted line corresponds to the reaction galena + hematite = pyrite + anglesite. The shaded region includes the normal range in fte2 and fs2 for hydrothermal ore deposits.

cause arsenopyrite is present in some telluride deposits, arsenic telluride is likely to be found ultimately as a mineral.

Experimental tie lines in the system Au-Ag-Te (Cabri, 1965) help estimate the relative stabilities

of ternary Au-Ag tellurides as a function offte=. Be- cause sylvanite may be formed by the breakdown of • (Au, Ag)•.gTe), the lower limit of sylvanite stability corresponds approximately to the hessite-• reaction, whereas the presence of a sylvanite-tellurium

-8

-16

-40 -35 -30 -25

LOg/O 2

FIG. 9. The stability of tellurides and oxides as a function of the fugacities of Te• and O• at 300øC. Heavy lines show the stable extensions of the corresponding reactions at • conditions of the pyrrhotite-pyrite buffer.


tie line indicates that sylvanitc is stable to the condensation of native tellurium (Fig. 3). Petzite is also formed by the breakdown of % but because the assemblage 'y-tellurium is not stable, the upper limit of petzite stability must be below the condensation of native tellurium. The instability of the assemblage krennerite-gold implies that the lower limit of krennerite stability must be above the gold-calaver- itc reaction. These relations suggest that the lower fTe2 limits for the stabilities of Au-Ag ditellurides (calaverite, krennerite, and sylvanitc) correspond roughly to the hessite-'y reaction.

fTe2-f02 diagrams The stabilities of metal oxides and tellurides are

shown in Figure 9 as a function Of fTe2 and fo2 at 300øC. The previously outlined limits on fT•2 along with the reactions fayalite 4- O2 = quartz 4- magnetite and galena + O2 = anglesite define the normal range offT•2 and fo2 in ore deposits. Except for Sn, Fe, and Co, it is clear that most metal oxides are unstable with respect to tellurides, consistent with the absence of all oxides except cassiterite, magnetite, or hematite from hypogene telluride ores. An- drushchuk et al. (1968) reported bismite (Bi203) from telluride ores in the Tyrnyauz district, but he did not indicate whether this mineral was supergene. The stability of bismite (Fig. 9) requires un- usually high oxygen fugacity, which suggests that bismite is secondary in origin. Tellurite (TeO2) is only stable at fo2 beyond the range of Figure 9, which suggests that tellurite will not occur as a hypogene mineral.

Discussion

Data from fT•2-J•2 and fT•2-f02 diagrams should be combined in order to characterize conditions of telluride deposition. For example, at fs• conditions of the pyrrhotite-pyrite buffer, most telluride-oxide reactions become unstable with respect to telluride-sulfide and sulfide-oxide reactions (Fig. 9). Other parameters must also be considered for particular situations. For example, the relative stabilities of tellurides with respect to arsenides or selenides may be evaluated by constructing fTe2-f^s or fT•2-J•2 diagrams. Although most natural tellurides are essentially stoichiometric, some form solid solu- tions with other tellurides, bismuthinides, and/or selenides, in which case the calculated univariant reactions should be shifted to account for reduced activities of the end members.

Conclusions

1. The important variables controlling telluride mineral assemblages for a given metallic composition are fTe2, fs2, and temperature.

2. Most oxides, except those of Sn and Fe, are

unstable with respect to sulfides and/or tellurides over the normal range in fo2 encountered in hydrothermal deposits.

3. The assemblage electrum-hessite is useful as a sliding-scale indicator offTe2 at relatively low values offTe2, where other useful univariant reactions are rare.

4. fT•2-J52 and fT•2-fo.• diagrams explain the occur- renee or absence of particular tellurides as minerals. Common tellurides, such as hessite, coloradoire, and altaire, are stable over the normal range in 752, fo2, and fT•2 in ore deposits. Some rare tellurides, such as the copper tellurides, require uncommonly low values of J•2 in order to be stable with respect to ehaleopyrite and bornite. The rarity of other tellurides, such as tellurantimony and mattagamite, is due to substitution of Sb and Co in more common

tellurides (tellurobismuth and frohbergite, respectively).

5. Minerals stable over a wide range in fTe2-J•2, such as altaite, are relatively common, while minerals stable over a narrow range in fT•2-JS•, such as frohbergite, are less common.

6. fT•2-JS= diagrams help explain or predict associated or antipathetic minerals. Pyrite, for example, is stable with a wide variety of telluride minerals, whereas the yet unrecorded assemblage argentite- altaire is thermodynamically unstable.

Acknowledgments Aspects of the early research were aided by sup-

port from National Science Foundation grants EAPt-80-09538 and EAPt-84-08169 (to EJE) and EAPt-80-S•5363 (to WCK). A.M.A. was partly sup- ported by the Saudi Arabian Deputy Ministry for Mineral Ptesourees. I. V. Hanel and P. L. Cloke

helped us with German and Ptussian references. We have benefited from discussions with P. L. Cloke, F. J. Haynes, D. Pt. Pea½or, E. U. Petersen, and L. J. Cabri. This manuscript has been improved by com- ments from two anonymous Economic Geology re- viewers.

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APPENDIX I

Gibbs Free Energies of Formation of Binary Tellurium Compounds

G in calories for Temperature Reaction T in øK • Accuracy 2 range (øC) References

4Ag + Ten -- 2Ag2Te -55,055 + 27.66T B 25-148 Mills (1974a) -50,197 + 16.32T B 148-527 Mills (1974a)

3.28Ag + Te• = 2Ag•.64Te -52,844 + 29.39T B 25-145 Mills (1974a) -50,421 + 23.63T B 145-296 Mills (1974a) -49,090 + 21.23T B 296-352 Mills (1974a)

4/3As + Ten = 2/3As•Tea -44,177 + 36.68T A 25-85 Pankratz et al. (1984) -43,523 + 34.97T A 85-382 Pankratz et al. (1984)

Au + Te• = AuTe• -41,943 + 37.08T A 25-464 Mills (1974a)

4/3Bi + Ten = 2/3Bi2Tea -50,452 + 37.22T A 25-272 Mills (1974a) -52,785 + 41.58T A 272-723 Mills (1974a)

1.67Co + Ten = 1.67CoTe•.• -53,089 + 33.46T A 25-427 Komarek et al. (1975) -51,460 + 31.11T A 427-477 Mills (1974b)

1.17Co + Ten = 1.17CoTe•.7 -54,749 + 37.62T A 25-427 Komarek et al. (1975) -53,134 + 35.29T A 427-477 Mills (1974b)

Co + Te• = CoTe• -55,290 + 39.43T A 25-427 Komarek et al. (1975) -54,112 + 37.69T A 427-477 Mills (1974b)

4Cu + Te• = 2Cu2Te -56,268 + 35.15T A 25-160 Abbasov et al. (1976) -55,295 + 32.91T A 160-258 Blachnik and Gunia (1978)

8/3Cu + Te• = 2/3Cu4Tea -52,730 + 35.98T B 25-270 Abbasov et al. (1976); Cp est. 2Cu + Te• -- 2CuTe -48,652 + 33.72T B 25-367 Abbasov et al. (1976), Mills and

Richardson (1973)

2H• + Ten = 2H•Te<s• 8,200 + 17.96T A 25-750 Mills (1974a) 2.22Fe + Te• = 2.22FeTeo.9 -49,763 + 31.23T A 25-760 Mills (1974a)

Fe + Te2 = FeTe• -54,781 + 42.26T A 25-649 Mills (1974a)

2Mn + Ten = 2MnTe -89,958 + 28.22T A 32-707 Mills (1974a), Gronvald et al. (1972) 1.82Ni + Ten = 1.82NiTe•.• -62,662 + 36.63T A 25-358 Geiderikh et al. (1980), Mills (1974b)

-61,075+ 34.19T A 358-727

Ni + Te• -- NiTe• -58,330 + 38.78T A 25-358 Geiderikh et al. (1980), Mills (1974b) -56,867 + 36.54T A 358-727

2Pb + Te• = 2PbTe -70,750 + 39.25T A 25-328 Mills (1974a) -72,131 + 41.57T A 328-727 Mills (1974a)

4/3Sb + Te• = 2/3Sb•Tea -46,299 + 34.40T A 25-550 Mills (1974a) 2Sn + Te• = 2SnTe -67,784 + 37.89T A 25-232 Mills (1974a)

-70,376 + 43.14T A 232-650 Mills (1974a)

Te• = 2Tel• • -37,504 + 35.89T A 25-450 Mills (1974a) Te• -- 2Telt • -25,415 + 19.28T A 450-1,000 Mills (1974a) Ten + 20• = 2TeO•l• • -190,028 + 119.77T A 25-733 Mills (1974a), Pashinkin et al. (1985)

Te• + 20• = 2TeO•i• • -66,737 + 28.65T B 25-750 Mills (1974a) 2Zn + Te• = 2ZnTe -94,462 + 42.28T A 25-727 Mills (1974a)

1 The free energy change is expressed as a linear function of temperature for each reaction as written and is referenced to a state of ideal Te•l• at I bar

• The estimated accuracy is referenced to formation of I mole of the compound from Te•l•; uncertainty is shown by "A" where within 1 Kcal, and by "B" where within 2 Kcal


APPENDIX II

Molar Thermodynamic Quantities for Selected Binary Tellurides

H•9s Compound (Kcal)

Heat capacity coefficients •

S•9s c X 10 -4 (cal/øK) a b X 10 a (øK) T range References

AgnTe -8.60 a 36.7 a

Ag• .64Te - 7.55 a 32.0 a

AshTea -9.0 _+ 2 54.1 _+ 1

AuTen -4.45 33.87

BinTea -18.75 _+ 0.5 62.4 _+ 2

CoTe•.n -9.29 a 23.02 a

CoTe•.7 -14.54 a 26.16 •

CoTen -17.58 • 27.88 •

CunTe -9.1 _+ 0.7 ' 28.8 _+ 0.2 •

Cu4Tea -23.9 _+ 1.3 a 65.5 _+ 0.3 a

CuTe -5.6 -+ 0.5 a 20.5 _+ 0.2 '

FeTe0.• -5.57 • 19.14 b

FeTe2 -17.3 _+ 1 23.94

HnTe(a ) 23.83 _+ 0.2 54.7 _+ 0.5

HgTe -7.6 -+ 1 27.0 _+ 0.5

MnTe -26.6 _+ 3 a 22.4 _+ 0.4 •

NiTeL• -13.7 _0.3 • 20.09 b

NiTen -20.5 __+ 0.5' 28.76 b

PbTe -16.4 26.3 _+ 0.5

SbnTea -13.5 _+ 0.5 58.9

SnTe -14.8 -+0.5 23.9 -+ 1

Te(,.•) 0 11.83 -+ 0.05

Ten(a) 38.33 -+ 0.3 61.87

TeOn(,) -77.1 -+ 1 ' 16.83 b

TeOn(g) -14.2 -+ 2 65.7 -+ 1

ZnTe -28.5 -+ 0.l 18.6 -+ 0.5

11.758 b 26.20 b 6.60 b 298-421 19.71 b -0.9958 b 27.47 b 421-550 20.08 b 0 0

9.629 b 29.46 b --0.270 b 298-418 24.72 b --6.533 b --0.2287 b 418-569 18.88 b 0 0 569-800

32.31 10.6 -44.44 298-648

15.20 8.93 4.12 298-737

25.81 13.2 0 298-850

11.41 b 6.51 b 0 298-750

13.87 b 7.92 b 0 298-750

18.05 b 3.35 b -10.31 b 298-750

18.00 b 58.1 b 0.013 b 298-433 1.82 b 31.4 b 98.87 b 433-531

-14.0 b 74.8 b -8.0 b 531-590 31.63 b 0 0 590-635

0.97 b 84.04 b 143.5 b 298-543

97.351 b -1,087.7 b 789.50 b 298-600

(d = -13,479)(e = 6.26 X 10 -4 ) 12.633 b 1.011 b -9.463 b

17.284 4.912 -10.02

298-1,200

298-933

8.48 2.88 -0.74 298-2,000

12.45 2.17 0 298-900

-1,526 b 3,495 b 4,464 b 298-305 2,037.1 b -1,793.8 b 2,695.2 b 305-700

(d = -3,204.2) (e = 7.93 X 10 -4) 12.5 b 4.01 b -4.34 b 298-750

17.76 b 4.96 b 10.0 b 298-750

11.28 2.69 0 298-1,000

26.98 12.7 0 298-892

11.46 2.82 0 298-1,079

4.57 5.28 0 298-723

9.00 0 0 723-1,300

8.28 1.581 -0.61 298-1,300

16.36 b 2.74 b -21.74 b 298-1,006

13.09 0.577 --28.28 298-2,000

10.54 4.48 0 298-1,300

•Mills (1972), bMills and Richardson (1973)

'Mills (1972), Richardson

•Mills (1972), Richardson

•Mills (1972), Richardson

Mills (1974a)

Mills (1974a)

Mills (1974a)

bMills and

(1973) bMills and

(1973) bMills and (1973)

aKomarek et al. (1975), bMills (1974b)

aKomarek et al. (1975), bMills (1974b)

•Komarek et al. (1975), bMills (1974b)

•Abbasov et al. (1976), bBlachnik and Gunia

(1978)

•Abbasov et al. (1976), bestimated

•Abbasov et al. (1976), bMills and Richardson (1973)

•Geiderikh et al. (1961), bMills (1974a)

•Geiderikh et al. (1961), bMills (1974a)

Mills (1974a)

Mills (1974a)

'Mills (1974a), bGronvald et al. (1972)

•Geiderikh et al. (1980), bMills (1974b)

•Geiderikh et al. (1980), bMills (1974b)

Mills (1974a)

Mills (1974a)

Mills (1974a)

Mills (1974a) Mills (1974a)

Mills (1974a)

aMills (1974a), bpashinkin et al. (198,5)

Mills (1974a)

Mills (1974a)

I Coefficients for the equation Cv(cal ) = a + bT + cT -n +dT -ø s + eTn; for most compounds only the first three terms are significant

Phase Relations Among Tellurides, Sulfides and Oxides I 88_Afifi-Essene

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