Minerals and calculated low-temperature phase equilibria in the

Mineralogy and Petrology (1995) 53:277-284
Mineralogy
mlCl
Petrology
© Springer-Verlag 1995
Printed in Austria
Minerals and calculated low-temperature phase
equilibria in the pseudoternary system
TI2S-As2Sa-Sb2S 3
R. J. G. Sobott
Mineralogisch-Petrographisches Institut, Heidelberg, Federal Republic of Germany
With 1 Figure
Received December 28, 1993;
accepted February 7, 1994
Summary
The experimental determination of phase relations in the pseudoternary system T12SAs2S3-Sb2S 3 below 200 °C is practically impossible, especially so under dry condensed
conditions. As thallium sulfosalts are naturally formed at low temperatures equilibrium
phase assemblages at 100 and 200 °C were calculated by application of thermochemical
approximations for the free enthalpies of formation of the sulfosalts. A comparison of
the calculated conode configurations with the results of syntheses under dry condensed
conditions at 200 °C yielded good agreement between experiment and calculations.
Zusammenfassung
M inerale und Phasenbeziehungen im pseudotern~ren System
Temper atur en
TI 2 S - A s 2 S 3 - S b 2 S 3 bei tiefen
Die Ableitung der Phasenbeziehungen im pseudotern/iren System TI2S-AszS3-Sb2S 3
ffir Temperaturen unterhalb 200 °C, insbesondere unter trockenen Bedingungen, ist auf
experimentellem Weg praktisch nicht m6glich. Da die Thalliumsulfosalze als tieftemperierte Mineralbildungen anzusehen sind, wurden die stabilen Gleichgewichtsassoziationen bei 100 und 200 °C unter Verwendung thermodynamischer N/iherungen fiir
die freie Bildungsenthalpie der Sulfosalze berechnet. Ein Vergleich der berechneten
Konodenverl/iufe mit den Ergebnissen von Versuchen unter trocken kondensierten
Bedingungen bei 200°C ergab gute lJbereinstimmung zwischen Experiment und
Berechnung.
Introduction
Since the beginning of systematic research on the stability and phase relations of
thallium sulfosalts some 18 years ago a lot of information has been gained on this
278
R.J.G. Sobott
rather exotic group of minerals. Experimental work, mainly carried out under dry
conditions in silica tubes and, to a lesser extent, by the hydrothermal recrystallization technique, pointed to the large number of intermediate compounds and helped
to elucidate the principal phase relations. Although most of the sulfide and certainly
all sulfosalt minerals are the product of precipitation of ionic species from aqueous
solutions, the results of experiments under dry condensed conditions, involving solid
state reactions, are nevertheless useful for the interpretation of natural sulfide
parageneses. Thermal stabilities of individual phases, univariant phase assemblages,
and the ranges of possible solid solution series may help to determine upper limits
of formation and/or reequilibration temperatures. However, due to slow reaction
rates below 250 °C not all phases known to exist as minerals could be synthesized
under dry condensed conditions. On the other hand, considering the physicochemical conditions of formation of corresponding mineral assemblages, it seems
to be more reasonable to discuss them on the basis of phase equilibria data for
temperatures much lower than 325 and 275 °C for which the isothermal sections of
the pseudoternary system T12S-Sb2S3-AsES 3 were established (Sobott, 1984).
Computation of phase equilibria
When experiments fail to determine low-temperature phase equilibria their calculation from thermochemical data is a feasible alternative. The change of free enthalpy
of a reaction at constant pressure and with a negligible difference between the molar
volumes of products and reactants is defined by
AG°r,T = AH°r,T - T*(AS°r,T)
with
AH°r,T = ( 2 AH°f,T)products -- (Z AH°f,T)reactants
and
AS°r,T = (~ S°T)products-- (~ S°T)reactants•
If it is negative, the reaction should proceed to the product side; if it is positive, the
reactants form the stable phase assemblage; if it is zero, products and reactants are
equally stable. It is important to note that the change of free enthalpy provides
information only on the direction into which the reaction will proceed but not at
what rate. Even reactions with great negative values for the change of free enthalpy
can be so sluggish that the equilibrium state is not reached in a reasonable period
of time or that they require high activation energies (catalysis) to accelerate the
reaction rate. The results of many low-temperature experiments under dry condensed conditions furnish good examples for this fact.
For the binary sulfide components of the system under consideration free
enthalpy, enthalpy of formation, entropy, and heat capacity data are available,
(Barin and Knacke, 1973; Robie et al., 1978). With respect to the T1 sulfosalts, no
such data exist and one has to resort to thermochemical approximations. Craig and
Barton (1973) proposed a method of calculating the standard free enthalpy of
formation AG°r,T for sulfosalts based on the concept of their formation by quasi
ideal mixing of the end-member sulfides.
Assuming that AHmixlng is zero and taking into account that measured
(AG/T)mixing values (per gram atom S) lie within the range of(1.2 + 0.8)R ~ N i In Ni,
(R: gas constant; Ni: mole fraction ofi-th end-member component), they derived the
Minerals and calculated low-temperaturephase equilibria
279
following formula:
AG°f,T = ZNiAGi + (1.2 ___0.8)RT~NilnN i
The authors stress the point that the precision of approximated free enthalpies
may not be sufficient for the prediction of complicated conode configurations. The
results obtained by this approximation can be improved by taking thermochemical
data of binary components in which the metal atoms have coordination polyhedra
identical to those in the polynary phase structure. If such binary compounds do not
exist so-called fictive data can be recalculated from corresponding polynary phases.
Robinson and Haas (1983) and Bente (1988) presented calculations for silicates and
oxides (tungstates, molybdates) and demonstrated that approximated data may
differ from measured data by less than 2 percent. However, the transfer of this
approach to calculations for (T1) sulfosalts meets fundamental problems. Unlike
arsenic which displays the trigonal pyramid as the characteristic coordination
polyhedron in binary and polynary sulfides, thallium and antimony atoms in
sulfosalt structures do not only exhibit coordination polyhedra differing from those
in the corresponding binary sulfides but also have different coordination polyhedra
in a given crystal structure. For example, in the structure of pierrotite (Engel and
Gostojic, 1983) antimony has SbS 3, SbS,, and SbS 5 coordination polyhedra. In
order to take the individual contributions of different polyhedra into account,
recalculation of fictive data from polynary phases is necessary but proved to be
impossible due to a lack of measured values. In the case of sulfosalts with the
'spiessglanz' type structure (e.g. T1Sb3Ss), however, this problem does not occur.
Keeping in mind that the contribution ofenthalpy to free energy at low temperatures
is much greater than that of entropy the application of approximated standard free
enthalpy values for the calculation of low-temperature phase equilibria in the
pseudoternary system pertaining to thallium sulfosalts seems reasonably justified.
And the discussion of this approximation against data for sulfosalts in the Ag-Bi-S
system measured by direct synthesis calorimetry by Bryndzia and Kleppa (1988)
gives it further credit.
Nevertheless, the reliability of such data has to be checked whenever possible
against experimental results and is enhanced by the corroboration of respective
natural phase assemblages.
For the laborious calculations of stable bivariant phase assemblages the author
of this paper wrote a computer programme called GIBBS, reminiscent of Josiah
Willard GIBBS who introduced free enthalpy as a new function of state, derived
the phase rule and lent his name to the phase triangle
Description of the computer programme GIBBS
The programme GIBBS was originally written in the computer language BASIC
and subsequently compiled by QUICK BASIC 4.5 into an EXE file running on IBM
compatible PCs. It comprises the following subroutines:
1.
2.
3.
4.
Calculation
Calculation
Calculation
Calculation
of H°T -- H°298 and SOTfor any compound
of AG°f,T for binary sulfides
of AG°f,T for sulfosalts
of AG°r,T for reactions of type ~ riRi ~ ~ pjPj
280
R.J.G. Sobott
For calculations within the system under discussion GIBBS uses thermochemical
data (Cp, AH°f,298, S°29s) for T1, As, Sb, $2, T12S, As2S3, and Sb2S 3 stored in a
separate data file.
The programme starts with the definition of the sulfides of monovalent (A2S;
A=Ag, Cu, T1) and/or bivalent (BS; B=Pb, Hg, Fe) and/or trivalent metals (C2S 3,
D 2 $3; C, D=As, Sb, Bi) making up the three-component system under consideration.
With input data for AH°f,298, S°298 and Cp-values (Cp = f(T)) it calculates AH°f,T,
S°T and AG°f,T for the binary sulfides in the temperature range where the polynoms
for Cp and Cp/T are applicable. (In the case of the thallium sulfosalt system thermochemical values are taken from the stored data file.)
The programme continues with the calculation of AG°f,T for sulfosalts using the
approximation proposed by Craig and Barton (1973). In order to normalize all
binary sulfide components to one sulfur atom total, thermochemical data for sulfides
of trivalent metals must be divided by 3. The mole fractions N i of the end-member
sulfides are defined by normalizing the sulfosalt formula also to one sulfur atom
total.
Example: Lorandite T1AsS 2
0.25"T12S + 0.75"Aso.666S ~ Tlo.sAso.sS
AGTIAsS: = 2* [(0.25*AGxl~s + 0.75*AGgso.666s)
+ ((1.2 _+ 0.8)*R'T*(0.25* ln(0.25) + 0.75* ln(0.75))]
= -53810 + 38.32"T
After the computation of the free enthalpies of formation for R i and Pi the calculus
requires the stoichiometric coefficients rl and pl according to the reaction scheme
rj R i ~ ~ pj Pj (i, j = 2). Finally, the change of free enthalpy of reaction is calculated
and the data output consists of the reaction equation, the value of AG°r,a-, and
indication of the stable assemblage.
Phases of the pseudoternary system
Except T13 SbS 3 and T1Sb 3S 5 all other phases within the system T12 S-As 2S 3 - S b 2 S 3
occur as minerals. Table 1 summarizes the thallium sulfosalts so far known.
The synthesis of imhofite, bernardite, chabourneite, and gillulyite has not been
reported. The occurrence of T1-Sb sulfosalts with chemically analoguous T1-As
compounds gives rise to the assumption that solid solution series (sss) exist between
them. Actually, there is a complete sss between the isotypic phases T13AsS 3 and
T13SbS 3. A limited sss exists between lorandite and weissbergite taking up about
41 m o l ~ lorandite at 280 °C (Sobott, 1981). The evaluation of XRD data for charges
of a phase mixture of parapierrotite and pierrotite (16.66 m o l ~ T12S, 65.91 m o l ~
Sb2S3) quenched from 325 and 275 °C yielded a single (parapierrotite) and a two
phase (parapierrotite and pierrotite) diffraction pattern, respectively, indicating that
there is also a limited sss between these phases.
From a crystal chemistry point of view the absence of more than one complete
sss was expected. The crystal structures of T13AsS3 (Gostojic, 1980) and T13SbS3
(Rey et al., 1984) are characterized by the occurrence of isolated coordination
Minerals and calculated low-temperature phase equilibria
281
Table 1. Thallium sulfosalts of the pseudoternary system TI2S-As2S3-Sb2S 3
Mineral name
Chemical
Formula
Melting
Point (°C)
Ellisite
Lorandite
Imhofite
T13AsS 3
T1AsS2
342 (c)
292 (c)
--
Gostojic (1980)
Fleet (1973)
Divjakovic and Nowacki (1976)
Bernardite
T1As5$8
T13SbS3
T1SbS z
TISb3S5
T1Sb5 $8
342 (c)
484 (c)
390
422 (i)
Pasava et al. (1989)
Sobott (1981)
Dickson and Radtke (1978)
Gostojic et al. (1982)
Johan et al. (1975)
Weissbergite
Parapierrotite
Pierrotite
Chabourneite
Rebulite
Gillulyite
T15.6 As 15 S25.3
T12As 2 Sb3 $8
TI42AssgSb98 S294
T15As8 Sb5 S22
TI 2(As, Sb)8 S ~3
---
-
< 200 (i)
Reference
Guillemin et al. (1970)
Johan et al. (1981)
Balic-Zunic et al. (1982)
Wilson et al. (t991)
(--) no data available; (c) congruent; (i) incongruent melting
polyhedra for As(III) and Sb(III) as trigonal pyramids with As or Sb at the apex.
In spite of different A s - S or S b - S b o n d lengths and S - A s - S or S - S b - S b o n d
angles, respectively, this structure tolerates a complete mutual substitution. With
an increasing brittle metal to sulfur ratio the coordination polyhedra are linked
to greater one-, two-, or even three-dimensional structural units forming chains
(T1AsS2, Fleet, 1973), sheets (T1SbS2, Rey et al., 1983; T1Sb 3 $5, Gostojic et al., 1982)
or frameworks (T1SbsS8, Enoel, 1980). The resulting structures are geometrically
m u c h more rigid and the mutual substitution of As by Sb is only possible on
a limited scale or even completely impossible.
Apart from the not yet discovered T13SbS 3 and T1SbS5 in nature, it is almost
certain that the n u m b e r of phases belonging to the thallium sulfosalt system exceeds
that presently known.
Calculated phase relations at 100 and 200 °C
The results of the phase equilibria computations are presented in Table 2. The
numerical values for AG°r should be viewed with the reservations discussed above
and taken for what they are, namely approximations. The agreement between
calculated and experimental findings, however, proves the correctness of the general
idea underlying the treatment of the problem.
The derived conode configurations at 200 °C agree very well with the phase
relations determined experimentally by Grzetic (pers. comm., 1991). Although some
differences occur due to the fact that it is difficult or even impossible to synthesize
imhofite, bernardite, and chabourneite under dry condensed conditions there is no
contradiction between the two presentations of the isothermal section at 200 and
100 °C in Fig. 1. As a matter of fact, they supplement each other so well that a
peritectic formation of gillulyite from orpiment and liquid below 200 °C can be
282
R.J.G. Sobott
Table 2. Calculated free enthalpies at 100 and 200 °C for
reactions of phases within the TI2S-As2S3-Sb2S 3 system
No.
AG°~( K J)
Reaction
100 °C
lel + l s t ~ 11o + 2we
51o+5ge~lor+lre
5pi + 4 o r ~ Ire + 10ge
41o+2pi~lre+lwe
l r e + 4 z ~ 5we + 4pi
2ge + l p p ~ 4 s t + lpi
14z + l c h ~ 14we + 42pi
200 °C
-5.0
17.6
- 26.4
-
-
13.4
-22.6
-511.5
-74.1
-6.3
-22.6
-33.1
-16.7
-28.5
-462.1
-92.5
(or orpiment; st stibnite; ge getchellite; el ellisite; lo lorandite;
re rebulite; we weissbergite; z T1Sb3Ss; pp parapierrotite;
pi pierrotite; ch chabourneite)
ird.te
stibrdte Sb2S 3
pterrotite p
getcheLlite AsSbS 3
TI2Sb6As4816
rebulife re
2 O0 o ~
o,
As2S 3 orpiment
T~sSbsAS8S22
chabourn~ite cb T142As84Sb989294
bernardite b T1AssS8
9Jllulyite gi
imhufite i
T12(As, Sb)8S13
T15.6As15S25.3
Fig. 1. Phase relations in the pseudoternary system T12 S - A s 2 S 3 - S b 2 S 3 at low temperatures.
Left Conode configuration at 200°C according to experimental results by Grzetic (1991).
Right Conode configuration at 100 °C according to thermochemical approximations
inferred. With respect to natural occurrences, the following phase assemblages, i.e.
phases in contact with each other, have been described unambiguously:
pierrotite-stibnite
weissbergite-stibnite
chabourneite-pierrotite
chabourneite-parapierrotite
bernardite-orpiment
gillulyite-orpiment
Guillemin et al. (1970)
Dickson and Radtke (1978)
Johan et al. (1975)
Johan et al. (1975)
Pasava et al. (1989)
Wilson et al. (1991)
Minerals and calculated low-temperature phase equilibria
283
The weissbergite/stibnite and chabourneite/parapierrotite assemblages do not
accord with experimental results and theoretical considerations. Because the run
products of mixtures containing weissbergite and stibnite show that these phases
can be equilibrated at 200 °C, the non-equilibrium phase assemblages must be
interpreted as the product of either a very low-temperature formation or of an
incomplete reaction due to rapid cooling. The verification of the computed phase
relations will be supported by studies of natural phase relations and the improvement of thermodynamical data. By application of the method by Schenck and
Pardun (1933) the approximated free enthalpies of formation for T1 sulfosalts can
be replaced by measured data.
References
Balic-Zunic T, Scavnicar S, Engel P (1982) The crystal structure of rebulite. Z Krist 160:
109-125
Bente K (1988) Strukturgeometrische Aspekte fiktiver thermodynamischer Daten zur
Prognostik freier Enthalpien. Chem Erde 48:55-60
Bryndzia LT, Kleppa OJ (1988) Standard enthalpies of formation of sulfides and sulfosalts
in the Ag-Bi-S system by high-temperature, direct synthesis calorimetry. Econ Geol 83:
174-181
Crai9 JR, Barton PB (1973) Thermochemical approximations of sulfosalts. Econ Geol 68:
493-506
Dickson FW, Rad~ke AS (1978) Weissbergite, T1SbS2, a new mineral from the Carlin gold
deposit, Nevada. Am Min 63:700-724
Divjakovic V, Nowacki W (1976) Die Kristallstruktur von Imhofit, T15.6As15S25.3. Z Krist
144:323-333
Engel P (1980) Die Kristallstruktur von synthetischem Parapierrotit. Z Krist 151:203-216
- - - - Gostojic M (1983) The crystal structure of pierrotite, T12(Sb, As)loS 6. Z Krist 165:
209-215
Fleet M E (1973) The crystal structure and bonding of lorandite. Z Krist 138: 147-160
Gostojic M (1980) Die Kristallstruktur yon synthetischem Ellisit, T13AsS3. Z Krist 151:
249-254
- - - - Nowacki W, Engel P (1982) The crystal structure of synthetic TISb3S 5. Z Krist 159:
217-224
Guillemin C, Johan Z, Laforet C, Picot P (1970) La pierrotite T12(Sb, As)loS17 une nouvelle
esp6ce min6rale. Bull Soc Fr Min6ral Cristallogr 93:66-71
Johan Z, Mantienne J, Picot P (1981) La chabourneite, un nouveau mineral thallifere. Bull
Mineral 104:10 15
- - - - Picot P, Hak J, Kvacek M (1975) La parapierrotite, un nouveau mineral thallifere
d'Allchar (Yougoslavie). Tschermaks Mineral Petrogr Mitt 22:200-210
Pasava J, Pertlik F, Stumpfl EF, Zeman J (1989) Bernardite, a new thallium arsenic
sulphosalt from Allchar, Macedonia, with a determination of the crystal structure. Min
Mag 53:531-538
Rey N, Jumas JC, Olivier-Fourcade J, Philippot E (1983) Sur le composes III-V-VI: Etude
structurale du disulfure d'antimoine et de thallium, T1SbS2. Acta Cryst C39:971-974
Robie RA, Heminyway BS, Fisher JR (1978) Thermodynamic properties of minerals and
related substances at 298.15 K and 1 bar (105 Pascals) pressure and at higher temperatures. Geol Survey Bull 1452, US Government Printing Office Washington, 456pp
Robinson GR, Haas J L (1983) Heat capacity, relative enthalpy, and calorimetric entropy of
silicate minerals: an empirical method of prediction. Am Min 68:541-553
284
R.J.G. Sobott: Minerals and calculated low-temperature phase equilibria
Schenck R, Pardun H (1933) Untersuchungen /fiber die chemischen Systeme der Lenardphosphore I. Z Anorg Allgem Chem 211:209-221
Sobott R (1981) Die Systeme T13SbSa-T13AsS3 und TISbS2-T1AsS2. Monatsh Chemie 112:
411-414
----(1984) Sulfosalts and T12S-As2 $3-Sb2 S 3 phase relations. N Jb Miner Abh 150:54-59
Wilson JR, Robinson PD, Wilson PN, Stanger LW, Salmon GL (1991) Gillulyite,
T12(As, Sb)8 S 13, a new T1-As sulfosalt from the Mercur gold deposit, Utah. Am Min 76:
653-656
Author's address: Dr. R. Sobott, c/o PREUSSAG AG Zentraltechnikum, Eddesserstrasse 1,
D-31234 Edemissen, Federal Republic of Germany