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The decarbonation and heat capacity of ZnCO3
0016-7037/87/33.00
Geuchimicn rt Co~mochimica Acta Vol. 51, PP. 261-265 0 pecgamoll Journ& Ltd. 1987. Printed in U.S.A.
+ .@a
The decarbonationand heat capacity of ZnC03 H. T. HASELTON* and JULIAN R. ~LDSMIT~** *U.S. Geological Survey, 959 National Center, Reston, VA 22092, U.S.A. **Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637, U.S.A. (Received August 8, 1986; accepted in revisedform November 11, 1986) Abstract-The decarbonation curve for ZnCOs has been determined in the pressure range 3-20 kbar by using a combination of cold-seal vessels and piston-cylinder apparatus with NaCl assemblies. Heat capacities for both synthetic and natural Z&Q samples were measured by ~ff~ti~ scanning calorimetry at temperatures ranging from 340 to 497 K. The results of these experiments indicate that the enthalpy of formation for smithsonite, AH, (1,298.l S), is approximately -8 17. kJ/mol. which is about 4 kJ more negative than most tabulated values. INTRODUCTION HARKER and HUT-IA (1956) determined the P(CO+ T curve for the reaction ZnCOS = ZnO + CO, by
reversed experiments using mixtnres of ZnCO, and ZnO under various pressures of CO, between 0.35 and 4.14 kbar. At 0.69, 2.07, and 3.45 kbar, their curve passes through 3 10,4 10, and 460°C, respectively. The initial part of the present study was carried out at pressures ranging from 6 to 20 kbar in piston-cylinder ap paratus. These new data appeared to be inconsistent with the work of HARKER and HUTTA (1956). To resolve this inconsistency, we extended our study to indude experiments in cold-seal apparatus at 3 kbar and heat capacity measurements (340-497 K} on Z&OS. EXPERIMENTAL PROCEDURES Materials. We used synthetic well-crystallized ZnCOs in most of the experiments. The synthetic carbonate was prepared by Alan Gaines by holding analytical grade basic zinc carbonate (2ZnCOs- 3Zn[OHh) at 2.50°C for 20 hours in a Money bomb; CO* was intr~u~ as dry ice. Hi-tern~mt~ heat capacities were measured for both the synthetic ~i#~nite and for a natural sample (U.S. National Museum of Natural History #108721, Tsumeb Mine, Namibia). The major impurity of the natural material was 0.35 wt.% Cu. Synthetic ZnO (Fisher’, reagent grade) was fired at 1 130°C for several hours. Tbe average grain size of the ZnCO, and ZnO was about 1 firn. Piston-cylinder experiments.For most of these experiments, the anhydrous carbonate was enclosed in Pt capsules pinched shut, but not sealed to permit the escape of CO, if the internal CO, pressure exceeded the conning pressure. Three-quarter inch, NaCl assemblies were used in the piston-cylinder ap paratus; chromel-alumel thermocouples, tipped with a thin film of alundum cement, were in contact with a single platinum capsule. Before heating, the NaCl assemblies were pressurized to a predetermined value based on prior experience; the large thermal expansion of the salt during heating brought the sample to the desired pressure and produced a “pistonout” condition. The sample was initially brought to a temperature 2%100°C below the Enal run temperature and was held for variable lengths of time to eliminate as much adsorbed
’ The use of tradenames is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey.
water from the unsealed capsule as possible. The temperature was then increased while the pressure was bled until the desired conditions were reached. Decarbonation was indicated by the presence of ZnO, identified by its diffraction peaks, (lOi), (002), and ( 100)being the strongest. In the ex~~rnen~ with unsealed capsulea a possibility exists that CO, diffuses out of the capsule ~ntinuo~ly and the CO, pressure is less than Pti. This condition would shit? the univariant boundary to lower temperatures. To check that the decarbonation experiments described above were not conducted in a low PCO, environment, the following experiments were performed. At 15 kbar, two sealed capsules, one containing ZnCOs + Ag&O, and the other one containing ZnO + Ag&Oh, were run in each of two experiments (Zn-16, Zn- 17). The Ag&O, decomposes (Ag metal + CO,) before run unctions are reached providing a CO, atmosphere. At the conclusion of these runs, the capsuleswere weighed pierced to release the COz, and then reweighed. Both the COz loss and X-ray scans were used to determine the direction of maction, Small amounts of AgrCOs were detected in the X-ray scans of these experiments. Some earlier experiments had been carried out by using talc pressure medium (piston-in approach) and, although not used to construct the P-T curve, are included in Table 1 for a “friction” comparison with NaCl. Cold-sealexperiments. For these experiments in standard cold-seal vessels, two perforated gold caps&s, one containing synthetic ZnCO, and the other containing ZnO, were sealed in a larger gold capsule to which Ag&O, had been added. As in the piston-cylinder experiments, the outer capsule was weighed, punctured, and reweighed to determine the extent and direction of reaction. A comparison of the calculated CO, yield from the Ag&O, and the measured loss gave the net CO, lost or gained by the two inner capsules. Though a loss by the ZnCOs capsule and a gain by the ZnO capsule could lead to confusion in some instances, optical or X-ray examination of the products is conclusive. Water was the pressure medium for the cold-seal runs. Temperatures were controlled using external thermocouples which had been previously compared to internal thermocouples. The results of these experiments are also given in Table 1. Dtfirerentialscanning calorimetry.High-temperature heat capacities were measured for both the synthetic and natural samples by using a Perkin Elmer DSC-2 differential scanning calorimeter. The samples were contained in unsealed gold pans and were measured at a heating rate of 1.26 W. Additional details of the DSC measurement procedure are given by HEMINGWAY ef al. (198 1). Preliminaty scan traq at a slower heating rate, of both the natural and synthetic samples showed no evidenceof decomposition,when compared with a standard sapphire trace, until a temperature of 595 K was reached. This temperature is approximately 170 K in excess of the
261
262
H. T. Haselton and J. R. Goldsmith Table
1.
Experimental
P kbar
Run F
W&et-pressure z-101*
T "C
on the decarbonation
Time hr.
Preheat oc
oP
ZnCO3
Preheat hr.
Result
madlum
3
z-102*
data
422
3
447
329
---
20a
0
---
0
zneog-->
znc03
2n0
zno + znc03
-->
znco3-->
NaCl
zno
+ znco3
--> zno
ZnO medium
p~e88w’e
Zn-15
6
503
4.75
4?5
17
All
Zn-Ii
6
520
3.0
475
15
ZnCOj
zn-8
8
563
6.0
525
18
All
Zn-7
8
573
2.0
550
18
ZnC03
+ some
Zn-6
a
599
2.5
575
5
ZnC03
+ strong
Zn-5
13
674
1.75
650
17
Zn-4
13
699
3.0
650
Zn-17s
15
720
3.0
ZnC03
All
ZnO 2n-lb*
15
755
___
2.0
ZnO -__
Zn-1
17
750
2.33
zn-3
I?
775
2.0
700
zn-2
17
800
1.0
700
Zn-9
20
813
1.25
750
ZnCO3
--> 2nCO3
znC53-->
0
+ Zn0
ZnCO3
+ Zn0
--f zn0
All ZnC03
0
All ZnCO3 znc03 All
1.5
+ zno
ZnC03
Zn-10
20
835
1.25
750
17
All
ZnCO3
Zn-13
20
855
1.25
750
4
All
ZnCO3
zn-14
20
875
1.0
750
4
ZnCO3
Talc? pressure
ZnO
+ zno
ZnCO3-->
0
2110
ZnC03
znc03
-__
trace-Zno
+
ZnC03
+ St?On8
ZnO
medium
G-20
10
580
1.0
350
1.0
All
G-25
10
590
1.3
400
1.1
ZnC03
+ some
ZnO
G-15
10
600
0.8
350
0.8
ZnC03
+ some
ZnO
G-42
15
700
1.1
600
1.1
All
G-44
15
720
1.2
600
1.0
ZnC03
G-46
15
730
1.0
600
1.0
ZnO + ZnCO3
in each
axperl8tent.
'Two
saaple
capsules
ww'e
run
calculated 1 bar decarhonation temperature. To avoid spmious thermal effects associated with decomposition, the C, scans were terminated at 500 K.
I
I
ZnCO3 + 96me
ZRO
t
I
I
II-
I /I
16
> < /
znco, b f:
1
f
20
RESULTS
The piston-cylinder data are listed in Table 1, and the pertinent points are plotted in Fii. 1. The data points at 6 and 8 kbar in Fig. 1 are indicated as having larger error bars than the higher pressure points, as discussed below. The P-T curve can be represented as a straight line between about 10 and 20 kbar, having a slope of 4 I bar/K. At lower pressures, we expect visible curvature in the decomposition boundary due to the rapidly changing properties of C& . The opposing arrows at 3 and 15 kbar represent the reversed experiments. The crosses in Fig 1 are points on the curve selected by HARKER and HUTTA( 1956). The C, data are listed in Table 2, and the values of smoothed thermodynamic functions are given in Table 3. The smoothed values were generated from the following C, polynomial:
’
ZnC03
12 /
Y
B
B -
Lf
/
E-
J’
1
zncl+cop
_
4.. r
'/ X
2'
Ftc. 1. P(CO&T curve for tbe reaction Z&O3 = ZnO + CO,, Solid symbols represent bracketing decarbonation experimcnts using unsealed capsules. Tke tips of the arrows represent tbe P-T conditions of rum with sealed capsules containing Ag&O, . The crosses indicate the chosen points from HARKER~~~HuTTA(~~S~).
The decarbonation and heat capacity of ZnC03 Table (scan
2. 1)
High-temperature and
natural
(scan
Heat capacity Jl(mo1 .K)
Temp K
2)
heat
smlthsonite;
Temp.
capacities formula
of
weight
Heat capeclty JI(mo1.K)
K
scan 1
C&J/mol.K)
experimental
263
synthetic -
125.389
Temp.
g/m01
Heat capacity JI(mo1.K)
K
440.3
96.1
379.4
90.1
450.3
97.1
309.2
91.2
340.2
83.9
468.2
98.0
399.0
92.3
350.1
66.2
478.2
96.0
408.8
93.4
359.9
07.2
408.1
99.7
418.6
94.3
369.6
88.7
497.1
100.1
420.4
95.4
379.4
09.4
438.3
96.4
309.2
90.3
448.3
97.5
399.0
91.4
458.3
98.1
400.8
92.3
340.3
85.3
468.2
99.3
418.6
93.2
350.1
86.6
470.2
100.2
420.4
94.4
359.9
88.9
488.1
101 .o
438.3
95.4
369.6
90.1
497.1
101.6
Scan
2
= 148.38 +0.028353 - 1419.T-“‘+4.796
X 105T-’
which was obtained by a least-squares fit to the DSC data, and, at temperatures higher than the measureme& the polynomial was constrained to follow the trend of the C, tkztion given by JACOBSet al. ( 198 1) for calcite. The generated polynomial is well-behaved to at least 1200 K, which is slightly above the highest temperature of the decarbonation experiments. The internal consistency of the thermodynamic and P(COr)-T data was tested by calculating an apparent enthalpy of formation, AH, (1 bar, 298.15 K), for
Table
Temp
3.
Smoothed
thermodynamic
lieat capacity
functiona
Entropy
smithsonite at each of the phase equilibrium by means of the equation
Lwfzncoy 1,298) = AHF”( 1,298) + A&?,-( 1,298) P
T +
s 298
AC&T-
TASr+
for
smlthsonlte
Enthalpy function
Gibbs energy function -(Co-H0 )/T T 298
JI(mo1.K)
298.15
60.05
81.19
0.000
81.19
300
80.29
81.69
0.494
81.20
106.4
21.935
84.47
127.9
36.856
91.10
400
91.77
500
101.0
s I
AV&‘+RTlnfcg
where AC, is the C,, of reaction, ASr is the entropy of reaction at temperature T, and AV, is the volume of reaction for the solids. Sources of thermodynamic data other than the CO2 fugacities are summarized in Table 4. Values of RTlnf(C02) were interpolated from the tables of BOITINGA and RICHET ( 198 1). Figure 2 is a plot of calculated the AHf( 1,298.15) values for smith-
(Ho-H0 j/T T 298 K
brackets
600
108.8
147.1
48.213
98.9
700
115.6
164.3
57.361
106.9
800
121.6
180.2
65.023
115.2
900
127.2
194.8
71.626
123.2
1000
132.3
208.5
77.443
131.1
1100
137.2
221.3
82.655
138.6
1200
141.8
233.5
07.392
146.1
264
H. T. Haselton and J. R. Goldsmith Table 4. Selected carbon dioxide.
thermodynamic
propartles
--_______ Parametef
oP smlthaonlte,
zincite,
~----_I_-----
SmithsOnit&
zineite
Carbon
____AHp(l,298), s2Yg
Diowfde ---__-
_--
kJlmo1
J/mol.K
.* b c
-350.46
-350.51
82.42'
43.16'
213.79
148.382
6.043
87.82
28.355
2.007
-2.644
-14.19
-2.924
-9.9886
d
4.7960
-3.2234
VI ,298 J/bar
2.8275
1.4338
-_-
a x 105/K
3.24
2.06
--_
8 x 106/K
0.825
0.707
___
All data
unless
+Cp(Jlmol-K) data).
2This
4Approximated
indicated
= a + bT study.
otherwise
+ c y 102T-'/2 3Hflla
by expansivity
compressibility
and
or magnetite
(1972)
IS Prom
Roble
+ d x 105Te2. refit
OP mangesite (Chriatsnaen,
et al.
(1978).
lgobie
to join
Robie
(Bayer,
1971).
1972).
7.064
(~~bli§bed
(unpublished
data).
5Approximated
6Taylor
by
(1984).
TEeternan 11962).
sonite from the reaction ZnC03 = ZnO + CO1 at the various pressures. The value listed by ROBIE et al. (1978) is shown. The three P-T points selected by HARKER and HUT-I-A (1956) to represent their determination ofthe deearbonation boundary at 0.69,2.07, and 3.45 kbar CQ are shown by circles and the present data are shown by shaded bars. If the phase ~~b~urn and thermodynamic data are internally consistent, the calculated enthalpy of formation would be the same at all pressures. The major source of uncertainty in the calculation relates to the high pressure properties of CO,. For example, the values of K%f(COz) predicted by the equations of BOTTINGA and RKHET ( 198 1) and KERRICK and JACOES (198 1) differ by about 1.5 kJ/mol at 20 kbar and 875*C. At lower pressures, the equations are better constrained by measured data and the differences between them become smaller. If the same calculation is repeated using values of Klnf(CO,) from KERRICK and JACOLS (1981), the calculated enthalpies of formation become increasingly less negative with increasing pressure. The CO* fugacities calculated from the B~ITINGA and RICHET (198 1) version of the modified Redlich Kwong equation are also more consistent with other d&nation equilibria at pressures greater than 10 kbar (HASELTON et al.(1978). The uncertainties arising from the smithsonite C, extrapolation and from the smithsonite expansivity and comp~bility approximation am probably smaller than those arising from the CO2 fugacity predictions. These uncertainties which are difficult to quantify are not included in Fig. 2. The experiments at pressures less than 10 kbar indicate that the AH,( 1,298.U) should be approximately -817. k.I/ mol, a value that is considerably more negative than the value listed by ROBIEet al. (1978).
A pressure correction has not been applied to the piston-cylinder data. If applied, a correction would shift the P-Tpoints to relatively higher pressures and would produce a discontinuity between the results of pistoncylinder and cold-seal experiments. There is evidence, however, that a pressure correction should be added to our lower pressure piston-cylinder data. In a study of the reaction 2 zoisite + kyanite f quartz = 4 anorthite + H20,
820 '----I
II
I II
81.4 t 81&
-812.78CzS3
, 812M.l 0
Robls atal.(1878) 4
a
I
' 12
' 18
'
' 20
1
Pleaawa,kbar
FIG. 2. Apparent AH? (1,298)for smi~nite
calcuiated
from the reaction ZnCO,, = ZnO + C@ and from additional data as indicated in the text. The circles are the chosen points of HARKER and HUTTA (1956)and the shaded bars represent the present data from cold-seal and piston cylinder experimen% The bars at 3 and 15 kbar represent the reversal runs in cold-seat vessels and piaton*liW ilppsratus, RSp%tiVety. If all of the data were internally con&tent, they c&d be connected by a ho&_onM line which would agne, within error, with the suggested value of IioBIEetal.(1978).
The decarbonation and heat capacity of ZnCO, using internally-heated argon vessels and piston-eylinder apparatus having %” and 1” NaCl assemblies, JENRINS et al. (1985) found #at the piston~y~nder results yielded pressures that were about 400 bars too high. The P-T regime of their study is close to that of the present work. Acknowledgements-This
research was partiaUy supported by
National Science Foundation grant EAR-8305904 We are indebted to AIan Gaines for the synthetic Z&O3 and to Richard A. Robie and Bruce S. Hemin~ay for discussions, the natural smi~~nite sample, and use of the DSC. We thank John L. Haas, Jr. for a simultaneous fit of the data with his program PHAS20. We greatly appreciate the critical reviews of I-Ming Chou, David M. Jenkins, Richard J. Reeder, and Richard A. Robie. Editorial handling: P. C. Hess RECENT BATEMANT. B. (1962) Elastic moduli of single-crystal zinc
oxide. J. Appl. Phys. 33, 3309-3312. BAYERG. (1971) Thermal expansion anisotropy ofdolomite-
type berates Me2+Mec+Bz0,.Zeit. Krist. 133, 85-90. BOTIINGA Y. and R~CHETP. ( 1981) High pressure and tem-
perature equation of state and calculations of the thermodynamic properties of gaseous carbon dioxide. Amer. J. Sci. 281,615-660. ~~~N~N N. I. (1972) Elastic propertiesof ~ly~ne
265
magnesium, iron, and manganese carbonates to 10 kiIobars. J. Geophys.Res. 77,369-372. HARKERR. I. and HUT~A J. J. (1956) The stability of smithsonite. Ectm. Geol. 51,375-38 1. HASELTONH. T. JR., SHARPW. E. and NEWTONR. C. f 1978) CO, fugacity at high temperatures and pressures from experimental decarbonation reactions. Geophys.Rex L.ett.5, 753-756. HEMINGWAYB. S., KRUPKA K. M. and Rolam R. A. (1981) Heat capacities of alkali feldspars between 350 and 1000 K from differential scanning calorimetry, the thermodynamic functions of the alkali feldspars from 298.15 to 1400 K, and the reaction quartz + jade&e = albite. Amer. ~~n~~. 66, 1202-1215. JACOBSG. K., KERRICKD. M. and KRUPKA K. M. (1981) The high-temperature heat capacity of natural calcite (CaCOr). Phys. C/rem. Minerals 7,55-59. JENKINSD. M., NEWTONR. C. and GOLDSMITHJ. R. (1985) Relative stability of Fe-free zoisite and clinozoisite. J. Geol. 93,663-612. _ KEWCK D. M. and JACOBSG. D. (198 1) A modifiedRed&hKwong equation for H20, CO,, and H&)-CO2 mixtures at elevated pressures and temperatures Amer. J. Sci. 281,735767. MILLSK. C. (1972) The heat capacities of Gaz03(c), TI,O,(c), ZnO(c), and CdO(c). High Temp. High Pres. 4,37 l-377. ROBIE R. A., HEMINGWAYB. S. and RSHER J. R. (1978) Thermodynamic properties of minerals and related sub stances at 298.15 K and 1 bar ( 10J pas&s) pressure and at higher temperatures. U.S. Geol. Surv. Bull. 1452. 456 p. TAYLORD. (1984) Thermal expansion data: I. Binary oxides with the sodium chloride and wurtzite structures, MO. Brit. Ceram. Sot., Trims. 83,5-9.
Geuchimicn rt Co~mochimica Acta Vol. 51, PP. 261-265 0 pecgamoll Journ& Ltd. 1987. Printed in U.S.A.
+ .@a
The decarbonationand heat capacity of ZnC03 H. T. HASELTON* and JULIAN R. ~LDSMIT~** *U.S. Geological Survey, 959 National Center, Reston, VA 22092, U.S.A. **Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637, U.S.A. (Received August 8, 1986; accepted in revisedform November 11, 1986) Abstract-The decarbonation curve for ZnCOs has been determined in the pressure range 3-20 kbar by using a combination of cold-seal vessels and piston-cylinder apparatus with NaCl assemblies. Heat capacities for both synthetic and natural Z&Q samples were measured by ~ff~ti~ scanning calorimetry at temperatures ranging from 340 to 497 K. The results of these experiments indicate that the enthalpy of formation for smithsonite, AH, (1,298.l S), is approximately -8 17. kJ/mol. which is about 4 kJ more negative than most tabulated values. INTRODUCTION HARKER and HUT-IA (1956) determined the P(CO+ T curve for the reaction ZnCOS = ZnO + CO, by
reversed experiments using mixtnres of ZnCO, and ZnO under various pressures of CO, between 0.35 and 4.14 kbar. At 0.69, 2.07, and 3.45 kbar, their curve passes through 3 10,4 10, and 460°C, respectively. The initial part of the present study was carried out at pressures ranging from 6 to 20 kbar in piston-cylinder ap paratus. These new data appeared to be inconsistent with the work of HARKER and HUTTA (1956). To resolve this inconsistency, we extended our study to indude experiments in cold-seal apparatus at 3 kbar and heat capacity measurements (340-497 K} on Z&OS. EXPERIMENTAL PROCEDURES Materials. We used synthetic well-crystallized ZnCOs in most of the experiments. The synthetic carbonate was prepared by Alan Gaines by holding analytical grade basic zinc carbonate (2ZnCOs- 3Zn[OHh) at 2.50°C for 20 hours in a Money bomb; CO* was intr~u~ as dry ice. Hi-tern~mt~ heat capacities were measured for both the synthetic ~i#~nite and for a natural sample (U.S. National Museum of Natural History #108721, Tsumeb Mine, Namibia). The major impurity of the natural material was 0.35 wt.% Cu. Synthetic ZnO (Fisher’, reagent grade) was fired at 1 130°C for several hours. Tbe average grain size of the ZnCO, and ZnO was about 1 firn. Piston-cylinder experiments.For most of these experiments, the anhydrous carbonate was enclosed in Pt capsules pinched shut, but not sealed to permit the escape of CO, if the internal CO, pressure exceeded the conning pressure. Three-quarter inch, NaCl assemblies were used in the piston-cylinder ap paratus; chromel-alumel thermocouples, tipped with a thin film of alundum cement, were in contact with a single platinum capsule. Before heating, the NaCl assemblies were pressurized to a predetermined value based on prior experience; the large thermal expansion of the salt during heating brought the sample to the desired pressure and produced a “pistonout” condition. The sample was initially brought to a temperature 2%100°C below the Enal run temperature and was held for variable lengths of time to eliminate as much adsorbed
’ The use of tradenames is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey.
water from the unsealed capsule as possible. The temperature was then increased while the pressure was bled until the desired conditions were reached. Decarbonation was indicated by the presence of ZnO, identified by its diffraction peaks, (lOi), (002), and ( 100)being the strongest. In the ex~~rnen~ with unsealed capsulea a possibility exists that CO, diffuses out of the capsule ~ntinuo~ly and the CO, pressure is less than Pti. This condition would shit? the univariant boundary to lower temperatures. To check that the decarbonation experiments described above were not conducted in a low PCO, environment, the following experiments were performed. At 15 kbar, two sealed capsules, one containing ZnCOs + Ag&O, and the other one containing ZnO + Ag&Oh, were run in each of two experiments (Zn-16, Zn- 17). The Ag&O, decomposes (Ag metal + CO,) before run unctions are reached providing a CO, atmosphere. At the conclusion of these runs, the capsuleswere weighed pierced to release the COz, and then reweighed. Both the COz loss and X-ray scans were used to determine the direction of maction, Small amounts of AgrCOs were detected in the X-ray scans of these experiments. Some earlier experiments had been carried out by using talc pressure medium (piston-in approach) and, although not used to construct the P-T curve, are included in Table 1 for a “friction” comparison with NaCl. Cold-sealexperiments. For these experiments in standard cold-seal vessels, two perforated gold caps&s, one containing synthetic ZnCO, and the other containing ZnO, were sealed in a larger gold capsule to which Ag&O, had been added. As in the piston-cylinder experiments, the outer capsule was weighed, punctured, and reweighed to determine the extent and direction of reaction. A comparison of the calculated CO, yield from the Ag&O, and the measured loss gave the net CO, lost or gained by the two inner capsules. Though a loss by the ZnCOs capsule and a gain by the ZnO capsule could lead to confusion in some instances, optical or X-ray examination of the products is conclusive. Water was the pressure medium for the cold-seal runs. Temperatures were controlled using external thermocouples which had been previously compared to internal thermocouples. The results of these experiments are also given in Table 1. Dtfirerentialscanning calorimetry.High-temperature heat capacities were measured for both the synthetic and natural samples by using a Perkin Elmer DSC-2 differential scanning calorimeter. The samples were contained in unsealed gold pans and were measured at a heating rate of 1.26 W. Additional details of the DSC measurement procedure are given by HEMINGWAY ef al. (198 1). Preliminaty scan traq at a slower heating rate, of both the natural and synthetic samples showed no evidenceof decomposition,when compared with a standard sapphire trace, until a temperature of 595 K was reached. This temperature is approximately 170 K in excess of the
261
262
H. T. Haselton and J. R. Goldsmith Table
1.
Experimental
P kbar
Run F
W&et-pressure z-101*
T "C
on the decarbonation
Time hr.
Preheat oc
oP
ZnCO3
Preheat hr.
Result
madlum
3
z-102*
data
422
3
447
329
---
20a
0
---
0
zneog-->
znc03
2n0
zno + znc03
-->
znco3-->
NaCl
zno
+ znco3
--> zno
ZnO medium
p~e88w’e
Zn-15
6
503
4.75
4?5
17
All
Zn-Ii
6
520
3.0
475
15
ZnCOj
zn-8
8
563
6.0
525
18
All
Zn-7
8
573
2.0
550
18
ZnC03
+ some
Zn-6
a
599
2.5
575
5
ZnC03
+ strong
Zn-5
13
674
1.75
650
17
Zn-4
13
699
3.0
650
Zn-17s
15
720
3.0
ZnC03
All
ZnO 2n-lb*
15
755
___
2.0
ZnO -__
Zn-1
17
750
2.33
zn-3
I?
775
2.0
700
zn-2
17
800
1.0
700
Zn-9
20
813
1.25
750
ZnCO3
--> 2nCO3
znC53-->
0
+ Zn0
ZnCO3
+ Zn0
--f zn0
All ZnC03
0
All ZnCO3 znc03 All
1.5
+ zno
ZnC03
Zn-10
20
835
1.25
750
17
All
ZnCO3
Zn-13
20
855
1.25
750
4
All
ZnCO3
zn-14
20
875
1.0
750
4
ZnCO3
Talc? pressure
ZnO
+ zno
ZnCO3-->
0
2110
ZnC03
znc03
-__
trace-Zno
+
ZnC03
+ St?On8
ZnO
medium
G-20
10
580
1.0
350
1.0
All
G-25
10
590
1.3
400
1.1
ZnC03
+ some
ZnO
G-15
10
600
0.8
350
0.8
ZnC03
+ some
ZnO
G-42
15
700
1.1
600
1.1
All
G-44
15
720
1.2
600
1.0
ZnC03
G-46
15
730
1.0
600
1.0
ZnO + ZnCO3
in each
axperl8tent.
'Two
saaple
capsules
ww'e
run
calculated 1 bar decarhonation temperature. To avoid spmious thermal effects associated with decomposition, the C, scans were terminated at 500 K.
I
I
ZnCO3 + 96me
ZRO
t
I
I
II-
I /I
16
> < /
znco, b f:
1
f
20
RESULTS
The piston-cylinder data are listed in Table 1, and the pertinent points are plotted in Fii. 1. The data points at 6 and 8 kbar in Fig. 1 are indicated as having larger error bars than the higher pressure points, as discussed below. The P-T curve can be represented as a straight line between about 10 and 20 kbar, having a slope of 4 I bar/K. At lower pressures, we expect visible curvature in the decomposition boundary due to the rapidly changing properties of C& . The opposing arrows at 3 and 15 kbar represent the reversed experiments. The crosses in Fig 1 are points on the curve selected by HARKER and HUTTA( 1956). The C, data are listed in Table 2, and the values of smoothed thermodynamic functions are given in Table 3. The smoothed values were generated from the following C, polynomial:
’
ZnC03
12 /
Y
B
B -
Lf
/
E-
J’
1
zncl+cop
_
4.. r
'/ X
2'
Ftc. 1. P(CO&T curve for tbe reaction Z&O3 = ZnO + CO,, Solid symbols represent bracketing decarbonation experimcnts using unsealed capsules. Tke tips of the arrows represent tbe P-T conditions of rum with sealed capsules containing Ag&O, . The crosses indicate the chosen points from HARKER~~~HuTTA(~~S~).
The decarbonation and heat capacity of ZnC03 Table (scan
2. 1)
High-temperature and
natural
(scan
Heat capacity Jl(mo1 .K)
Temp K
2)
heat
smlthsonite;
Temp.
capacities formula
of
weight
Heat capeclty JI(mo1.K)
K
scan 1
C&J/mol.K)
experimental
263
synthetic -
125.389
Temp.
g/m01
Heat capacity JI(mo1.K)
K
440.3
96.1
379.4
90.1
450.3
97.1
309.2
91.2
340.2
83.9
468.2
98.0
399.0
92.3
350.1
66.2
478.2
96.0
408.8
93.4
359.9
07.2
408.1
99.7
418.6
94.3
369.6
88.7
497.1
100.1
420.4
95.4
379.4
09.4
438.3
96.4
309.2
90.3
448.3
97.5
399.0
91.4
458.3
98.1
400.8
92.3
340.3
85.3
468.2
99.3
418.6
93.2
350.1
86.6
470.2
100.2
420.4
94.4
359.9
88.9
488.1
101 .o
438.3
95.4
369.6
90.1
497.1
101.6
Scan
2
= 148.38 +0.028353 - 1419.T-“‘+4.796
X 105T-’
which was obtained by a least-squares fit to the DSC data, and, at temperatures higher than the measureme& the polynomial was constrained to follow the trend of the C, tkztion given by JACOBSet al. ( 198 1) for calcite. The generated polynomial is well-behaved to at least 1200 K, which is slightly above the highest temperature of the decarbonation experiments. The internal consistency of the thermodynamic and P(COr)-T data was tested by calculating an apparent enthalpy of formation, AH, (1 bar, 298.15 K), for
Table
Temp
3.
Smoothed
thermodynamic
lieat capacity
functiona
Entropy
smithsonite at each of the phase equilibrium by means of the equation
Lwfzncoy 1,298) = AHF”( 1,298) + A&?,-( 1,298) P
T +
s 298
AC&T-
TASr+
for
smlthsonlte
Enthalpy function
Gibbs energy function -(Co-H0 )/T T 298
JI(mo1.K)
298.15
60.05
81.19
0.000
81.19
300
80.29
81.69
0.494
81.20
106.4
21.935
84.47
127.9
36.856
91.10
400
91.77
500
101.0
s I
AV&‘+RTlnfcg
where AC, is the C,, of reaction, ASr is the entropy of reaction at temperature T, and AV, is the volume of reaction for the solids. Sources of thermodynamic data other than the CO2 fugacities are summarized in Table 4. Values of RTlnf(C02) were interpolated from the tables of BOITINGA and RICHET ( 198 1). Figure 2 is a plot of calculated the AHf( 1,298.15) values for smith-
(Ho-H0 j/T T 298 K
brackets
600
108.8
147.1
48.213
98.9
700
115.6
164.3
57.361
106.9
800
121.6
180.2
65.023
115.2
900
127.2
194.8
71.626
123.2
1000
132.3
208.5
77.443
131.1
1100
137.2
221.3
82.655
138.6
1200
141.8
233.5
07.392
146.1
264
H. T. Haselton and J. R. Goldsmith Table 4. Selected carbon dioxide.
thermodynamic
propartles
--_______ Parametef
oP smlthaonlte,
zincite,
~----_I_-----
SmithsOnit&
zineite
Carbon
____AHp(l,298), s2Yg
Diowfde ---__-
_--
kJlmo1
J/mol.K
.* b c
-350.46
-350.51
82.42'
43.16'
213.79
148.382
6.043
87.82
28.355
2.007
-2.644
-14.19
-2.924
-9.9886
d
4.7960
-3.2234
VI ,298 J/bar
2.8275
1.4338
-_-
a x 105/K
3.24
2.06
--_
8 x 106/K
0.825
0.707
___
All data
unless
+Cp(Jlmol-K) data).
2This
4Approximated
indicated
= a + bT study.
otherwise
+ c y 102T-'/2 3Hflla
by expansivity
compressibility
and
or magnetite
(1972)
IS Prom
Roble
+ d x 105Te2. refit
OP mangesite (Chriatsnaen,
et al.
(1978).
lgobie
to join
Robie
(Bayer,
1971).
1972).
7.064
(~~bli§bed
(unpublished
data).
5Approximated
6Taylor
by
(1984).
TEeternan 11962).
sonite from the reaction ZnC03 = ZnO + CO1 at the various pressures. The value listed by ROBIE et al. (1978) is shown. The three P-T points selected by HARKER and HUT-I-A (1956) to represent their determination ofthe deearbonation boundary at 0.69,2.07, and 3.45 kbar CQ are shown by circles and the present data are shown by shaded bars. If the phase ~~b~urn and thermodynamic data are internally consistent, the calculated enthalpy of formation would be the same at all pressures. The major source of uncertainty in the calculation relates to the high pressure properties of CO,. For example, the values of K%f(COz) predicted by the equations of BOTTINGA and RKHET ( 198 1) and KERRICK and JACOES (198 1) differ by about 1.5 kJ/mol at 20 kbar and 875*C. At lower pressures, the equations are better constrained by measured data and the differences between them become smaller. If the same calculation is repeated using values of Klnf(CO,) from KERRICK and JACOLS (1981), the calculated enthalpies of formation become increasingly less negative with increasing pressure. The CO* fugacities calculated from the B~ITINGA and RICHET (198 1) version of the modified Redlich Kwong equation are also more consistent with other d&nation equilibria at pressures greater than 10 kbar (HASELTON et al.(1978). The uncertainties arising from the smithsonite C, extrapolation and from the smithsonite expansivity and comp~bility approximation am probably smaller than those arising from the CO2 fugacity predictions. These uncertainties which are difficult to quantify are not included in Fig. 2. The experiments at pressures less than 10 kbar indicate that the AH,( 1,298.U) should be approximately -817. k.I/ mol, a value that is considerably more negative than the value listed by ROBIEet al. (1978).
A pressure correction has not been applied to the piston-cylinder data. If applied, a correction would shift the P-Tpoints to relatively higher pressures and would produce a discontinuity between the results of pistoncylinder and cold-seal experiments. There is evidence, however, that a pressure correction should be added to our lower pressure piston-cylinder data. In a study of the reaction 2 zoisite + kyanite f quartz = 4 anorthite + H20,
820 '----I
II
I II
81.4 t 81&
-812.78CzS3
, 812M.l 0
Robls atal.(1878) 4
a
I
' 12
' 18
'
' 20
1
Pleaawa,kbar
FIG. 2. Apparent AH? (1,298)for smi~nite
calcuiated
from the reaction ZnCO,, = ZnO + C@ and from additional data as indicated in the text. The circles are the chosen points of HARKER and HUTTA (1956)and the shaded bars represent the present data from cold-seal and piston cylinder experimen% The bars at 3 and 15 kbar represent the reversal runs in cold-seat vessels and piaton*liW ilppsratus, RSp%tiVety. If all of the data were internally con&tent, they c&d be connected by a ho&_onM line which would agne, within error, with the suggested value of IioBIEetal.(1978).
The decarbonation and heat capacity of ZnCO, using internally-heated argon vessels and piston-eylinder apparatus having %” and 1” NaCl assemblies, JENRINS et al. (1985) found #at the piston~y~nder results yielded pressures that were about 400 bars too high. The P-T regime of their study is close to that of the present work. Acknowledgements-This
research was partiaUy supported by
National Science Foundation grant EAR-8305904 We are indebted to AIan Gaines for the synthetic Z&O3 and to Richard A. Robie and Bruce S. Hemin~ay for discussions, the natural smi~~nite sample, and use of the DSC. We thank John L. Haas, Jr. for a simultaneous fit of the data with his program PHAS20. We greatly appreciate the critical reviews of I-Ming Chou, David M. Jenkins, Richard J. Reeder, and Richard A. Robie. Editorial handling: P. C. Hess RECENT BATEMANT. B. (1962) Elastic moduli of single-crystal zinc
oxide. J. Appl. Phys. 33, 3309-3312. BAYERG. (1971) Thermal expansion anisotropy ofdolomite-
type berates Me2+Mec+Bz0,.Zeit. Krist. 133, 85-90. BOTIINGA Y. and R~CHETP. ( 1981) High pressure and tem-
perature equation of state and calculations of the thermodynamic properties of gaseous carbon dioxide. Amer. J. Sci. 281,615-660. ~~~N~N N. I. (1972) Elastic propertiesof ~ly~ne
265
magnesium, iron, and manganese carbonates to 10 kiIobars. J. Geophys.Res. 77,369-372. HARKERR. I. and HUT~A J. J. (1956) The stability of smithsonite. Ectm. Geol. 51,375-38 1. HASELTONH. T. JR., SHARPW. E. and NEWTONR. C. f 1978) CO, fugacity at high temperatures and pressures from experimental decarbonation reactions. Geophys.Rex L.ett.5, 753-756. HEMINGWAYB. S., KRUPKA K. M. and Rolam R. A. (1981) Heat capacities of alkali feldspars between 350 and 1000 K from differential scanning calorimetry, the thermodynamic functions of the alkali feldspars from 298.15 to 1400 K, and the reaction quartz + jade&e = albite. Amer. ~~n~~. 66, 1202-1215. JACOBSG. K., KERRICKD. M. and KRUPKA K. M. (1981) The high-temperature heat capacity of natural calcite (CaCOr). Phys. C/rem. Minerals 7,55-59. JENKINSD. M., NEWTONR. C. and GOLDSMITHJ. R. (1985) Relative stability of Fe-free zoisite and clinozoisite. J. Geol. 93,663-612. _ KEWCK D. M. and JACOBSG. D. (198 1) A modifiedRed&hKwong equation for H20, CO,, and H&)-CO2 mixtures at elevated pressures and temperatures Amer. J. Sci. 281,735767. MILLSK. C. (1972) The heat capacities of Gaz03(c), TI,O,(c), ZnO(c), and CdO(c). High Temp. High Pres. 4,37 l-377. ROBIE R. A., HEMINGWAYB. S. and RSHER J. R. (1978) Thermodynamic properties of minerals and related sub stances at 298.15 K and 1 bar ( 10J pas&s) pressure and at higher temperatures. U.S. Geol. Surv. Bull. 1452. 456 p. TAYLORD. (1984) Thermal expansion data: I. Binary oxides with the sodium chloride and wurtzite structures, MO. Brit. Ceram. Sot., Trims. 83,5-9.