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The gypsum-anhydrite equilibrium by solubility measurements
The gypsum-anhydrite
equilibrium by solubility measurements G. INNORTA
Istituto Chlmico
Ciamician,
University
of Bologna,
via Selmi 2. 40127 Bologna,
Italy
E. RABBI Istituto
di Geologia
e Paleontolgia
dell’Universit&
via Zamboni
67. 40127 Bologna,
Italy
and L. T~MADIN
Laboratorio di Geologia
Marina
del Consiglio
Nazionale
delle Ricerche,
wa Zamboni
65, 40127 Bologna,
Italy
Abstract-Solubility measurements have been used to establish the gypsum-anhydrite equilibrium in the CaSO,-H,O system at atmospheric pressure. The saturation equilibrium has been approached both from undersaturated and supersaturated solutions. The invariant point temperature has been found to be 49.5 k 2.5 C
(1) Calcium sulphate (Merck, Ar. 2161). (2) Selenitic gypsum (natural, from Borgo Tossignano. Bologna, Italy). (3) Selenitlc gypsum (natural, from Farento. Bologna, Italy). (4) Anhydrite [obtained from gypsum (1) by heating at 500°C for 3 hrl (5) Crystalline anhqdrite (natural. from Costa Volplno. Bergamo, Italy).
INTRODUCTION
SINCE the pioneering work by VANT‘HOFF (1903), the CaS04~-H20 system has received much attention: this first work stated the equilibrium point of the system at 63.5’C. Subsequent measurements, based on the water solubility determination of gypsum and anhydrite, led to a transition temperature of about 40°C (POSNJAK, 1938, 1940). This value was generally accepted to 1966. Reviews of this problem were given by ZEN (1965) and by HARDIE (1967) with appropriate references. A method of indirect deduction was used by HARDIE (1967): the value found for the transition temperature (58 2 2 C) caused a renewed interest in this subject. Since then, there have been claims for a transition temperature around 40 C (D’ANs, 1968; CRUFT and CHAO. 1970: GRIWRIEV and SHAMAEV, 1976), as well as for a transition temperature near that measured by Hardie (BLOUNT and DICKSON, 1973; KNACKE and GANS, 1977). A new experimental determination of the transition temperature is clearly needed. Direct measurements of gypsum and anhydrite solubility was carried out for this purpose. Some of the previous measurements of this kind have been criticized because the equilibrium solubility was approached from the undersaturation side only. In this work we attempt to approach the saturation equilibrium both from UnderSaturated and supersaturated solutions. To accomplish this. a solution in the presence of anhydrite or gypsum was gradually heated; solubility was measured continuously. At a higher temperature the solution was cooled, with continued measurement of the solubility. which proved to be reversible. EXPERIMENTAL The folIowIng work
:
materials
MATERIALS
hake been used throughout
the
The experimental substances were examined by powder X-Ray diffraction using filtered CuKa radiation. The gypsum (1). (2) and (3) gave powder spectra whose d-spacings were in agreement with Card 6-0046 of the Joint Committee on Powder Diffraction Standards. The anhydrite spectra corresponded to the listing of Card 6-0226. Intensities as well as d-spacings were generally in agreement with the J.C.P.D.S. Cards. Some minor intensity-differences, sometimes observed, can be ascribed. as usual. to the powder packing, to variable crystallinity of the mineral samples, etc. The materials used in the dissolution experiments have the following unit cell parameters: Gypsum : u = 5.6774 b = 15.1872 (’ = 6.5274 2 =7=90 fi = 118.39’
f 0.07 A i 0.04 a _t 0.07 A * 0.05
Anhydrite: tl = 6.2389 k 0.06 A h = 6.9928 k 0.06 h c’ = 6.9920 + 0.05 A sc=/j=;‘=9() The medium particle size is 0.7 mm for gypsum and 0.1 mm for anhydrite. To check the composition of the solid phase during the solubllity measurements, a work diagram for gypsum (1) anhydrite (4) mixtures was prepared. This diagram allowed us to ascertain the composition of the solid phase down to 0.05”,, content of one of the two components. The diagram (Fig. I ) has been obtained by plotting log /I 3.49 A (Anhydrite);/? 4.27 A (Gypsum) vs the anhydrite content. ‘The peaks heights have been measured in the following conditlons:
1931
1932 Table I. Solubihty of gypsum in pure water (1 atm pressure) % CaSO4
tot
0.208 0.208 0.210 0.199 0.194 0.192
25 30 45 50.5 55 60.5
Data are in ?,CaS04 in solution. All values reported in Tables I, 2, 3 are averages of at least three measurements.
corresponded to that of gypsum, solution kinetics of this material.
RESULTS
I
40
20
60
80
ANHYDRl
100 “lo TE
Fig. 1. Calibration curve for X-ray diffraction determination of gypsum and anhydrite contents in their mixtures.
20(CuKr Gypsum Anhydrite
20.80 25.53
Ni)
(i(A)
h/,/
4.21 3.49
l2i 020
Gypsum (1) and anhydrite (4) were used for the solubility measurements. The crystalline form of these two materials is shown in Fig. 2. SOLUBILITY
MEASUREMENTS
Solubility measurements have been performed with the aid of the apparatus shown in Fig. 3. Gypsum or anhydrite were placed in the container with bi-distilled water; the system was stirred mechanically (60 rpm) and the temperature control, within +O.l C, was achieved by water circulation in an external Jacket. Two reactors operated simultaneously and the temperature of the solutions within the two reactors did not differ by more than 0.3 C at the higher temperatures (> 50-C). Small amounts of the solution were sucked through a sintered glass septum to avoid the presence of solid particles. The Ca content of the solution, after dilution, was determined by atomic absorption spectrophotometry with a Perkin-Elmer Model 603 instrument. For each determination, the temperature of the system was kept constant until successive readings gave constant concentration (the longest runs of about 1 week’s duration were necessary for anhydrite). After saturation was confirmed, the sample was checked by X-ray diffraction. Data were collected, and henceforth accepted, hhen there was no variation of the solid phase within the limit of the analytical curve of Fig. 1; they thus refer to the solubility of the initial substance even if in a metastable region. Sometimes variations in the solid phase (both from gypsum to anhydrite and, more frequently, from anhydrite to gypsum) were observed; in these cases, the measured solubility
AND
because
of the faster dis-
DISCUSSION
A first set of measurements was performed by measuring the gypsum and anhydrite solubility separately. In this case, the same material was present in the two reactors; this allowed also an evaluation of the experimental error, which was usually less than value. The results of these S”, of the measured measurements are reported in Table 1 for gypsum and Table 2 for anhydrite. The order in the tables is the same of the temperature variation along the measurements; it is noteworthy that, after inversion of the temperature variation direction, the solubility data for anhydrite were in agreement with those taken in the runs with decreasing temperature. From inspection of Fig. 4, it is apparent that the equilibrium temperature must be substantially lower than 58°C as reported by HAKDIE (1967); however, these data alone do not allow precise determination of the equilibrium temperature. To reduce the errors attributable to temperature differences or to analytical procedure which are possible in long lasting experiments, gypsum was placed in a reactor and anhydrite in an other one. This allowed the simultaneous measurement of the solubility of the two substances. The results of this second set of measurements are reported in Table 3, where the data are presented in the same order in which they have been taken. The solubility variation was therefore studied, for the two materials, both from undersaturated and supersaturated solutions.
Table
2. Solubility of anhydrite pure water (1 atm pressure)
tOc
% CaS04
60.5 59.5 53 52.5 55 57
0.174 0.174 0.190 0.189 0.185 0.180
in
Fig. 2. Electron microscope photomicrographs of crystals of gypsum (1) showing the typical monocline form and of anhydrite (4) pseudomorph on gypsum (Photograph by V. LANDUZZI).
Fig. 3. The experimental
apparatus
used for the solubility
I934
measurements
(Photograph
by P. FERRIERI).
The gypsum-
anhydrite
equilibrium
by solubility
1935
measurements
0.20 -
0.19 -
0.18 -
a
0.16 -
50
40 Fig. 4. Solubility
Table 3. Solubility
of gypsum
of gypsum
(g) and anhydrite
and anhydrite
% CaSOr
% CaSOk
(anhydrite)
( gyps urn)
60
(a) in water as a function
64.5 57.5 56.0 54.0
0.187 0.188 0.196 0.194
0.163 0.177 0.187 0.185
[correlation
52.0
0.195
0.192
drite the corresponding
39.5
0.207
0.225
45.0 51.5
0.203 0.196
0.208 0.194
s = 8.69’ lo-‘t2
40
a correlation
50
- 1.73. lo-“t
equation -
equal
(1) anhy-
is:
3.667.10m3r
coefficient
60
+ 0.2624
= 0.95151 and for the
coefficient
s = 1.14.10-5t2 with
30
of the temperature
The experimental data have been fitted, by means of a least square procedure, to a second degree relationship for solubility, s. in wt”f(; CaS04, as a function of temperature, f. For gypsum, the relation is:
in pure water (1 atm pressure) tot
t°C
+ 0.3517 to 0.9938.
t’C
Fig. 5. Solubility of gypsum (g) and anhydrite (a) in water as a function of the temperature. The continuous lines are the equilibrium curves calculated according eqns (1) and (2). The discontinuous lines are the relative standard deviations.
(2)
1936
G. INN~RTAet crl.
The standard deviation of the observations is 0.00206 for gypsum and 0.0022 for anhydrite. The results are summarized in Fig. 5 where the continuous lines are the eqns (1) and (2); the polygon of error is defined by standard deviation. According to these data the equilibrium temperature is 49.5 & 2.5 ‘C. It is hard to find a satisfactory explanation for the difference between the present data and literature values for the equilibrium temperature since the crystallinity and grain dimensions of our samples are comparable with those of the samples used by several workers. We wish to note that no extrapolation procedure has been used throught the work and, more important. the solubility measurements proved to be reversible with respect to the direction of temperature variation.
REFERENCES
BLOUNTC. W. and DICKSONF. W. (1973) Gypsum-Anhydrite equilibria in systems CaSO,- Hz0 and CaSO,--NaCI-H,O. Am. Minercll. 58, 3233331. CRUFTE. F. and CHAOP. C. (1970) Nucleation kinetics of the gypsum-anhydrite system. 3rd Symp. Salt, 1969, pp. 109-l 18. D’ANs S. (1968) Der cbergangspunkt GipssAnhydrite. Kali Steins& I, 17-38. GRIGOR’EVA. P. and SHAMAEVP. P. (1976) Determination of gypsum-anhydrite equilibrium temperature. IX. Sib. Otd. A/ml. Nuuk SSSR. Ser. Khim. NM.& 104. HARDIEL. A. (1967) The gypsum-anhydrite equilibrium at one atmosphere pressure. Am. Minrrul. 52, 171~200. HOFF S. H. VAN’T,ARMSTRONC; E. F.. HINRICHESENW., WEI~ERTF. and JUST G. (1903) Gips and anhydrite. Z. Phys. Chem. 45, 257-306.
KNAKE0. and GAYSW. (1977) The thermodynamics system CaSO,--H,O. Z. Phys. Chew. 104, 41 48. A~,l\nn~~lrdgrmenr.sThe authors are indebted to ProPOSNJAKE. (1938) The system CaSO,-HZO. 4m. fessors E. BONATTI,A. CASTELLARIN and F. Rtcc~ LUCCHI 35A, 2477272. for the critical reading of the manuscript and for helpful ZEN E.-AN. (1965) Solubility measurements in the discussions, and to Mr A. PINI for the determination of the CaSO,-NaCl-HZ0 at 35’, 50 and 70 C and one unit cell parameters. phere pressure. J. Prtrol. 6, 124-164.
of the J. %i. system atmos-
equilibrium by solubility measurements G. INNORTA
Istituto Chlmico
Ciamician,
University
of Bologna,
via Selmi 2. 40127 Bologna,
Italy
E. RABBI Istituto
di Geologia
e Paleontolgia
dell’Universit&
via Zamboni
67. 40127 Bologna,
Italy
and L. T~MADIN
Laboratorio di Geologia
Marina
del Consiglio
Nazionale
delle Ricerche,
wa Zamboni
65, 40127 Bologna,
Italy
Abstract-Solubility measurements have been used to establish the gypsum-anhydrite equilibrium in the CaSO,-H,O system at atmospheric pressure. The saturation equilibrium has been approached both from undersaturated and supersaturated solutions. The invariant point temperature has been found to be 49.5 k 2.5 C
(1) Calcium sulphate (Merck, Ar. 2161). (2) Selenitic gypsum (natural, from Borgo Tossignano. Bologna, Italy). (3) Selenitlc gypsum (natural, from Farento. Bologna, Italy). (4) Anhydrite [obtained from gypsum (1) by heating at 500°C for 3 hrl (5) Crystalline anhqdrite (natural. from Costa Volplno. Bergamo, Italy).
INTRODUCTION
SINCE the pioneering work by VANT‘HOFF (1903), the CaS04~-H20 system has received much attention: this first work stated the equilibrium point of the system at 63.5’C. Subsequent measurements, based on the water solubility determination of gypsum and anhydrite, led to a transition temperature of about 40°C (POSNJAK, 1938, 1940). This value was generally accepted to 1966. Reviews of this problem were given by ZEN (1965) and by HARDIE (1967) with appropriate references. A method of indirect deduction was used by HARDIE (1967): the value found for the transition temperature (58 2 2 C) caused a renewed interest in this subject. Since then, there have been claims for a transition temperature around 40 C (D’ANs, 1968; CRUFT and CHAO. 1970: GRIWRIEV and SHAMAEV, 1976), as well as for a transition temperature near that measured by Hardie (BLOUNT and DICKSON, 1973; KNACKE and GANS, 1977). A new experimental determination of the transition temperature is clearly needed. Direct measurements of gypsum and anhydrite solubility was carried out for this purpose. Some of the previous measurements of this kind have been criticized because the equilibrium solubility was approached from the undersaturation side only. In this work we attempt to approach the saturation equilibrium both from UnderSaturated and supersaturated solutions. To accomplish this. a solution in the presence of anhydrite or gypsum was gradually heated; solubility was measured continuously. At a higher temperature the solution was cooled, with continued measurement of the solubility. which proved to be reversible. EXPERIMENTAL The folIowIng work
:
materials
MATERIALS
hake been used throughout
the
The experimental substances were examined by powder X-Ray diffraction using filtered CuKa radiation. The gypsum (1). (2) and (3) gave powder spectra whose d-spacings were in agreement with Card 6-0046 of the Joint Committee on Powder Diffraction Standards. The anhydrite spectra corresponded to the listing of Card 6-0226. Intensities as well as d-spacings were generally in agreement with the J.C.P.D.S. Cards. Some minor intensity-differences, sometimes observed, can be ascribed. as usual. to the powder packing, to variable crystallinity of the mineral samples, etc. The materials used in the dissolution experiments have the following unit cell parameters: Gypsum : u = 5.6774 b = 15.1872 (’ = 6.5274 2 =7=90 fi = 118.39’
f 0.07 A i 0.04 a _t 0.07 A * 0.05
Anhydrite: tl = 6.2389 k 0.06 A h = 6.9928 k 0.06 h c’ = 6.9920 + 0.05 A sc=/j=;‘=9() The medium particle size is 0.7 mm for gypsum and 0.1 mm for anhydrite. To check the composition of the solid phase during the solubllity measurements, a work diagram for gypsum (1) anhydrite (4) mixtures was prepared. This diagram allowed us to ascertain the composition of the solid phase down to 0.05”,, content of one of the two components. The diagram (Fig. I ) has been obtained by plotting log /I 3.49 A (Anhydrite);/? 4.27 A (Gypsum) vs the anhydrite content. ‘The peaks heights have been measured in the following conditlons:
1931
1932 Table I. Solubihty of gypsum in pure water (1 atm pressure) % CaSO4
tot
0.208 0.208 0.210 0.199 0.194 0.192
25 30 45 50.5 55 60.5
Data are in ?,CaS04 in solution. All values reported in Tables I, 2, 3 are averages of at least three measurements.
corresponded to that of gypsum, solution kinetics of this material.
RESULTS
I
40
20
60
80
ANHYDRl
100 “lo TE
Fig. 1. Calibration curve for X-ray diffraction determination of gypsum and anhydrite contents in their mixtures.
20(CuKr Gypsum Anhydrite
20.80 25.53
Ni)
(i(A)
h/,/
4.21 3.49
l2i 020
Gypsum (1) and anhydrite (4) were used for the solubility measurements. The crystalline form of these two materials is shown in Fig. 2. SOLUBILITY
MEASUREMENTS
Solubility measurements have been performed with the aid of the apparatus shown in Fig. 3. Gypsum or anhydrite were placed in the container with bi-distilled water; the system was stirred mechanically (60 rpm) and the temperature control, within +O.l C, was achieved by water circulation in an external Jacket. Two reactors operated simultaneously and the temperature of the solutions within the two reactors did not differ by more than 0.3 C at the higher temperatures (> 50-C). Small amounts of the solution were sucked through a sintered glass septum to avoid the presence of solid particles. The Ca content of the solution, after dilution, was determined by atomic absorption spectrophotometry with a Perkin-Elmer Model 603 instrument. For each determination, the temperature of the system was kept constant until successive readings gave constant concentration (the longest runs of about 1 week’s duration were necessary for anhydrite). After saturation was confirmed, the sample was checked by X-ray diffraction. Data were collected, and henceforth accepted, hhen there was no variation of the solid phase within the limit of the analytical curve of Fig. 1; they thus refer to the solubility of the initial substance even if in a metastable region. Sometimes variations in the solid phase (both from gypsum to anhydrite and, more frequently, from anhydrite to gypsum) were observed; in these cases, the measured solubility
AND
because
of the faster dis-
DISCUSSION
A first set of measurements was performed by measuring the gypsum and anhydrite solubility separately. In this case, the same material was present in the two reactors; this allowed also an evaluation of the experimental error, which was usually less than value. The results of these S”, of the measured measurements are reported in Table 1 for gypsum and Table 2 for anhydrite. The order in the tables is the same of the temperature variation along the measurements; it is noteworthy that, after inversion of the temperature variation direction, the solubility data for anhydrite were in agreement with those taken in the runs with decreasing temperature. From inspection of Fig. 4, it is apparent that the equilibrium temperature must be substantially lower than 58°C as reported by HAKDIE (1967); however, these data alone do not allow precise determination of the equilibrium temperature. To reduce the errors attributable to temperature differences or to analytical procedure which are possible in long lasting experiments, gypsum was placed in a reactor and anhydrite in an other one. This allowed the simultaneous measurement of the solubility of the two substances. The results of this second set of measurements are reported in Table 3, where the data are presented in the same order in which they have been taken. The solubility variation was therefore studied, for the two materials, both from undersaturated and supersaturated solutions.
Table
2. Solubility of anhydrite pure water (1 atm pressure)
tOc
% CaS04
60.5 59.5 53 52.5 55 57
0.174 0.174 0.190 0.189 0.185 0.180
in
Fig. 2. Electron microscope photomicrographs of crystals of gypsum (1) showing the typical monocline form and of anhydrite (4) pseudomorph on gypsum (Photograph by V. LANDUZZI).
Fig. 3. The experimental
apparatus
used for the solubility
I934
measurements
(Photograph
by P. FERRIERI).
The gypsum-
anhydrite
equilibrium
by solubility
1935
measurements
0.20 -
0.19 -
0.18 -
a
0.16 -
50
40 Fig. 4. Solubility
Table 3. Solubility
of gypsum
of gypsum
(g) and anhydrite
and anhydrite
% CaSOr
% CaSOk
(anhydrite)
( gyps urn)
60
(a) in water as a function
64.5 57.5 56.0 54.0
0.187 0.188 0.196 0.194
0.163 0.177 0.187 0.185
[correlation
52.0
0.195
0.192
drite the corresponding
39.5
0.207
0.225
45.0 51.5
0.203 0.196
0.208 0.194
s = 8.69’ lo-‘t2
40
a correlation
50
- 1.73. lo-“t
equation -
equal
(1) anhy-
is:
3.667.10m3r
coefficient
60
+ 0.2624
= 0.95151 and for the
coefficient
s = 1.14.10-5t2 with
30
of the temperature
The experimental data have been fitted, by means of a least square procedure, to a second degree relationship for solubility, s. in wt”f(; CaS04, as a function of temperature, f. For gypsum, the relation is:
in pure water (1 atm pressure) tot
t°C
+ 0.3517 to 0.9938.
t’C
Fig. 5. Solubility of gypsum (g) and anhydrite (a) in water as a function of the temperature. The continuous lines are the equilibrium curves calculated according eqns (1) and (2). The discontinuous lines are the relative standard deviations.
(2)
1936
G. INN~RTAet crl.
The standard deviation of the observations is 0.00206 for gypsum and 0.0022 for anhydrite. The results are summarized in Fig. 5 where the continuous lines are the eqns (1) and (2); the polygon of error is defined by standard deviation. According to these data the equilibrium temperature is 49.5 & 2.5 ‘C. It is hard to find a satisfactory explanation for the difference between the present data and literature values for the equilibrium temperature since the crystallinity and grain dimensions of our samples are comparable with those of the samples used by several workers. We wish to note that no extrapolation procedure has been used throught the work and, more important. the solubility measurements proved to be reversible with respect to the direction of temperature variation.
REFERENCES
BLOUNTC. W. and DICKSONF. W. (1973) Gypsum-Anhydrite equilibria in systems CaSO,- Hz0 and CaSO,--NaCI-H,O. Am. Minercll. 58, 3233331. CRUFTE. F. and CHAOP. C. (1970) Nucleation kinetics of the gypsum-anhydrite system. 3rd Symp. Salt, 1969, pp. 109-l 18. D’ANs S. (1968) Der cbergangspunkt GipssAnhydrite. Kali Steins& I, 17-38. GRIGOR’EVA. P. and SHAMAEVP. P. (1976) Determination of gypsum-anhydrite equilibrium temperature. IX. Sib. Otd. A/ml. Nuuk SSSR. Ser. Khim. NM.& 104. HARDIEL. A. (1967) The gypsum-anhydrite equilibrium at one atmosphere pressure. Am. Minrrul. 52, 171~200. HOFF S. H. VAN’T,ARMSTRONC; E. F.. HINRICHESENW., WEI~ERTF. and JUST G. (1903) Gips and anhydrite. Z. Phys. Chem. 45, 257-306.
KNAKE0. and GAYSW. (1977) The thermodynamics system CaSO,--H,O. Z. Phys. Chew. 104, 41 48. A~,l\nn~~lrdgrmenr.sThe authors are indebted to ProPOSNJAKE. (1938) The system CaSO,-HZO. 4m. fessors E. BONATTI,A. CASTELLARIN and F. Rtcc~ LUCCHI 35A, 2477272. for the critical reading of the manuscript and for helpful ZEN E.-AN. (1965) Solubility measurements in the discussions, and to Mr A. PINI for the determination of the CaSO,-NaCl-HZ0 at 35’, 50 and 70 C and one unit cell parameters. phere pressure. J. Prtrol. 6, 124-164.
of the J. %i. system atmos-