- Email: [email protected]
Phase relations in the system NaCl-KCl-H2O II: Differential thermal analysis of the halite liquidus in the NaCl-H2O binary above 450°c
1X316-7037/83/05863-lISO3.00/0
Geochimco PI Cosmochimica Ada Vol. 47, pp. 863 to 873 0 Pergamon Pres Ltd. 1983. Printed inU.S.A.
Phase relations in the system NaCl-KCl-Hz0 II: Differential thermal analysis of the halite liquidus in the NaCl-Hz0 binary above 450°C W. D. GUNTER'~~, I-MING CHOU* and SVEN GIRSPERGER’ ‘lnstitut fuer Kristallographie und Petrographic, ETH-Zentrum, CH-8092, Switzerland *U.S. Geological Survey, 959 National Center, Reston, Virginia, 22092, U.S.A. ‘Present address: Oil Sands Research Dept., Alberta Research Council, I I3 1587th Ave. Edmonton, Alberta, T6G 2C2, Canada (Received June 22, 1982; accepted in revised form January 2 I, 1983)
Abstract-Thermal analysis of the halite liquidus in the system NaCI-H20 has been conducted for NaCl mole fractions (XNacl)greater than 0.25 (i.e., >50 wt. % NaCI) at pressures between 0.3 and 4. I kb and temperatures greater than 450°C. The position of the liquidus was located by differential thermal analysis (DTA) of cooling scans only, as heating scans did not produce definitive DTA peaks. The dP/dT slope of the liquidus is positive and steep at high pressures, but at high X NaCland pressures below 0.5 kb it appears to reverse slope and intersects the three-phase curve (liquid-halite-vapour) at a shallow angle. However, due to the complex nature of the DTA signal when P 5 0.5 kb, there is considerable doubt about exactly what event has been recorded in the experiments conducted at these low pressures. The solubility of halite can be expressed as a function of the mole-fractional-based activity of NaCl in the liquid phase (L) in temperature (T, “K) and pressure (P, bars) In aNacI,L.T.Pj = -19.884 - 0.001275T - 1388/T + 3.2305 In (T) - O.O7574P/T Our liquidus data (based on IO compositions) above 500 bars for these brines were combined with this equation to generate activity coefficients of NaCl which were fit within their experimental uncertainties to the following one parameter Margules equation In -rNaCI(L.T.P, = (0.7268 - 695.7/T - 0.12 17P/T)( I - XNacJ2. Concentrated solutions of NaCl show negative deviations from ideality which rapidly increase in magnitude with decreasing XNacl. INTRODUCTION MASS TRANSPORT in crustat rocks is largely through a fluid medium by diffusion or infiltration if deformational mechanisms are not important. In order to formulate quantitative transport models, the thermodynamic properties of these fluids must be known. The two most common components in natural fluids are HI0 and CO*. HELGESON and KIRKHAM ( 1974a, 1974b, 1976) and HELGESON et al. (198 1) have analyzed the available data for aqueous fluids and have predicted the properties of the important solutes to 600°C and 5 kb, using an electrostatic model. However the thermodynamic properties of CO*-rich solutions are not as well known. The only pertinent experimental studies, aside from the binary COZH20, is on the ternary NaCI-CO>-H20, by TAKENOUCHI and KENNEDY (1965) and GEHRIG (1980) (assessed by BOWERS and HELGESON, 1981). Even the behavior of the common natural solutes in water is not well known at high concentrations, yet the concentration of total salts found in fluid inclusions ranges from dilute to greater than 70 wt. % (ROEDDER, 1967; EASTOE, 1978).The phase boundaries of the most well-known system, NaCLH20, are not well established at high temperatures and pressures. SouRIRAJAN and KENNEDY (1962) measured the compositions of coexisting gases and liquids between 863
350” and 700°C but they were unable to obtain data for fluids that contained more than 26 wt. % NaCI. DEVIL (I 942) reported values of XNacl for the point of intersection of the NaCMiquidus with the threephase boundary (halite-liquid-vapour) to a maximum temperature of 650°C and a maximum pressure of 0.39 kb. These measurements have been corroborated to within 5% by URUSOVA (1974) at high concentrations and by POTTER et al. (1977) at low concentrations. There have been no determinations of the position of the halite Iiquidus at higher pressures. The reaction
is nonquenchable and therefore ideal for study by differential thermal analysis (DTA). This contribution reports on the location of the halite liquidus above 450°C and 50 wt. % NaCl to 4.1 kb, as determined by DTA in two different laboratories. EXPERIMENTAL
PROCEDURE
Two different experimental approaches were used for the DTA. The first, a classical DTA configuration, involved the use of two thermocouples, and the voltage-difference signal between the two thermocouples was recorded as a function of temperature. This experimental set up has been fully
W. D. Gunter,
864 described
in earlier
publications
(CHOII
and
I-M. Chou
and S. Girsperger
EUGSTER,
1981; CHOIJ, 1982).
The other technique, in which only one thermocouple was used, is more correctly referred to as thermal analysis (TA), and is described in detail here. The sample holder (see CHOU, 1982, for a more detailed description) consists of a 4.0 cm long, 0.50 cm OD large gold tube into one end of which has been welded a 0.13 cm OD small gold tube to form a I .5 cm long well, which contains the tip of a 0.05 cm OD inconel-sheathed chrome]-alumel “sample” thermocouple. The other end of the large gold tube was left open until after the sample (NaCI plus H,O) was introduced and then was welded shut by a plasma arc. The sample container, with the thermocouple in place, was sealed in a cold-seal pressure-vessel, pressurized under argon. and placed in a horizontal tube furnace. Cooling rates, heating rates, and collection of the data were controlled by a PDP-I I computer linked to a D/A power supply (Hewlett Packard model 59501a’). In a heating run, the dc voltage from the power supply was incremented by the computer at the desired rate and directed to the temperature controller of the tube furnace so that it was opposed to that of the controller-thermocouple (see Fig. 1). Therefore, at each voltage increment, the controller would be “tricked” into supplying additional power to the furnace because the sum of the voltage of the controller-thermocouple and the D/A power supply would be low. In a cooling scan, the dc voltage from the D/A power supply was decremented stepwise to achieve the desired cooling rate. Although cooling scans were linear, heating scans departed about 1“C from linearity in a smooth fashion over a 125°C scan. We are limited to a maximum heating rate of lO”C/ min and a maximum cooling rate of 2”C/min in our standard tube furnaces. However, the maximum cooling rate was raised to lO”C/min by replacing the furnace insulation around the heating coil of the furnace with a coil of copper tubing through which cold water was circulated (= cooling furnace). Sample thermocouples were calibrated to an accuracy of f 1“C using a standard Pt/PtwRdio thermocouple from the Swiss National Bureau of Standards. Pressure was measured by a Brosa transducer, which was calibrated frequently against a standard 7 kb Heise bourdon-tube gauge and was accurate to ?5 bars. Temperatures and pressures were measured by a digital voltmeter (Hewlett Packard model 3456a) with a sensitivity of 0.1 microvolt, and recorded on the floppy disk of the PDP-I I (see Fig. 1). During heating and cooling scans, the sample temperatures were taken 25 times per degree centigrade. The temperature gradients across the sample position in the pressure vessel were measured at pressure using six chrome]-alumel thermocouples, positioned 2 cm apart. Under “isothermal” conditions, the gradients were found to be less than 5°C over the sample. During cooling scans, the gradient decreased by a factor of 2. whereas during heating scans, it became larger (see Fig. 2) for the normal tube furnace. The total temperature uncertainty for cooling runs is *3”C, increasing to a maximum of &6”C for heating runs in our standard tube furnaces. In the cooling furnace. the gradient reached a maximum of 6°C during cooling. The error introduced from the sample composition is only that introduced from the accuracy of weighing, ?50 micrograms, which corresponds to an error of fO.l wt. % for a 50-milligram sample. A thermal scan consisted of heating or cooling at a fixed rate, either 0.25, 0.5, 1, 2, 4.6, or 8.1”C/min over a 200°C range. Temperature readings were made over a 125°C “linear” portion of the scan. The sample temperatures viewed on a coarse scale reflect only the rate of power input lo the
’ Any use of trade names in this report is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey.
CONTROLLER
PRESSURE TRANSOUCER
PROGRAMMABLE
7kb
PDP-I1
L FIG. 1. Schematic ment system.
PUMP
COMPUTER
I diagram
of our control
and measure-
furnace, but on a fine scale they are sensitive to changes in the thermal properties of the sample. In order to construct the DTA peaks (i.e.. magnify the fine structure), part of the temperature-time TA-curve was fit by least squares to a straight line. The difference between the measured temperature-time curve and the linear fit over the 125°C “linear” part of the scan is the DTA fit (see Fig. 3) and is similar to that which would be measured in a standard DTA configuration using two thermocouples. A typical heating scan would start in the halite + liquid region, cross the liquidus, and end in the liquid-only region; a cooling scan would proceed in exactly the opposite direction. Obviously, the best region to make the linear fit would be the one-phase liquid region where the absorption or release of heat from the sample also would be expected to vary in a linear fashion. However, we found that in locating the onset of the DTA peak, it made no difference whether the linear fit was made in the one-phase liquid region or over the complete 125°C interval. As the latter technique was faster, it was used to obtain all the data presented in this paper. The DTA peak resolution of this technique was 4 microvolts (i.e., 0.1 “C), which limited our investigation of the halite liquidus to X,,, greater than 0.25. DTA peaks could not be resolved at lower concentrations. The heating scans did not produce a sharp DTA peak as the halite liquidus was crossed, except for pure NaCl, and so they could not be used definitively. In contrast the cooling scans involved a nucleation event at the liquidus, and the DTA analysis yielded well-defined peaks (Fig. 3). If no temperature gradients exist in the sample and the sample is homogeneous, the temperature estimates of the liquidus boundary from the cooling scans (see Table I) must represent minimum values, as stable nucleii can form only after supersaturation. An estimate of the equilibrium position of the liquidus was made by noting the shift in the position of the DTA peak as a function of cooling rate up to lO”C/ min. When the cooling rate is plotted as a square-root function, a straight line can be fit to the data and extrapolated
NaCl-KCI-HZ0
865
system
20. _
(a)
-
T=50S°C 510-620
before heating in 10' steps
15.
heating
rate
P = 2. 0 kb
G “,
= Z’/mln
10.
5 i+
5.
0. t 0.
2. DISTANCE
(b)
-
T=708"C 700-560
(cm)
4.
6.
before cooling in 10O steps
15.
cooling
rote
= 2O/rn1” P = 0.5
10.
0.
2. DISTANCE
(cm)
kb
Y
4.
FIG. 2. Temperature gradients along a typical TA capsule. End of bomb at “0” capsules were 4 cm long and were positioned 1 cm from the end of the bomb. (a) thermal gradient before the heating scan was initiated and solid lines represent the during heating at Z”C/min. (b) Dotted line is the thermal gradient before the cooling and solid lines represent the thermal gradient during cooling at Z”C/min.
to zero cooling rate (i.e., equilibrium). When this is done the lower rates of cooling (up to 1“C/min) are found to be within 1“C of the equilibrium position (see Fig. 4). The temperatures of the NaCI-HZ0 liquidus based on cooling scans reported by CHOU and EUGSTER (198 1) are about 15°C higher than the values reported here. This discrepancy is due to the difference in orientation of the furnace during experimentation. In the vertical furnace used by CHOU and EUGSTER (1981), the vertical dimension of the sample capsule is 25 mm while the horizontal dimension is 4 mm; consequently the sample in the capsule may not be homogenous owing to incomplete mixing of the highly viscous liquid formed by melting of the halite. Poor mixing can be explained by gravitational segregation of the twophase mixture, halite + liquid, below the liquidus coupled with slow diffusion rates in the liquid above the liquidus. In the horizontal tube furnace used in the present study, the vertical dimension of the sample capsule is only 4 mm; therefore, gravitational segregation of the sample and the distance of the vertical-diffusion path for complete mixing were minimized. Because the slope of the liquidus is steep, a two mole % difference in composition can account for the 15’C difference in temperature. Temperatures and DTA signals obtained in a heating-cooling cycle for X,,, = 0.588 at 2 kb in a vertical and horizontal furnace are compared
6.
position. All TA Dotted line is the thermal gradient scan was initiated
in Fig. 5. The inhomogeneity of the sample in the vertical furnace is demonstrated in the cooling cycle by the broad DTA signal, the presence of a second peak in the DTA curve, and the absence of a detectable break in the sample temperature curve. To improve the rate of mixing, a vibrator was attached to the cool head of the cold-seal vessel to agitate the sample during DTA scans in horizontal furnaces. No detectable difference in the position of the liquidus determined from agitated and unagitated runs could be discerned.
ISOPLETHS
IN THE
NaCI-H,O
BINARY
Because each of our experiments involved determining the halite liquidus position over a range of temperature and pressure for a constant XNacl, the geometry of each isopleth of concern must be understood in order to correctly interpret the DTA peaks. In Fig. 6, isopleths for X,,r of 0.25 (52 wt. %) and 0.75 (91 wt. %) have been constructed following the approach of ERWOOD et al. ( 1979). The main elements of the isopleths shown in Fig. 6 are
W. D. Gunter, I-M. Chou and S. Girsperger
866
heating
29.
rote
= 2’/mln
30. TEMP.
C,":;
32.
33.
31.
:
50.
40.
P s
. 30.
P = 1.0
kb
cooling
rote
= 1°C/nll"
9 L
20. -=z !I g
10.
29. TEMP.
C,"v"i
FIG. 3. DTA peaks from linear data fitting at 1 kb pressure. XNaCl= 0.745 (90.5 wt.% NaCI). Each point represents a single measurement. For a K-type thermocouple, 1“C = 0.04 mv. Note difference in scales, millivolts versus microvolts. (a) Heating scan at Z”C/min. “U” shape reflects nonlinearity of heating scans. (b) Cooling scan at 1“C/min.
the two three-phase curves. The first three-phase curve, liquid + ice + halite (ADAMS and GIBSON, 1930; ADAMS, 193 1), has a steep slope and lies at low temperatures. The phase relations of the dihydrate, NaCl . 2H20, are only important at lower temperatures and are not shown. The other three-phase curve, liquid + vapour + halite (I&EVIL, 1942; HAAS, 1976; POTTER et al., 1977), is touched by the apex of the “V” formed by the intersection of the halite liquidus, H + L/L, and the liquid + vapour boundary curve, L/L + V, and is finally truncated by the liquid + vapour boundary curve, L + V/V, at high temperature. For isopleths of intermediate to high concentration, these truncations are stacked against the high-temperature end of the three-phase curve (SOURIRAJAN and KENNEDY, 1962) and consequently are not very evident on Fig. 6. The apex of this “V” sweeps along the three-phase curve, attaining higher temperatures as the isopleths increase in concentration. In the two-phase regions, the phase compositions must fall off the plane of the isopleth. These
two-phase regions can be contoured and are identical for all isopleths. The liquid + vapour boundary, L/L + V and L + V/V, for each isopleth defines a constant composition line which becomes a fixed contour of the L + V fields for other bulk compositions if the contour falls within their L + V fields. Vapour contours for the H + V and the L + V fields have been presented in SOURIRAJAN and KENNEDY (1962) for low concentrations in their Figs. 10 and 18, respectively. The three-phase curve separates the two fields, and each vapour contour passes smoothly through it. The liquid-vapour boundary for each isopleth lies inside the critical curve except at one point, the critical point, where it is tangent to the critical curve. The critical points for the isopleths shown in Fig. 6 are unknown. The vapour contour changes to a liquid contour as it passes through a critical point. Between adjacent critical points, the liquid contour emanating from one critical point must cross the vapour contour from the other. Construction of these contours has not been attempted because the data
NaCI-KCI-H20 system Table 1.
OTA and TA data from isobaric scans across the halite liquidus and the J-phase boundary of N&l-H20. Phase
Fate of
Onset
No.
,","dundary (:b) method
$;;;,,
;r;f'
::a",
0.262 (53.5)
Liquidus TA
0.51 0.98 1.03
0.5-Z 0.::)
450-449 440 450-449
2.02 3.01
0.5-l 1.
454-453 451
;
0.339
Liquidus
0.41
-R
510
2
OTA
0.51 0.61 0.71 1.00 1.01 1.50 1.51
- 8 - 8 - 8 - 8 -8 -8
509 510 510 510 513 515-514 515
1 i
1.92 1.53 1.93 1.98
- 8 - 8 - 8
514 520 518 520
(62.4)
7 1 3
1 1 2 1
1
1 1
YNaCl 0.663 (86.5) O.R42 0.792 0.798 0.724 0.665 0.916 0.903 0.899 0.890 0.866 0.878 0.847 0.846 0.840 0.835 O.R25 0.830
0.496 (76.2)
Liquidus TA
0.58 1.00 2.01 3.55
0.25-Z 0.25-10 0.5-Z l-2
609-606 613-609 626-624 645-643
7 15 6 5
0.979 0.961 0.932 0.888
0.502 (76.6)
Liquidus TA
1.27 2.06 4.07
2 0.5-Z 1
616 628-626 655
1 2 1
0.940 0.925 0.077
0.526 (78.3)
Liquidus OTA
0.51 0.57 0.59 0.61 0.70 0.80 0.90 1.00 1.02 1.50 1.51 1.96
-8 -8 -8 -8 -8 -8 -P -6 -8 -8 -8 -8
623-621 622 622 622-621 623 624 625 626 626 634 634 641
3 1 1 2 1 1 1 1 1 1 1 1
0.985 0.976 0.974 0.973 0.969 0.965 0.961 0.957 0.955 0.948 0.947 0.939
2.00 2.02
-8 -8
641 641
1 1
0.936 0.935
649 656-655 665 672
1 2 2 4
0.979 0.965 0.960 0.948
690-685 680-675 670-665 662-661 658-657 654-651 659-656 680-678 667-665 649-648 618-612 673-675 666-668 650-653 619-629
11 11 4 4 3 7 9 7 7 7 7 6 6 6 6
1.152 1.109 1.067 1.034 1.016 0.993 0.971
703-697 699 694-690 690 684 680 683 681 682 683 685 690 691-690 699
2 1 2 1 1 1 1 1 1 1 1 1 2 1
1.076 I.060 1.039 1.023 1.000 0.984 0.988 0.979 0.976 0.979 0.971 0.973 0.976 0.968
0.588 (82.3)
Liquidus OTA
0.50 1.00 1.50 2.00
0.591 (82.4)
Llquidus TA
0.32 0.34 0.36 0.38 0.40 0.50 1.01 0.32 0.34 0.36 0.38 0.32 0.34 0.36 0.38
3-phase TA
0.663 (86.5)
867
Liquidus OTA
0.32 0.33 0.35 0.37 0.38 0.40 0.49 0.51 0.59 0.60 0.80 1.01 1.02 1.49
1; -8 -8 0.5-Z 0.5-Z l-2 l-2 l-2 0.5-Z 0.5-Z 0.5-Z 0.5-Z 0.5-Z 0.5-Z /'1::\
-8 -8
-8 -6 -8 -8 -8 -6 -8 -8 -8 -8 -I3 -0
(i.e., the positions of L/L + V and L + V/V) needed to construct liquid or vapour contours in the L + V field are incomplete, particularly at high XNaa (>26 wt. %). In Fig. 6, the position of each liquid + vapour curve boundary is constrained only by its two intersections with the liquid + vapour + halite curve, and by the fact that the slope of the boundary must approach the low slope of the liquid + vapour curve,
Liquidus OTA 3-phase (hi;; T)
3-phase (law T) OTA 0.745 (90.5)
Liquidus TA 3-phase (high T)
1.000 (100.)
Liquidus TA
1.51 1.97 2.00 0.32 0.33 0.35 0.37 0.38 0.32 0.37 0.30 0.51 1.00 2.02 0.30 0.30
- 8 - 8 - 8 - 8 - 8 - 8 - 8 - 8 -R - 8 0.25-Z 0.25-Z 0.25-Z 0.25-Z 0.25-Z (0.5-10)
1.02 0.25-3 0" l-2 1.02 (0.25-Z) O"\ (l-2)
699 708 708 6~33-678 666 656-652 630 625 490 542
0.967
718-716 718-716 726-722 W;;;
1.010 0.994 0.984 0.979
0.964
0.961
690-692 825-824
870
824-825 800
1.000 1.000 1.000 1.oon
*Rates of heating scans are given in parentheses; all ~;pe;t;lues In this column are for cooling scans.
L/V of the NaCl unary for concentrated isopleths such as these. Consequently, the placement of our boundaries for the liquid + vapour field is very uncertain for these two concentrated NaCl isopleths. Also, each H f L/L boundary defines a constant liquid-composition line, which is a fixed contour for the L + H field regardless of bulk composition. All these liquid contours are truncated at the three-phasecurve. The positions of the constant liquid-composition contours in the liquid + halite field are determined in this paper. In our cooling scans, the scan was always initiated above the halite liquidus. Because the liquid field narrows at low pressure until it is truncated by the three-phase curve, the possibility exists that several phase boundaries are intersected in an isobaric cooling scan; if so, several DTA peaks would be expected to appear. Intersection ofthe three-phase curve would produce a large peak, because the liquid phase must be completely transformed to halite + vapour before the boundary can be crossed. Crossing of the L/L + V boundary might not be recognizable because the scan would start in the L + V field. The disappearance of vapour to form liquid during cooling would be a gradual process whose rate would be dependent on the spacing of liquid-vapour contours in P-T space (see SOURIRAJAN and KENNEDY, 1962, Fig. 18); this process would not be conducive to producing a welldefined peak. This is analogous to heating scans crossing the halite liquidus where no peak is discernible (see Fig. 3). On the contrary, for heating scans that cross the liquid-vapour boundary, a sharp peak would be expected, because in going from the liquid to the L + V field, vapour must nucleate. LOW-PRESSURE
EXPERIMENTS
We have chosen to discuss the results of our experiments in two different sections because at high
W. D. Gunter, I-M. Chou and S. Girsperger
868
X(thCl>
= 0. 50
P = 1.0
kb
25. 37.
25. 27, 0.
1. ROOT
SQ.
2. OF COOLING
3. RATE
FIG. 4. Extrapolation to equilibrium (i.e.. zero cooling rate) at 1 kb pressure. Hexagons represent the positions of onset temperatures for the halite liquidus at different scan rates for XNr(.,= 0.496 (76.2 wt.% NaCI). Cooling rate units = “C/minute.
pressures (>500 bars), the position of the DTA peak attributed to the halite liquidus is relatively insensitive to pressure. At lower pressures, the shape of the DTA signal is more complex, and its position is very sensitive to small changes in pressure. Extrapolation of the high-pressure liquidii of halite along lines of constant X,,, to the three-phase curve intersect it -25°C below their points of intersection as reported by KEEVIL (1942). In order to investigate this discrepancy we performed several experiments at low pressure. The position of the three-phase curve
I
/ I \
500 -
- 0
I
T, t
AT
\
\ \
i
\ \, AT
0.2
--__.
i
[
(mv)
‘\ \_/=-~----
FIG. 5. Temperatures (solid curves) and differential temperatures (dashed curves) from the two-thermocouple configuration experiments for XNacl = 0.588 (82.3 wt.% NaCI) at 2 kb in a vertical tube furnace (top) and a horizontal tube furnace (bottom). The time offsets of the solid and dashed curves are indicated by vertical lines.
(liquid + vapour + halite) was reversed on heating and cooling runs (see Fig. 7 and Table I) and agrees reasonably well with that determined by SOURIRAJAN and KENNEDY( 1962). Fluids with XNaClof 0.59 (82 wt. % NaCI) and 0.66 (87 wt.% NaCI) were chosen for study because their liquidii lie at higher temperatures than the pressure maximum on the three-phase-curve. A liquidus intersecting the three-phase curve below the maximum could not be determined. because during a cooling scan, the initial DTA peak from the three-phase curve (see Fig. 6) would mask the much smaller, succeeding liquidus peak. In Fig. 7, the position of the halite liquidus determined from cooling scans has been plotted to show the sharp reversal in slope of the liquidus below 500 bars as it approaches the threephase curve. However, the interpretation of the position of the liquidus was complicated by ‘peak-splitting’ at these low pressures (Fig. 8). Both doublets and triplets occur, which only shrink to a single peak above 400 bars. The onset position of the first peak has been used to plot the liquidus position in Fig. 7. We cannot satisfactorily explain the splitting of the liquidus peak at low pressures. We do not think that the first peak represents the intersection of the liquidvapour boundary, as the peak doublets are not reproducible from one scan to another, and they increase in temperature with decreasing pressure. Even ifthis interpretation is incorrect, the event is probably recorded by one of the peaks of the doublets or triplets and therefore would not alter our conclusion that the liquidus moves to higher temperatures at low pressures as it approaches the three-phase boundary. At such low pressures, the system may no longer be binary. The hydrolysis reaction (HAAS, pers. commun.) NaCl,c, + HzO,rj -
NaOH,(
or ,_I+ HCIp,
where “C” stands for crystal, “L” for liquid, and “V”
NaCl-KCI-H20 system
869
(a) X (N&1)=0.25
,’ ,’ ,’ _’ ,’
Liquid
+ Hollte
Llquld
I’
,’ ,’
,I’
L lquld
+ Vopour :
5
100.
00.
300.
500.
TEMPERATURE
700.
900.
1
('C)
2000.,
I
I
(b) 3
1500. t
X(NaC 1)=0.75
:
Llquld
+ Halite
I
I :
.*‘
fiae.
100.
300.
500.
TEMPERATURE
_
,*
.I
700.
L + V _-
-
i-00.
("C)
FIG. 6. Isopleths in the NaCl-H,O binary. Dashed lines are extrapolations (see text). (a) Xvac., = 0.25 (52.0 wt.% NaCl) (b) XNaCl= 0.75 (90.7 wt.% NaCI).
for vapour, might be the reason for the slope reversal of the isopleth curves. If HCl and NaOH are present in appreciable amounts, the double or triple DTA
signal in a cooling scan at low pressure and high XNaCl might reflect the complex phase relations in the reciprocal system NaCl-H20-HCl-NaOH. R. 0. FOUR-
1000.
Hal lte
0.
I
P
+
7
Vapour
I
600.
620.
640.
Temperature
660.
'C
680.
700.
FIG. 7. P-T plot of our low-pressure data (“Y”) for the location of the three-phase curve (horizontal solid line and extrapolated dotted line are based on KEEVIL, 1942, and SOURIRAJAN and KENNEDY, 1962) and the liquidus isopleths for XN,c, = 0.591 (82.4 wt.% NaCI) and XNaCl= 0.663 (86.5 wt.% NaCl). Data points for each liquidus isopleth are joined in order of descending pressure by dashed lines. The small irregularities in the line reflect the uncertainties in the data.
W. D. Gunter.
870 200.
and S. Girsperger
vapour phase. It is also possible that the air trapped in our DTA capsules may exsolve at low pressure and contribute to the DTA peak shape. The capsule filling with halite + water was less in the low-pressure experiments because of the larger coefficient of thermal expansion for aqueous fluids compared with higher pressures. This smaller (halite + water)/air would enhance this exsolution. Finally, the reversal of slope of the halite liquidus below 500 bars may be the result of the partial molal volume of NaCl decreasing rapidly at low pressures because of the large increase in the partial molal volume of water. Obviously, many more experiments must be completed at these low pressures before all of these questions can be resolved. Our intent here is to point out the uncertainty in the interpretation of these simple experiments based on DTA. Hopefully these lowpressure phase equilibria will be elucidated soon.
.-
1
X(NoC1)
I-M. Chou
= 0. 59
P (kb)
150.
s ,1 2 100. : 2 _1 ; 50.
HIGH-PRESSURE
0. 2 4.
I
25.
26. TEMP.
27. (mV)
28.
+
2
FIG. 8. DTA peaks from linear data fitting of cooling scans at low pressures. XNeCl = 0.591 (82.4 wt.% NaCI), P = 0.32 to 1.O 1 kb. Diamonds mark the positions of threephase-boundary peaks while arrows point to the onset temperatures for the first DTA peak. Each data point plotted represents a single measurement. 1°C = 0.04 mv. Note difference in scales; microvolts verws millivolts.
NIER
(unpublished data for alkali chloride brines) has found that the chloride/total-alkali ratio is one in the fluid at high pressures and increases rapidly at low pressures in the vapour in the two-phase region, which would correlate with an increase of HCI in the
EXPERIMENTS
Our results for fluids between XNaCl of 0.25 and I are presented in Fig. 9 and Table 1. We believe that the liquidus temperatures are within 5°C of equilibrium values, for the following reasons: 1. CHOU ( 1982) found that his DTA results from cooling scans for the location of the liquidus in the binary NaCl-KC1 agreed to within 5°C of most of the data reported by earlier investigators who used a variety of different techniques. 2. The position of the three-phase boundary, liquid-vapour-halite, determined by us in heating scans. is essentially identical to that obtained from cooling scans (see Table 1). 3. Extrapolation of the DTA peak positions to zero cooling rate is always within I o of the positions for the slower cooling rates. 4. Finally, our data are consistent with those of
2500.
500.
700. 600. Temperature 'C
800.
900.
FIG. 9. P-T plot of our high-pressure (>0.5 kb) data for the location of the halite liquidus. Each nearlyvertical solid line is a liquidus contour whose position was calculated from a one-parameter Margules fit of our data above 500 bars and Eqn. (3). The dashed lines at low pressure join the point of intersection of each contour with the three-phase curve (reported by WEEVIL, 1942) to the tip of our high pressure contour. The dashed end of the three-phase curve is extrapolated. The width of a hexagon data symbol is 8°C.
NaCl-KC&H20 system URUSOVA
(1975) who at 1000 bars pressure found only liquid at 50 wt. % NaCl and 450°C and at 60 wt. % NaCl and 550°C (see Fig. 10). ‘The mixing properties of NaCl and Hz0 are determined by the intermolecular forces between solute and solvent, and by the melting point and the enthaipy of fusion of halite. These thermodynamic reiationships are embodied in the expression for equilibrium between NaCl-Hz0 liquid and pure crystalline halite: viz, &MW .T.P)+- RT In
(YN~cI(L.T,P)XN~CI(L.T.P))
where 7‘ = T(K), P = P(bars), and the superscript “0’” is used to indicate a pure phase consisting of a single component. Assuming that the difference in heat-capacity between liquid and crystal can be expressed as a function of the farm LX&,
= AL+__c)+ Ab&T,
that the volume difference between Iiquid and crystal (AV~,__,,) is independent of temperature and pressure, and that no solid-solution occurs in the crystalline halite, and, finally, choosing a standard state of pure crystal and pure metastable liquid NaCl at the temperature and pressure of interest, Eqn. (1) can be used to describe the solubility of halite at any temperature and pressure; viz., -RT In _= AH" +
(YNaCI(L.T.P)XNaCI(L.T.P)) rn.NaC.~.--C.Tm.d 1 - T/Tm)
.&z$_~(T - Tm - T In (TfTm))
- O.SAh~t_.c(T - Tm)” f AVyL_,l(P - I)
(2)
The heat of fusion at 1 bar (AHO,,NacI(L-~,Tm,I))r the fusion temperature at 1 bar (Tm), and the heat-capacity functions for NaCl have been tabulated by ROW er ul. (1978). The difference in volume be-
871
tween liquid and crystal has been estimated from the Clausius-Clapeyron slope for the halite-liquid boundary in the NaCl unary. Therefore substituting these quantities into Eqn. (2) yields 111(YN~CCI(L.T,P~XNaCI(L,.T,P)) = -19.884 -
- 0a001275T
1388/T + 3.2305 In (T) - O.O7574P/T
Because the only unknown in this equation is the interaction term between NaCl and HzO. ~N~cr(r.T,p), an activity coethcient can be calculated at each experimentally determined point on the liquidus, and these values are listed in Table 1. The effects of errors in the thermodynamic properties for pure NaCl on the calculated value of ~N~cl(r,T,p)were estimated in the following manner. -&cr(r,T.P) was recalculated and compared with the original values assuming that: 1. Ac~~-~, = Au$_-~~. The maximum difference in ~~~~~~~~~~~~ due to this a~umption was 0.02, and it occurred at the lowest values of XH,. Generally, however, the differences were less than 0.01. 2. AC&~, = 0. The maximum difference was greater; i.e., up to 0.06, but most were less than 0.02. 3. An error of 2°C in T,. The difference was always less than 0.0 1. 4. An error of 100 calories in 4H~,NaCI~L_C,Tm,l~. The difference at LyNaer< 0.5 was as high as 0.02, but at higher XNaClthe differences were always less than 0.0 1. 5. AV& = a constant. This assumption introduces negligible error because the Clausius-Clap~yron slope is constant (24”C/kb) to within i “C when estimated at SOO-bar intervals between 1 and 2000 bars on the basis of data in Table 1 and the data of CHOW ( 1982). Consequently, for )y,,,, < 0.5, calculated values of YNacI(L,T,P) are probably within 0.1 of the correct values and they should be even closer to the correct
P = 1000
bars
/I-.Halite
4”0i.~
0. 4
(3)
m
X (NaICl)
+ Llquld
1.0
FIG. 10. Marguies fit of our data at I kb pressure. The triangles illustrate one-phase (liquid) data from URUSOVA( 1975). The data of SOURIRAJAN and KENNEDY( 1962) are plotted as the solid boundary of
the liquid + vapour field (t + V). Our data for the halite liquidus are represented by hexagons.
W. D. Gunter,
872
I-M. Chou
and S. Girsperger
\ Hollte 500.
+ Vapour
1.
600. 700. Temperature "C
FIG. II. P-T plot (20.5 kb) for the location of the halite liquidus of the liquidus contours were calculated from Eqns. (3) and (4).
values at higher mole fractions, provided that the position of the liquidus is located accurately, and that there are negligible errors in the data used for NaCl from ROBIE et al. (1978). The activity coefficients were fit to a one-parameter Margules solution model ln
YNaCI(L.T.P)
=
MIT)(
1 -
XNaCI(L,T.P)?
(4)
where A was assumed to be a linear function of temperature and pressure. This formulation predicts -rNaCI(L,r,Pj to within 1% of the values determined from experiment, listed in Table 1. The fit equation for the data above 500 bars is A = 0.7268T
- 695.7 - 0.1217P
(5)
and was combined with Eqns. (3) and (4) to calculate the positions of the halite liquidus. When compared with the experimental data plotted in Fig. 9, except for one data point at low pressure, the maximum difference between experiment and the fit prediction is 4°C or 1% in the XNaCl value, which is within the 1
I.
-.._ 800.
900.
at X Nacl of 0.05 intervals.
Position
uncertainty of the experimental data. This also applies to the three high-pressure data points between 3 and 4 kb, which are not plotted in Fig. 9. Hence, a more complicated model is unwarranted at this time. The equations used above to locate the position of the liquidus can be confidently applied to higher pressures, as the variation is close to linear, but they should not be extrapolated to lower temperatures because the curvature of the liquidus changes rapidly at low lyNacI (Fig. 10). Liquidus isopleths were calculated at regular XNac.r intervals and are plotted in Fig. 1I. Equation (3) was used to calculate the smoothed values for ~N~cr(t_T,p) as a function of XNac,, T, and P. Each point on Fig. 12 ItpreSentS yNacI(L,T,P) at a SpeCifiC point on the liquidus. Although the simple one-parameter Margules mixing model predicts accurate values for ~~~cr(L,r,p) on the liquidus, there is no way of estimating how far this model’s predictions can be extended into the one phase liquid field on any isobaric-isothermal sec-
01
w
0.
8_
0. 2
Llquld
+ Halite
0. 6 0. 4 X(NaC1)
0. 8
1. 0
FIG. 12. Values of yNac,(L.T,Pjfor halite saturated NaCI-HZ0 solutions calculated from the one-parameter Margules fit; contoured for pressure (solid lines) and temperature (dashed lines). Temperature contours are spaced at 100°C intervals.
NaCI-KCI-HZ0 without other data for comparison. The model cannot be correct in the more dilute regions, as it does not predict the miscibility gap between liquid and vapour that has been recorded by SOURIRAJAN and KENNEDY (1962) and plotted in Fig. 10. However, there is little doubt that concentrated solutions of NaCl show negative departures from ideality which rapidly increase in magnitude with decreasing X,,c,.
tions,
CONCLUSIONS
Unfortunately, the sensitivity of the DTA technique used here for the detection of the halite liquidus was not sufficient to allow extension of our measurements to XNaCl< 0.25. Therefore we were unable to construct complete isopleths because our data do not overlap with the liquid-vapour boundary determinations made by SOURIRAJAN and KENNEDY (1962). Even though our data fit was based on a thermodynamic model, the model is too simple to extrapolate with any confidence to lower temperatures and more dilute liquids. Rather, it should be used to interpolate between our experimental points for the position of the liquidus (Fig. 11) and to calculate yNaclci,l,Pj over this region (Fig. 12). At low pressure, the reversal of slope for the position of the DTA peak onsets and the more complex nature of the DTA signal has been established for these high NaCl concentrations. Fluid inclusion compositions determined from homogenization temperatures resulting from disappearance of halite should be reexamined in the light
of these
new
data.
In this paper we have demonstrated that differential thermal analysis is an experimental technique which permits rapid determination of phase boundaries in concentrated aqueous solutions. We are working on several different experimental techniques to cover the more dilute regions and also the lower pressure regions, and we hope to report on these in the future. ilc,knowl~dRmmts-This study was initiated at the Johns Hopkins University and supported by NSF grant EAR7300391 AOZ/Eugster, while one of us (I.M.C.) was a postdoctoral fellow. The equipment which enabled W.D.G. and SC. to complete this study was purchased through special funds from E.T.H. Comments by J. L. Hass, Jr. and R. J. Bodnar of the U.S.G.S. were helpful. We thank J. L. Haas, Jr., and R. 0. Fournier (U.S.G.S.) and Urs Raz (E.T.H.) for stimulating discussions, B. Buhlmann (E.T.H.) for assistance with the drafting, and J. G. Blencoe, (Pennsylvania State Univ.) D. A. Crerar (Princeton Univ.) and R. W. Potter II (Occidental Research Corp.) for the’ final review of this paper. REFERENCES ADAMS L. H. (1931) Equilibrium in binary systems under pressure. 1. An experimental and thermodynamic investigation of the system NaCI-Hz0 at 25°C. Amer. Chem.
Sac. .I. 53, 3769-38 13. ADAMS L. H. and GIBSON R. E. (1930) The melting curve of sodium chloride dihydrate. Amer. Chem. Sot. J. 52,
4252-4264. BOWERS T. S. and HELGESON H. C. (198 1) Thermodynamic
system and geochemical consequences system HzO-C02-NaCl. (abstr.)
873 of immiscibility in the Geol Sec. Amer. Ahstr.
Programs 13, 414. CHOU I-M. (1982) Phase relations in the system NaCI-KCIHz0 Part 1: Differential thermal analysis of the NaCIKCI liquidus at I atmosphere and 500, 1000, 1500 and 2000 bars. Gcochim. Cosmochim. Acta 46, 1957- 1962. CHOU I-M. and EUGSTER H. P. (1981) DTA studies of the phase relations in the system H,O-NaCl-KCI at elevated P and T. (abstr.) EOS 62, 4 10. EASTOE C. J. (1978) A fluid inclusion study of the Panguna porphyry copper deposit, Bougainville, Papua New Guinea. Econ. Geol. 13, 72 l-748. ERWOOD R. J., KESLER S. E. and CLOKE P. L. (1979) Compositionally distinct saline hydrothermal solutions. Naica. Chihuahua, Mexico. Ecc’on. Geol. 74, 95-108. GEHRIG M. (1980) Phasengleichgewichte und pVT-Daten ternaerer Mischungen aus Wasser, Kohlendioxid und Natriumchlorid bis 3 kbar und 550°C. Ph.D. thesis. Univ. Karlsruhe, West Germany. HAAS J. L. JR. (1976) Physical properties of the coexisting phases and thermochemical properties of the Hz0 component in boiling NaCl solutions. U.S. Geol SurvcL’ Hull. 1421-A. 13 p. HFLGESON H. C. and KIRKHAM D. H. (1947a) Theoretical prediction of the thermodynamic properties of aqueous electrolytes at high pressures and temperatures, I. Summary of the thermodynamic/electrostatic properties of the solvent. .4mrTr. J. Sci. 274, 1089-I 198. HELGESON H. C. and KIRKHAM D. H. (1974b) Theoretical prediction of the thermodynamic properties of aqueous electrolytes at high pressures and temperatures. II. DebyeHuckel parameters for activity coefficients and relative partial properties. Amer. J. SCI. 274, I 199- I26 I, HELGESON H. C. and KIRKHAM D. H. (1976) Theoretical prediction of the thermodynamic properties of aqueous electrolytes at high pressures and temperatures. III. Equation of state for aqueous species at infinite dilution. Amer.
J. Sci. 276, 97-240. HELC;ESON H. C., KIRKHAM D. H. and FLOWERS G. C. (I 98 1) Theoretical prediction of the thermodynamic properties of aqueous electrolytes at high pressures and temperatures. IV. Calculation of activity coefficients, osmotic coefficients, and apparent molal and standard and relative partial molal properties to 600°C and 5 kb. Amer
J. Sri. 281, 1249-1516. KEEVIL N. B. (1942) Vapour pressures of aqueous solutions at high temperatures. Amer. Chem. Sot. J. 64, 841-850. POTTER R. W. II, BABCOCK R. S. and BROWN, D. L. (1977) A new method for determining the solubility of salts in aqueous solution at elevated temperatures. C7.S. C;c~)l Surv., J. Res. 5, 389-395. ROBIE R. S., HEMINGWAY B. S. and FISHER J. R. (1978) Thermodynamic properties of minerals and related substances at 298.15”C and 1 bar (IO’ pascals) pressure and at higher temperatures. U.S. Geol. Surv. Bull. 1452.456~. ROEDDER E. (1967) Fluid inclusions as samples of ore fluids. In Geochemistry of Hydrothermal Ore Deposits (ed. H. L. Barnes) Holt, Rinehart and Winston, Inc., p. 515-574. SOURIRAJAN S. and KENNEDY G. C. (1962) The system H20-NaCI at elevated temperatures and pressures. Amer J. Sri. 260, 115-141. TAKENOUCHI S. and KENNEDY G. C. (1965) The solubility of carbon dioxide in NaCl solutions at high temperatures and pressures. Amer. J. Sci. 263, 445-454. URUSOVA M. A. (1974) Phase equilibria in the sodium hydroxide-water and sodium chloride-water systems at 350550°C. Russian J. Inorg. Chem. 19, 450-454. URUSOVA M. A. (1975) Volume properties of aqueous SOlutions of sodium chloride at elevated temperatures and pressures. Russian J. Inorg. Chem. 20, 17 17- 172 1.
Geochimco PI Cosmochimica Ada Vol. 47, pp. 863 to 873 0 Pergamon Pres Ltd. 1983. Printed inU.S.A.
Phase relations in the system NaCl-KCl-Hz0 II: Differential thermal analysis of the halite liquidus in the NaCl-Hz0 binary above 450°C W. D. GUNTER'~~, I-MING CHOU* and SVEN GIRSPERGER’ ‘lnstitut fuer Kristallographie und Petrographic, ETH-Zentrum, CH-8092, Switzerland *U.S. Geological Survey, 959 National Center, Reston, Virginia, 22092, U.S.A. ‘Present address: Oil Sands Research Dept., Alberta Research Council, I I3 1587th Ave. Edmonton, Alberta, T6G 2C2, Canada (Received June 22, 1982; accepted in revised form January 2 I, 1983)
Abstract-Thermal analysis of the halite liquidus in the system NaCI-H20 has been conducted for NaCl mole fractions (XNacl)greater than 0.25 (i.e., >50 wt. % NaCI) at pressures between 0.3 and 4. I kb and temperatures greater than 450°C. The position of the liquidus was located by differential thermal analysis (DTA) of cooling scans only, as heating scans did not produce definitive DTA peaks. The dP/dT slope of the liquidus is positive and steep at high pressures, but at high X NaCland pressures below 0.5 kb it appears to reverse slope and intersects the three-phase curve (liquid-halite-vapour) at a shallow angle. However, due to the complex nature of the DTA signal when P 5 0.5 kb, there is considerable doubt about exactly what event has been recorded in the experiments conducted at these low pressures. The solubility of halite can be expressed as a function of the mole-fractional-based activity of NaCl in the liquid phase (L) in temperature (T, “K) and pressure (P, bars) In aNacI,L.T.Pj = -19.884 - 0.001275T - 1388/T + 3.2305 In (T) - O.O7574P/T Our liquidus data (based on IO compositions) above 500 bars for these brines were combined with this equation to generate activity coefficients of NaCl which were fit within their experimental uncertainties to the following one parameter Margules equation In -rNaCI(L.T.P, = (0.7268 - 695.7/T - 0.12 17P/T)( I - XNacJ2. Concentrated solutions of NaCl show negative deviations from ideality which rapidly increase in magnitude with decreasing XNacl. INTRODUCTION MASS TRANSPORT in crustat rocks is largely through a fluid medium by diffusion or infiltration if deformational mechanisms are not important. In order to formulate quantitative transport models, the thermodynamic properties of these fluids must be known. The two most common components in natural fluids are HI0 and CO*. HELGESON and KIRKHAM ( 1974a, 1974b, 1976) and HELGESON et al. (198 1) have analyzed the available data for aqueous fluids and have predicted the properties of the important solutes to 600°C and 5 kb, using an electrostatic model. However the thermodynamic properties of CO*-rich solutions are not as well known. The only pertinent experimental studies, aside from the binary COZH20, is on the ternary NaCI-CO>-H20, by TAKENOUCHI and KENNEDY (1965) and GEHRIG (1980) (assessed by BOWERS and HELGESON, 1981). Even the behavior of the common natural solutes in water is not well known at high concentrations, yet the concentration of total salts found in fluid inclusions ranges from dilute to greater than 70 wt. % (ROEDDER, 1967; EASTOE, 1978).The phase boundaries of the most well-known system, NaCLH20, are not well established at high temperatures and pressures. SouRIRAJAN and KENNEDY (1962) measured the compositions of coexisting gases and liquids between 863
350” and 700°C but they were unable to obtain data for fluids that contained more than 26 wt. % NaCI. DEVIL (I 942) reported values of XNacl for the point of intersection of the NaCMiquidus with the threephase boundary (halite-liquid-vapour) to a maximum temperature of 650°C and a maximum pressure of 0.39 kb. These measurements have been corroborated to within 5% by URUSOVA (1974) at high concentrations and by POTTER et al. (1977) at low concentrations. There have been no determinations of the position of the halite Iiquidus at higher pressures. The reaction
is nonquenchable and therefore ideal for study by differential thermal analysis (DTA). This contribution reports on the location of the halite liquidus above 450°C and 50 wt. % NaCl to 4.1 kb, as determined by DTA in two different laboratories. EXPERIMENTAL
PROCEDURE
Two different experimental approaches were used for the DTA. The first, a classical DTA configuration, involved the use of two thermocouples, and the voltage-difference signal between the two thermocouples was recorded as a function of temperature. This experimental set up has been fully
W. D. Gunter,
864 described
in earlier
publications
(CHOII
and
I-M. Chou
and S. Girsperger
EUGSTER,
1981; CHOIJ, 1982).
The other technique, in which only one thermocouple was used, is more correctly referred to as thermal analysis (TA), and is described in detail here. The sample holder (see CHOU, 1982, for a more detailed description) consists of a 4.0 cm long, 0.50 cm OD large gold tube into one end of which has been welded a 0.13 cm OD small gold tube to form a I .5 cm long well, which contains the tip of a 0.05 cm OD inconel-sheathed chrome]-alumel “sample” thermocouple. The other end of the large gold tube was left open until after the sample (NaCI plus H,O) was introduced and then was welded shut by a plasma arc. The sample container, with the thermocouple in place, was sealed in a cold-seal pressure-vessel, pressurized under argon. and placed in a horizontal tube furnace. Cooling rates, heating rates, and collection of the data were controlled by a PDP-I I computer linked to a D/A power supply (Hewlett Packard model 59501a’). In a heating run, the dc voltage from the power supply was incremented by the computer at the desired rate and directed to the temperature controller of the tube furnace so that it was opposed to that of the controller-thermocouple (see Fig. 1). Therefore, at each voltage increment, the controller would be “tricked” into supplying additional power to the furnace because the sum of the voltage of the controller-thermocouple and the D/A power supply would be low. In a cooling scan, the dc voltage from the D/A power supply was decremented stepwise to achieve the desired cooling rate. Although cooling scans were linear, heating scans departed about 1“C from linearity in a smooth fashion over a 125°C scan. We are limited to a maximum heating rate of lO”C/ min and a maximum cooling rate of 2”C/min in our standard tube furnaces. However, the maximum cooling rate was raised to lO”C/min by replacing the furnace insulation around the heating coil of the furnace with a coil of copper tubing through which cold water was circulated (= cooling furnace). Sample thermocouples were calibrated to an accuracy of f 1“C using a standard Pt/PtwRdio thermocouple from the Swiss National Bureau of Standards. Pressure was measured by a Brosa transducer, which was calibrated frequently against a standard 7 kb Heise bourdon-tube gauge and was accurate to ?5 bars. Temperatures and pressures were measured by a digital voltmeter (Hewlett Packard model 3456a) with a sensitivity of 0.1 microvolt, and recorded on the floppy disk of the PDP-I I (see Fig. 1). During heating and cooling scans, the sample temperatures were taken 25 times per degree centigrade. The temperature gradients across the sample position in the pressure vessel were measured at pressure using six chrome]-alumel thermocouples, positioned 2 cm apart. Under “isothermal” conditions, the gradients were found to be less than 5°C over the sample. During cooling scans, the gradient decreased by a factor of 2. whereas during heating scans, it became larger (see Fig. 2) for the normal tube furnace. The total temperature uncertainty for cooling runs is *3”C, increasing to a maximum of &6”C for heating runs in our standard tube furnaces. In the cooling furnace. the gradient reached a maximum of 6°C during cooling. The error introduced from the sample composition is only that introduced from the accuracy of weighing, ?50 micrograms, which corresponds to an error of fO.l wt. % for a 50-milligram sample. A thermal scan consisted of heating or cooling at a fixed rate, either 0.25, 0.5, 1, 2, 4.6, or 8.1”C/min over a 200°C range. Temperature readings were made over a 125°C “linear” portion of the scan. The sample temperatures viewed on a coarse scale reflect only the rate of power input lo the
’ Any use of trade names in this report is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey.
CONTROLLER
PRESSURE TRANSOUCER
PROGRAMMABLE
7kb
PDP-I1
L FIG. 1. Schematic ment system.
PUMP
COMPUTER
I diagram
of our control
and measure-
furnace, but on a fine scale they are sensitive to changes in the thermal properties of the sample. In order to construct the DTA peaks (i.e.. magnify the fine structure), part of the temperature-time TA-curve was fit by least squares to a straight line. The difference between the measured temperature-time curve and the linear fit over the 125°C “linear” part of the scan is the DTA fit (see Fig. 3) and is similar to that which would be measured in a standard DTA configuration using two thermocouples. A typical heating scan would start in the halite + liquid region, cross the liquidus, and end in the liquid-only region; a cooling scan would proceed in exactly the opposite direction. Obviously, the best region to make the linear fit would be the one-phase liquid region where the absorption or release of heat from the sample also would be expected to vary in a linear fashion. However, we found that in locating the onset of the DTA peak, it made no difference whether the linear fit was made in the one-phase liquid region or over the complete 125°C interval. As the latter technique was faster, it was used to obtain all the data presented in this paper. The DTA peak resolution of this technique was 4 microvolts (i.e., 0.1 “C), which limited our investigation of the halite liquidus to X,,, greater than 0.25. DTA peaks could not be resolved at lower concentrations. The heating scans did not produce a sharp DTA peak as the halite liquidus was crossed, except for pure NaCl, and so they could not be used definitively. In contrast the cooling scans involved a nucleation event at the liquidus, and the DTA analysis yielded well-defined peaks (Fig. 3). If no temperature gradients exist in the sample and the sample is homogeneous, the temperature estimates of the liquidus boundary from the cooling scans (see Table I) must represent minimum values, as stable nucleii can form only after supersaturation. An estimate of the equilibrium position of the liquidus was made by noting the shift in the position of the DTA peak as a function of cooling rate up to lO”C/ min. When the cooling rate is plotted as a square-root function, a straight line can be fit to the data and extrapolated
NaCl-KCI-HZ0
865
system
20. _
(a)
-
T=50S°C 510-620
before heating in 10' steps
15.
heating
rate
P = 2. 0 kb
G “,
= Z’/mln
10.
5 i+
5.
0. t 0.
2. DISTANCE
(b)
-
T=708"C 700-560
(cm)
4.
6.
before cooling in 10O steps
15.
cooling
rote
= 2O/rn1” P = 0.5
10.
0.
2. DISTANCE
(cm)
kb
Y
4.
FIG. 2. Temperature gradients along a typical TA capsule. End of bomb at “0” capsules were 4 cm long and were positioned 1 cm from the end of the bomb. (a) thermal gradient before the heating scan was initiated and solid lines represent the during heating at Z”C/min. (b) Dotted line is the thermal gradient before the cooling and solid lines represent the thermal gradient during cooling at Z”C/min.
to zero cooling rate (i.e., equilibrium). When this is done the lower rates of cooling (up to 1“C/min) are found to be within 1“C of the equilibrium position (see Fig. 4). The temperatures of the NaCI-HZ0 liquidus based on cooling scans reported by CHOU and EUGSTER (198 1) are about 15°C higher than the values reported here. This discrepancy is due to the difference in orientation of the furnace during experimentation. In the vertical furnace used by CHOU and EUGSTER (1981), the vertical dimension of the sample capsule is 25 mm while the horizontal dimension is 4 mm; consequently the sample in the capsule may not be homogenous owing to incomplete mixing of the highly viscous liquid formed by melting of the halite. Poor mixing can be explained by gravitational segregation of the twophase mixture, halite + liquid, below the liquidus coupled with slow diffusion rates in the liquid above the liquidus. In the horizontal tube furnace used in the present study, the vertical dimension of the sample capsule is only 4 mm; therefore, gravitational segregation of the sample and the distance of the vertical-diffusion path for complete mixing were minimized. Because the slope of the liquidus is steep, a two mole % difference in composition can account for the 15’C difference in temperature. Temperatures and DTA signals obtained in a heating-cooling cycle for X,,, = 0.588 at 2 kb in a vertical and horizontal furnace are compared
6.
position. All TA Dotted line is the thermal gradient scan was initiated
in Fig. 5. The inhomogeneity of the sample in the vertical furnace is demonstrated in the cooling cycle by the broad DTA signal, the presence of a second peak in the DTA curve, and the absence of a detectable break in the sample temperature curve. To improve the rate of mixing, a vibrator was attached to the cool head of the cold-seal vessel to agitate the sample during DTA scans in horizontal furnaces. No detectable difference in the position of the liquidus determined from agitated and unagitated runs could be discerned.
ISOPLETHS
IN THE
NaCI-H,O
BINARY
Because each of our experiments involved determining the halite liquidus position over a range of temperature and pressure for a constant XNacl, the geometry of each isopleth of concern must be understood in order to correctly interpret the DTA peaks. In Fig. 6, isopleths for X,,r of 0.25 (52 wt. %) and 0.75 (91 wt. %) have been constructed following the approach of ERWOOD et al. ( 1979). The main elements of the isopleths shown in Fig. 6 are
W. D. Gunter, I-M. Chou and S. Girsperger
866
heating
29.
rote
= 2’/mln
30. TEMP.
C,":;
32.
33.
31.
:
50.
40.
P s
. 30.
P = 1.0
kb
cooling
rote
= 1°C/nll"
9 L
20. -=z !I g
10.
29. TEMP.
C,"v"i
FIG. 3. DTA peaks from linear data fitting at 1 kb pressure. XNaCl= 0.745 (90.5 wt.% NaCI). Each point represents a single measurement. For a K-type thermocouple, 1“C = 0.04 mv. Note difference in scales, millivolts versus microvolts. (a) Heating scan at Z”C/min. “U” shape reflects nonlinearity of heating scans. (b) Cooling scan at 1“C/min.
the two three-phase curves. The first three-phase curve, liquid + ice + halite (ADAMS and GIBSON, 1930; ADAMS, 193 1), has a steep slope and lies at low temperatures. The phase relations of the dihydrate, NaCl . 2H20, are only important at lower temperatures and are not shown. The other three-phase curve, liquid + vapour + halite (I&EVIL, 1942; HAAS, 1976; POTTER et al., 1977), is touched by the apex of the “V” formed by the intersection of the halite liquidus, H + L/L, and the liquid + vapour boundary curve, L/L + V, and is finally truncated by the liquid + vapour boundary curve, L + V/V, at high temperature. For isopleths of intermediate to high concentration, these truncations are stacked against the high-temperature end of the three-phase curve (SOURIRAJAN and KENNEDY, 1962) and consequently are not very evident on Fig. 6. The apex of this “V” sweeps along the three-phase curve, attaining higher temperatures as the isopleths increase in concentration. In the two-phase regions, the phase compositions must fall off the plane of the isopleth. These
two-phase regions can be contoured and are identical for all isopleths. The liquid + vapour boundary, L/L + V and L + V/V, for each isopleth defines a constant composition line which becomes a fixed contour of the L + V fields for other bulk compositions if the contour falls within their L + V fields. Vapour contours for the H + V and the L + V fields have been presented in SOURIRAJAN and KENNEDY (1962) for low concentrations in their Figs. 10 and 18, respectively. The three-phase curve separates the two fields, and each vapour contour passes smoothly through it. The liquid-vapour boundary for each isopleth lies inside the critical curve except at one point, the critical point, where it is tangent to the critical curve. The critical points for the isopleths shown in Fig. 6 are unknown. The vapour contour changes to a liquid contour as it passes through a critical point. Between adjacent critical points, the liquid contour emanating from one critical point must cross the vapour contour from the other. Construction of these contours has not been attempted because the data
NaCI-KCI-H20 system Table 1.
OTA and TA data from isobaric scans across the halite liquidus and the J-phase boundary of N&l-H20. Phase
Fate of
Onset
No.
,","dundary (:b) method
$;;;,,
;r;f'
::a",
0.262 (53.5)
Liquidus TA
0.51 0.98 1.03
0.5-Z 0.::)
450-449 440 450-449
2.02 3.01
0.5-l 1.
454-453 451
;
0.339
Liquidus
0.41
-R
510
2
OTA
0.51 0.61 0.71 1.00 1.01 1.50 1.51
- 8 - 8 - 8 - 8 -8 -8
509 510 510 510 513 515-514 515
1 i
1.92 1.53 1.93 1.98
- 8 - 8 - 8
514 520 518 520
(62.4)
7 1 3
1 1 2 1
1
1 1
YNaCl 0.663 (86.5) O.R42 0.792 0.798 0.724 0.665 0.916 0.903 0.899 0.890 0.866 0.878 0.847 0.846 0.840 0.835 O.R25 0.830
0.496 (76.2)
Liquidus TA
0.58 1.00 2.01 3.55
0.25-Z 0.25-10 0.5-Z l-2
609-606 613-609 626-624 645-643
7 15 6 5
0.979 0.961 0.932 0.888
0.502 (76.6)
Liquidus TA
1.27 2.06 4.07
2 0.5-Z 1
616 628-626 655
1 2 1
0.940 0.925 0.077
0.526 (78.3)
Liquidus OTA
0.51 0.57 0.59 0.61 0.70 0.80 0.90 1.00 1.02 1.50 1.51 1.96
-8 -8 -8 -8 -8 -8 -P -6 -8 -8 -8 -8
623-621 622 622 622-621 623 624 625 626 626 634 634 641
3 1 1 2 1 1 1 1 1 1 1 1
0.985 0.976 0.974 0.973 0.969 0.965 0.961 0.957 0.955 0.948 0.947 0.939
2.00 2.02
-8 -8
641 641
1 1
0.936 0.935
649 656-655 665 672
1 2 2 4
0.979 0.965 0.960 0.948
690-685 680-675 670-665 662-661 658-657 654-651 659-656 680-678 667-665 649-648 618-612 673-675 666-668 650-653 619-629
11 11 4 4 3 7 9 7 7 7 7 6 6 6 6
1.152 1.109 1.067 1.034 1.016 0.993 0.971
703-697 699 694-690 690 684 680 683 681 682 683 685 690 691-690 699
2 1 2 1 1 1 1 1 1 1 1 1 2 1
1.076 I.060 1.039 1.023 1.000 0.984 0.988 0.979 0.976 0.979 0.971 0.973 0.976 0.968
0.588 (82.3)
Liquidus OTA
0.50 1.00 1.50 2.00
0.591 (82.4)
Llquidus TA
0.32 0.34 0.36 0.38 0.40 0.50 1.01 0.32 0.34 0.36 0.38 0.32 0.34 0.36 0.38
3-phase TA
0.663 (86.5)
867
Liquidus OTA
0.32 0.33 0.35 0.37 0.38 0.40 0.49 0.51 0.59 0.60 0.80 1.01 1.02 1.49
1; -8 -8 0.5-Z 0.5-Z l-2 l-2 l-2 0.5-Z 0.5-Z 0.5-Z 0.5-Z 0.5-Z 0.5-Z /'1::\
-8 -8
-8 -6 -8 -8 -8 -6 -8 -8 -8 -8 -I3 -0
(i.e., the positions of L/L + V and L + V/V) needed to construct liquid or vapour contours in the L + V field are incomplete, particularly at high XNaa (>26 wt. %). In Fig. 6, the position of each liquid + vapour curve boundary is constrained only by its two intersections with the liquid + vapour + halite curve, and by the fact that the slope of the boundary must approach the low slope of the liquid + vapour curve,
Liquidus OTA 3-phase (hi;; T)
3-phase (law T) OTA 0.745 (90.5)
Liquidus TA 3-phase (high T)
1.000 (100.)
Liquidus TA
1.51 1.97 2.00 0.32 0.33 0.35 0.37 0.38 0.32 0.37 0.30 0.51 1.00 2.02 0.30 0.30
- 8 - 8 - 8 - 8 - 8 - 8 - 8 - 8 -R - 8 0.25-Z 0.25-Z 0.25-Z 0.25-Z 0.25-Z (0.5-10)
1.02 0.25-3 0" l-2 1.02 (0.25-Z) O"\ (l-2)
699 708 708 6~33-678 666 656-652 630 625 490 542
0.967
718-716 718-716 726-722 W;;;
1.010 0.994 0.984 0.979
0.964
0.961
690-692 825-824
870
824-825 800
1.000 1.000 1.000 1.oon
*Rates of heating scans are given in parentheses; all ~;pe;t;lues In this column are for cooling scans.
L/V of the NaCl unary for concentrated isopleths such as these. Consequently, the placement of our boundaries for the liquid + vapour field is very uncertain for these two concentrated NaCl isopleths. Also, each H f L/L boundary defines a constant liquid-composition line, which is a fixed contour for the L + H field regardless of bulk composition. All these liquid contours are truncated at the three-phasecurve. The positions of the constant liquid-composition contours in the liquid + halite field are determined in this paper. In our cooling scans, the scan was always initiated above the halite liquidus. Because the liquid field narrows at low pressure until it is truncated by the three-phase curve, the possibility exists that several phase boundaries are intersected in an isobaric cooling scan; if so, several DTA peaks would be expected to appear. Intersection ofthe three-phase curve would produce a large peak, because the liquid phase must be completely transformed to halite + vapour before the boundary can be crossed. Crossing of the L/L + V boundary might not be recognizable because the scan would start in the L + V field. The disappearance of vapour to form liquid during cooling would be a gradual process whose rate would be dependent on the spacing of liquid-vapour contours in P-T space (see SOURIRAJAN and KENNEDY, 1962, Fig. 18); this process would not be conducive to producing a welldefined peak. This is analogous to heating scans crossing the halite liquidus where no peak is discernible (see Fig. 3). On the contrary, for heating scans that cross the liquid-vapour boundary, a sharp peak would be expected, because in going from the liquid to the L + V field, vapour must nucleate. LOW-PRESSURE
EXPERIMENTS
We have chosen to discuss the results of our experiments in two different sections because at high
W. D. Gunter, I-M. Chou and S. Girsperger
868
X(thCl>
= 0. 50
P = 1.0
kb
25. 37.
25. 27, 0.
1. ROOT
SQ.
2. OF COOLING
3. RATE
FIG. 4. Extrapolation to equilibrium (i.e.. zero cooling rate) at 1 kb pressure. Hexagons represent the positions of onset temperatures for the halite liquidus at different scan rates for XNr(.,= 0.496 (76.2 wt.% NaCI). Cooling rate units = “C/minute.
pressures (>500 bars), the position of the DTA peak attributed to the halite liquidus is relatively insensitive to pressure. At lower pressures, the shape of the DTA signal is more complex, and its position is very sensitive to small changes in pressure. Extrapolation of the high-pressure liquidii of halite along lines of constant X,,, to the three-phase curve intersect it -25°C below their points of intersection as reported by KEEVIL (1942). In order to investigate this discrepancy we performed several experiments at low pressure. The position of the three-phase curve
I
/ I \
500 -
- 0
I
T, t
AT
\
\ \
i
\ \, AT
0.2
--__.
i
[
(mv)
‘\ \_/=-~----
FIG. 5. Temperatures (solid curves) and differential temperatures (dashed curves) from the two-thermocouple configuration experiments for XNacl = 0.588 (82.3 wt.% NaCI) at 2 kb in a vertical tube furnace (top) and a horizontal tube furnace (bottom). The time offsets of the solid and dashed curves are indicated by vertical lines.
(liquid + vapour + halite) was reversed on heating and cooling runs (see Fig. 7 and Table I) and agrees reasonably well with that determined by SOURIRAJAN and KENNEDY( 1962). Fluids with XNaClof 0.59 (82 wt. % NaCI) and 0.66 (87 wt.% NaCI) were chosen for study because their liquidii lie at higher temperatures than the pressure maximum on the three-phase-curve. A liquidus intersecting the three-phase curve below the maximum could not be determined. because during a cooling scan, the initial DTA peak from the three-phase curve (see Fig. 6) would mask the much smaller, succeeding liquidus peak. In Fig. 7, the position of the halite liquidus determined from cooling scans has been plotted to show the sharp reversal in slope of the liquidus below 500 bars as it approaches the threephase curve. However, the interpretation of the position of the liquidus was complicated by ‘peak-splitting’ at these low pressures (Fig. 8). Both doublets and triplets occur, which only shrink to a single peak above 400 bars. The onset position of the first peak has been used to plot the liquidus position in Fig. 7. We cannot satisfactorily explain the splitting of the liquidus peak at low pressures. We do not think that the first peak represents the intersection of the liquidvapour boundary, as the peak doublets are not reproducible from one scan to another, and they increase in temperature with decreasing pressure. Even ifthis interpretation is incorrect, the event is probably recorded by one of the peaks of the doublets or triplets and therefore would not alter our conclusion that the liquidus moves to higher temperatures at low pressures as it approaches the three-phase boundary. At such low pressures, the system may no longer be binary. The hydrolysis reaction (HAAS, pers. commun.) NaCl,c, + HzO,rj -
NaOH,(
or ,_I+ HCIp,
where “C” stands for crystal, “L” for liquid, and “V”
NaCl-KCI-H20 system
869
(a) X (N&1)=0.25
,’ ,’ ,’ _’ ,’
Liquid
+ Hollte
Llquld
I’
,’ ,’
,I’
L lquld
+ Vopour :
5
100.
00.
300.
500.
TEMPERATURE
700.
900.
1
('C)
2000.,
I
I
(b) 3
1500. t
X(NaC 1)=0.75
:
Llquld
+ Halite
I
I :
.*‘
fiae.
100.
300.
500.
TEMPERATURE
_
,*
.I
700.
L + V _-
-
i-00.
("C)
FIG. 6. Isopleths in the NaCl-H,O binary. Dashed lines are extrapolations (see text). (a) Xvac., = 0.25 (52.0 wt.% NaCl) (b) XNaCl= 0.75 (90.7 wt.% NaCI).
for vapour, might be the reason for the slope reversal of the isopleth curves. If HCl and NaOH are present in appreciable amounts, the double or triple DTA
signal in a cooling scan at low pressure and high XNaCl might reflect the complex phase relations in the reciprocal system NaCl-H20-HCl-NaOH. R. 0. FOUR-
1000.
Hal lte
0.
I
P
+
7
Vapour
I
600.
620.
640.
Temperature
660.
'C
680.
700.
FIG. 7. P-T plot of our low-pressure data (“Y”) for the location of the three-phase curve (horizontal solid line and extrapolated dotted line are based on KEEVIL, 1942, and SOURIRAJAN and KENNEDY, 1962) and the liquidus isopleths for XN,c, = 0.591 (82.4 wt.% NaCI) and XNaCl= 0.663 (86.5 wt.% NaCl). Data points for each liquidus isopleth are joined in order of descending pressure by dashed lines. The small irregularities in the line reflect the uncertainties in the data.
W. D. Gunter.
870 200.
and S. Girsperger
vapour phase. It is also possible that the air trapped in our DTA capsules may exsolve at low pressure and contribute to the DTA peak shape. The capsule filling with halite + water was less in the low-pressure experiments because of the larger coefficient of thermal expansion for aqueous fluids compared with higher pressures. This smaller (halite + water)/air would enhance this exsolution. Finally, the reversal of slope of the halite liquidus below 500 bars may be the result of the partial molal volume of NaCl decreasing rapidly at low pressures because of the large increase in the partial molal volume of water. Obviously, many more experiments must be completed at these low pressures before all of these questions can be resolved. Our intent here is to point out the uncertainty in the interpretation of these simple experiments based on DTA. Hopefully these lowpressure phase equilibria will be elucidated soon.
.-
1
X(NoC1)
I-M. Chou
= 0. 59
P (kb)
150.
s ,1 2 100. : 2 _1 ; 50.
HIGH-PRESSURE
0. 2 4.
I
25.
26. TEMP.
27. (mV)
28.
+
2
FIG. 8. DTA peaks from linear data fitting of cooling scans at low pressures. XNeCl = 0.591 (82.4 wt.% NaCI), P = 0.32 to 1.O 1 kb. Diamonds mark the positions of threephase-boundary peaks while arrows point to the onset temperatures for the first DTA peak. Each data point plotted represents a single measurement. 1°C = 0.04 mv. Note difference in scales; microvolts verws millivolts.
NIER
(unpublished data for alkali chloride brines) has found that the chloride/total-alkali ratio is one in the fluid at high pressures and increases rapidly at low pressures in the vapour in the two-phase region, which would correlate with an increase of HCI in the
EXPERIMENTS
Our results for fluids between XNaCl of 0.25 and I are presented in Fig. 9 and Table 1. We believe that the liquidus temperatures are within 5°C of equilibrium values, for the following reasons: 1. CHOU ( 1982) found that his DTA results from cooling scans for the location of the liquidus in the binary NaCl-KC1 agreed to within 5°C of most of the data reported by earlier investigators who used a variety of different techniques. 2. The position of the three-phase boundary, liquid-vapour-halite, determined by us in heating scans. is essentially identical to that obtained from cooling scans (see Table 1). 3. Extrapolation of the DTA peak positions to zero cooling rate is always within I o of the positions for the slower cooling rates. 4. Finally, our data are consistent with those of
2500.
500.
700. 600. Temperature 'C
800.
900.
FIG. 9. P-T plot of our high-pressure (>0.5 kb) data for the location of the halite liquidus. Each nearlyvertical solid line is a liquidus contour whose position was calculated from a one-parameter Margules fit of our data above 500 bars and Eqn. (3). The dashed lines at low pressure join the point of intersection of each contour with the three-phase curve (reported by WEEVIL, 1942) to the tip of our high pressure contour. The dashed end of the three-phase curve is extrapolated. The width of a hexagon data symbol is 8°C.
NaCl-KC&H20 system URUSOVA
(1975) who at 1000 bars pressure found only liquid at 50 wt. % NaCl and 450°C and at 60 wt. % NaCl and 550°C (see Fig. 10). ‘The mixing properties of NaCl and Hz0 are determined by the intermolecular forces between solute and solvent, and by the melting point and the enthaipy of fusion of halite. These thermodynamic reiationships are embodied in the expression for equilibrium between NaCl-Hz0 liquid and pure crystalline halite: viz, &MW .T.P)+- RT In
(YN~cI(L.T,P)XN~CI(L.T.P))
where 7‘ = T(K), P = P(bars), and the superscript “0’” is used to indicate a pure phase consisting of a single component. Assuming that the difference in heat-capacity between liquid and crystal can be expressed as a function of the farm LX&,
= AL+__c)+ Ab&T,
that the volume difference between Iiquid and crystal (AV~,__,,) is independent of temperature and pressure, and that no solid-solution occurs in the crystalline halite, and, finally, choosing a standard state of pure crystal and pure metastable liquid NaCl at the temperature and pressure of interest, Eqn. (1) can be used to describe the solubility of halite at any temperature and pressure; viz., -RT In _= AH" +
(YNaCI(L.T.P)XNaCI(L.T.P)) rn.NaC.~.--C.Tm.d 1 - T/Tm)
.&z$_~(T - Tm - T In (TfTm))
- O.SAh~t_.c(T - Tm)” f AVyL_,l(P - I)
(2)
The heat of fusion at 1 bar (AHO,,NacI(L-~,Tm,I))r the fusion temperature at 1 bar (Tm), and the heat-capacity functions for NaCl have been tabulated by ROW er ul. (1978). The difference in volume be-
871
tween liquid and crystal has been estimated from the Clausius-Clapeyron slope for the halite-liquid boundary in the NaCl unary. Therefore substituting these quantities into Eqn. (2) yields 111(YN~CCI(L.T,P~XNaCI(L,.T,P)) = -19.884 -
- 0a001275T
1388/T + 3.2305 In (T) - O.O7574P/T
Because the only unknown in this equation is the interaction term between NaCl and HzO. ~N~cr(r.T,p), an activity coethcient can be calculated at each experimentally determined point on the liquidus, and these values are listed in Table 1. The effects of errors in the thermodynamic properties for pure NaCl on the calculated value of ~N~cl(r,T,p)were estimated in the following manner. -&cr(r,T.P) was recalculated and compared with the original values assuming that: 1. Ac~~-~, = Au$_-~~. The maximum difference in ~~~~~~~~~~~~ due to this a~umption was 0.02, and it occurred at the lowest values of XH,. Generally, however, the differences were less than 0.01. 2. AC&~, = 0. The maximum difference was greater; i.e., up to 0.06, but most were less than 0.02. 3. An error of 2°C in T,. The difference was always less than 0.0 1. 4. An error of 100 calories in 4H~,NaCI~L_C,Tm,l~. The difference at LyNaer< 0.5 was as high as 0.02, but at higher XNaClthe differences were always less than 0.0 1. 5. AV& = a constant. This assumption introduces negligible error because the Clausius-Clap~yron slope is constant (24”C/kb) to within i “C when estimated at SOO-bar intervals between 1 and 2000 bars on the basis of data in Table 1 and the data of CHOW ( 1982). Consequently, for )y,,,, < 0.5, calculated values of YNacI(L,T,P) are probably within 0.1 of the correct values and they should be even closer to the correct
P = 1000
bars
/I-.Halite
4”0i.~
0. 4
(3)
m
X (NaICl)
+ Llquld
1.0
FIG. 10. Marguies fit of our data at I kb pressure. The triangles illustrate one-phase (liquid) data from URUSOVA( 1975). The data of SOURIRAJAN and KENNEDY( 1962) are plotted as the solid boundary of
the liquid + vapour field (t + V). Our data for the halite liquidus are represented by hexagons.
W. D. Gunter,
872
I-M. Chou
and S. Girsperger
\ Hollte 500.
+ Vapour
1.
600. 700. Temperature "C
FIG. II. P-T plot (20.5 kb) for the location of the halite liquidus of the liquidus contours were calculated from Eqns. (3) and (4).
values at higher mole fractions, provided that the position of the liquidus is located accurately, and that there are negligible errors in the data used for NaCl from ROBIE et al. (1978). The activity coefficients were fit to a one-parameter Margules solution model ln
YNaCI(L.T.P)
=
MIT)(
1 -
XNaCI(L,T.P)?
(4)
where A was assumed to be a linear function of temperature and pressure. This formulation predicts -rNaCI(L,r,Pj to within 1% of the values determined from experiment, listed in Table 1. The fit equation for the data above 500 bars is A = 0.7268T
- 695.7 - 0.1217P
(5)
and was combined with Eqns. (3) and (4) to calculate the positions of the halite liquidus. When compared with the experimental data plotted in Fig. 9, except for one data point at low pressure, the maximum difference between experiment and the fit prediction is 4°C or 1% in the XNaCl value, which is within the 1
I.
-.._ 800.
900.
at X Nacl of 0.05 intervals.
Position
uncertainty of the experimental data. This also applies to the three high-pressure data points between 3 and 4 kb, which are not plotted in Fig. 9. Hence, a more complicated model is unwarranted at this time. The equations used above to locate the position of the liquidus can be confidently applied to higher pressures, as the variation is close to linear, but they should not be extrapolated to lower temperatures because the curvature of the liquidus changes rapidly at low lyNacI (Fig. 10). Liquidus isopleths were calculated at regular XNac.r intervals and are plotted in Fig. 1I. Equation (3) was used to calculate the smoothed values for ~N~cr(t_T,p) as a function of XNac,, T, and P. Each point on Fig. 12 ItpreSentS yNacI(L,T,P) at a SpeCifiC point on the liquidus. Although the simple one-parameter Margules mixing model predicts accurate values for ~~~cr(L,r,p) on the liquidus, there is no way of estimating how far this model’s predictions can be extended into the one phase liquid field on any isobaric-isothermal sec-
01
w
0.
8_
0. 2
Llquld
+ Halite
0. 6 0. 4 X(NaC1)
0. 8
1. 0
FIG. 12. Values of yNac,(L.T,Pjfor halite saturated NaCI-HZ0 solutions calculated from the one-parameter Margules fit; contoured for pressure (solid lines) and temperature (dashed lines). Temperature contours are spaced at 100°C intervals.
NaCI-KCI-HZ0 without other data for comparison. The model cannot be correct in the more dilute regions, as it does not predict the miscibility gap between liquid and vapour that has been recorded by SOURIRAJAN and KENNEDY (1962) and plotted in Fig. 10. However, there is little doubt that concentrated solutions of NaCl show negative departures from ideality which rapidly increase in magnitude with decreasing X,,c,.
tions,
CONCLUSIONS
Unfortunately, the sensitivity of the DTA technique used here for the detection of the halite liquidus was not sufficient to allow extension of our measurements to XNaCl< 0.25. Therefore we were unable to construct complete isopleths because our data do not overlap with the liquid-vapour boundary determinations made by SOURIRAJAN and KENNEDY (1962). Even though our data fit was based on a thermodynamic model, the model is too simple to extrapolate with any confidence to lower temperatures and more dilute liquids. Rather, it should be used to interpolate between our experimental points for the position of the liquidus (Fig. 11) and to calculate yNaclci,l,Pj over this region (Fig. 12). At low pressure, the reversal of slope for the position of the DTA peak onsets and the more complex nature of the DTA signal has been established for these high NaCl concentrations. Fluid inclusion compositions determined from homogenization temperatures resulting from disappearance of halite should be reexamined in the light
of these
new
data.
In this paper we have demonstrated that differential thermal analysis is an experimental technique which permits rapid determination of phase boundaries in concentrated aqueous solutions. We are working on several different experimental techniques to cover the more dilute regions and also the lower pressure regions, and we hope to report on these in the future. ilc,knowl~dRmmts-This study was initiated at the Johns Hopkins University and supported by NSF grant EAR7300391 AOZ/Eugster, while one of us (I.M.C.) was a postdoctoral fellow. The equipment which enabled W.D.G. and SC. to complete this study was purchased through special funds from E.T.H. Comments by J. L. Hass, Jr. and R. J. Bodnar of the U.S.G.S. were helpful. We thank J. L. Haas, Jr., and R. 0. Fournier (U.S.G.S.) and Urs Raz (E.T.H.) for stimulating discussions, B. Buhlmann (E.T.H.) for assistance with the drafting, and J. G. Blencoe, (Pennsylvania State Univ.) D. A. Crerar (Princeton Univ.) and R. W. Potter II (Occidental Research Corp.) for the’ final review of this paper. REFERENCES ADAMS L. H. (1931) Equilibrium in binary systems under pressure. 1. An experimental and thermodynamic investigation of the system NaCI-Hz0 at 25°C. Amer. Chem.
Sac. .I. 53, 3769-38 13. ADAMS L. H. and GIBSON R. E. (1930) The melting curve of sodium chloride dihydrate. Amer. Chem. Sot. J. 52,
4252-4264. BOWERS T. S. and HELGESON H. C. (198 1) Thermodynamic
system and geochemical consequences system HzO-C02-NaCl. (abstr.)
873 of immiscibility in the Geol Sec. Amer. Ahstr.
Programs 13, 414. CHOU I-M. (1982) Phase relations in the system NaCI-KCIHz0 Part 1: Differential thermal analysis of the NaCIKCI liquidus at I atmosphere and 500, 1000, 1500 and 2000 bars. Gcochim. Cosmochim. Acta 46, 1957- 1962. CHOU I-M. and EUGSTER H. P. (1981) DTA studies of the phase relations in the system H,O-NaCl-KCI at elevated P and T. (abstr.) EOS 62, 4 10. EASTOE C. J. (1978) A fluid inclusion study of the Panguna porphyry copper deposit, Bougainville, Papua New Guinea. Econ. Geol. 13, 72 l-748. ERWOOD R. J., KESLER S. E. and CLOKE P. L. (1979) Compositionally distinct saline hydrothermal solutions. Naica. Chihuahua, Mexico. Ecc’on. Geol. 74, 95-108. GEHRIG M. (1980) Phasengleichgewichte und pVT-Daten ternaerer Mischungen aus Wasser, Kohlendioxid und Natriumchlorid bis 3 kbar und 550°C. Ph.D. thesis. Univ. Karlsruhe, West Germany. HAAS J. L. JR. (1976) Physical properties of the coexisting phases and thermochemical properties of the Hz0 component in boiling NaCl solutions. U.S. Geol SurvcL’ Hull. 1421-A. 13 p. HFLGESON H. C. and KIRKHAM D. H. (1947a) Theoretical prediction of the thermodynamic properties of aqueous electrolytes at high pressures and temperatures, I. Summary of the thermodynamic/electrostatic properties of the solvent. .4mrTr. J. Sci. 274, 1089-I 198. HELGESON H. C. and KIRKHAM D. H. (1974b) Theoretical prediction of the thermodynamic properties of aqueous electrolytes at high pressures and temperatures. II. DebyeHuckel parameters for activity coefficients and relative partial properties. Amer. J. SCI. 274, I 199- I26 I, HELGESON H. C. and KIRKHAM D. H. (1976) Theoretical prediction of the thermodynamic properties of aqueous electrolytes at high pressures and temperatures. III. Equation of state for aqueous species at infinite dilution. Amer.
J. Sci. 276, 97-240. HELC;ESON H. C., KIRKHAM D. H. and FLOWERS G. C. (I 98 1) Theoretical prediction of the thermodynamic properties of aqueous electrolytes at high pressures and temperatures. IV. Calculation of activity coefficients, osmotic coefficients, and apparent molal and standard and relative partial molal properties to 600°C and 5 kb. Amer
J. Sri. 281, 1249-1516. KEEVIL N. B. (1942) Vapour pressures of aqueous solutions at high temperatures. Amer. Chem. Sot. J. 64, 841-850. POTTER R. W. II, BABCOCK R. S. and BROWN, D. L. (1977) A new method for determining the solubility of salts in aqueous solution at elevated temperatures. C7.S. C;c~)l Surv., J. Res. 5, 389-395. ROBIE R. S., HEMINGWAY B. S. and FISHER J. R. (1978) Thermodynamic properties of minerals and related substances at 298.15”C and 1 bar (IO’ pascals) pressure and at higher temperatures. U.S. Geol. Surv. Bull. 1452.456~. ROEDDER E. (1967) Fluid inclusions as samples of ore fluids. In Geochemistry of Hydrothermal Ore Deposits (ed. H. L. Barnes) Holt, Rinehart and Winston, Inc., p. 515-574. SOURIRAJAN S. and KENNEDY G. C. (1962) The system H20-NaCI at elevated temperatures and pressures. Amer J. Sri. 260, 115-141. TAKENOUCHI S. and KENNEDY G. C. (1965) The solubility of carbon dioxide in NaCl solutions at high temperatures and pressures. Amer. J. Sci. 263, 445-454. URUSOVA M. A. (1974) Phase equilibria in the sodium hydroxide-water and sodium chloride-water systems at 350550°C. Russian J. Inorg. Chem. 19, 450-454. URUSOVA M. A. (1975) Volume properties of aqueous SOlutions of sodium chloride at elevated temperatures and pressures. Russian J. Inorg. Chem. 20, 17 17- 172 1.