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The ionization constants of aqueous MgCl2 at elevated temperatures and pressures—a revision
NOTE
The ionization constants of aqueous MgCl, at elevated temperatures and pressures-a revision JOHN D. FRANTZ Geophysical
Laboratory.
2801 Upton St., N.W., Washington.
DC 20008, U.S.A.
and ROBERT K. POPP
Department of Geology.
Texas A and M University TX 77843. U.S.A.
Abstract- -The ionization constants of aqueous MgCI, ments of electrical conductance. from the calculations
~~~AR~C~RIZATI(~~ of Imetamorphic fluids containing dissolved metals is an important step towards understanding material transport in the earth’s crust. Computation of the distribution of species in fluids coexisting with mineral assemblages requires ionization constants as well as mineral solubility constants. In Part I of the series “Mineral-solution equilibria.. .” (FRANTZ and POPP. 1979), an attempt was made to calculate the equilibrium constant for the following reaction as a function of temperature at 2000 bars: MgCI,
= Mg’ - c 2c1-
College
Station,
have been revised. in the light of recent measurereported previously by FRANTZ and POPP (1979).
(b) The activity coefficients of all aqueous are unity under these conditions.”
species
Recent electrical conductance studies by FRANTZ and MARSHALL (1981) have demonstrated that the first assumption is invalid. The species MgCl” must be considered as a major magnesium complex and the ionization constant of reaction (1) cannot be determined by the method shown in Part I. Frank and Marshall determined the constants (K,. K,) for the following two stepwise ionization reactions:
(1) MgC& !=’ MgCl-
reaction is a combination of two stepwise ionization constants. The purpose of this note is to revise these calculations in light of recent electrical conductance measurements. FRANTZ and POPP (1979) used the variation of the total magnesium concentration as a function of the concentration of associated hydrogen chloride in a fluid equilibrated with the mineral assemblage talc + quartz to determine the predominant aqueous magnesium species. They found that at temperatures above 500 C at 2000 bars. associated MgCl, was the major species in fluids with total magnesium concentrations ranging from 0.1 to 4 modal. At tem~ratures below 500 C. however, charged magnesium species appeared to become increasingly important and Mg’+ was concluded to be the dominant magnesium species below 400 C. Frank and Popp used these data to compute the constant for reaction (1). Their results (0~. dt., Fig. 5) were based on the following assumptions (op cit., p. 1232):
+ Cl
The
MgCI+ “2 Mg2+ + Cl-
I
I
1.3 I/TIK)
“(a) Mg’+ and MgCI, are the only predominant magnesium species existing in the fluid at the temperature- pressure ranges studied.
1
I
1.4
1.5 x IO3
Fig. I. Logarithm of the first, second, and combined ionization constants of MgClz at 2000 bars as a function of temperature.
2511
2512
Notes
at temperatures to 600-C and pressures to 4000 bars. Their data at 2ooO bars (summarized in Fig. 1) can be used to compute the constant of reaction (1) by multiplying K, by K,. The values of this constant (shown in Fig. 1) differ considerably from those given in Part 1. In summary, FRANTZ and Pw~ (1979) correctly demonstrated that associated MgCI, was the dominant magnesium complex at temperatures above 500 C at 2000 bars in fluids containing 0.1 to 4 molal total magnesium. They also showed that charged magnesium species become ~ncreasjngly import~~nt between 500 and 4OO’C. Due to incorrect assignment of species, however. their calculations of the ionization constant for reaction (1) computed from solubility studies are invalid and equation (19) and Figs 5 and 8 of their 1979 paper (op. cit.) should not be used in subsequent
work. The results of the calculation
of the free energy
differences between associated MgCI, and HCI were fit using the reversed buffered experiments obtained at temperatures above 500°C and are not affected by this revision.
REFERENCES FHAN.rzJ. D. and POPP R. K. (1979) Mineral-solutmn cyuilibria 1. An experimental study of complex& and
43, 122% 1239.- - FRANTZJ. D. and MARSHALLW. t. (IY82) Electrrcal conductances and ionization constants of calcium chloride and magnesium chloride in aqueous solutions at trmperatures to 600% and pressures to 4000hars. Im. .I. Sci. (in press).
The ionization constants of aqueous MgCl, at elevated temperatures and pressures-a revision JOHN D. FRANTZ Geophysical
Laboratory.
2801 Upton St., N.W., Washington.
DC 20008, U.S.A.
and ROBERT K. POPP
Department of Geology.
Texas A and M University TX 77843. U.S.A.
Abstract- -The ionization constants of aqueous MgCI, ments of electrical conductance. from the calculations
~~~AR~C~RIZATI(~~ of Imetamorphic fluids containing dissolved metals is an important step towards understanding material transport in the earth’s crust. Computation of the distribution of species in fluids coexisting with mineral assemblages requires ionization constants as well as mineral solubility constants. In Part I of the series “Mineral-solution equilibria.. .” (FRANTZ and POPP. 1979), an attempt was made to calculate the equilibrium constant for the following reaction as a function of temperature at 2000 bars: MgCI,
= Mg’ - c 2c1-
College
Station,
have been revised. in the light of recent measurereported previously by FRANTZ and POPP (1979).
(b) The activity coefficients of all aqueous are unity under these conditions.”
species
Recent electrical conductance studies by FRANTZ and MARSHALL (1981) have demonstrated that the first assumption is invalid. The species MgCl” must be considered as a major magnesium complex and the ionization constant of reaction (1) cannot be determined by the method shown in Part I. Frank and Marshall determined the constants (K,. K,) for the following two stepwise ionization reactions:
(1) MgC& !=’ MgCl-
reaction is a combination of two stepwise ionization constants. The purpose of this note is to revise these calculations in light of recent electrical conductance measurements. FRANTZ and POPP (1979) used the variation of the total magnesium concentration as a function of the concentration of associated hydrogen chloride in a fluid equilibrated with the mineral assemblage talc + quartz to determine the predominant aqueous magnesium species. They found that at temperatures above 500 C at 2000 bars. associated MgCl, was the major species in fluids with total magnesium concentrations ranging from 0.1 to 4 modal. At tem~ratures below 500 C. however, charged magnesium species appeared to become increasingly important and Mg’+ was concluded to be the dominant magnesium species below 400 C. Frank and Popp used these data to compute the constant for reaction (1). Their results (0~. dt., Fig. 5) were based on the following assumptions (op cit., p. 1232):
+ Cl
The
MgCI+ “2 Mg2+ + Cl-
I
I
1.3 I/TIK)
“(a) Mg’+ and MgCI, are the only predominant magnesium species existing in the fluid at the temperature- pressure ranges studied.
1
I
1.4
1.5 x IO3
Fig. I. Logarithm of the first, second, and combined ionization constants of MgClz at 2000 bars as a function of temperature.
2511
2512
Notes
at temperatures to 600-C and pressures to 4000 bars. Their data at 2ooO bars (summarized in Fig. 1) can be used to compute the constant of reaction (1) by multiplying K, by K,. The values of this constant (shown in Fig. 1) differ considerably from those given in Part 1. In summary, FRANTZ and Pw~ (1979) correctly demonstrated that associated MgCI, was the dominant magnesium complex at temperatures above 500 C at 2000 bars in fluids containing 0.1 to 4 molal total magnesium. They also showed that charged magnesium species become ~ncreasjngly import~~nt between 500 and 4OO’C. Due to incorrect assignment of species, however. their calculations of the ionization constant for reaction (1) computed from solubility studies are invalid and equation (19) and Figs 5 and 8 of their 1979 paper (op. cit.) should not be used in subsequent
work. The results of the calculation
of the free energy
differences between associated MgCI, and HCI were fit using the reversed buffered experiments obtained at temperatures above 500°C and are not affected by this revision.
REFERENCES FHAN.rzJ. D. and POPP R. K. (1979) Mineral-solutmn cyuilibria 1. An experimental study of complex& and
43, 122% 1239.- - FRANTZJ. D. and MARSHALLW. t. (IY82) Electrrcal conductances and ionization constants of calcium chloride and magnesium chloride in aqueous solutions at trmperatures to 600% and pressures to 4000hars. Im. .I. Sci. (in press).