Phase equilibria in the system calcium carbonate-water

Geochimica

et Cosmochimlca

Acta,

1975. Vol. 39. pp. I193 to 1197. Pergamon

Press.

PrInted in GreatBritain

NOTE Phase equilibria in the system calcium carbonate-water GREGGMARLAND Department of Geology, Indiana State University, Terre Haute, Indiana, 47809, U.S.A. (Received

10 June

1974; accepted

in revised form 21 November

1974)

Abstract-High pressure experiments and thermodynamic calculations have explored the pattern phase equilibria in the system CaC03-H,O with particular emphasis on the phase CaC03.H,0. The data sueeest that CaCOa .H,O is unstable with respect to argonite plus water everflhere the region c&idered (%14o”e anh 1 bar-20 kb).

WITHTHErecently available data on the stability and thermochemical parameters for CaCO, .6H,O (MARLAND,1975), CaCO,. Hz0 (HULL and TURNBULL, 1973) and vaterite (CaC03) (TURNBLJLL,1973) it becomes enticing to try to construct the pattern of phase equilibria in the binary system CaC03-H,O. The polymorphs of calcium carbonate (calcite and aragonite) have often been evaluated dry in spite of the fact that the most important natural occurrences are associated with aqueous solutions. MARLAND (1975) has now shown that an invariant point for calcite, aragonite, CaC03. 6H20 and water exists at about 3.02 kb and - 2.O”Cand that much of the stability field generally attributed to aragonite is in fact occupied by CaCO, .6Hz0 in the water saturated system. MARLAND (1972) employed the geometric method of Schreinemakers (see ZEN, 1966) to describe qualitatively where the stability field for C&O3 .HzO must be if it exists at all and calculations by HULL and TURNBULL (1973) have suggested that CaC03 .H,O is stable with respect to aragonite + water at 25°C when the total pressure is greater than 11.5 kb. ALBRIGHT(1971) has recently suggested that vaterite is the stable phase at very low temperatures (less than 10°C) at 1 bar pressure, although his conclusions are challenged by TURNBULL(1973). METHODS AND RESULTS A number of high pressure experiments have been conin an attempt

to describe

in

by MARLAND (1975), while those at higher pressures were

INTRODU~ION

ducted

of

a stability

field for the

conducted in a piston-cylinder apparatus (BOETTCHER and WYLLIE, 1968). The piston-cylinder runs contained 3(r50 wt. y0 water plus the solid carbonate in welded platinum capsules. Starting monohydrate samples were prepared according to a recipe from M. N. A. Peterson (1962, personal communication): 40ml 0.5 M Na,C03 mixed with 20ml 0.1 M MgCl, plus 20 ml 0.1 M CaCl, and allowed to stand 5 days at room temperature until the first formed gel crystallized into monohydrate spherulites. Starting aragonites were; (a) precipitated from Na,C03-CaCI, solutions at elevated temperatures (all cold-seal runs), (b) precipitated as in the monohydrate recipe above but in an instance when the monohydrate failed to form (most piston-cylinder runs), or (c) pure, well-crystallized natural samples from Livermore, Calif. (as noted in Table 1). It should be noted first that the prevailing tendency of the monohydrate is to convert to aragonite (see also MARSCHNER, 1969; BRINKS et al., 1959; KINSMAN and HOLLAND, 1969). This is true under a great variety of conditions, although it does convert to calcite at temperatures greater than loo” (e.g. MARSCHNER, 1969), in NH3 solution (MARSCHNER, 1969), and apparently at 0.97atm CO2 (MALONE and TOWE, 1970). Observation of this prevailing tendency led MARLAND (1970) to speculate that the monohydrate to aragonite conversion (and similarly the hexahydrate to calcite conversion) is favored structurally and will take place in preference to other reactions that would be permissible on free energy considerations. Reaction kinetics are clearly a problem in the low-temperature region under consideration, in that many reactions approach equilibrium so slowly that they are not amenable to experimental determination. The data (Table 1 and Fig. 1) reflect inability to produce CaCO, .H,O or to reverse any reaction in this region. The most significant distinction is the suggestion of a boundary below which CaCOs .H,O converts to aragonite plus water and above which &CO, .H,O + H,d converts to either CaCO,.6H,O or CaCO,.6H,O + aragonite. In only one of the pision-cylinder experiments (PC3) did the initially sperulitic aragonite recrystallize into a better crystalline aragonite (0.1 mm needles). I.

calcium carbonate monohydrate and thus complete the basic outlines of the CL&O,. H,O system. Runs at less than 5 kb were made in cold-seal rod bombs as described

1193

1194

Notes Table 1. Data from high pressure experiments

1-o

3.525

3.7li.06

“+

1.8

3.62

EL+*+”

261

1.5f.5

2.07

LIc*nl

167

-5 to -6

2.37-2.46

*+“+m*“or

143

14.8

3.65

A+”

67%

4-c

120

4-3-B 9-1-B 15-D

unchanged

A

w ice

1-A

6%

2322

1.98

n+

increased

l-c

68

3.5f.5

2.09

e+

increased

A

4-o

119%

1.8

0.99

n+

infreased

A

26-A

7aq

11.7

1.98-2.03

at minor

9-c-5

4Bllr

5.1

2.13

A

19-B

112

10.3

2.23-2.36

A

19-D

114

14.3

3.88

A

19-c-z

1124

14.3-14.6

4.15

A

PC-1

12

23

12.0

A

PC-2

24

80

12.0

A

PC-3

24

140

12.5

A.

PC-4

244

80

PC-5

24

140

20.0

A

PC-6

24

140

20.0

A

PC-8

15

140

20.0

A

PC-9

24

140

12.5

A

PC-10

24

140

16.0

A

PC-11

24

140

20.0

A

.

a.75

veil

I

A

A

cryse.3llired

A

Times are given to the nearest half hour and include the time required for the bomb to equilibrate with the temperature bath. In the cold-seal runs (less than 5 kb) temperature and pressure are judged accurate to within &02”C and @02 kb, respectively, except as noted. In the piston-cylinder runs, pressures have been corrected for frictional effects and temperatures are believed accurate within f 5°C (BOETRXER and Wyurn, 1968). C = calcite I, A = aragonite, M = CaCOs . HzO, H = CaCO3 .6H,O, I = ice I. DISCUS!SION The calculations by HULL and TURNBULL(1973) suggesting that the monohydrate becomes stable with respect to aragonite plus water at 115 kb and 25°C is in error and needs to be reevaluated. Hull and Turnbull derived the standard free energy of the monohydrate from solubility data and attempted to use the relationship

d AG = AVdP at constant

T

(1)

to determine the pressure at which the three phases would be in equilibrium. Their neglect of compressibilities does not create any serious error with regard to the solids, but water is highly compressible over the range O-1 1.5 kb. If one attempts to solve the equation by graphically integrating AV dP, it becomes apparent that AV rapidly decreases in magnitude as

a function of pressure and in tact changes sign near 10.4 kb (still neglecting the compressibilities of the solids). If we include the compressibility of aragonite (BIRCH, 1966) and use the gypsum (CaSO_+.2H,O) data (BIRCH, 1966) as a likely upper limit for the compressibility of the monohydrate, it is still true that dAG/dP decreases rapidly with P and there is no value of P which satisfies equation (1). This suggests that either the monohydrate never becomes stable at increasing pressure or that Hull and Turnbull’s free energy value is too large (less likely possibilities are that the molar volume data are in error or that the monohydrate is vastly more compressible than the doubly-hydrated gypsum for which data are available). In order that the monohydrate becomes stable with respect to aragonite + water at some pressure in the order of l&12 kb, AGRxwould

Notes

I

0

20

I

40

1195

A

MONOHYDRATE -D

0 0

MOMtlYMATE ARAGONITE

A

CALCITE

I

60

--)

I

I

80

TEMPERATURE

loo (‘C

HEXAHYDRATE

-D ARAGONITE SURVIVED ARAGONITE

I

I20

I

I40

)

Fig. 1. Phase equilibria in the low temperature portion of the system CaC03-H,O. Also shown are the high pressure experimental runs which were conducted. Numbers in parentheses indicate replicate experiments at the same temperature and pressure. Metastable phase boundaries are shown dashed. The calcite-aragonite boundary is from CRAWFORD and HOER~CH(1972), and the hexahydrate-

aragonite + water boundary is from MARLAND (1975). have to be less than 50 per cent of the value derived by Hull. and Turnbull. The phase boundary will be at 10 kb if AGRXis 42 per cent of Hull and Turnbull’s value. (Data on water from ROBIE and WALDBAUM, 1968, and BURNHAMet al., 1969; aragonite data based on ROBIE and WALDBALJM,1968; LANGMUIR,1968; STAVEUYand L~NDFORD,1969). Another approach to the phase boundaries is the use of equation (1) to determine the pressure at which CaCOs .H20 plus water is in equilibrium with CaC03 .6Hz0. The resulting value is 3.07 kb (at 25°C) if the free energy value of Hull and Turnbull is used and the solids are incompressible. The exact position of this line is very sensitive to changes in Ak&,rids.Neglecting the effect of pressure on the entropy and volume of the solids, the Clapeyron equation, (dP/dT) = (AS/Al’), predicts a slope of 55.7 bars/deg for this line. While this line (dashed in Fig. 1) is thus seen to be consistent with my experimental

conversions of monohydrate to hexahydrate, it fails to converge upward with the aragonite + water% hexahydrate line (slope 76.4 bars/deg; MARLAND, 1975) over this region and their intersection must occur at several hundred degrees, if at all. The metastable boundary for CaC03 .H20 + 5Hz0 % C+03 .6Hz0 shown in Fig. 1 is the upper limit obtained by using the Hull and Turnbull value for the free energy of the monohydrate and ignoring the compressibilities of the solids. Note that the linear extrapolations to high temperature shown in Fig. 1 are perhaps longer than the data justify. While they should be reasonably accurate and serve to illustrate the pattern of phase equilibria, they do not acknowledge that the true lines will increase slightly in slope as temperature increases. The point should be made that phase boundaries are demonstrated only by reversing reactions. While our experiments do not prove the existence and/or

Notes

I I96

position of a metastable CaCO, . H,O + 5H20% CaCO, .6H,O phase bouhdary. they do corroborate the conclusions reached from thermochemical arguments and the free energy data of HULL and TURNBULL ( 1973). The final phase that needs to be considered is vaterite. ALBRIGHT(1971) measured the conductivity difference for solutions in equilibrium with vaterite and calcite. extrapolated to zero difference, and concluded that vaterite was stable at I bar pressure below 10°C. TURNBULL’S (1973. p. 1600) calculations show that “no reasonable value of (entropy) could make vaterite stable at IO’C.... .” It should be pointed out that Albright’s own data would seem to postulate that aragonite is stable with respect to calcite at temperatures less than about lo”C, an observation in striking conflict with the bulk of data. My observations on vaterite arc clearly not definitive. Nonetheless. all observations of the phase both in my work and in the literature seem to be consistent with generation as an intermediate in the hexahydratc to calcite conversion [DASGUPTA’S (1965) unusual conversion from ankerite is an obvious exception]. I tentatively suggest that vaterite is thermodynamically unstable with respect to calcite and aragonite in all geologically reasonable environments and occurs only as a precursor to calcite and only when favored for epitaxial reasons. CONCLUSIONS

This investigation was begun with the intent of describing a stability field for CaCO, .H,O at low temperature and thus completing the basic outlines of the CaC03~-H20 system. Early experiments in cold-seal bombs and later ones in piston-cylinder apparatus failed to yield the monohydrate in the predicted P-T region. Inclusion of recently available thermodynamic data reinforces the conclusion that the monohydrate is not stable at low temperature and high pressure and Fig. 1 presents the apparent phase relations. The CaCO, .H,O observed in natural and laboratory environments* is unstable with respect to calcite and aragonite under all conditions investigated and will generally convert to aragonite at low temperatures and calcite at temperatures in excess of 100°C. It appears likely, as suggested by MARSCHNER (1969) and MARLAND (1970). that the low temperature precipitation of aragonite may be preceded by a monohydrate precursor. * In addition to the references already cited, see BARON and PESNEAU.1956; LIPPMAN. 1959; SAPOZHNIKOVand TSVETKOV, 1960; VAN TASSEL. 1962; CARLSTR~~M. 1963: SEMENOV, 1964; DUEDALLand B~JCKLEY. 1971.

Similarly, for vaterite.

the data do not suggest a stability

field

4ckno~[r&mentsThe author acknowledges the encouragement and assistance of D. L. GRAF and H. T. HALL. throughout this work. A. L. BOETTCHER graciously made his piston-cylinder lab available and D. K. SMITHsupplied the hand-picked aragonite from Livermore, California. The research was supported by NSF grants GA-453 and GA1651 with D. L. Graf as principal investigator, the National Speleological Society R. W. Stone Research Fund and the Indiana State University Faculty Research Fund.

REFERENCES

ALUREHT J. N. (1971) Vatcrite stability. Amer. Mineral. 56, 620-624. BARONG. and PESNI:AC M. (1956) Sur l’existence et un mode de preparation du monohydrate de carbonate de calcium. Compt. Rend. 243, 1217.~1219. BIRCH F. (1966) Compressibility. elastic constants. In Handbook of PhysicalConstams, (editor S. P. Clark. Jr.). Geol. Sot. Amer: Mrm. 96, 97. 193. BOETTCHER A. L. and WYLLIE P. J. (1968) The calcite-

aragonite transition

measured in the system CaCO,-

CO;-H,O. J. Geol. 76, 314-330. BRCJOKS R.. CLARK L. M. and TH~RSTON. E. F. ( 1950) Calcium carbonate and its hydrates. Phil. Trans. Rev. Sot. London. A, 243, 145.~1/67.BURNHAM C. W.. HOLI.OWAY J. R. and DAVIS N. F. (1969)

Thermodynamic properties of water to 1,000°~ and 10.000 bars. Geol. Sot. .4mrr. Spec. Puper 132, 96 pp. CARLSTRBM D. (1963) A crystallographic brate otoliths. Bull. Mar. Biol. Lab.

study of verteWoods Hole, 125.

44 I --463. CRAWFORD W. A. and HO~~RSCH A. L. (1972) Calcitearagonite equilibrium from 50°C to 150°C. Amer. Mineral. 57, 995-998. DASGUPTA D. R. (1965) The transformation of ankerite during thermal treatment. Mineral Mag. 35, 634639. DUEDALL 1. W. and BUCKLEY D. E. (1971) Calcium carbonate monohydrate in seawater. Nature Phys. Sci. 234, 39-40. HULL H. and TURNRULL A. G. (1973) A thermochemical study of monohydrocalcite. Geochim. Cosmochim. Actu 37, 68s-694. KINSMAN D. J. J. and HOLLAND H. D. (1969) The coprecipitation of cations with CaCO,-IV. The co-

precipitation of Sr’+ with aragonite between 16; and 96°C. Geochim. Cosmochim. Acta 33, l- 17. LANCMUIRD. (1968) Stability of calcite based on aqueous solubility measurements. Geochim. Cosmochim. Acta 32, 83%851. L~PPMAN F. (1959) Darstellung und kristallographische Daten von CaCO, .H,O. Naturwiss. 46, 553- 554. MALONE P. G. and TOWE K. M. (1970) Microbia carbonate and phosphatz precipitation from seawater cultures. Mar. Geol. 9, 301-309.

MARLANDG. (1970) Precursors to calcite and aragonite precipitation (abs.). Trans. Amer. Geophys. Union 51, 83 I. MARLAND G. (1972) Phase relations

H,O. Ph.D. Dissertation, Minneapolis, 131 pp.

in the system CaCO,

University

of Minnesota.

Notes

MARLANDG. (1975) The stability of CaCO, .6H,O (ikaite). Geochim. C&m&him. Acta 39, 83-91. _ _ MAR~CHNERH. (1969) Hydrocalcite (CaCOa.H,O) and nesquehonite (MgCO, .3H,O) in’ carbonate scales. Science 165, 1119-1120. ROBIE R. A. and WALDBAUM D. R. (1968) Thermodynamic properties of minerals and related substances at 298.15” K (250°C) and one atmosphere (1.013 bars) pressure and at higher temperature. LI.S. Geol. Suru. Bull. 1259, 256 pp. SAFQZHNIKOV D. G. and TSVETKOVA. L. (1960) Precipitation of hydrous calcium carbonate on the bottom-of Lake Issyk Kul. Dokl. Akad. Nauk SSSR Earth Sci. 124, 131-133.

1197

SEMENOV

E. I. (1964) Hydrated carbonates of sodium and calcium. Soviet Phys. ~Cryst. 1964, 88-90. STAAVELEY L. A. K. and LINDFORDR. G. (1969) The heat capacity and entropy of calcite and aragonite, and their interpretation. J. Chem. Thermodynamics, 1, l-11. TURNBULLA. G. (1973) A thermochemical study of vaterite. Geochim. Cosmochim. Acta 37, 1593-1602. VAN TASSEL R. (1962) Carbonatniederschllge aus calcium-magnesiumchloridliisungen. Z. gemischten Anorg. Allgem. Chem. 319, 107-112. ZEN E. (1966) Construction of pressure-temperature diagrams for multi-component systems after the method of Schreinemakers-a geometric approach. U.S. Geol. Surv. Bull. 1225, 56 pp.

Geochimica et Cosmochimica Acta. 1975, Vol. 39, pp. I197 to 1201. Pergamon Press. Printed in Great Brrtain

NOTE

Limits on the effect of pressure on isotopic fractionation ROBERT N. CLAYT~N,*~$ JULIAN R. GOLDSMITH,~KARIN J. KAREL,$$ TOSHIKO K. MAYEDA* and ROBERT C. NEWToNt Enrico Fermi Institute,* Department of Geophysical Sciences,? and Department of Chemistry,$ The University of Chicago, 5630 Ellis Avenue, Chicago, Illinois 60637, U.S.A. (Received 27 August 1974; accepted in revised

form 9 December

1974)

Abstract-The equilibrium distribution of oxygen isotopes between calcium carbonate and water was determined at 500°C at pressures from 1 to 20 kbar and at 700°C at pressures of 0.5 and 1 kbar. At both temperatures, the pressure-dependence of the fractionation’factor was below the limit of detection. The experimental results are consistent with theoretical estimates of the volume change due to isotope substitution. Application of the theory to silicate systems leads to the conclusion that pressure effects on oxygen isotopic fractionation between silicates are <02x, at pressures of tens of kilobars. Thus the observed large variations of O1*/O’6 ratio in kimberlitic eclogites cannot be attributed to the effect of pressure. INTRODUCTION

of stable isotope geothermometry that the effect of pressure on isotopic fractionation is negligible. This assumption is based on the fact that isotopic substitution makes only a minute change in the molar volume of solids and liquids. However, neither theoretical calculations nor prior experiments have ruled out the possibility that measurable pressure effects might exist at pressures of tens or hundreds of kilobars, with significant consequences for the isotopic compositions of mantle rocks

IT IS A basic assumption

and mantle-derived

rocks.

4 Present address: Department of Chemistry, Princeton University, Princeton, N.J., U.S.A.

The effect of pressure on oxygen isotope fractionation between calcium carbonate and water has been estimated theoretically (JOY and LIBBY, 1960). If their treatment were correct, isotopic fractionations in the lower crust and upper mantle would be dominated by pressure effects rather than temperature effects. The only published experimental search for pressure effects on isotopic fractionation was carried out on the reaction between water and dissolved bicarbonate ion (HOERING, 1961). The fractionation factor at 4 kbar was not measurably different from that at one bar. In only one instance have isotopic pressure effects been adduced to account for observed oxygen isotope abundance in rocks (GARLICK et al., 1971). The authors concluded that crystal-liquid fractionations