THE DEGREE OF SATURATION
OF MAGNESIUM AND CALCIUM CARBONATE MINERALS
IN NATURAL WATERS (*)
P. B. HOSTETLER
U.S. Geological Survey, Mcnlo Park, Calif.
ABSTRACT
Thermodynamic data bearing on the extent of complexing between the common
ionic constituents of natural waters were used to calculate the distribution of Mg+~,
Ca-^+,and CO3— in certain published analyses of natural waters and hence to evaluate
the degree of saturation of these waters with respect to pure Mg and Ca carbonate
minerals. Waters considered include ground waters (and a few surface waters) from
limestones, dolomites, ultramafic rocks, and alkaline soda lakes and springs. The
following table summarizes the degree of saturation of these four water types
(l.Ox is thermodynamic saturation) :
Saturation ranges
Water type
Limestone ground waters
Dolomite ground waters
Ground and surface waters
from ultramafic rocks
Soda alkaline lakes
Calcitc
Dolomite
0.9A- - 8 A :
0.5A: - 5 x
0.4A- - 2 A :
0.6A- - 7 A 0.5A- - 1 4 A -
O.lx
-2A-
5x - l O . v
15A- - 8 0 A -
Magnesite
6
:•
0.03A- - 2 A 0.1A- - 2 A 0.3A- - 4 0 A -
H
ydromagnesite
—
0.03A- -
8A- - 1 3 0 A -
9A:
3A--45A-
Ground waters from limestones are usually supersaturated with respect to calcitc
and undersaturated with respect to magnesite; the reverse is true of water derived from
ultramafic rocks. All three types of ground waters are generally supersaturated with
respect to dolomite. Alkaline soda lakes, a favorable environment for precipitation
of dolomite, calcite, and hydromagnesite, are strongly supersaturated with respect
to all four carbonate minerals, and these supersaturation factors persist even in
shallow, well-mixed water overlyjng primary bottom carbonates. Because the minerals
in the surficial layers of precipitated bottom carbonate sediments are in a higher
free-energy state than that of their pure, coarsely crystalline counterparts, higher
solubility is to be expected; thus, it seems likely that many lake waters are in near
equilibria with these minerals. Greater free energy would result from a number of
factors, including fine grain size, metastable ionic substitution, and crystal defects.
RESUME
Des donn£es thermo-dynamiques applicables, dans unc certaine mesure, aux
composants ioniqucs habituels des eaux
naturelles, ont 6te utilisdes en premier lieu pour
calculer la distribution des ions Mg ++ , Ca+*" ct COs"" dans quelques analyses d"eaux
naturelles deja publides, ensuite pour ^valuer le degre de saturation des ccs eaux en
carbonates purs de Mgct deCa. Les eaux examinees comprenent des eaux souterraines
(ainsi que quelques eaux de surface) preleve'es dans des calcaires, des dolomies, des
roches ultra-basiques, des lacs sodiques alcalins, enfin dans des sources. Le tableau
ci-dessous montre le degre de saturation des eaux prelev6es dans ces divers environnements (1,0 x est la saturation thermo-dynamique).
(*) Publication authorized by the Director, U.S. Geological Survey.
34
Dcgre de saturation
Types d'eaux naturclles
Eaux souterraines dans les
calcaires
Eaux souterraincs dans les
dolomies
haux souterraines et dc
surface dans lcs roehes
ultra-basiques
Eaux des lacs sodiqucs
alcalins
Calcitc
Dolomic
Magnesite
0.9.V - 8.v
"
0.4A: - 9A-
I
. 0.03A- - 2A
0.5A- - 5 A
0.6.Y - 7A-
0.1X-2A
0.1A-2AT
0 . 5 A - 14A
0.3A- 40A-
5A
- 1 0A- !
15.v - 80.Y !
8A-
yui L»-
j
magnesitc
T
0.03A- - 9.v
- -1 30A!
3A-
- 45A
Lcs eaux souterraines prelcvees dans lcs calcaires sont gen6ralement sursaturecs
en calcitc ct sous-saturees en magncsite, tandis que la proposition inverse est realisee
dans lcs eaux prelevees dans lcs roches ultra-basiqucs. Chacunc des trois varictcs
d'eaux souterraines sc trouve sursaturee en dolomie. Lcs lacs sodiqucs alcalins, qui
forment un milieu favorable a la precipitation de dolomie, de calcite et d'hydromagnesite, montrent unc forte sursaturation en ces quatre mineraux. Etccttesursaturation
persiste jusque dans les eaux moins profondes ct agitces qui recouvrcnt lcs carbonates
primaires tapissant lc fond. Les mineraux appartenant aux couches supcrficicllcs des
sediments carbonates du fond ayant une energic librc plus grande que des mineraux
purs plus largcment cristallisSs, doivent faire preuve d'unc solubility plus dev6c. II
est done probable que la plupart des eaux lacustres sont presque en equilibre avec
ces mineraux. Un certain nombrc de facteurs tels que finesse des grains, substitutions
ioniqucs metastables, et defauts cristallins seraient la cause de cette augmentation
d'energie libre.
1. INTRODUCTION
The rates of formation from solution of many carbonates are sufficiently rapid
at surface temperatures to encourage gcochemists to attempt correlations between
field observations, laboratory studies, and thermochemical data. In this respect there
has been and is now a great deal of interest concerning the mode of origin and stability
of calcium and magnesium carbonates in natural waters that are saturated or supersaturated with respect to these minerals. Thus, Holland and his coworkers (20) have
discussed the mode of origin of calcite and aragonite from certain supersaturated
cave waters; Garrels and others ( u ) have noted that representative ocean water is
supersaturated by as much as 300 percent with respect to pure calcite; and Schmalz
and Chavc (34) report the nearshore Bermuda marine waters to be greatly oversaturated with respect to calcite, somewhat less oversaturatcd with respect to aragonite
and moderately magnesian calcite, and perhaps only slightly oversaturatcd with respect
to highly magnesian calcite.
The prerequisite conditions for dolomite formation at surface temperatures are
not yet clear, partly because of our inability to synthesize this mineral at low temperatures, except in high I'cO2 environments. Fortunately, in recent years there have been
several reports (1> 15> 35> 36) of natural primary dolomite together with chemical
analyses of the associated water. Generally these dolomites (35> 3fi) are similar to the
"protodolomites" synthesized by Graf and Goldsmith (17), which are characterized
by abnormally high CaCC>3 content and a lack of superstructure X-ray reflections,
indicating that the ordered succession of Mg-Ca-Mg-Ca planes in the crystalline
lattice is not consistently maintained.
Our knowledge of formation conditions for the magnesium carbonates — magnesite, hydromagnesitc(Mg4(CO3)3(OH)2.3H2O), and nesquehonite (MgCO3.3H2O)—
is also scanty. One commonly encounters the concept in the literature (see, for instance,
Baron and Favrc (5)) that nesquehonite is the stable magnesium carbonate at surface
.35
temperatures and hydromagnesite at somewhat higher temperatures (~100°C), yet
thermodynamic data (13, 24) indicate that at 25"C a solution saturated with nesquehonitc is greatly supersaturated with respect to magnesite and that in the presence of
water hydromagnesite is stable relative to magnesite only at Pco« values less than
2 x 10~° atm. However, in natural environments at surface temperatures both nesquehonitc and hydromagnesite will precipitate directly from solution, whereas it has not
yet been definitely established that magnesite can form in this way. In an aqueous or
moist environment hydromagnesite will alter more or less readily to magnesite; the
hydromagnesite deposits around the Atlin, Cariboo, and Kamloops districts of British
Columbia (u> 29) are good examples. Perhaps the best, but not conclusive, evidence
of primary, low-temperature magnesite has been reported by Alderman and von der
Borch (2> 3) from lagoonal environments around the Coorong district of South Australia. Here, in one lagoon, an aragonite-hydromagnesite assemblage was found from
the sediment-water interface to a depth of 9 inches, whereas in another lagoon some
8 miles distant, a magnesite-dolomite assemblage was reported from the interface to
a depth of 10 inches. There have been a number of other descriptions (4> 9- 18> 28> 31>
33, 37, 38) o f magnesite allegedly formed at surface temperatures. In general such
magnesite is found either in evaporite facies ( 18 ' 33 ' 37 ), where [the reduced activity
of H2O due to very high salinity may have enhanced the probability of direct magnesite
precipitation by dehydration of the solvated Mg ions, or as a weathering alteration
product of ultramafic masses (4> 9- 2 8 ).
In the present report analyses of waters derived from a number of rock types and
environments were studied and the chemical activity of each pertinent ion calculated
with a view towards: a) determining the degree of saturation of carbonates in waters
derived from parent carbonate rocks and b) relating such relative saturation to that of
waters from which the same carbonate minerals are found to have precipitated at
surface temperatures. Waters considered from the standpoint of (a) include ground
waters derived from limestones, dolomites, add ultramafic rocks. Waters derived
from ultramafic rocks were considered because of the nonexistence of analyses of
waters derived from magnesites or other magnesium carbonates. Some, at least, of the
reported ultramafic rocks contain appreciable amounts of magnesium carbonates.
Waters considered from standpoint (b) are alkaline soda waters (mainly lakes), '
which are a favorable environment for the precipitation of calcite, dolomite, and
hydromagnesite. These waters arc typically characterized by large concentrations of
sodium, significant concentrations of sulfate and chloride ions, and distinctly alkaline
pH values (pH 5; 8.5).
2. RESULTS
Sixty-two water analyses were individually recalculated with the aid of available
thermochemical data (12> 13- 24) to determine the molal concentrations of the various
aqueous species that are present in each sample. Calculations revealed that for many
waters some or all of the following complex species are significant: CaCO,V\ CaHCOj,
CaSCV, MgCo3°, MgHCOij, MgSO4°, NaCOg, NaHCO3°, NaSOl, and KSO^.
The dissociation constants used for determining molal concentration of these complex
species are those reported by Garrels and Thompson (12) except that of MgHCOJ;
the value used was the one recently reported by the author (21). These data, including
the pH of each water and its determined ionic strength, were used to calculate the
activities of Mg++, Ca++, and CO" r in each sample. Calculation methods, which are
rather cumbersome, are not reported here as they are essentially the same as those
reported by Garrels and Thompson (12) in determining a chemical model for sea water.
The ionic strength of sea water is 0.7 (12), but none of the samples used in this study
had an ionic strength greater than 0.45.
36
TABLE I
Distribution of Ca, Mg and CO3 in natural waters
g(
Location
- l o g ( 3 ) I -log(3) I— log(3) j - I o g ( 4 ) 1 - l 0 g ( 4 )
!
1
pH
PCO2
I
\
j 2.13
i 2.49
1 2.30
2.73
! 2.78
| 2.18
1.88
2.27
2.71
I 2.27
I 2.36
j 2.29
7.0
7.7
7.5
8.0
8.2
7.8
7.3
7.6
7.9
7,6
7.61
7.55
0.0018
0.004
0.005
0.005
0.009
I 0.012
0.015
1
0.016
0.004
0.006
0.0045
0.0047
3.51
3.05
2.97
3.15
3.02
2.74
2.86
2.75
3.14
2.96
3.00
2.99
3.65
3.75
4.02
3.35
3.05
3.17
3.06
3.07
3.52
3.58
3.92
3.82
6.25
5.21
5.42
4.80
4.50
4.70
5.40
5.19
5.02
5.19
5.26
5.31
9.76
8.26
8.39
7.95
7.52
7.44
8.26
7.94
8.16
8.15
8.26
8.30
9.90
8.96
9.44
8.15
7.55
7.87
8.46
8.26
8.54
8.77
9.18
9.13
19.66
17.22
17.83
16.10
15.07
15.31
16.72
16.20
16.70
16.92
17.44
17.43
Ground waters from dolomites
West Allis, Wisconsin
Copper Ridge, Alabama
Irene, Pretoria, Transvaal
Bainbridge, Ohio
Fort Recovery, Ohio
•
! 2.86
I 2.34
1 2.36
! 2.06
I 2.04
8.2
7.5
7.9
7.6
7.4
0.008
0.004
0.009
0.010
0.03
3.28
3.20
2.97
3.36
2.75
3.08
3.37
2.99
2.73 I
2.85 i
4.58
5.46 I
4.68 I
4.98
5.36 I
7.86 I
8.66 ,
7.65
8.34 I
8.11 I
7.66
8.83
7.67
7.71
8.21
15.52
17.49
15.32
16.05
16.32
Ground and surface waters from ultramafic rocks
Lydenburg, Transvaal, 413, ground water
pyroxenite
Lydenburg, Transvaal, 414, ground water
pyroxenite
Lydenburg, Transvaal, 415, ground water,
peridotitc
Rustenburg, Transvaal, 307, ground water,
gabbro or pyroxenite
Pretoria, Transvaal, 302, ground water,
gabbro or pyroxenite
Krantzberg, Transvaal, 445, ground water
serpentine and pyroxenite
Marico, Western Transvaal, 538, spring in
gabbro and pyroxenite
Shasta Valley, Calif., ground water,
serpentine, 41/5-4N1
Shasta Valley, Calif., 4I/5-4D1, ground
water, serpentine
Shasta Valley, Calif., 41/5—9F3, ground
water, sepentine
Shasta Valleyr Calif., 42/5—33 Ml, ground
water, ser.pcntinc
Shasta Valley, Calif., 4 2 / 6 - 10JI, ground
water, serpentine
Shasta Valley, Calif., 4 1 / 5 - 9P, surface
water, serpentine
Clear Creek, San Benito County, California,
drains serpentine — June 13, 1955
Clear Creek, San Benito County, California
drains serpentine — June 28, 1959
1
'
]
I
I
I
2.01 j
I
I
I
.
I
I
1.52 I
.
2.28 1
7.2
0.012
8.0
0.013
1.83
7.2
0.0065
3.65
Alkaline soda springs and lakes
Kecne Wonder Springs, Inyo County, California,
sample 1
Keene Wonder Springs, Inyo County, California,
sample 2
Spring Water, 141 Mile House, Caribou Road,
British Columbia
Spring Water near North Fork of Riskc
Creek, British Columbia
St. Andra, lake, Seewinkel, Burgenland,
Austria
Langc-Lacke, lake, Seewinkel, Burgenland,
Austria
Darscho, lake Seewinkel Burgenland,
Austria, - July 8, 1957
Darscho, lake, Seewinkel, Burgenland, Aug. 5, 1958
Halbjochlacke, lake, Seewinkel, Burgenland,
Austria —July 8, 1957
Halbjochlacke, lake, Seewinkel, Burgenland,
Austria — J u n e 18, 1958
Fuchslochlacke, lake, Seewinkel, Burgenland,
Austria
lllmitzer Zicksee, lake, Seewinkel,
Burgenland, Austria —July 8, 1957
lllmitzer Zicksee, lake, Seewinkel,
Burgenland, Austria —June 18, 1958
Neusicdlersee-Podersdorf, lake, Seewinkel,
Burgenland, Austria—June 18, 1958
Neusiedlersee-Podersdorf, lake, Scewinkel,
Burger.land, Austria— July 1, 1958
Neusiedlcrsee-Podersdorf, lake, Seewinkel,
Burgenland, Austria — August 5, 1958
Neusiedlersee-Podersdorf, lake, Seewinkel,
Burgenland, Austria — September 1, 1958
Big Soda Lake, Nevada
Pyramid Lake, Nevada
Walker Lake, Nevada
Abert Lake, Oregon
Borax Lake, California
Little Borax Lake, California
Lake Balkhash, USSR, western pool, southern part
Lake Balkhash, USSR, western pool, northern part
Lake Balkhash, USSR, middle pool, western part
Lake Balkhash, USSR, Lepsinskij pool
Lake Balkhash, USSR, Biurliuj-Tupinskij, pool, west
Lake Balkhash, USSR, Biurliuj-Tupinskij, pool, east
Lake Kingston, South Astralia
I
I
3.50
2.69 I
3.60
2.62 j
4.00 j
2.62 I
2.88
5.55 j
3.69 I
2.36
7.6
2.15
7.9
0.014 i
3.72
I
2.17
8.0
0.016 I
4.19
j
2.47
8.0
0.010
3.88
i
I
i
0.010 I
0.005 I
I
•
2.98 j
! 0.005
'
4.14
1
7.6
I 0.007
,
« 7 j 2.82 J
8.4
J 0.008
2.17
'
2.94 '
I
1
| 2.99 I
I
! 3.01
1
I
' 2.33 '
I 0.004
4.19
8.7
I 0.016 1
0.017 1
I
I
I
3.82 i
8.2
8.1
j
I
2.89 I
2.94 I
2.78
!
3.02
4.68 J
2.57 1
4.13 I
2.52 I
1.1
37.66
42.86
38.20
38.86
40.68
3.0
0.48
4.9
1.0
1.7
! 16.05 ' 38.35
0.81
,
I
I
,
1.1
0.60
0.60
39
39
39
39
39
39
39
39
39
39
27
27
1.6
5.0
5.5
0.58
6.9
3.0
2.2
0.36
0.02
0.26
0.18
0.06
39
39
39
39
39
2.0
3.0
0.24
6
1.1
0.28
0.45
0.23
0.14
0.05
0.06
1.4
1.8
0.12
1.7
5.24
,
8.84 i
7.86
16.70
I 39.80
0.32
I.I
0.11
6
,
"•I
7.03
I 15.44
! 35.71
0.85
7.6
6.0
1.1
6
8.43
j 17.63
I 41.77
0.14
0.30
0.48
0.03
6
8.26
! 17.23
40.56
0.23
0.45
0.76
0.07
6
7.4
7.6
1.0
6
7J.
1.7
.6.
5.28
!
4.47
:
4.81 j
9.20 I
8.97 ' I
8.19
'
8.48 1
8.47 I
7.04
6.81
7.33
7.70
8.79 I
5.09 J
8.86
i
7.91
4.26 1
8.08
j
7.04
'
7.73
4.71
|
35.89
I 15.29
1
15.80
I 34.95
0.73
' 36.73
0.74
3.8
4.0
0.60
26
I 38.19
0.63
1.6
2.4
0.26
26
8.90
3.73
I
I
,
8.41
6.30
4.25
I
8.38
6.77
I
i
1.4
1
I
7.59
4.65
15.23
'
I
16.24
I 16.38
I 16.77
[ 15.12
! 16.63
13.
I 37.31
0.36
2.1
2.0
0.44
26
I 39.35
0.30
1.0
1.3
0.14
26
1.8
7.4
8.7
1.6
26
1.5
1.5
0.33
26
8.9
10
8.3
2.0
10
4.2
0.08
c
39
1.0
c
39
35.10
I 37.81
0.28
14.
32.07
0.85
41.
34.63
0.91
14.
1
I 15.76
j 14.63
40.21
3.2
28.63
140.
65.
c, h
29
30.34
140.
24.
c, h
29
I 14.71
I 15.15
3.02
3.25
3.08
9.45
I 0.15
5.51
4.20
2.30 I
7.81
6.50
I 14.31
I 32.80
3.4
9.36
J 0.16
5.09
4.19
2.34
7.43
6.53
13.96
i 33.06
8.1
24.
9.31
I 0.10
5.18
4.07
2.48
7.66
6.55
14.21
j 33.10
4.8
23.
2.45
7.47
5.96
13.43
I 30.49
7.4
89.
I 60.
22.
12.
1.39
I
I
!
i
3.73
44.11
1
1.6
2.45
4.33
2.58
2.97
2.90
i
i
1
0.06
3.12
]
3.22
4.71
'
J 0.06
3.77
!
3.34
3.76
I
i
7.83
7.93
7.53 I
7.10
35.84
6.5
! 0.07
3.52
2.25 I
5.77
I
I
1 0.04
I
j 0.019
2.46
3.30 1
5.76
I
3.50
2.76
3.83 I
7.33
;
6.59
I 13.92
33.27
I 0.019
3.98
3.07
3.76 I
7.74
I
6.83
1
14.57
34.18
4.0
j 0.033
0.032
4.37
4.57
3.29
3.43
3.14
3.00
7.51
7.57
6.43
6.43
i 13.94
I 14.00
32.58
32.36
6.8
5.9
—
0.95
I
| 2.98 |
1
1
3.23 ,
I
I 3.00 I
.
I
] 3.25
i
i
1 3.17 |
1
I
, 3.27 |
!
j
:
I
,
3.30
3.76
3.00
3.30
3.82
3.64
I 3.94
1 2.90
! 2.93
I 3.50
I 3.55
| 3.59
| 3.53
j 4.67
I
i
!
I
1
:
I
j
1
I
9.45
9.16
8.85
8.81
8.88
8.94
9.6
8.9
9.3
9.8
9.7
10.0
8.3
8.4
9.0
9.15
9.2
9.15
9.2
I
I 0.19
I 0.09
I 0.03
I
30.
30.
23
34.
32.
6.6
7.6
23
23
5.9
23
5.0
23
4.9
23
i 22.
I
' 33.
I
I 25.
5.02
3.51
4.87
3.37
2.79
7.66
6.16
13.82
i 31.53
4.8
56.
; 39.
3.79
2.79
3.67
7.46
6.46
13.92
32.47
7.6
28.
I 35.
3.90
32.63
5.9
27.
3.67
7.57
6.48
14.05
2.82
6.45
14.03
32.41
5.8
I 0.45
I 0.08
0.12
I 0.18
I 0.18
0.28
0.025
0.18
| 0.03
1 0.05
0.08
I 0.09
0.09
0.45
2.83
3.11
2.93
3.23
4.46
4.13
4.39
2.87
2.78
2.71
2.65
2.60
2.59
2.18
3.63
I
3.54
2.68
3.32 I
2.83
2.34
2.36
2.06
7.58
3.95
4.72
4.33
4.64
5.14
5.15
5.09
3.38
3.38
3.73
4.02
4.10
4.13
3.03
6.37
5.79
6.25
6.06
6.80
6.49
6.45
7.28
7.03
6.33
6.02
5.91
5.94
6.57
13.86
13.19
13.90
13.53
14.28
14.00
13.60
15.07
14.66
13.68
13.41
13.32
13.42
13.99
32.06 I
29.28
31.88
3.35
4.39
7.49
7.40
7.65
7.47
7.48
7.51
7.15
7.79
7.63
7.35
7.39
7.41
7.48
7.42
29.93
30.11
31.49
7.1
8.7
4.9
7.4
7.3
6.8
15.
3.6
5.1
9.8
8.9
8.5
7.3
8.3
0.7
2.58
1.77
5.33
7.91
7.10
15.01
34.87
2.7
0.03
23
2.6
!
2.81
1 0.03
4.5
17.
10.
3.95
I 0.03
15.
6.5
I
I
I
2.94 |
ence
0.001
0.02
0.004
0.11
0.44
0.15
0.3
0.06
0.04
0.02
0.008
0.009
0.01
0.09
0.03
0.58
2.3
•
I
I
1
! 8.4
1
[ 10.1
I
1 8.7
1
I 8.63
I
| 8.69
i
| 9.00
| 9.18
1
,
1
|
|
i
I
I
I
|
,
0.007 1
1.5
1.2
0.05
0.78
0.38
2.8
9.1
6.9
1.4
2.5
I Rcfer-
I
4.41
4.59 I
8.2
2.5
(C)
1.4
2.74
7.9
1.2
phase 0 )
,)*;
apm/Kspm
8.43
4.29 I
2.49
,
I
1 2.93 1
0.89
2.5
6.6
7.9
\
'•
4.93
2.57 j
2.52 I
!
1 40.65
1 41.34
0.04
1.2
42.69
44.24
I
7.62
I
1
1
I
47.35
43.23
45.34
39.80
37.30
39.18
41.84
Precipitated
"~!
! ape/Kspe
"CaH
Ground waters from limestones and marbles
Gainesville, Florida, limestone
Brooksville, Florida, limestone
Irondalc, Alabama, limestone
Lake City, Florida, limestone
Bardstown, Kentucky, limestone
Nit. Juliet, Tennessee, limestone
Birmingham, Alabama, limestone
Roswell, New Mexico, limestone
Sylacauga, Alabama, marble
Baltimore County, Maryland, marble
Black chasm Is cave, Volcano, Calif, lake surface
Black chasm Is cave, Volcano, Calif. 40' below surface
7.6
Departure from saturation( 5 )
I
4.41
4.25
3.62
3.37
3.31
30.81
33.26
32.20
31.74
36.11
35.07
31.70
30.41
35.
130.
46.
71.
13.
I
I
I
I
i
I
23
23
7.1
Cd
23
6.5
Cd
23
30.
7.4
Cd
23
37.
79,
36.
55.
23.
32.
50.
9.1
15.
8.9
45.
10.
18.
4.5
8.3
11.
0.87
1.6
11.
23.
30.
28.
13.
c, d
23
40
40
40
40
40
40
15
15
15
15
15
15
1
30.
26.
29.
4.3
7.6
38.
78.
100.
93.
22.
46.
63.
69.
62.
32.
6.5
10.
c(7J
c (?)
c, d
c, d
C d, h(?)
c, d,h(?)
c.d, h(?)
c,d
cean water
Representative sea water
(')
(2)
(3)
(45)
()
(6)
. 3.30
8.10
For example, Atmospheric COo —- lO" 3 - 40 atm., —log PCO2 •= 3.40.
Ionic strength.
Calculated activities for these ions.
Subscripts c, m, d, and
h represent calcite, magncsite, dolomite, and hydromagnesite respectively, apc - («ca + "') («C()3~~), apm - (WMK1""1") («CO3—), apt - («ca + + ) ( a « 8 T + ) Ocos—) 2 aph — (aM g + +) 4 , (aco3—) 3
1"OH~Y" Kspc - 10~ 8 - 34 , Kspm -• 10" 7 - 91 , Ksp& -- 10 17 - 00 , Ksp\, - - IO~35-87. Value for quotient of 1.0 is thermodynamic saturation; < 1.0 is undcrsaturation, > 1.0 is supersaturation.
Phases precipitated from springs or lakes at surface temperatures.
12
Four activity products — (aca+-) (acof), (oMgT+) (nct>3~~), («ca'*) (a.Mg+-)
(oco3~~)2, and (aMg+')4("co3" ~)3(aon~)2 — were derived from the calculated ionic
activities for each sample. These values, abbreviated as apc, apm, apa, and apn, may be
compared respectively to the thcrmodynamic solubility products (Ksp) of calcite,
magncsitc (or nesquehonite), dolomite, and hydromagnesite to determine the extent
of departure of the sample from thermodynamic saturation. For instance, at 15UC,
K sp calcite is 10~8-34 (24), and a solution is thermodynamically saturated with respect
to calcite when the activity product (aca++) ("co3~") is equal to lO"8-34, whatever the
invidual activities may be for Ca^+ and CO" " ions. The important point is that thermodynamic saturation represents the value towards which the appropriate activity
product (ap) should tend to approach if the solution is supersaturated or is undersaturated and in contact with a carbonate mineral. If this tendency is not apparent,
the amount of departure from thermodynamic saturation gives us some information
concerning the persistence of metastability and (or) conditions for incipient precipitation of the carbonate mineral itself.
The quotient ap/KSp is a convenient measure of the amount of departure from
thermodynamic saturation; values for the quotient of greater than 1 represent supersaturation, and values less than 1 represent undcrsaturation. These values will be
frequently referred to in this report and the reader should note that they refer to the
excess (supersaturation) or deficiency (undersaturation) of only one of the compositional ions of the carbonate. That this is so can be discerned from the solubility product of calcite («ca~+) (aco3" ~) =- 10" 8-34, where a change of, say, an order of magnitude in either Ca^"1" or CO^~ will be reflected by a change of an order of magnitude
in the solubility product. To facilitate comparisons between carbonates, especially
cationcomparison.thequotients^prt/K^j,^)1.'2 and (apn/Kspn)114 are used here for dolomite and hydromagnesite respectively. Thus, a quotient of 5 for calcite indicates that
either aca ++ or oco 3 , is 5 times that required for thermodynamic saturation, a quotient
of 5 for dolomite indicates that cither o c o a " or the product (oca++) («Mg++) is 5
times that required for saturation, but for hydromagnesite a quotient of 5 applies
only to oM*'+ (KsPh - (aM g + -) 4 («co 3 ) 3 («oir) 2 ).
The value for Ksp magncsite is 10 7-91 (24). The ZlF° hydromagncsite value given
by Garrels and others (13), and the free-energy of formation values for Mg~*~+, COj~,
and OH~ listed by Latimer (24) were used to derive a K sp iiydromaRnesite value of
10-35.87 A KSJ) neSqUehonite value of 10~5-51, which is somewhat lower than that
given by Latimer (2/1), is consistent with the more recent solubility data for this mineral
of Kazakov and others (22). There is considerable discrepancy between reported /IF
values for dolomite at 25°C, and none of the available thermochemical data (l3>
19, 30) w a s used_ Instead, a Ksp dolomite value of 10~17-00was used. This figure was
arbitrarily chosen by Holland and his coworkers (20) as being most consistent with
analyses of cave waters derived from solution of dolomitic rocks and is very similar
to a Ksp<t of 10~1(i-78 derived from Halla's (19) value for zlF dolomiteKsp values are, of course, temperature dependent, but the Ksp values given above
are for 25 °C. The temperatures of most of the water samples used in this study varied
from 17" to 25 °C. A few samples were slightly cooler, one was slightly warmer, and
for a considerable number of surface samples, collected during the summer months,
no temperatures were listed in the original sources. The solubility of calcite at I7UC
is about 20 percent greater than at 25°C (2r>); the other carbonates are probably
similar. This sort of error, not significant for the purposes of this study, is roughly
comparable to an error of 0.1 pH unit between true and measured pH. Most pH values
reported for samples used here were undoubtedly determined in the laboratory; consequently, many pH measurements are probably no more accurate than : j- 0.2 pH unit
of the true pH of the sample at the time of collection. The pH measurements for alkaline soda lakes are apt to be the most accurate because these samples were nearly in
equilibrium with atmospheric CO2 at the time of collection (no notable gain or loss
37
of COo from field to laboratory). An error of | 0.2 pH unit yields an error of about
40 percent for the stated quotient ap/Ksp.
The results are shown in table 1. Note that fi represents the determined ionic
strength and that Pco2 values, individual ionic activities, and calculated activity products (ap"s) are expressed as negative logarithms. Much of the data of table 1 is best
illustrated by separate figures.
In figure 1 pH is plotted against calculated Pco > for each sample, and the buffering
effect of the carbonate-bicarbonate couple on the pH of natural waters is illustrated
by the linear trend, or "belt", of all the analyses. Groundwatcrs from limestones,
dolomites, and ultramafic rocks appear to have similar ranges of pH and Pco2 values,
with the latter varying from 10" :)-no to lO"1-50 atm (3 to 100 times atmospheric CO2).
The alkaline soda lakes, having Pco 2 values ranging from 10~ 300 to 10 4-(l0, are, as
mentioned above, more nearly in equilibrium with atmospheric CO2. Most of these
lakes are slightly supersaturated with respect to atmospheric CO2, but a few very
alkaline ones (pH > 9.5) are slightly undersaturated. Slight supersaturation is probably common because of subsurface discharge into lakes of ground waters with
higher Poo.> values such as those plotted from limestones, dolomites, and ultramafic
rocks. In general this effect should be less noticeable at higher pH values because of
the progressively greater total carbonate content of the lakes. The very low Pco>
value for Lake Kingston (pH 9.2, Pcoj ~ 10 4-67) is apparently due to photosynthesis
effects (1) in a very shallow body of water.
The heavy solid lines in figure I separate ionic "domains"; in each domain one
of the lettered aqueous species, either M+- (M — Mg or Ca), MHCOf, or MCO30,
accounts for most of the aqueous M, and along a line separating two domains the
activities of adjoining species are equal. Actually there should be separate but parallel
lines for distinguishing the domains of the calcium and magnesium aqueous species,
but the dissociation constants for MgHCO^ and CaHCOg and those for MgCCV
and CaCO.i0 are sufficiently similar (see Garrels and Thompson C12)) that the separation is insignificant for the purposes of this study.
Thus, in groundwaters from limestones, dolomites, and ultramafic rocks most,
if not essentially all, of the aqueous Ca and Mg is present as Ca++ and Mg+~. In the
alkaline soda lakes, however, where total carbonate greatly exceeds aqueous Ca and
Mg, significant amounts of Ca and Mg (as much as 90 percent) are present mainly as
MCO3'1 plus some MHCCKj. Sulfate complexing as MSO40 may also be important,
so that as little as 5 percent of the total aqueous Ca and Mg is present as Ca ++ and
Mg++. These data are given to emphasize the importance of quantitatively accounting
for such complexes in attempting to explain saturation conditions in alkaline waters
where carbonates may precipitate.
The extent to Which the natural waters studied here are saturated with respect
to calcile, dolomite, and magnesite is illustrated respectively in figures 2, 3, and 4.
In every figure, the abscissa is «co3~" ; in figure 2, the ordinate is flca"+, in figure 3
[(«Ca~~) (0Mg"l+)]I/2.,and in figure 4, OMg'1"". In figures 2 and 4 a solid line represents
Kip at 25"C. In figure 3 the range of Ksp values for dolomite, using the values of
Halla (10), Robie (30), and Garrels and others (13) is illustrated by dashed lines, and
the value of Holland and his coworkers (20), from which the apjKsv quotients were
computed, is shown by a solid line.
Perhaps the most striking gross feature of these figures is the high slate of" supersaturation that exists in many natural waters, and especially in alkaline soda waters,
with respect to calcite, dolomite, and magnesite. Table 1 indicates that a similar situation exists with respect to hydromagnesite. No waters were encountered which were
saturated with respect to nesquehonitc, although several approached this value. Table
2 summarizes the range of quotients for each water type.
38
Fig. 1 — Pcoo- pH range of natural water types considered. The letters c, d, and h
represent, respectively, calcite, dolomite, and hydromagnesite. Heavy solid lines
separate ionic "domains"; in each domain the lettered species, either M'~+
(M -- Mg or Ca), MHCCb*, or MCO30, accounts for most of the aqueous M. The
nature of the bottom sediments in many alkaline soda lakes is unknown, but a
comparison of Na/Ca ratios in the lakes and their tributaries provides indirect evidence of calcite precipitation. Where this evidence is available, the analysis plots
are designated by a half-filled circle (Cl). The dot-dash line joining two analysis
plots from Kecne Wonder Spring illustrates drop in fcoo and increase in pH between 150 feet and 1,000 feet downstream from vent. Representative ocean water
(12) is designated by a cross (-1 ).
39
TABLE 2
Range of each water type in amount of departure from saturation with respect to calcite, dolomite, magnesite, nesquehonite, and hydromagnesite
Range (*)
Water type
Calcite
Dolomite
Ground waters from limestones and marbles
from limestones
from marbles
limestone cave water
0.9 — 8
1.5
1.1
0.4 — 9
1.2
Ground waters from dolomites
0.5 — 5
Ground and surface waters from ultramafics
ground waters
surface waters
Soda alkaline springs and lakes
spring water precipitating calcite
spring water precipitating hydromagnesite
, lakes
Representative ocean water
Magnesite
Nesquehonite
0.03 — 2
0.2
0.06
—
0.6 — 7
0.1 — 2
—
0.1 — 2
0.3 — 1.0
0.5 — 9
1.5 — 14
0.3 — 13
1.5 — 40
0.001 — 0.05
0.006 — 0.2
3 —6
4—15
5 — 10
15 — 80
1.0 — 6
140
8 — 130
0.6
0.03 — 0.6
2.7
0.6
10
(•) Amount of departure from saturation is expressed by the quotient ap/K.sp.
6.5
Hydromagnesite
—
—
—
0.03 — 2
0.3 — 9
40
3 — 45
1.8
~\
1
f
1=1
•
.
e
5
1
o
/
«
X
g sS c^
-
g e t
a.
"
1
—i
11"» ^ 5
3
o
.£>
—
:ti t> 2 ""
13 < x ;]<^o
a
S °
A
,
x
'
x
X
X
o ••••4-
/b
D
<3
+
n
y
/ _
/
X
/
X
X
X
/
/
/
/
-
X
V
/
/
/
/
/
-
/
u
-
/
c A' /,{
, -+•
_
/
^f
£
_
/
/
/
e
.^
'" /ft
Fig. 2 — Departure from saturation
with respect to calcite. Thermodynamic saturation along line Kspc = 10~8-34. The letters c,d, and h represent calcite, dolomite,
and hydromagnesite. Four analysis plots from Neusiedlersee are connected by a
dotted line, six analysis plots from west to east of Lake Balkhash are connected
by a dashed line, and two analysis plots from Kcene Wonder Spring are joined
by a dot-dash line. Representative ocean water (12) is designated by a cross (4 ).
41
/
•/
/
_ y ,«y
9^y
/
/
i
x
<
<
y
/
•
1
x
/
_
xf
/ \
/./
1
'
///
•>
1
A
4
- //
y
X
v
/
X
u
•? /
/
/
Ji //
/
/
/
/
/
/
|f"i
•B
a
S
o
/
a.
.
•n
/
a
o
•*•
%%%
V*
*•
ill
XI
Q
^
« 1B % - o o
G -3 X = <>o
-«##-•
i
6o,
•-
Fig. 3 — Departure from saturation with respect to dolomite. The letters c, el, and h
represent calcite, dolomite, and
hydromagnesite. Presumed thcrmodynamic
saturation
along line30Kspa — 10! 17-00, but three 13
other dashed Kspa lines (work of
1!l
Halla ( ), ROBIE ( ), and Garrels and others ( )) possibly represent thermodynamic saturation more correctly. Four analysis plots from Neusiedlcrsee are
connected by a dotted line, six analysis plots from west to east of Lake Balkhash
are connected by a dashed line, and two analysis plots from Keene
Wonder Springs
are joined by a dot-dash line. Representative ocean water (12) is designated by
a cross (-••).
42
From figure 2 it is apparent that alkaline soda lakes, from many of which calcitc
precipitates, are regularly and uniformly supersaturated with respect to this mineral
by a factor of 5 to 10 times. Ground waters from limestones and dolomites are much
more variable than the alkaline soda waters with respect to calcitc saturation. Most
of these ground waters arc supersaturated, a few by factors as high as those encountered
for alkaline soda waters. It is perhaps significant that analyses of ground waters from
two marbles, the calcites of which have been presumably rccrystallizcd to a coarser,
more thermodynamically stable variety, indicate only slight supersaturation. Not
surprisingly, waters from ultramafic rocks are generally undcrsaturatcd with respect
to calcite.
Supersaturation of 10 times apparently constitutes a definite ceiling for calcite,
and this is illustrated in figure 2 by the dashed line connecting six analyses of waters
from Lake Balkhash, U.S.S.R. The analyses, shown in Table 1, were of waters from
six different stations, or pools, ranging from the western (no carbonate precipitation)
to the eastern end (considerable carbonate precipitation) of the lake. From west to
east, there is a marked increase in pH, salinity, magnesium and carbonate content,
and a decrease in calcium (15). According to Sapozhnikov (a2) the shallow character
of the lake (nowhere greater than 90 feet) does not permit a vertical zonation of the
waters.
Turning to dolomite saturation, it is apparent from figure 3 that all water types
under discussion show a complete disregard for the various postulated thermodynamic
solubility products of this mineral. Waters from dolomites, limestones, and ultramafic
rocks all indicate roughly similar ranges of supersaturation, and this supersaturation
commonly amounts to 5 to 10 times the value of Holland (20) or Halla (1!)). These
supersaturation values would be even higher, of course, if the values of Robie ('1())
or Garrels and others (13) are more nearly correct. The supersaturation values for the
alkaline soda waters are still greater, ranging from 15 to 80 times.These supersaturation values are surprising for waters derived from limestones and dolomites, and
although the evidence is scanty, the suggestion is that ground waters do not equilibrate readily with dolomites.
For those alkaline soda lakes that precipitate dolomite (or prolodolomite) the
apt is 1O~)4-00 or higher, but in no case higher than 10~13-nn. The solubility product
(K<sP) at 25°C for protodolomite probably lies within this range of values. Although
opa has a fairly narrow range, from 10"1'1-00 to 10 13-(l() for those alkaline soda lakes
in which dolomite is forming, it is not clear whether the 1O~13-00 figure represents a
supersaturation ceiling for this mineral. If there is a supersaturation ceiling for dolomite, the assemblage calcite plus dolomite should effectively limit the buildup of
magnesium in alkaline carbonate waters. The formation of hydromagnesite, magnesite,
or nesquehonite would also limit magnesium buildup. The occurrence of calcitehydromagnesite assemblages in British Columbia (u> 29) and calcite-dolomite-magnesium carbonate (hydromagnesite ?) assemblages in the sediment at the eastern end
of Lake Balkhash (15) suggest that a supersaturation ceiling for dolomite is not always
effective in limiting an increase in aqueous magnesium. Kinetic factors in a dynamic
environment are probably important.
The highest determined supersaturation values arc those with respect to magnesite
(fig. 4). In alkaline soda waters these range from minimum values near 10 times to
more than 100 times. Most of the waters from ultramafic rocks are also supersaturated,
some quite strongly, whereas almost all those from limestones are undersaturated.
Four of the five waters from dolomites hover near magnesite saturation. Lack of
evidence of direct magnesite precipitation at 25°C, at least from the waters studied,
indicates that Mg!"' and COg~ may build up in solution until («Mg++) (tico's )
reaches a value of about 10 5 ' 50 , the KS7) for nesquehonite. In distinctly alkaline
waters (pH > 8.5), hydromagnesite will probably precipitate before nesquehonite.
From table 1 the apn's, for the five waters probably precipitating hydromagnesite are
43
Fig. 4 — Departure from saturation with respect to magnesite and ncsquehonite.
Thh letters c, d, h, m, and n represent calcite, dolomite, hydromagnesite, magnesite, and nesquehonite.
Thermodynamic saturation along lines Kspm —10~7-91
and Kspn = 10"5-51. Four analysis plots from Neusiedlersee are connected by a
dotted line, six analysis plots from west to east of Lake Balkhash are connected
by a dashed line, and two analysis plots from Keene
Wonder Springs are joined
by a dot-dash line. Representative ocean water (12) is designated by a cross .( + )
44
15- .01 =
1
l
s
1| | . . I
*
t
o
s
:
4,•f
o —
•
! ! ;
a
«
S
C
-" i
O I-
•
X
m
0©
D
• ••
©
C
\
X
-
K
©
X
X
X
/
X
0
+
^ '
/
/
X
/
/
\
/
X
X
X
\
/
\
\
/
/
X
/
16-i- 01
/
/
/
\
\
•<
X
/
/
<^
\
y
/
a
:
a
\
"
c
• P.
-
/
\
/
x Ll
/
u
/
/
D
P.
/
/
-
-
/ ^ \
G
; /
a
^
/
i
D
1
D
o«
^\
I
6O|-
Fig. 5 — Separation of water types by departure from saturation with respect to
calcitc and magnesitc. The letters c, d, h, m, and n represent calcite, dolomite,
hydromagnesite, magnesite, and nesquehonite. Area of "residual concentration"
(alkaline soda waters
precipitating two or more Ca-Mg carbonates) is bounded
by apo's of 10- 7S4 and lO"7-34, apa's of 10-14-00 and 10" 1300 , and Kspn- The
ap's determined by Holland and others (20) for cave waters in limestones and
dolomites are represented
by half-filled squares (Is) and triangles (dolo) Representative ocean water (12) is designated by a cross (+).
45
all around 10 30-00 or slightly higher. This figure corresponds to a supersaturation of
30 times with respect to hydromagnesite, and such a high degree of supersaturation
may be necessary to induce precipitation. Only one other water, that from Big Soda
Lake, Nevada, is as highly supersaturated, but the nature of precipitated carbonates,
if any, from this lake is not known.
On figure 5, a summary plot of apc against apm, the four water types are clearly
separated on the basis of their relative degree of saturation with respect to both calcite
and magnesite. Most of theplotsof alkaline soda waters, including all thosethatareprecipitating calcite plus dolomite and (or) hydromagnesite, fall in an area of "residual
concentration", which is shown on figure 5 by heavy solid lines. This area is bounded
by ape values of 10~ 7 M and 10"7-3'1 (5and 10 times calcite supersaturation), «/?<* values
of 10"14-00 and lO""-011, and the solubility product of nesquehonite (10 " 5 - sl ). As
mentioned previously, the pH's of most alkaline soda waters are sufficiently high to
induce precipitation of hydromagnesite before an apa value of 10 l3-()n, or the solubility product of nesquehonite, is reached. This area illustrates the probable limits
of supersaluration with respect to calcite and dolomite that can be attained in alkaline
soda waters and very possibly other waters, such as sea water of comparable ionic
strength.
Activity products for calcite and dolomite as determined by Holland and others
(20) for cave waters of Virginia and Pennsylvania are also plotted on figure 5. The
moderate to high supersaturation of these cave waters with respect to calcite and
dolomite is in good general agreement with those reported here from ground waters
from limestones and dolomites. Holland and his coworkers (20), however, point out
that those cave waters which have ample opportunity to equilibrate with cave air,
and consequently have ceased precipitating calcite, are much less apt to show significant supersaturation with respect to this mineral. Waters from the Black Chasm
limestone caves (see table 1) also fall in this category.
3. CONCLUSIONS
The supersaturation values encountered in various water types have been largely
considered without reference to rate of thermodynamic equilibration or persistence
of metastable supersaturation, but it seems that for magnesite at least, high supersataration values, probably more than 100 times, may persist for years. If hydromagnesite
is precipitated, it should convert to magnesite more or less rapidly (from a geologic
standpoint) in a wet or moist environment, but whether supersaturation with respect
to magnesite can be sensibly reduced by contact with magnesite formed in this way
is doubtful.
There is also no evidence that waters from which some form of dolomite precipitates become less supersaturated with respect to this mineral. As will be explained
below, an apa of 10"1'1-00, roughly the K$v for protodolomite,. probably can be
maintained under certain conditions.
The possibility of long-term metastable persistence of calcite supersaturation is
not unequivocal. It was mentioned above that cave waters in limestones and dolomites,
which arc characterized by relatively high Pco 2 values, low ionic strengths, slight
seasonal fluctuations in temperature, and simple compositions (largely Ca"1"*, Mg+"",
and HCO3 ions), do tend to reduce their supersaturation and equilibrate with pure,
coarse calcite. This tendency is not noted in the much more dynamic environment of
the alkaline soda lakes. Pco a values in these lakes are similar to that of the atmosphere;
there are seasonal temperature fluctuations and variations in the rate of supply of
dissolved matter; ionic strength is moderate to high (0.02-0.5 and even higher); and
aqueous sodium is much greater than calcium or magnesium, although aqueous
magnesium is somewhat greater than that encountered in dolomite cave water.
46
The waters of Neusiedlcrsee-Podersdorf, on the Austrian salt steppes, offer compelling evidence of persistent calcitc supersaturation. Both calcite andprotodolomite
are precipitating in this lake (35). According to Knic (23) the lake, although 320 square
kilometers in extent, averages only 4 feet in depth, and the whole steppe area is characterized by strong winds and high evaporation rates. Under such conditions the lake
waters probably are thoroughly mixed.
Four separate analyses (see table 1) of the lake water in summer months indicate
constant, uniform supersaturation with respect to both calcite and dolomite. In the
winter, though, with pH conditions nearly identical with those of the summer (8.84 vs.
a summer average of 8.87) (2a), the calcium content of the lake water increases by a
factor of 1.7, which is just about what one would expect from the increased solubility
of calcite near O"C (25). Aqueous magnesium shows no corresponding increase.
Furthermore, analyses of the lake water during the time of spring rains (-3), with
concomitant dilution of lake water and lowering of pH (about 0.3 pH units), indicate
a further increase in aqueous calcium, although the concentrations of all other aqueous
constituents, including magnesium, are diluted to about 65-70 percent of their January
level. This behavior strongly suggests that the shallow and well-mixed waters of
Neusiedlcrsee do tend to seasonally equilibrate with primary calcitc and, during the
summer months only, with primary protodolomite. This calcite, however, must have
a much greater free-energy than that determined for pure, well crystallized material
because of the uniform supersaturation (6 times to 9 times) of the lake water during
the entire year. Other relatively shallow lakes from which both calcite and dolomite
(or protodolomite) are precipitating indicate virtually identical activity products for
calcite and dolomite. Examples arc Balkhash, U.S.S.R., Borax Lake, California, and
Lake Kingston, South Australia.
It is tempting to appeal to a high magnesian content in the primary calcites of
the above lakes as being the principal reason for higher and apparently persistent
solubility. Although there is a lack of chemical analyses for such calcites (physical
separation from very fine grained dolomites and clay minerals is exceedingly difficult),
Skinner (3fl) has used the X-ray cell-edge technique of Graf and Goldsmith (I(i) to
show that the primary calcites of Lake Kingston and adjacent lagoonal environments
contain 16 to 22 percent MgCOs,. Recently Chave and others (7) have demonstrated
by two different experimental methods that the solubility of skeletal marine calcites
increases with increasing MgCOs, content. In the range of 15-25 percent MgCOs,,
solubility increases from about 3 to about 10 times that of pure, coarse calcite. The
indicated supersaturation of the Lake Kingston sample (table 1) with respect to calcite
is 8.3 times.
It seems likely that extensive ionic substitution may also increase the solubility of
primary dolomite and even hydromagnesite. Skinner (3S) found that the protodolomite of Lake Kingston contains 56 percent CaCOa,. In addition to metastable ionic
substitution, greater surface energy due to fine grain size and crystal defects or dislocations (for example, the random positioning of Ca and Mg atoms in protodolomite)
should also play an important role in increasing carbonate solubility. For alkaline soda
lakes these factors presumably account for the seemingly high degree of supersaturation
necessary for carbonate precipitation, but, as indicated above for Neusiedlersee, these
waters arc probably not supersaturated with respect to the very fine grained nonstoichiometric carbonates that actually are precipitated. Persistence of these metastable
carbonates below the sediment-water interface (as much as 3 feet for the magnesian
calcites and calcian dolomites of Lake Kingston and nearby lagoonal environments
(36)) tends to confirm this suggestion. In a similar vein Schmalz and Chave (34) suggest
that nearshore Bermuda marine waters also tend to equilibrate with the most soluble,
highly magnesian calcite present in the sediment.
The alkaline soda lakes are more or less intermediate between cave water and
ocean water with regard to ionic strength and aqueous magnesium content, but are
47
as effectively supersaturated with respect to both calcite and aragonitc as cither (at
25"C, aragonite is about 13 percent more soluble than calcite (13)). Aragonite, commonly encountered in caves and warm-water shallow ocean sediments, is apparently
rare in alkaline soda lakes. The formation of aragonite rather than calcite in caves
may be due to a rapid release of CO2 to cave air, but the mechanisms that trigger
precipitation of inorganic aragonite in ocean water are poorly known (see, for instance,
the discussion in Cloud (8), p. 103-105) Whatever these mechanisms may be, the
nonformation of aragonite and the abundance.of primary, highly magnesian calcite
in alkaline soda lakes suggest that the mechanisms are not related to a high Mg/Ca
ratio in solution or to a lower solubility for aragonite than for magnesian calcite.
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14
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49