+
,
i
ree ree eae
a
*
1
.
ae
.
ee
2 shows, most of the points for the Lake
Magadi, Alkali Valley, and Abert Lake
areas fall near a straight line having a
slope of approximately 45° over the
entire range of sodium concentration
than by dissolution of free SiO., such
as opal, cristobalite, or quartz. A schematic reaction such as
(Table 1). The brine pool north of
Abert Lake is saturated with trona and
NaAlSi3s03 + 6144H:eO + COg =
ppm). The most concentrated brines
(/6). The at-
show definite SiO, enrichment. compared with CO.:SiO, trend set by
evaporative concentration; this enrichment is probably caused by the fractional crystallization of sodium carbonate minerals.
Asin Fig. 2, the silica value for Aqua
quires a direct relation between Nat:H*
de Ney Springs plots well above the
in feldspar
Na* + HCOs. + 3Si(OH): + %AlL0s
from 2400 to 132,000 ppm. The great-
est scatter is in the points for the most
dilute brines; they represent inflow
waters not yet subjected to much evaporation. A number of the interstitial
solutions, having SiO, contents below
the general evaporative trend, may result from silica sorption during longterm contact with silicate solids, The
highest sodium values reached are between 100,000 and 132,000 ppm and
are for brines saturated with respect to
sodium carbonate minerals. In these
saturated brines, silica may be further
enriched through fractional crystallization of trona or of natron and halite,
orof all three.
The data for the Oregon and Kenya
brines, in which carbonate and bicarbonate are the dominant anionic species,
were examined by simple-regression
analysis. The equations for the lines of
best fit were (ppm):
halite and has still less CO, (43,300
in
illustrates this process
clay
mosphere supplies the CO, necessary to
Maintain ionic balance during silicate
breakdown. This simple reaction reand SiO.,. However, field pH’s apparently are affected by factors other than
the CO.-SiO., equilibria, and a plot of
Nat:H* versus SiO, shows great scatter.
Saturation of solutions with respect
to sodium carbonate minerals limits the
CO, enrichment; the limits are dependent upon other anions present, such as
chloride. The Alkali Valley brines have
the highest total CO,:Cl ratio throughout and reach the greatest concentration
in SiO, and CO, species at saturation.
In contrast, the CO, values of the
Magadi brines do not exceed 70,000
ppm because they contain much Cl
trend for sodium carbonate brines. The
sodium and silica contents of these
waters probably result from the hydrolysis of silicate in the absence of COQ,.
In summary, evaporative concentration of sodium carbonate waters leads
to high-pH_ brines, which can retain all
the silica initially in solution. The final
values of SiO., pH, and Na+ and CO,
species depend on the initial solution
composition, on the extent of equilibration with the atmosphere and silicate
muds, or with both, and on the frac-
tional precipitation of sodium carbonate
minerals. The amount of silica stored
3000
Na+ = 8180 + 127 Si0e
for the Lake Magadi area
;
(a = 53,400 ppm Nat, r = .94)
Nat = 6316 + 113 SiOe2 for the combined
Abert Lake and Alkali Valley systems
(o = 29,000 ppm Na‘, r = .80)
HCO,- and CO,--, such as those from
Deep Springs and Owens and Searles
lakes (16), plot well below these lines;
they do not reach the high pH values
necessary to keep large amounts of
SiO. in solution. Conversely, waters
from Lake Nakuru, another closed
basin in the rift valley 270 miles north
of Lake Magadi, and Aqua de Ney
Springs are high in silica relative to
sodium, presumably because of initially
higher pH vaiues.
Figure 3 shows the correlation between SiO, and total CO,. The equation
for the regression line obtained from
the combined Oregon and Magadi data,
aX
1000+
Brines that contain much sulfate, chlo-
ride, and other anions in addition to
.
N. Abert pend
—
z
~~
Qo
[Aqua de Ney Springs, Calif.
=
rc
—
co
Ld
a.
—
cc
wn
«
<
a
Z
100;-
CO
”
|
—
s
fF
=
|
Sf
.
7
@ Abert Lake basin, Ore.
—
Deep Springs Valley, Calif.
(a = 25,300 ppm total COz, r = .92)
naeae
4 Lake Magadi, Kenya
x Alkali Valley, Ore.
Total COz = 1542 + 79SiO>2
8 DECEMBER 1967
—_—
500
exclusive of the most supersaturated
pools, is (ppm)
Again, evaporative concentration can
account for the trend shown. The good
correlation between total CO, and SiO,
content of the brines also suggests that
SiO, is derived from hydrolysis of
silicates in the presence of CO., rather
Yo 7
10
1000
|
i
LJ)
10,000
5000
@ Closed sag pond
O Lake area
|
|
|
fit!
50,000 100,000
CO, IN PARTS PER MILLION
Fig. 3. Total COzg versus SiOz for alkaline brines.
1313