+ , 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