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The densities of the binary solutions Nad-H20, KCI-H20, MgClj-HjO, and CaCI2-H20 were measured at 1 atm, from 25 to 45 °C and from 0.5 to 6.1 m, by using a commercially available oscillating tube-type densitometer. The data sets were described by using least-squares polynomial regression analysis. Statistical evaluation of the experimental data Indicates that uncertainties In the concentrations of the solutions due to adsorption of water by the salts during weighing Is the major source of error In the density measurements. The magnitude of this error Is greater for MgCI2 and CaCI2 solutions because of the hygroscopic nature of these salts. The measured densities of NaCI and KCI solutions have a standard deviation of 5.515 X 10~3 g/cm3. Standard deviations for MgCI2 and CaCI2 solutions range from 2.329 X 10~4 to 2.334 X 10"4 g/cm3 and from 2.796 X 10'4 to 2.839 X 1CT4 g/cm3, respectively. The experimental density data compare well with published values.

We propose that Precambrian iron-formations may have formed in the evaporative setting of a playa-lake complex. Hydrologically and sedimentologically such a complex would be similar to the depositional environment suggested for the Green River Formation by Eugster and Surdam (1973). Chemical and mineralogical differences can be accounted for if we assume a Precambrian atmosphere free of oxygen during the depositional interval of banded iron-formations.Following a suggestion of Eugster (1969), magadiite or a sodium silicate gel are considered to be the most likely precursors of the bedded cherts. Magadiite solubility curves have a minimum with respect to silica at a pH of 8.7 and saturation can occur either through evaporative concentration or through flooding of silicarich, high pH brines with fresher waters. Precipitation can take place in a fairly dilute lake. Most Precambrian magadiite must have been converted to chert during early diagenesis, while remaining magadiite interacted with iron minerals to produce riebeckite, a mineral common to iron-formations. Other reasons why magadiite or a sodium silicate gel are attractive as primary precipitates are their plasticity and high water content. These properties may be responsible for some of the diagenetic features observed in iron-formations by Trendall and Blockley (1970), Gross (1972), and Dimroth and Chauvel (1972, 1973).Iron solubiliw and transport have been calculated using the newest set of thermodynamic data available for 25øC. Fe(OH)a is extremely insoluble at pH values considered here (8-9), except at very low oxygen fugacities, such as those of the hematite-magnetite pair. The siderite stability data are conflicting, but siderite is considerably less soluble than calcite at pH below 10. A greenalite stability constant can be estimated using data on other phyllosilicates. Greenalite appears to be very insoluble. Neither siderite nor greenalite are likely to precipitate under equilibrium conditions, but probably require considerable supersaturation.Precipitation of iron minerals occurs in response to changes in pH and fcot at an fo• level between those of the hematite q-magnetite and magnetite q-quartz q-fayalite buffers. Fe(OH)a is precipitated whenever free oxygen is available. The precursor precipitate for magnetite is not clear. Magnetite could form either from Fe(OH)•., a mixture of Fe(OH)• and Fe(OH)a, or by reduction of Fe(OH)a with organic matter, as suggested by Perry and Tan (1973).In the playa-lake model, inflow into the lake is primarily through springs, with precipitation and brine evolution proceeding in the playa flats fringing an alkaline lake. This accounts for the paucity of clastics found in banded iron-formations. Water compositions were of the bicarbonate or of the hydroxide type. The latter are attractive because the lower organic activity of Precambrian soils would have provided less COs for weathering reactions and because the alteration of the abundantly exposed basic and ultrabasic rocks (Petrijohn, 1973) would have produced hig...