Evaporite salt

Minerals derived from evaporation of saline solutions have not been extensively examined. Evaporitic salts from the western U.S. Great Basin concentrated Li (5 Li = +31.8 to +32.6 Tomascak et al. 2003). Evaporation of thermal waters from Yellowstone yielded travertine with extremely low 5 Li (-4.8 Sturchio and Chan 2003), offset from its coexisting fluid by -ll.l%o. Such evaporites, where preserved in the sedimentary record, could afford an opportunity to examine variations in spring water compositions and hence information on hydrothermal processes over time. 

Evaporites are important sources of economic minerals that have been exploited for at least the past 6000y. For example, the evaporite mineral trona (NaHCOj -Na2C03 2H2O) was used by the ancient Egyptians to preserve mummies. Evaporite salts continue to be used for food preservation, construction, road deicing, and in industrial processes. The marine evaporites of Saskatchewan (Canada) are the world s largest source of potash (KCl), which is used as an agricultural fertilizer. In the United States,... 

The sea itself turned saltier, and the former offshore terrains that became exposed were covered with evaporitic salts. These two features must have increased the salt transport to the land (counteracting the effect of the receding shore) and changed the chemical composition of the sea spray to brine-spray. 

TABLE 3.3 Precipitation Sequence of Seawater Evaporitic Salts... 

Gibson GW (1962) Geological investigations in southern Victoria Land, Antarctica 8. Evaporite salts in the Wctoria Valley region. New Zealand J Geol Geophys 5 361-374... 

Smith GJ (1965) Evaporite salts from the dry valleys of Victoria Land, Antarctica. New Zealand J Geol Geophys 8 381-382 Speden IG (1962) Fossiliferous Quaternary marine deposits in the McMurdo Sound region, Antarctica. New Zealand J Geol Geophys 5 746-777... 

Chlorides - The usual sources of chloride ions in phosphate concentrates are the evaporite salts found in some sedimentary ores or from seawater or brackish water that might have been used in beneficiation. Chlorides sharply increase the corrosion rates of steel equipment during acidulaiion. In general, chloride contents higher than 200-300 ppmw cannot be tolerated because corrosion rates for common stainless steels then become excessive 1361. With special steels, use of plastics, and special concrete construction, higher Cl contents can be tolerated. 

The precipitation of evaporites from marine and brine sources depends upon a number of faetors. Prominent among these are the concentrations of the evaporite ions in the water and the solubility products of the evaporite salts. The presence of a common ion decreases solubility for example, CaS04 precipitates more readily from a brine that eontains Na2S04 than it does from a solution that contains no other source of sulfate. The presence of other salts that do not have a common ion increases solubility because it decreases activity coefficients. Differences in temperature result in significant differences in solubility. 

Resources for Potash Fertilizers. Potassium is the seventh most abundant element in the earth s cmst. The raw materials from which postash fertilizer is derived are principally bedded marine evaporite deposits, but other sources include surface and subsurface brines. Both underground and solution mining are used to recover evaporite deposits, and fractional crystallization (qv) is used for the brines. The potassium salts of marine evaporite deposits occur in beds in intervals of haUte [14762-51-7] NaCl, which also contains bedded anhydrite [7778-18-9], CaSO, and clay or shale. The K O content of such deposits varies widely (see Potassium compounds). 

Common names have been given to sodium sulfate as a result of manufacturiag methods. In rayon production, by-product sodium sulfate is separated from a slurry by filtration where a 7—10-cm cake forms over the filter media. Thus rayon cake was the term coiaed for this cake. Similarly, salt cake, chrome cake, phenol cake, and other sodium sulfate cakes were named. Historically, sulfate cakes were low purity, but demand for higher purity and controlled particle size has forced manufacturers either to produce higher quaUty or go out of busiaess. Sodium sulfate is mined commercially from three types of mineral evaporites thenardite, mirabilite, and high sulfate brine deposits (see Chemicals FROMBRINe). 

Evaporite Basin Sulfur Deposits. Elemental sulfur occurs in another type of subsurface deposit similar to the salt-dome stmctures in that the sulfur is associated with anhydrite or gypsum. The deposits are sedimentary, however, and occur in huge evaporite basins. It is befleved that the sulfur in these deposits, like that in the Gulf Coast salt domes, was derived by hydrocarbon reduction of the sulfate material and assisted by anaerobic bacteria. The sulfur deposits in Italy (Sicily), Poland, Iraq, the CIS, and the United States (western Texas) are included in this category. 

AH of these metallic and nonmetallic ions join together in a compHcated array of salts and minerals called evaporites. Several evaporites usually crystalline simultaneously in a mixture. This often makes separation into pure chemicals difficult. A Hst of some of the mote common evaporites is given in Table 1, which also shows their chemical formulas and other mineral names. 

Chlorine is the twentieth most abundant element in crustal rocks where it occurs to the extent of 126 ppm (cf. nineteenth V, 136 ppm, and twenty-first Cr, 122 ppm). The vast evaporite deposits of NaCl and other chloride minerals have already been described (pp. 69, 73). Dwarfing these, however, are the inconceivably vast reserves in ocean waters (p. 69) where more than half the total average salinity of 3.4 wt% is due to chloride ions (1.9 wt%). Smaller quantities, though at higher concentrations, occur in certain inland seas and in subterranean brine wells, e.g. the Great Salt Lake, Utah (23% NaCl) and the Dead Sea, Israel (8.0% NaCl, 13.0% MgCU, 3.5% CaCU). 

These deposits would result in carbonate rock (e.g., limestone). A third source rock possibility would be evaporite rocks (e.g., salt, gypsum, anhydrite), which often contain large organic concentrations when originally deposited [26-29]. 

The present sources to the ocean are the weathering of old evaporites (75% of river flux) and CP carried by atmospherically cycled sea-salts (25% of river flux). Loss from the ocean occurs via aerosols (about 25%) and formation of new evaporites. This last process is sporadic and tectonically controlled by the closing of marginal seas where evaporation is greater than precipitation. The oceanic residence time is so long for CP ( 100Myr) that an imbalance between input and removal rates will have little influence on oceanic concentrations over periods of less than tens of millions of years. 

Evaporite deposition is a much more episodic process and thus difficult to quantify. Because seawater is significantly undersaturated with respect to common evaporitic minerals, like gypsum and halite, evaporites are only formed when restricted circulation develops in an ocean basin in which evaporation exceeds precipitation. A geologically recent example is the Mediterranean Sea of 5-6 Myr ago. At this time excess evaporation exceeded the supply of ocean water through shallow inlet(s) from the Atlantic Ocean. As salinity increased, first CaS04, then NaCl precipitated. Over time, salt deposits 2-3 km thick formed. This thickness represents about 40 desiccations of the entire... 

The evaporite source is characterized by covariation of sulfate (from gypsum) and chloride (from halite). That elements can be recycled from the ocean to land by movement of saltbearing aerosols (so-called "cyclic salts") has confused the interpretation of river flux data somewhat. While this cycling generally follows the ratio of salts in the sea, the S/Cl ratio is an exception. Taking the S/Cl ratio of the cyclic component to be 2 (based on compositional data for marine rains) and assuming that all chloride in rivers is cyclic, an upper limit for the cyclic influence can be calculated. 

Solution of halite present as bedded or domal Na-Cl salt-evaporite deposits. Na-Ca-Cl... 

Perthuisot, J. P, 1980, Sebkha el Melah near Zarzis a recent paralic salt basin (Tunisia). In G. Busson (ed.), Evaporite Deposits, Illustration and Interpretation of Some Environmental Sequences, Editions Technip, Paris, pp. 11-17, 92-95. 

On the early Earth, ions were mobilized from volcanic rocks by chemical weathering. Rivers and hydrothermal emissions transported these chemicals into the ocean, making seawater salty. These salts are now recycled within the crustal-ocean-atmosphere fectory via incorporation into sediments followed by deep burial, metamorphosis into sedimentary rock, uplift, and weathering. The last process remobilizes the salts, enabling their redelivery to the ocean via river runoff and aeolian transport. In the case of sodium and chlorine, evaporites are the single most important sedimentary sink. This sedimentary rock is also a significant sink for magnesium, sulfate, potassium, and calcium. 

Because of their role as an elemental sink, the formation and weathering of evaporites has the potential to affect the salinity of seawater. This can in turn alter climate, because the heat capacity of seawater is a function of its salt content. Changes in the salt content of seawater also have the potential to affect survival of marine biota, particularly the calcifiers. 

---