Evaporite

Ion River input Sea-air fluxes Evaporites CEC- clay CaC03 Opaline silica Sulphides MOR 

CEC-clay, cation exchange on estuarine clay minerals MOR, mid-ocean ridge and other seawater-basalt interactions. 

Despite these complications, the main removal mechanisms of major ions from seawater are known (Table 6.2). Quantifying the importance of each mechanism is less easy and the uncertainty of data in Table 6.2 should not be forgotten. The amount of removal is compared with the riverine inputs resulting in a geochemical budget that helps constrain the quality of the data. In the following section we outline the important removal processes for major ions in seawater. 

Sea-to-air fluxes of major ions are caused by bubble bursting and breaking waves at the sea surface. These processes eject sea-salts into the atmosphere, the majority of which immediately fall back into the sea. Some of these salts are, however, transported over long distances in the atmosphere and contribute to the salts in riverwater (see Section 5.3). These airborne sea-salts are believed to have the same relative ionic composition as seawater and their flux out of the oceans is estimated by measuring the atmospheric deposition rates on the continents. In terms of global budgets, airborne sea-salts are an important removal process only for Na+ and Cl from seawater removal of other major ions by this route is trivial. 

Evaporation of seawater will precipitate the constituent salts, the so-called evap-orite minerals, in a predictable sequence (Box 6.2). This sequence starts with the least soluble salts and finishes with the most soluble (see Box 4.12). If approximately half (47%) of the water volume is evaporated, CaC03 precipitates (see 

All figures are available on the companion website in color (if applicable). 

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. 

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, 

15 to 20 million tons of rock salt is mined annually. Petroleum geochemists use sedimentary evaporites as indicators of the likely presence of recoverable petroleum because similar depositional environments are required for their formation. These salt deposits also act to concentrate oil and gas, as evaporite minerals present an impermeable barrier behind which migrating petrolevun tends to pool. 

Although calcium carbonate formation in soil is a result of high evapotranspiration rates relative to precipitation rates, the term evaporite is usually restricted to compounds more soluble than CaCC 3. Where drainage water from surrounding soils accumulates and where the amount of percolated water is small compared to the amount of water evaporated, soluble salts tend to accumulate. This subject is dealt with in more detail in Chapter 11. The present section is restricted to the extreme case of natural salt flats and play as (former and intermittent lake beds). 

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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). 

There are two basic classes of potash-containing evaporites (2), those deposits that are rich ia magnesium sulfate, such as polyhaUte and kieserite, and those that are poor ia magnesium sulfate. The primary source of potash ia the magnesium-rich deposits is camakite, whereas both camakite and sylvite are found ia the magnesium-poor deposits. 

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. 

The efficiency of the weathering of rocks in using carbonic acid produced in the carbon cycle is affected by various hydrologic, environmental, and cultural controls. The fact that the principal anion in fresh surface water worldwide almost always is bicarbonate attests to the overriding importance of this process. Exceptions are systems in which evaporite minerals are available for dissolution by groundwater or where human activities are major sources of sulfate or chloride inflow. 

Calcareous minerals and evaporite minerals (haUdes, gypsum) are very soluble and dissolve rapidly and, in general, congmendy, ie, yielding upon dissolution the same stoichiometric proportions in the solution as the proportions in the dissolving mineral and without forming new soHd phases (Fig. 

Their contribution to the total dissolved load in rivers can be estimated by considering the mean composition of river water and the relative importance of various rocks to weathering. Estimates (18) indicate that evaporites and carbonates contribute approximately 17% and 38%, respectively, of the total dissolved load in the wodd s rivers. The remaining 45% is the result of the weathering of siUcates, underlining the significant role of these minerals in the overall chemical denudation of the earth s surface. 

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. 

Boron is found in two underground ores, ulexite and colemanite. Research and pilot-plant studies have been completed to solution mine these ores, but as yet it is not done commercially. Boron is found in many different evaporite deposits (17). 

Figure 5 illustrates some principal brine evaporites and their derivative products. Some of these chemicals find appHcation in thousands of household items. Sodium chloride alone has over 14,000 different uses (68). 

K. G. Papke, Evaporites and Brines of Nevada, Bull. 87, Nevada Bureau of Mines Geology, 1976. 

The great evaporite basin deposits of elemental sulfur in Poland were discovered only in 1953 but have since had a dramatic impact on the economy of that country which, by 1985, was one of the world s leading producers (p. 649). The sulfur occurs in association with secondary limestone, gypsum and anhydrite, and is believed Ui be derived from hydrocarbon reduction of sulfates assisted 1 bacterial action. The H2S so formed is consumed by other bacteria to produce sulfur as waste — this accumulates in the bodies of the bacteria until death, when the sulfur remains. 

Gulf Coast region of the USA and Mexico, and from the evaporite basin deposits in west Texas, Poland, the former USSR and Iran. 

On this definition, 5 S is zero for meteroritic troilite dissolved sulfate in ix ean water is enriched +207° in S, as are contemporary evaporite sulfates, whereas sedimentary sulhdes are depleted in by as much as —507° due to fraclionation during bacterial reduction to HiS. 

Sodium chloride, by far the most abundant compound of chlorine, occurs in extensive evaporite deposits, saline lakes and brines, and in... 

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]. 

Chemogenic Volcanogenic Polygenic <10 >50 <10 Iron-manganese nodules, glauconite, phosphorite, nodules, phUlipsite, palagonite, celestobarite, and evaporites Pyroclastic material 38 Red clay... 

Where easily weathered lithologies such as carbonates and evaporites are near the surface, such as in the lower Amazon valley, their contribution to the rivers appears minor, probably because thick residual soil covers have devel-... 

Holland, H. D. (1974). Marine evaporites and the composition of sea water during the Phanerozoic. In "Studies in Paleo-oceanography (W. W. Hay, ed.), pp. 187-192. Society of Economic Paleontologists and Mineralogists, Tulsa, Oklahoma, Special Publication 20. 

Evaporation of river water will not make seawater. Instead, evaporation of the nearly neutral Na -Ca -HCOi" river water produces a highly alkaline Na-HCO -COf water such as foimd in the evaporitic lake beds of eastern California (Carrels and MacKenzie, 1967). In addition, on comparing the amount of material supplied to the ocean with the amoimt in the ocean, it may be seen that most of the elements could have been replaced many times (Table 10-12). Thus some chemical reactions must be occurring in the ocean to consume the river flux. 

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. 

However, not all the chloride is cyclic, a fact first appreciated in recent years. An example comes from a detailed study of river geochemistry conducted in the Amazon Basin. In the inland regions, rains typically have a chloride content of 10 pM, while major inland tributaries have chloride contents of 20-100 pM. These data suggest that only 25% of the Cl is cyclic, whereas 75% is derived by weathering of evaporites. Indeed, 90% of this 75% can be shown to have its origin in the Andean headwaters, derived from evaporites that make up only 2% of the area of the Amazon Basin (Stallard and Edmond, 1981). As the ratio of sulfate to chloride in evaporite deposits is generally much higher... 

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