Evaporites are chemogenic sediments in sedimentary rock types that have been precipitated from water after concentration of dissolved salts by evaporation. This takes place in both marine and non-marine waters (lakes, lagoons).
Although there are a large number of different chemical salts dissolved in seawater, their relative abundances and solubilities allow very few common evaporitic minerals to precipitate naturally.
How are evaporites formed?
The evaporite s are the products of evaporation from salt water. A good example is Utah’s Bonneville Salt Flats. The salar (evaporites) were formed when an ancient salt lake evaporated. Under the heat of the Sun, the water turned to steam and drifted up into the atmosphere, but the salt that had dissolved in the water was left behind and formed the evaporites.
Salt precipitation occurs when salt water becomes supersaturated, meaning it has exceeded its capacity to hold more dissolved ions. In supersaturated saltwater, ions stick together to form solid grains that settle out of the water or grow on the floor of the water body.
Supersaturated saltwater develops where evaporation removes water from a body of water (for example, a lake) faster than the rate at which new water enters. This process takes place in desert lakes and on the margins of restricted seas (figure 1).
For thick salt deposits to form, large volumes of water must be evaporated. Because salt deposits form as a result of evaporation, geologists refer to them as evaporites.
The specific type of salt minerals that an evaporite comprises depends on the amount of evaporation. When 80% of the water evaporates, gypsum forms; and when 90% of the water evaporates, it is the halite that precipitates.
Types of evaporites
The evaporites they include halite, gypsum, anhydrite, and to a lesser extent dolomite and potassium salts (or bitterns). Prolonged evaporation leads to a significant accumulation of evaporated minerals, and the resulting sedimentary deposit is usually named for the dominant mineral.
In thick successions, there is commonly a cyclic repetition of evaporites from less soluble to more soluble, ie dolomite, gypsum, anhydrite, halite, bittern. The evaporites Nonmarine are also often dominated by halite, gypsum, and anhydrite, although a broader range of minor salts exists.
This is because the chemical compositions of the original waters vary considerably depending on the composition of the rocks with which they interact. Such non-marine evaporites include trona, mirabilite, glauberite, borax, epsomite, thenardite, gaylusite, and bloedite. Table 1.
| TYPES OF EVAPORITES | |||
| Types | subtypes | Composition | Nature |
| Marine evaporites (underwater and subaerial precipitation) | |||
| chlorides | halite | NaCl | rock salt |
| Silvita | KCl | ||
| carnalite | KMgCl3.6H2O | ||
| Sulfates | langbeinite | K2Mg2(SO4)3 | Potash salts or bitterns |
| polyhalite | K2Ca2Mg(SO4) 4.2H2O | ||
| Kainite | KMg(SO4)Cl.3H2O | ||
| Anhydrite | CaSO4 | ||
| Cast | CaSO4 2H2O | ||
| kieserite | MgSO4.H2O | ||
| carbonates | calcite | CaCO3 | Inorganic carbonates |
| magnesite | mgCO3 | ||
| Dolomite | CaMg(CO3)2 | ||
| non-marine evaporites | |||
| chlorides | halite | NaCl | |
| Rinneite | FeCl2.NaCl.3KCl | ||
| Sulfates | Cast | CaSO42H2O | |
| Anhydrite | CaSO4 | ||
| epsomite | MgSO4.7H2O | ||
| mirabilita | Na2SO4.10H2O | ||
| Thenardite | Na2SO4 | ||
| bloedite | Na2SO4.MgSO4.4H2O | ||
| Glauberite | CaSO4.Na2SO4 | ||
| carbonates | Natronite | Na2CO3.10H2O | |
| high chair | NaHCO3.Na2CO3.2H2O | ||
| gaylussite | Na2CO3.CaCO3.5H2O | ||
| borates | Borax | Na2B4O5(OH)48H2O | |
| silicates | Magadite | NaSi7O13(OH)3.3H2O |
Sedimentary characteristics of evaporites
Stratum
A wide range of strata styles are possible, depending on the depositional environment, as well as postdeposit diagenesis and diapirism.
The thickness of the layer varies from very thin intercalations, for example, within a peritidal or fluvial-lacustrine succession, to more or less thick structureless units, especially where postdepositional changes have eliminated the original traces of the layer.
Structures
Nodular evaporites occur as discrete masses within mudrocks and also more compact, with only fine, irregular strings of sediment in between.
They are typical of supratidal environments.
Crystalline forms of evaporite minerals also occur as isolated euhedral crystals in the bottom sediment, typically mudstone, and as fully crystalline beds.
Some twinned gypsum (selenite) crystals can be 1 m in length.
These are typical of lagoonal and intertidal environments, where they are often associated with signs of periodic desiccation: polygonal cracks, megapolygons, and deep wedge cracks, cavities, and with microbial-evaporitic stromatolites.
Parallel lamination, cross lamination, and wavy and anastomosed bed/laminate indicate shallow to deep water evaporites.
Graded turbidite and desbrite structures are recognized in deep-sea evaporites.
Dissolution of evaporite minerals can result in a very vuggy rock or, where replacement has occurred, to give rise to pseudomorphs of the original nodule or crystal.
Texture
Crystal size and shape vary considerably within evaporite successions.
Bottom-growing gypsum crystals, on the floors of lakes, lagoons, and shallow restricted shelves, commonly grow vertically as well-formed, clear selenitic crystals.
They can reach spectacular sizes, over 1m in length, sometimes with curved glass faces giving a webbed or cauliflower effect.
Crystals precipitated in the evaporating surface waters of lakes or seas are mostly very fine-grained (10–100 μm).
However, because evaporites are particularly prone to diagenetic modifications, including dehydration/rehydration reactions, dissolution, cementation, recrystallization, replacement, and deformation, their original texture is often destroyed.
A fine, crystalline, sugar-textured material known as alabastrin gypsum is a common diagenetic replacement for anhydrite.
Larger crystals that grow on finer landmass are called porphyrotopic gypsum.
Pseudomorphic replacements, by evaporite or other minerals such as quartz, will reflect the original size of the crystal.
Settled evaporites, including debris and turbidites, are unusual in retaining at least part of their original textures.
Composition
The evaporites they are largely composed of individual evaporite minerals, with variable trace impurities.
Gypsum (CaSO4.2H2O) is the dominant sulfate mineral in modern evaporites, but it dehydrates to anhydrite (CaSO4) during burial diagenesis so that anhydrite dominates at depths greater than 600m.
Trace elements present (1 to 100 ppm) include Br, Sr, B, F, and Si. Strontium can also be present in much larger amounts, in excess of 1000 ppm.
Other impurities include organic carbon, clay minerals, quartz, feldspar, and sulfur. Their presence can affect the usual white, cream, or light gray color of evaporites: darker shades of gray are caused by trace amounts of organic carbon, pinkish shades by iron, and yellowish shades by sulfur.