Evaporites: Types, Properties and Uses

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.

See also  A Closer Look at the Rock Cycle: Igneous, Sedimentary, and Metamorphic Rocks

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   
TypessubtypesCompositionNature
Marine evaporites (underwater and subaerial precipitation)   
chlorideshaliteNaClrock salt
 SilvitaKCl 
 carnaliteKMgCl3.6H2O 
SulfateslangbeiniteK2Mg2(SO4)3Potash salts or bitterns
 polyhaliteK2Ca2Mg(SO4) 4.2H2O 
 KainiteKMg(SO4)Cl.3H2O 
 AnhydriteCaSO4 
 CastCaSO4 2H2O 
 kieseriteMgSO4.H2O 
carbonatescalciteCaCO3Inorganic carbonates
 magnesitemgCO3 
 DolomiteCaMg(CO3)2 
non-marine evaporites   
chlorideshaliteNaCl 
 RinneiteFeCl2.NaCl.3KCl 
SulfatesCastCaSO42H2O 
 AnhydriteCaSO4 
 epsomiteMgSO4.7H2O 
 mirabilitaNa2SO4.10H2O 
 ThenarditeNa2SO4 
 bloediteNa2SO4.MgSO4.4H2O 
 GlauberiteCaSO4.Na2SO4 
carbonatesNatroniteNa2CO3.10H2O 
 high chairNaHCO3.Na2CO3.2H2O 
 gaylussiteNa2CO3.CaCO3.5H2O 
boratesBoraxNa2B4O5(OH)48H2O 
silicatesMagaditeNaSi7O13(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.

See also  Exploring the Different Metamorphic Facies: Zeolite to Granulite

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.

See also  Understanding Shallow Foundations: Types and Characteristics

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.

Leave a Comment

Your email address will not be published. Required fields are marked *