Evaporite

Evaporite ( /ɪˈvæpəraɪt/) is the term for a water-soluble mineral sediment that results from concentration and crystallization by evaporation from an aqueous solution.[1] There are two types of evaporite deposits: marine, which can also be described as ocean deposits, and non-marine, which are found in standing bodies of water such as lakes. Evaporites are considered sedimentary rocks and are formed by chemical sediments.

HaliteEncrustedCobbleDeadSea
Cobble encrusted with halite evaporated from the Dead Sea, Israel

Formation of evaporite rocks

Although all water bodies on the surface and in aquifers contain dissolved salts, the water must evaporate into the atmosphere for the minerals to precipitate. For this to happen, the water body must enter a restricted environment where water input into this environment remains below the net rate of evaporation. This is usually an arid environment with a small basin fed by a limited input of water. When evaporation occurs, the remaining water is enriched in salts, and they precipitate when the water becomes supersaturated.

Evaporite depositional environments

Marine evaporites

Anhydrite HMNH1
Anhydrite

Marine evaporites tend to have thicker deposits and are usually the focus of more extensive research.[2] They also have a system of evaporation. When scientists evaporate ocean water in a laboratory, the minerals are deposited in a defined order that was first demonstrated by Usiglio in 1884.[2] The first phase of the experiment begins when about 50% of the original water depth remains. At this point, minor carbonates begin to form.[2] The next phase in the sequence comes when the experiment is left with about 20% of its original level. At this point, the mineral gypsum begins to form, which is then followed by halite at 10%,[2] excluding carbonate minerals that tend not to be evaporites. The most common minerals that are generally considered to be the most representative of marine evaporites are calcite, gypsum and anhydrite, halite, sylvite, carnallite, langbeinite, polyhalite, and kainite. Kieserite (MgSO4) may also be included, which often will make up less than four percent of the overall content.[2] However, there are approximately 80 different minerals that have been reported found in evaporite deposits (Stewart,1963;Warren,1999), though only about a dozen are common enough to be considered important rock formers.[2]

Non-marine evaporites

Non-marine evaporites are usually composed of minerals that are not common in marine environments because in general the water from which non-marine evaporite precipitates has proportions of chemical elements different from those found in the marine environments.[2] Common minerals that are found in these deposits include blödite, borax, epsomite, gaylussite, glauberite, mirabilite, thenardite and trona. Non-marine deposits may also contain halite, gypsum, and anhydrite, and may in some cases even be dominated by these minerals, although they did not come from ocean deposits. This, however, does not make non-marine deposits any less important; these deposits often help to paint a picture into past Earth climates. Some particular deposits even show important tectonic and climatic changes. These deposits also may contain important minerals that help in today's economy.[3] Thick non-marine deposits that accumulate tend to form where evaporation rates will exceed the inflow rate, and where there is sufficient soluble supplies. The inflow also has to occur in a closed basin, or one with restricted outflow, so that the sediment has time to pool and form in a lake or other standing body of water.[3] Primary examples of this are called "saline lake deposits".[3] Saline lakes includes things such as perennial lakes, which are lakes that are there year-round, playa lakes, which are lakes that appear only during certain seasons, or any other terms that are used to define places that hold standing bodies of water intermittently or year-round. Examples of modern non-marine depositional environments include the Great Salt Lake in Utah and the Dead Sea, which lies between Jordan and Israel.

Evaporite depositional environments that meet the above conditions include:

  • Graben areas and half-grabens within continental rift environments fed by limited riverine drainage, usually in subtropical or tropical environments
  • Graben environments in oceanic rift environments fed by limited oceanic input, leading to eventual isolation and evaporation
    • Examples include the Red Sea, and the Dead Sea in Jordan and Israel
  • Internal drainage basins in arid to semi-arid temperate to tropical environments fed by ephemeral drainage
  • Non-basin areas fed exclusively by groundwater seepage from artesian waters
    • Example environments include the seep-mounds of the Victoria Desert, fed by the Great Artesian Basin, Australia
  • Restricted coastal plains in regressive sea environments
  • Drainage basins feeding into extremely arid environments
    • Examples include the Chilean deserts, certain parts of the Sahara, and the Namib

The most significant known evaporite depositions happened during the Messinian salinity crisis in the basin of the Mediterranean.

Evaporitic formations

SaltCrystalCasts
Hopper crystal cast of halite in a Jurassic rock, Carmel Formation, southwestern Utah

Evaporite formations need not be composed entirely of halite salt. In fact, most evaporite formations do not contain more than a few percent of evaporite minerals, the remainder being composed of the more typical detrital clastic rocks and carbonates. Examples of evaporite formations include occurrences of evaporite sulfur in Eastern Europe and West Asia.[4]

For a formation to be recognised as evaporitic it may simply require recognition of halite pseudomorphs, sequences composed of some proportion of evaporite minerals, and recognition of mud crack textures or other textures.

Economic importance of evaporites

Evaporites are important economically because of their mineralogy, their physical properties in-situ, and their behaviour within the subsurface.

Evaporite minerals, especially nitrate minerals, are economically important in Peru and Chile. Nitrate minerals are often mined for use in the production on fertilizer and explosives.

Thick halite deposits are expected to become an important location for the disposal of nuclear waste because of their geologic stability, predictable engineering and physical behaviour, and imperviousness to groundwater.

Halite formations are famous for their ability to form diapirs, which produce ideal locations for trapping petroleum deposits.

Halite deposits are often mined for use as salt.

Major groups of evaporite minerals

Calcite-k270c
Calcite

This is a chart that shows minerals that form the marine evaporite rocks, they are usually the most common minerals that appear in this kind of deposit.

Mineral class Mineral name Chemical Composition
Chlorides Halite

Sylvite

Carnallite

Kainite

NaCl

KCl

KMgCl3 * 6H2O

KMg(SO4)Cl * 3H2O

Sulfates Anhydrite

Gypsum

Kieserite

Langbeinite

Polyhalite

CaSO4

CaSO4 * 2H2O

MgSO4 * H2O

K2Mg2(SO4)3

K2Ca2Mg(SO4)6 * H2O

Carbonates Dolomite

Calcite

Magnesite

CaMg(CO3)2

CaCO3

MgCO3

Hanksite
Hanksite, Na22K(SO4)9(CO3)2Cl, one of the few minerals that is both a carbonate and a sulfate

Evaporite minerals start to precipitate when their concentration in water reaches such a level that they can no longer exist as solutes.

The minerals precipitate out of solution in the reverse order of their solubilities, such that the order of precipitation from sea water is:

  1. Calcite (CaCO3) and dolomite (CaMg(CO3)2)
  2. Gypsum (CaSO4-2H2O) and anhydrite (CaSO4).
  3. Halite (i.e. common salt, NaCl)
  4. Potassium and magnesium salts

The abundance of rocks formed by seawater precipitation is in the same order as the precipitation given above. Thus, limestone (calcite) and dolomite are more common than gypsum, which is more common than halite, which is more common than potassium and magnesium salts.

Evaporites can also be easily recrystallized in laboratories in order to investigate the conditions and characteristics of their formation.

See also

References

  1. ^ Jackson, Julia A., 1997, Glossary of Geology 4th edition, American Geologic Institute, Alexandria Virginia
  2. ^ a b c d e f g Boggs, S., 2006, Principles of Sedimentology and Stratigraphy (4th ed.), Pearson Prentice Hall, Upper Saddle River, NJ, 662 p.
  3. ^ a b c Melvin, J. L.(ed) 1991, Evaporites, petroleum and mineral resources; Elsevier, Amsterdam
  4. ^ C.Michael Hogan. 2011. Sulfur. Encyclopedia of Earth, eds. A.Jorgensen and C.J.Cleveland, National Council for Science and the environment, Washington DC Archived October 28, 2012, at the Wayback Machine
Anhydrite

Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the orthorhombic barium (baryte) and strontium (celestine) sulfates, as might be expected from the chemical formulas. Distinctly developed crystals are somewhat rare, the mineral usually presenting the form of cleavage masses. The Mohs hardness is 3.5, and the specific gravity is 2.9. The color is white, sometimes greyish, bluish, or purple. On the best developed of the three cleavages, the lustre is pearly; on other surfaces it is glassy. When exposed to water, anhydrite readily transforms to the more commonly occurring gypsum, (CaSO4·2H2O) by the absorption of water. This transformation is reversible, with gypsum or calcium sulfate hemihydrate forming anhydrite by heating to around 200 °C (400 °F) under normal atmospheric conditions. Anhydrite is commonly associated with calcite, halite, and sulfides such as galena, chalcopyrite, molybdenite, and pyrite in vein deposits.

Aragonite

Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, CaCO3 (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including precipitation from marine and freshwater environments.

The crystal lattice of aragonite differs from that of calcite, resulting in a different crystal shape, an orthorhombic crystal system with acicular crystal. Repeated twinning results in pseudo-hexagonal forms. Aragonite may be columnar or fibrous, occasionally in branching stalactitic forms called flos-ferri ("flowers of iron") from their association with the ores at the Carinthian iron mines.

Baryte

Baryte or barite (UK: , ) is a mineral consisting of barium sulfate (BaSO4). Baryte is generally white or colorless, and is the main source of barium. The baryte group consists of baryte, celestine (strontium sulfate), anglesite (lead sulfate), and anhydrite (calcium sulfate). Baryte and celestine form a solid solution (Ba,Sr)SO4.

Bittern (salt)

Bittern (pl. bitterns) is a bitter-tasting solution that remains after precipitation of halite (common salt) from brines and/or seawater. It is rich in magnesium chlorides, sulfates, bromides, iodides, and other chemicals present in the original waters.

Calcite

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 3 as "calcite".

Other polymorphs of calcium carbonate are the minerals aragonite and vaterite. Aragonite will change to calcite over timescales of days or less at temperatures exceeding 300 °C, and vaterite is even less stable.

Carnallite

Carnallite (also carnalite) is an evaporite mineral, a hydrated potassium magnesium chloride with formula KMgCl3·6(H2O). It is variably colored yellow to white, reddish, and sometimes colorless or blue. It is usually massive to fibrous with rare pseudohexagonal orthorhombic crystals. The mineral is deliquescent (absorbs moisture from the surrounding air) and specimens must be stored in an airtight container.

Carnallite occurs with a sequence of potassium and magnesium evaporite minerals: sylvite, kainite, picromerite, polyhalite, and kieserite. Carnallite is an uncommon double chloride mineral that only forms under specific environmental conditions in an evaporating sea or sedimentary basin. It is mined for both potassium and magnesium and occurs in the evaporite deposits of Carlsbad, New Mexico; the Paradox Basin in Colorado and Utah; Stassfurt, Germany; the Perm Basin, Russia; and the Williston Basin in Saskatchewan, Canada. These deposits date from the Devonian through the Permian Periods. In contrast, both Israel and Jordan produce potash from the Dead Sea by using evaporation pans to further concentrate the brine until carnallite precipitates, dredging the carnallite from the pans, and processing to remove the magnesium chloride from the potassium chloride.Carnallite was first described in 1856 from its type location of Stassfurt Deposit, Saxony-Anhalt, Germany. It was named for the Prussian mining engineer Rudolf von Carnall (1804–1874).

Celestine (mineral)

Celestine or celestite is a mineral consisting of strontium sulfate (SrSO4). The mineral is named for its occasional delicate blue color. Celestine and the carbonate mineral strontianite are the principal sources of the element strontium, commonly used in fireworks and in various metal alloys.

Dolomite

Dolomite ( ) is an anhydrous carbonate mineral composed of calcium magnesium carbonate, ideally CaMg(CO3)2. The term is also used for a sedimentary carbonate rock composed mostly of the mineral dolomite. An alternative name sometimes used for the dolomitic rock type is dolostone.

Epsomite

Epsomite is a hydrous magnesium sulfate mineral with formula MgSO4·7H2O.

Epsomite crystallizes in the orthorhombic system as rarely found acicular or fibrous crystals, the normal form is as massive encrustations. It is colorless to white with tints of yellow, green and pink. The Mohs hardness is 2 to 2.5 and it has a low specific gravity of 1.67.

Epsomite is the same as the household chemical, Epsom salt, and is readily soluble in water. It absorbs water from the air and converts to hexahydrate with the loss of one water molecule and a switch to monoclinic structure.

Gypsum

Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard chalk and wallboard. A massive fine-grained white or lightly tinted variety of gypsum, called alabaster, has been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, the Byzantine Empire and the Nottingham alabasters of Medieval England. Gypsum also crystallizes as beautiful translucent crystals of selenite (mineral). It also forms as an evaporite mineral and as a hydration product of anhydrite.

The Mohs scale of mineral hardness defines hardness value 2 as gypsum based on scratch hardness comparison.

Halite

Halite ( or ), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride (NaCl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, pink, red, orange, yellow or gray depending on the amount and type of impurities. It commonly occurs with other evaporite deposit minerals such as several of the sulfates, halides, and borates. The name halite is derived from the Ancient Greek word for salt, ἅλς (háls).

Madison Group

The Madison Limestone is a thick sequence of mostly carbonate rocks of Mississippian age in the Rocky Mountain and Great Plains areas of western United States. The rocks serve as an important aquifer as well as an oil reservoir in places. The Madison and its equivalent strata extend from the Black Hills of western South Dakota to western Montana and eastern Idaho, and from the Canada–United States border to western Colorado and the Grand Canyon of Arizona.

Magnesite

Magnesite is a mineral with the chemical formula MgCO3 (magnesium carbonate). Iron, manganese, cobalt and nickel may occur as admixtures, but only in small amounts.

Manitoba Group

The Manitoba Group is a stratigraphical unit of middle to late Devonian age in the Western Canadian Sedimentary Basin.

It takes the name from the province of Manitoba, and was first defined by A.D. Baillie in 1953.

Prairie Evaporite Formation

The Prairie Evaporite Formation, also known as the Prairie Formation, is a geologic formation of Middle Devonian (Givetian) age that consists primarily of halite (rock salt) and other evaporite minerals. It is present beneath the plains of northern and eastern Alberta, southern Saskatchewan and southwestern Manitoba in Canada, and it extends into northwestern North Dakota and northeastern Montana in the United States.The formation is a major source of potash, most of which is used for fertilizer production. Salt is also produced from the formation, and solution caverns are created in its thick salt beds for natural gas storage.

Salt dome

A salt dome is a type of structural dome formed when a thick bed of evaporite minerals (mainly salt, or halite) found at depth intrudes vertically into surrounding rock strata, forming a diapir. It is important in petroleum geology because salt structures are impermeable and can lead to the formation of a stratigraphic trap.

Salt mining

A salt mine is a mine from which salt is extracted. The mined salt is usually in the form of halite (commonly known as rock salt), and extracted from evaporite formations.

Sylvite

Sylvite, or sylvine, is potassium chloride (KCl) in natural mineral form. It forms crystals in the isometric system very similar to normal rock salt, halite (NaCl). The two are, in fact, isomorphous. Sylvite is colorless to white with shades of yellow and red due to inclusions. It has a Mohs hardness of 2.5 and a specific gravity of 1.99. It has a refractive index of 1.4903. Sylvite has a salty taste with a distinct bitterness.

Sylvite is one of the last evaporite minerals to precipitate out of solution. As such, it is only found in very dry saline areas. Its principal use is as a potassium fertilizer.

Sylvite is found in many evaporite deposits worldwide. Massive bedded deposits occur in New Mexico and western Texas, and in Utah in the US, but the largest world source is in Saskatchewan, Canada. The vast deposits in Saskatchewan, Canada were formed by the evaporation of a Devonian seaway. Sylvite is the official mineral of Saskatchewan.

Sylvite was first described in 1832 at Mt. Vesuvius near Napoli in Italy and named for the Dutch chemist, François Sylvius de le Boe (1614–1672).Sylvite, along with quartz, fluorite and halite, is used for spectroscopic prisms and lenses.

Trona

Trona (trisodium hydrogendicarbonate dihydrate, also sodium sesquicarbonate dihydrate, Na2CO3•NaHCO3•2H2O) is a non-marine evaporite mineral. It is mined as the primary source of sodium carbonate in the United States, where it has replaced the Solvay process used in most of the rest of the world for sodium carbonate production.

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