Evaporites: Type, Formation, Uses, Occurrence
Evaporite sedimentary rocks are rocks that form from the evaporation of water bodies, leaving behind concentrated minerals. They are layered crystalline rocks and can be found in various colors depending on the mineral composition.
Evaporites are a type of chemical sedimentary rock. Although there are a very large number of different chemical salts dissolved in seawater, their relative abundances and solubilities allow only very few common evaporite minerals to precipitate naturally. These include halite, gypsum, anhydrite and, to a lesser extent, dolomite and potassium salts (or bit- terns). Prolonged evaporation leads to significant accumulation of evaporite minerals, and the resulting sedimentary deposit is generally named after the dominant mineral.
In thick successions there is commonly a cyclic repetition of evaporites from less to more soluble – that is, dolomite, gypsum, anhydrite, halite, bitterns.
Non-marine evaporites are also typically dominated by halite, gypsum and anhydrite, although a wider range of minor salts do occur. This is because the chemical compositions of the original waters vary considerably according to the composition of the rocks with which they interact. Such non-marine evaporites include trona, mirabilite, glauberte, borax, epsomite, thenardite, gaylussite and bloedite.
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| Evaporites in Zagros, Iran Photo: Mohammad.Alinia.53/Instagram |
Formation: How do evaporite rocks form?
Evaporites form through a process of concentration and precipitation as a result of intense evaporation in bodies of water. Here's a breakdown of the key steps:
High Evaporation Rate: The process starts with a body of water, like a lake or a shallow sea, located in an arid or semi-arid environment. In these areas, evaporation happens much faster than the water is replenished by rainfall or inflow from rivers.
Salinity Increase: As water evaporates, the remaining water becomes more concentrated with dissolved minerals. This increases the salinity of the water.
Mineral Saturation: With increasing salinity, the water eventually reaches a saturation point for certain minerals. This means the water can no longer hold these minerals in solution.
Precipitation: Once saturated, these minerals start to come out of solution and solidify as crystals. The order of precipitation depends on the mineral's solubility. Less soluble minerals like calcium sulfate (gypsum or anhydrite) will precipitate first, followed by more soluble minerals like sodium chloride (halite) as the water becomes even more concentrated.
Sedimentation and Lithification: The precipitated crystals settle down at the bottom of the water body as sediments. Over time, the weight of these sediments compacts them together. They may also be further cemented by additional mineral precipitation filling the gaps. Eventually, these sediments transform into solid rock – the evaporite.
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| Evaporites successions (gypsum, halite)-Iran Photo: Mohammad.Alinia.53/Instagram |
Evaporites Depositional Environments
Evaporites are sedimentary rocks that are formed from the precipitation of minerals from concentrated solutions. They are typically associated with environments where evaporation exceeds precipitation, leading to the concentration of dissolved minerals in water bodies. Some common depositional environments for evaporites include:
Salty Lakes or Playas: These are ideal settings due to their closed drainage and high evaporation rates in arid climates. Dissolved minerals concentrate as the water level shrinks, leading to precipitation of halite and gypsum.
Sabkhas: These coastal environments experience fluctuating water tables and salinity levels due to tidal influences and evaporation. Evaporite minerals like gypsum and anhydrite precipitate from the concentrated seawater or brackish water.
Saline Panes: Temporary bodies of water form in these shallow depressions after rainfall events. As the water evaporates entirely, it leaves behind concentrated salt deposits, often halite and various other salts. These are another example of an arid environment conducive to evaporite formation.
Saline Mudflats: Similar to sabkhas, these coastal settings experience fluctuating water levels and salinity. Mud flats within these areas may contain gypsum and halite due to evaporation processes.
Evaporitic Basins: These large-scale environments represent extensive deposits of evaporites. Restricted marine basins, where evaporation significantly outweighs freshwater inflow, are prime locations for these formations. Thick sequences of various evaporite minerals accumulate over time.
Salt Diapirs: Under specific tectonic forces, evaporite deposits can be uplifted and even pierce through overlying rock layers. These structures, called salt diapirs, are a result of the buoyant nature of salt compared to surrounding rocks.
These environments are typically associated with arid to semi-arid climates where evaporation rates are high and precipitation is low, leading to the concentration of dissolved minerals and the formation of evaporite deposits.
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| Types of Evaporites |
Types of Evaporites
The principal types of evaporites can be categorized based on their chemical composition. Here's a breakdown of the most common ones:
Halides:
These are the most abundant evaporite minerals and consist primarily of chloride salts.
The main types include:
- Halite (Rock Salt): NaCl (sodium chloride) - The most common evaporite, used as table salt.
- Sylvite: KCl (potassium chloride) - A valuable source of potassium for fertilizers.
- Carnallite: KMgCl₃·6H₂O (potassium magnesium chloride hexahydrate) - Another source of potassium and magnesium for fertilizers.
Sulfates:
These evaporites are composed of sulfate minerals. Common types include:
- Gypsum: CaSO₄·2H₂O (calcium sulfate dihydrate) - Used in construction materials like plaster and drywall.
- Anhydrite: CaSO₄ (calcium sulfate) - A dehydrated form of gypsum, used in some cements.
- Kieserite: MgSO₄·H₂O (magnesium sulfate monohydrate) - Used in some fertilizers and fire retardants.
- Thenardite: Na₂SO₄ (sodium sulfate) - Used in the pulp and paper industry.
Carbonates:
Less common than halides and sulfates, these evaporites contain carbonate minerals. Examples include:
- Calcite: CaCO₃ (calcium carbonate) - Primarily a component of limestone, but can occur in some evaporite deposits.
- Dolomite (dolostone): CaMg(CO₃)₂ (calcium magnesium carbonate) - Similar to calcite, but with magnesium replacing some calcium.
Borates:
Found in specific evaporitic environments, these minerals contain boron. Examples include:
- Borax: Na₂B₄O₇·10H₂O (sodium tetraborate decahydrate) - Used in detergents, cleaning products, and some fire retardants.
- Ulexite: NaCaB₅O₆(OH)₆·5H₂O (sodium calcium borate octahydrate) - Used in some fire retardants and fiberglass production.
Nitrates:
Relatively rare, these evaporites consist of nitrate minerals. Examples include:
- Nitratine (Soda Niter): NaNO₃ (sodium nitrate) - Used historically as a fertilizer but now has limited applications.
- Niter: KNO₃ (potassium nitrate) - Primarily a historical component of gunpowder.
It's important to remember that the specific types of evaporites found in a deposit depend on the initial water body's composition and the evaporation conditions.
Common Evaporite Minerals - Rocks
There are several common types of evaporite sedimentary rocks:
Halite (Rock Salt)
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| Halite (Rock Salt) |
Composition: Rock salt is primarily composed of the mineral halite, which is sodium chloride (NaCl). It usually forms cubic crystals but can also occur as granular or massive aggregates. Additionally, it may contain impurities such as gypsum, anhydrite, clay minerals, and organic matter.
Grain Size/Texture: The grain size of rock salt can vary, ranging from fine-grained to coarse-grained, depending on factors such as the rate of crystallization and subsequent deformation. In general, it tends to have a crystalline texture, often exhibiting the characteristic cleavage planes of halite.
Uses:
Culinary Use: Rock salt is widely used as a seasoning and preservative in food preparation. It is often employed in cooking, curing meats, pickling, and preserving fish.
Industrial Applications: Rock salt is utilized in various industrial processes. It is a key ingredient in the production of chlorine, sodium hydroxide (caustic soda), and other chemicals through the electrolysis of brine (sodium chloride solution). Additionally, it is employed in water softening, de-icing roads and highways during winter, and as a raw material in the chemical and pharmaceutical industries.
Animal Husbandry: Rock salt serves as a supplement for livestock and other animals, providing essential minerals such as sodium and chloride. Salt licks, blocks, and loose salt are commonly provided to livestock to meet their nutritional requirements and prevent deficiencies.
Medicinal Use: In some cultures, rock salt is used for medicinal purposes, such as in salt therapy (halotherapy) for respiratory conditions and in salt baths for skin ailments.
Decorative and Crafts: Crystalline specimens of rock salt are prized by collectors and used in decorative items, jewelry, and crafts.
Gypsum
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| Gypsum |
Composition: Gypsum is a mineral composed of calcium sulfate dihydrate (CaSO₄·2H₂O). It contains calcium, sulfur, oxygen, and water molecules. Its chemical formula indicates that each molecule of gypsum contains two molecules of water. Gypsum often occurs as transparent to translucent crystals or fibrous masses. It may also be found as massive granular aggregates.
Grain Size/Texture: The grain size of gypsum can vary, ranging from fine-grained to coarse-grained. It commonly exhibits a granular texture, but it can also display fibrous or satin-like textures, especially in massive deposits. Crystals of gypsum typically form elongated prisms or tabular shapes with well-developed cleavage planes.
Uses:
Construction Materials: Gypsum is widely used in the construction industry for manufacturing plaster, drywall (also known as gypsum board or sheetrock), and other building materials. Plaster is made by heating gypsum to remove its water content, resulting in a fine powder that can be mixed with water to form a workable paste for plastering walls and ceilings. Gypsum board is used as a wall covering material in residential and commercial construction projects.
Fertilizer: Gypsum is utilized in agriculture as a soil amendment to improve soil structure and fertility. It provides calcium and sulfur, essential nutrients for plant growth. Gypsum can help in reducing soil compaction, improving water infiltration, and mitigating soil salinity.
Industrial Applications: Gypsum is employed in various industrial processes, including the manufacture of cement, pottery, ceramics, and as a filler in paper and textiles. It is also used in making molds for casting metals and ceramics.
Medical Use: Certain medical-grade gypsum products are used in dentistry for making dental impressions and casts.
Art and Craft: Gypsum is used in art and crafts for sculpting, modeling, and creating decorative objects due to its workability and ease of carving.
Anhydrite
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| anhydrite |
Composition: Anhydrite is another calcium sulfate mineral (CaSO₄), but unlike gypsum, it doesn't contain water molecules. Anhydrite can form from gypsum when it loses its water content at higher temperatures.
Grain Size/Texture: Anhydrite can exhibit various grain sizes, ranging from fine-grained to coarse-grained, depending on factors such as the rate of crystallization and subsequent alteration. It commonly has a granular texture but may also occur as massive aggregates. Anhydrite crystals often form tabular or prismatic shapes with well-defined cleavage planes.
Occurrence: It can also occur as veins or nodules within sedimentary rocks, often replacing pre-existing gypsum or forming through the dehydration of gypsum.
Uses:
Construction Materials: Anhydrite is used in the construction industry as a substitute for gypsum in certain applications. It can be ground into a fine powder and used as a binder in the production of cement and as a component in various construction materials such as floor screeds and self-leveling compounds.
Industrial Applications: Anhydrite is utilized in industrial processes such as the production of sulfuric acid, a key chemical used in various industries including fertilizers, chemicals, and pharmaceuticals. It can also be used as a desiccant and drying agent due to its ability to absorb moisture.
Oil and Gas Exploration: Anhydrite can serve as an indicator mineral for hydrocarbon exploration. It often occurs in association with salt domes and other geological structures that may contain oil and gas reservoirs. The presence of anhydrite layers can help geologists identify potential hydrocarbon-bearing formations.
Environmental Remediation: Anhydrite can be used for environmental remediation purposes, such as soil stabilization and the treatment of acid mine drainage. It can help neutralize acidic soils and improve soil structure.
Occurrence: Famous Evaporite Deposits Around the World
Dead Sea (Israel and Jordan): This hypersaline lake is famous for its unique evaporite deposits. The Dead Sea's high evaporation rate and lack of an outlet to the ocean lead to a constant state of concentrated brines. Thick layers of halite (rock salt) and minerals like carnallite blanket the lakebed. The mineral composition allows people to float effortlessly due to the high buoyancy.
Paradox Basin (United States): Spanning parts of Colorado, Utah, New Mexico, and Arizona, the Paradox Basin is renowned for its evaporite deposits formed during the Pennsylvanian and Permian periods. These evaporites are particularly interesting for their coexistence with rich uranium deposits, a geological oddity.
Stassfurt Basin (Germany): This basin in central Germany holds some of the world's largest deposits of potassium salts, formed during the Zechstein Sea (late Permian period). These evaporite deposits are commercially significant, being a source of potash fertilizer.
Mount Agung (Indonesia): This active volcano is an interesting example of how volcanic processes can contribute to evaporite formation. Steam vents on Mount Agung release volcanic gases like sulfur dioxide. These gases react with atmospheric water vapor to form sulfuric acid, which condenses and can lead to the precipitation of sulfate evaporite minerals like alunite.
Salar de Uyuni (Bolivia): Nicknamed the "Salar of Mirrors," Salar de Uyuni in Bolivia is the world's largest salt flat. This vast expanse, covering over 10,000 square kilometers, formed from the transformation of a prehistoric lake. The remaining brine is rich in lithium, making Salar de Uyuni a potential key player in the battery metals industry.
Searles Lake (United States): Located in California, Searles Lake is a remnant of a much larger Pleistocene lake. Today, it's a dry lakebed famous for its diverse evaporite minerals. These include borates, carbonates, sulfates, and chlorides, making Searles Lake a valuable resource for the mining industry.
Messina Formation (Italy): This geologic formation in Sicily, Italy, is known for its spectacular outcrops of gypsum and anhydrite. The rhythmic layering and contorted bedding within the Messina Formation provide valuable insights into the cyclic nature of evaporitic environments.
Zagros, Iran: This mountain range in southwestern Iran is known for its thick sequences of Paleozoic and Miocene evaporites. The Hormuz Formation, a well-studied example, consists primarily of salt deposits (halite) and carbonates (dolomite) formed during the Ediacaran to Early Cambrian period. The movement of these evaporite layers due to tectonic forces has also played a role in the geological evolution of the Zagros region.
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| Salar de Uyuni (Bolivia) |
Uses and Applications of Evaporites
Evaporites are a hidden gem beneath our feet, offering a wealth of natural resources with diverse applications across various industries. Here's a glimpse into the many uses and applications of evaporites:
Salt Production
The most prominent use of evaporites is in salt production. Halite (rock salt) is the primary component of these deposits and is mined or extracted from brines for various purposes.
- Food Processing: Salt is a fundamental ingredient in food processing, used for seasoning, preservation, and enhancing flavor.
- Chemical Manufacturing: The chemical industry utilizes sodium chloride from evaporites as a raw material in numerous processes, including chlorine and caustic soda production.
- Water Treatment: Salt plays a role in water treatment for softening and disinfection purposes.
- De-icing: Rock salt is a common de-icing agent used on roads and walkways during winter months to improve traction and safety.
Potash Fertilizers
Evaporite deposits often contain potassium-rich minerals like sylvite (KCl). These minerals are essential for plant growth and are mined to produce potash fertilizers, which are crucial for agriculture to maintain soil fertility and boost crop yields.
- Construction Materials: Gypsum, another prevalent evaporite mineral, finds numerous applications in the construction industry:
- Plaster and Drywall: When heated and processed, gypsum transforms into plaster, a versatile material used for wall and ceiling construction. Gypsum also forms the core of drywall panels, widely used in modern building practices.
- Cement: Gypsum serves as a setting agent in cement, a crucial component of concrete, a fundamental building material.
Chemical Industry Beyond Salt
Evaporites offer a broader range of chemical compounds besides sodium chloride. For example:
- Sodium Carbonate (Soda Ash): Extracted from trona, a sodium carbonate-rich evaporite mineral, soda ash has applications in glass production, detergents, and various chemical processes.
- Sodium Bicarbonate (Baking Soda): Similar to soda ash, sodium bicarbonate can be obtained from evaporites and finds uses in baking, pharmaceuticals, and cleaning products.
- Borates: Some evaporite deposits contain borate minerals like borax, which have applications in fiberglass production, ceramics, and even fire retardants.
Oil and Gas Exploration
While not a direct use, evaporite deposits can be significant for oil and gas exploration. Evaporite layers, particularly halite and sylvite, can be relatively impermeable and act as caprocks, trapping hydrocarbons within the underlying rock formations. Understanding the distribution and properties of evaporites is crucial for successful exploration efforts.
In conclusion, evaporites are a valuable natural resource with far-reaching applications in our daily lives. From the salt on our tables to the building materials around us, these minerals play a significant role in various industries. As we continue to explore and utilize evaporite deposits responsibly, they will remain a cornerstone of a sustainable future.
Conclusion
In conclusion, evaporites are not only intriguing geological formations but also valuable resources with diverse industrial applications. Their study continues to provide valuable insights into the Earth's history and the processes that shape our planet.