Evaporite

An evaporite is a type of sedimentary rock formed through the concentration and crystallisation of dissolved minerals as water evaporates from an aqueous solution. Evaporites are classified as chemical sedimentary rocks, as their formation results from chemical precipitation rather than the mechanical accumulation of clastic material. These deposits provide critical evidence of past climatic, hydrological, and tectonic conditions and play an important role in both geological interpretation and economic geology.
Evaporites form in environments where evaporation exceeds water inflow, leading to increasing salinity until minerals can no longer remain in solution. Such conditions are most commonly associated with arid and semi-arid climates.

Definition and General Characteristics

Evaporites are rocks composed of minerals that precipitate from saline water during evaporation. All natural waters contain dissolved salts, but evaporite formation requires restricted basins in which water input is limited and evaporation rates are high. As water volume decreases, dissolved ions become increasingly concentrated, eventually reaching saturation and crystallising as solid minerals.
Although the term “evaporite” is often associated with rock salt (halite), most evaporite formations contain only small proportions of halite, with the majority consisting of gypsum, anhydrite, carbonates, and associated clastic sediments. Over 80 different evaporite minerals have been identified, though only a small number are common and volumetrically significant.

Formation Processes

Evaporite formation occurs when a body of water becomes hydrologically restricted, meaning that inflow is insufficient to balance evaporation. This typically happens in:

• Arid or semi-arid climates
• Closed or poorly drained basins
• Shallow seas with limited connection to the open ocean

As evaporation progresses, the remaining water becomes increasingly saline. Minerals precipitate in a predictable sequence controlled by their relative solubility. This process has been demonstrated experimentally by evaporating seawater under controlled laboratory conditions.
The order of precipitation from seawater generally follows this sequence:

• Carbonates (such as calcite and dolomite)
• Sulphates (gypsum followed by anhydrite)
• Chlorides (halite)
• Potassium and magnesium salts (such as sylvite and carnallite)

This sequence explains why limestone is more abundant than gypsum, gypsum more abundant than halite, and halite more abundant than potassium-rich salts in the geological record.

Depositional Environments

Evaporite deposits are broadly divided into marine and non-marine types, depending on the source of the saline water.

Marine Evaporites

Marine evaporites form from seawater in restricted marine basins. These deposits tend to be thick and laterally extensive and are among the most intensively studied evaporite systems. Formation typically occurs when shallow seas become isolated due to tectonic uplift, sea-level fall, or barrier formation.
Common marine evaporite minerals include:

• Calcite
• Gypsum
• Anhydrite
• Halite
• Sylvite
• Carnallite
• Polyhalite
• Kainite

Marine evaporites often develop in sequences interbedded with carbonates and clastic sediments. One of the most significant marine evaporite events in Earth history occurred during the Messinian salinity crisis, when much of the Mediterranean basin experienced extreme evaporation, leading to massive salt accumulation.

Non-marine Evaporites

Non-marine evaporites form in continental settings where saline waters accumulate in standing bodies such as lakes. These environments differ chemically from marine systems, resulting in a distinct mineral assemblage.
Common non-marine evaporite minerals include:

• Borax
• Trona
• Mirabilite
• Thenardite
• Glauberite
• Gaylussite
• Epsomite

Non-marine deposits may also contain halite, gypsum, and anhydrite, though their presence does not imply a marine origin. These deposits are particularly valuable for reconstructing past climates and tectonic settings.
Typical non-marine environments include:

• Saline lakes (perennial or seasonal)
• Playa lakes
• Internal drainage basins
• Groundwater-fed seepage mounds
• Closed continental rift basins

Modern examples include the Great Salt Lake in Utah and the Dead Sea between Israel and Jordan.

Evaporitic Sedimentary Settings

Evaporites commonly form in specific geological environments, including:

• Continental rift valleys and half-grabens with restricted drainage
• Isolated oceanic rift basins such as the Red Sea
• Arid internal drainage basins such as deserts
• Restricted coastal plains in regressive seas, forming sabkha deposits
• Extremely arid basins fed by ephemeral rivers

These environments allow prolonged evaporation and repeated cycles of flooding and drying, producing thick evaporite sequences.

Evaporitic Formations

Evaporite formations rarely consist entirely of evaporite minerals. Instead, evaporite layers are commonly interbedded with mudstones, sandstones, and carbonates. A formation may be classified as evaporitic if it contains:

• Halite pseudomorphs
• Evaporite-rich sedimentary sequences
• Mud cracks and other desiccation features
• Diagnostic evaporite microstructures

Examples include Jurassic evaporite-bearing formations in North America and sulphur-bearing evaporites in parts of Eastern Europe and West Asia.

Economic Importance

Evaporites are of major economic significance due to their mineral content and physical properties.
Key economic uses include:

• Halite mined for salt and food preservation
• Gypsum used in plaster, cement, and drywall
• Potash minerals essential for fertiliser production
• Nitrate minerals mined for fertilisers and explosives, particularly in Chile and Peru

Evaporite formations are also important in subsurface geology. Thick halite layers are geologically stable, impermeable to groundwater, and predictable in mechanical behaviour, making them suitable for nuclear waste storage. Additionally, halite commonly forms diapirs, which create structural traps for petroleum and natural gas.

Major Groups of Evaporite Minerals

Evaporite minerals can be grouped according to their chemical composition:

• Carbonates: calcite, dolomite
• Sulphates: gypsum, anhydrite, barite
• Chlorides: halite, sylvite
• Nitrates: nitratine, niter
• Borates: borax
• Mixed salts: trona, hanksite

Some minerals, such as hanksite, belong to more than one chemical group, reflecting complex depositional chemistry.

Experimental and Planetary Evaporites

Evaporites can be readily reproduced in laboratory settings, allowing geologists to study precipitation sequences and environmental conditions. Beyond Earth, evidence from remote sensing and experimental simulations suggests that evaporite-like deposits may exist on Titan, Saturn’s largest moon. Titan hosts lakes of liquid methane and ethane, where soluble hydrocarbons may evaporate and crystallise in a manner analogous to salt pans on Earth.
These findings extend the concept of evaporite formation beyond terrestrial water-based systems and highlight its relevance to planetary geology.