(45) Earth Science

Formation of an Evaporite



Sandstone is made up of mineral grains (mostly quartz) cemented together by silica, iron oxide, or calcium carbonate. Sandstones are commonly white, gray, brown, or red. Iron oxide impurities give the red and brown color to the darker colored sandstones. Most sandstones are gritty, while some are easily crushed ( friable) and break apart to form sand. The pores or spaces between the separate grains of sand in sandstones controls how porous the sandstone is. The amount and size of this spacing is called porosity. The porosity of sandstone allows sandstone to serve as good reservoirs for oil and natural gas. Petroleum engineers and geologists oftenlook for these natural resources in sandstone areas.

Sandstone is made up of one or more of the following:

* Silt (grain size 1/256 to 1/16mm (gritty)),

* Siltstone,

* Clay (grain size less than 1/256mm (smooth)),

* Shale (most abundant of sedimentary rock types),

* Claystone,

* Mudstone (a mixture of silt and clay or mudshale if it fractures along sedimentary lines), and

* Ironstone (clay ironstone).

Sandstones are very resistant to erosion and form bluffs, cliffs, ridges, rapids, arches, and waterfalls. Loose sands have many colors, but are commonly seen as white to light brown. Silica (quartz-rich) sands and sandstones of high purity (white color) are used widely in the glass industry for making window glass, light bulbs, vases, and utility containers. Tightly cemented sandstone is often used as a building stone. Sand sediments are moved along by medium-speed currents like those of rivers, shoreline waves, and the wind. These can be rounded, which tells a geologist that they have traveled far (probably by water), or rough edged which usually points to shorter treks.

The amount of sand grains sorting in one area is another clue as to sediment origins. The average size of grains tells a lot about the strength of the current that carried them to a new spot and the type of parent crystal the grain was originally part of. If the grain sizes in sandstone are all the same, they are well sorted. If many grains are larger, with a lot of smaller and inbetween grains, then they are poorly sorted. Sorting takes place during the sand’s travels. Well-sorted sand grains commonly come from beaches, while poorly sorted grains are often the result of glacial travels.


Rock asphalt is a medium- to coarse-grained sandstone with asphalt (bitumen) filling the pore spaces. It is squishy to solid, brown to black, has a pitchy to resinous luster, and is very sticky when completely saturated.

Bitumen is made up of various mixtures of liquid, viscous, flammable, or solid naturally occurring hydrocarbons, excluding coal, that are soluble in carbon disulfide

Rock asphalt deposits were formed when erosion of the surface rocks exposed oil-bearing rocks and allowed the more volatile hydrocarbons to escape. The asphalt-based crude gradually thickened until only a heavy tar or asphalt remained. The bitumen content of commercial rock asphalt varies from 3 to 15%. When asphalt is produced as part of some petroleum refining processes, it is called artificial asphalt.

Rock asphalt was once mined widely in Kentucky in the eastern United States. Large deposits were found in several areas. During the early 1980s, attempts were made to recover the petroleum in the rock asphalt by heat treatment, distillation, and other processes. Rock asphalt is used most commonly for surfacing streets and roads. It is also used for roofing, waterproofing, and mixing with rubber.


Iron oxide sediments are sedimentary rocks containing more than 15% iron in the form of iron oxides, iron silicates, and iron carbonates. Geologists think that most sedimentary iron was formed early in the Earth’s history when there was less oxygen in the air and iron was more soluble. When soluble iron was carried to the ocean, it formed iron oxides and other compounds that then settled in layers to the bottom.

Ironstone, heavy, compact fine-grained stone is found mostly in nodules and in uneven beds with carboniferous and other rocks. It has 20–30% iron content and a clay-like texture, with large amounts of iron oxide, mostly limonite, in nodular form. Much of the iron produced in the United Kingdom is made from ironstone.


This group includes the evaporites, the carbonates (limestones and dolostone), and the siliceous rocks. Evaporites form from chemical elements dissolved in seawater. These compounds can be removed from saltwater and crystallized into rock by chemical processes or through biological processes (such as shell growth). Sometimes it’s tough to sort between the two (carbonates and siliceous rocks), so evaporites are commonly grouped as chemical/biochemical.

Evaporites that form as elements become more and more concentrated in an evaporating solution (usually seawater).

Marine evaporites are the sediments and sedimentary crystalline rocks formed as seawater becomes more and more salty through evaporation. Some marine evaporites are hundreds of meters thick. Huge amounts of seawater would have to evaporate for this amount of crystal formation to have been possible.

The most common types of evaporites include the following:

* Carbonates – mostly calcite and dolomite by diagenesis,

* Gypsum – made up of calcium sulfate (CaSO4 and water),

* Halite (rock salt) – made up of sodium chloride (NaCl), and

* Travertine – made up of calcium carbonate (CaCO3) (forms in caves and around hot springs).

Geologists have found that three things must happen in a bay for large amounts of evaporites to form. They are: (1) freshwater that flows into the bay from rivers and streams is limited, (2) connections to the open sea are constricted, and (3) the climate is parched and dry. In these bays, seawater evaporates constantly, but is replenished at a steady rate remaining supersaturated all the time. Evaporite minerals then settle steadily to the floor of the bay in sedimentary layers.

Phosphorite is another marine evaporite formed from chemical and biochemical sediments. It is made up mostly of calcium phosphate from places along the continental margins where ocean water is cold and deep. The phosphorite forms from an interaction between phosphate-rich seawater and muddy or carbonate-containing sediments. Land (nonmarine) evaporites form in areas usually far from the sea. These are found in desert-region lakes with little or no river outlet. In these places, minerals come into the lake from chemical weathering and erosion, but without water current can’t move on. One of the best known examples of this is the Great Salt Lake in Utah in the western United States. Rivers bring ions into the lake, but there they stay when the water evaporates. The concentrated dissolved ions in the Great Salt Lake make it one of the saltiest places on Earth, with levels eight times saltier than seawater. 

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