Concretion

A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin "(act of) compacting, condensing, congealing, uniting", itself from "con" meaning 'together' and "crescere" meaning "to grow". Concretions form within layers of sedimentary strata that have already been deposited. They usually form early in the burial history of the sediment, before the rest of the sediment is hardened into rock. This concretionary cement often makes the concretion harder and more resistant to weathering than the host stratum.
There is an important distinction to draw between concretions and nodules. Concretions are formed from mineral precipitation around some kind of nucleus while a nodule is a replacement body.
Descriptions dating from the 18th century attest to the fact that concretions have long been regarded as geological curiosities. Because of the variety of unusual shapes, sizes and compositions, concretions have been interpreted to be dinosaur eggs, animal and plant fossils (called pseudofossils), extraterrestrial debris or human artifacts.
Origins.
Detailed studies have demonstrated that concretions form after sediments are buried but before the sediment is fully lithified during diagenesis. They typically form when a mineral precipitates and cements sediment around a nucleus, which is often organic, such as a leaf, tooth, piece of shell or fossil. For this reason, fossil collectors commonly break open concretions in their search for fossil animal and plant specimens. Some of the most unusual concretion nuclei, are World War II military shells, bombs, and shrapnel, which are found inside siderite concretions found in an English coastal salt marsh.
Depending on the environmental conditions present at the time of their formation, concretions can be created by either concentric or pervasive growth. In concentric growth, the concretion grows as successive layers of mineral precipitate around a central core. This process results in roughly spherical concretions that grow with time. In the case of pervasive growth, cementation of the host sediments, by infilling of its pore space by precipitated minerals, occurs simultaneously throughout the volume of the area, which in time becomes a concretion. Concretions are often exposed at the surface by subsequent erosion that removes the weaker, uncemented material.
Appearance.
Concretions vary in shape, hardness and size, ranging from objects that require a magnifying lens to be clearly visible to huge bodies three meters in diameter and weighing several thousand pounds. The giant, red concretions occurring in Theodore Roosevelt National Park, in North Dakota, are almost in diameter. Spheroidal concretions, as large as in diameter, have been found eroding out of the Qasr El Sagha Formation within the Faiyum depression of Egypt. Concretions are usually similar in color to the rock in which they are found. Concretions occur in a wide variety of shapes, including spheres, disks, tubes, and grape-like or soap bubble-like aggregates.
Composition.
They are commonly composed of a carbonate mineral such as calcite; an amorphous or microcrystalline form of silica such as chert, flint, or jasper; or an iron oxide or hydroxide such as goethite and hematite. They can also be composed of other minerals that include dolomite, ankerite, siderite, pyrite, marcasite, barite and gypsum.
Although concretions often consist of a single dominant mineral, other minerals can be present depending on the environmental conditions that created them. For example, carbonate concretions, which form in response to the reduction of sulfates by bacteria, often contain minor percentages of pyrite. Other concretions, which formed as a result of microbial sulfate reduction, consist of a mixture of calcite, barite, and pyrite.
Occurrence.
Concretions are found in a variety of rocks, but are particularly common in shales, siltstones, and sandstones. They often outwardly resemble fossils or rocks that look as if they do not belong to the stratum in which they were found. Occasionally, concretions contain a fossil, either as its nucleus or as a component that was incorporated during its growth but concretions are not fossils themselves. They appear in nodular patches, concentrated along bedding planes, protruding from weathered cliffsides, randomly distributed over mudhills or perched on soft pedestals.
Small hematite concretions or Martian spherules have been observed by the "Opportunity" rover in the Eagle Crater on Mars.
Types of concretion.
Concretions vary considerably in their compositions, shapes, sizes and modes of origin.
Septarian concretions.
 Septarian concretions (or septarian nodules) are carbonate-rich concretions containing angular cavities or cracks (septaria; "", from the Latin "partition, separating element", referring to the cracks / cavities separating polygonal blocks of hardened materials). Cracks are highly variable in shape and volume, as well as the degree of shrinkage they indicate. Although it has commonly been assumed that concretions grew incrementally from the inside outwards, the fact that radially oriented cracks taper towards the margins of septarian concretions is taken as evidence that in these cases the periphery was stiffer while the inside was softer, presumably due to a gradient in the amount of CaCO3 precipitate progressively cementing the mud porosity from outside to inside. 
The combination of natural processes giving rise to the formation of the carbonate-rich septaria remains unclear, but likely involves microbial activity and oxidation of the organic matter in the clay sediment as an internal source of carbonates. The calcium ions present in seawater, or in the pore water of the surrounding clay sediments, slowly diffuse towards the center of the initially soft concretion and progressively precipitate in contact with the carbonate anions present along their path and produced by the decaying organic matter. Indeed, when the pore water of the clay sediment becomes locally saturated with respect to calcium carbonate, this latter precipitates and progressively starts to cement the porosity of the initial medium (decaying marine organisms?). The cementation process seems to proceed from outside (harder) to inside (softer) along with the Ca2+ ions diffusion transport. 
Many mechanisms have also been proposed to explain the formation of the very characteristic internal cracks (or cavities) pattern called "septa". It includes the desiccation of clay-rich, gel-rich, or organic-rich, cores leading to the shrinkage of the concretion's softer center. Some theories suggest the expansion of gases (CO2, CH4) produced by the decay of organic matter. Less satisfying theories even consider the shrinkage of the concretion interior by sediment compaction or its brittle fracturing by earthquakes. Several mechanisms might also be combined. In the Oligocene Boom Clay Formation (Rupelian age: ~ 34 – 29 Ma) in Belgium, flattened septaria are found in horizontal clay layers enriched in calcium carbonate, so their spatial distribution suggests they formed at the bottom of the ancient sea under special conditions. 
Septaria often contain tiny crystals of calcite precipitated from the sediment pore solution at the surface of the walls of the cracks and cavities. Sometimes it is also possible to observe in large Boom Clay septaria small stalactites grown inside the open cavities. As stalactites are always slowly deposited drop by drop by gravity-driven water flow under unsaturated conditions it is a sign of a more recent evolution when the clay formation was no longer immersed in the sea. Siderite or pyrite coatings are also occasionally observed on the walls of the cavities present in the septaria, giving rise respectively to a panoply of bright reddish and golden colors with characteristic iridescences.
A spectacular example of boulder septarian concretions, which are as much as in diameter, are the Moeraki Boulders. These concretions are found eroding out of Paleocene mudstone of the Moeraki Formation exposed along the coast near Moeraki, South Island, New Zealand. They are composed of calcite-cemented mud with septarian veins of calcite and rare late-stage quartz and ferrous dolomite. The much smaller septarian concretions found in the Kimmeridge Clay exposed in cliffs along the Wessex coast of England are more typical examples of septarian concretions.
Cannonball concretions.
Cannonball concretions are large spherical concretions, which resemble cannonballs. These are found along the Cannonball River within Morton and Sioux Counties, North Dakota, and can reach in diameter. They were created by early cementation of sand and silt by calcite. Similar cannonball concretions, which are as much as in diameter, are found associated with sandstone outcrops of the Frontier Formation in northeast Utah and central Wyoming. They formed by the early cementation of sand by calcite. Somewhat weathered and eroded giant cannonball concretions, as large as in diameter, occur in abundance at "Rock City" in Ottawa County, Kansas. Large and spherical boulders are also found along Koekohe beach near Moeraki on the east coast of the South Island of New Zealand. The Moeraki Boulders, Ward Beach boulders and Koutu Boulders of New Zealand are examples of septarian concretions, which are also cannonball concretions. Large spherical rocks, which are found on the shore of Lake Huron near Kettle Point, Ontario, and locally known as "kettles", are typical cannonball concretions. Cannonball concretions have also been reported from Van Mijenfjorden, Spitsbergen; near Haines Junction, Yukon Territory, Canada; Jameson Land, East Greenland; near Mecevici, Ozimici, and Zavidovici in Bosnia-Herzegovina; in Alaska in the Kenai Peninsula Captain Cook State Park on north of Cook Inlet beach and on Kodiak Island northeast of Fossil Beach; Reports of cannonball concretions have also come from Bandeng and Zhanlong hills near Gongxi Town, Hunan Province, China.
Hiatus concretions.
Hiatus concretions are distinguished by their stratigraphic history of exhumation, exposure and reburial. They are found where submarine erosion has concentrated early diagenetic concretions as lag surfaces by washing away surrounding fine-grained sediments. Their significance for stratigraphy, sedimentology and paleontology was first noted by Voigt who referred to them as "Hiatus-Konkretionen". "Hiatus" refers to the break in sedimentation that allowed this erosion and exposure. They are found throughout the fossil record but are most common during periods in which calcite sea conditions prevailed, such as the Ordovician, Jurassic and Cretaceous. Most are formed from the cemented infillings of burrow systems in siliciclastic or carbonate sediments.
A distinctive feature of hiatus concretions separating them from other types is that they were often encrusted by marine organisms including bryozoans, echinoderms and tube worms in the Paleozoic and bryozoans, oysters and tube worms in the Mesozoic and Cenozoic. Hiatus concretions are also often significantly bored by worms and bivalves.
Elongate concretions.
Elongate concretions form parallel to sedimentary strata and have been studied extensively due to the inferred influence of phreatic (saturated) zone groundwater flow direction on the orientation of the axis of elongation. In addition to providing information about the orientation of past fluid flow in the host rock, elongate concretions can provide insight into local permeability trends (i.e., permeability correlation structure; variation in groundwater velocity, and the types of geological features that influence flow.
Elongate concretions are well known in the Kimmeridge Clay formation of northwest Europe. In outcrops, where they have acquired the name "doggers", they are typically only a few metres across, but in the subsurface they can be seen to penetrate up to tens of metres of along-hole dimension. Unlike limestone beds, however, it is impossible to consistently correlate them between even closely spaced wells.
Moqui Marbles.
Moqui Marbles, also called Moqui balls or "Moki marbles", are iron oxide concretions which can be found eroding in great abundance out of outcrops of the Navajo Sandstone within south-central and southeastern Utah. These concretions range in shape from spheres to discs, buttons, spiked balls, cylindrical forms, and other odd shapes. They range from pea-size to baseball-size. They were created by the precipitation of iron, which was dissolved in groundwater.
Kansas pop rocks.
Kansas pop rocks are concretions of either iron sulfide, "i.e." pyrite and marcasite, or in some cases jarosite, which are found in outcrops of the Smoky Hill Chalk Member of the Niobrara Formation within Gove County, Kansas. They are typically associated with thin layers of altered volcanic ash, called bentonite, that occur within the chalk comprising the Smoky Hill Chalk Member. A few of these concretions enclose, at least in part, large flattened valves of inoceramid bivalves. These concretions range in size from a few millimeters to as much as in length and in thickness. Most of these concretions are oblate spheroids. Other "pop rocks" are small polycuboidal pyrite concretions, which are as much as in diameter. These concretions are called "pop rocks" because they explode if thrown in a fire. Also, when they are either cut or hammered, they produce sparks and a burning sulfur smell. Contrary to what has been published on the Internet, none of the iron sulfide concretions, which are found in the Smoky Hill Chalk Member were created by either the replacement of fossils or by metamorphic processes. In fact, metamorphic rocks are completely absent from the Smoky Hill Chalk Member. Instead, all of these iron sulfide concretions were created by the precipitation of iron sulfides within anoxic marine calcareous ooze after it had accumulated and before it had lithified into chalk.
Iron sulfide concretions, such as the Kansas Pop rocks, consisting of either pyrite and marcasite, are nonmagnetic (Hobbs and Hafner 1999). On the other hand, iron sulfide concretions, which either are composed of or contain either pyrrhotite or smythite, will be magnetic to varying degrees. Prolonged heating of either a pyrite or marcasite concretion will convert portions of either mineral into pyrrhotite causing the concretion to become slightly magnetic.
Claystones, clay dogs, and fairy stones.
Disc concretions composed of calcium carbonate are often found eroding out of exposures of interlaminated silt and clay, varved, proglacial lake deposits. For example, great numbers of strikingly symmetrical concretions have been found eroding out of outcrops of Quaternary proglacial lake sediments along and in the gravels of the Connecticut River and its tributaries in Massachusetts and Vermont. Depending the specific source of these concretions, they vary in an infinite variety of forms that include disc-shapes; crescent-shapes; watch-shapes; cylindrical or club-shapes; botryoidal masses; and animal-like forms. They can vary in length from to over and often exhibit concentric grooves on their surfaces. In the Connecticut River Valley, these concretions are often called "claystones" because the concretions are harder than the clay enclosing them. In local brickyards, they were called "clay-dogs" either because of their animal-like forms or the concretions were nuisances in molding bricks. Similar disc-shaped calcium carbonate concretions have also been found in the Harricana River valley in the Abitibi-Témiscamingue administrative region of Quebec, and in Östergötland county, Sweden. In Scandinavia, they are known as "marlekor" ("fairy stones").