What are carbonate concretions?

Carbonate concretions are common features of sedimentary rocks of all geological ages. They are most obvious in sandstones and mudstones as ovoid bodies of rock that protrude from natural outcrops: clearly harder or better cemented than their host rocks.

How are concretions formed in geology?

Concretions are masses of mineral matter embedded within rock layers, including limestone, sandstone, and shale. They often take shape when minerals precipitating (settling) out of water collect around a nucleus, such as a pebble, leaf, shell, bone, or fossil.

Do concretions contain fossils?

Concretions are commonly misunderstood geologic structures. Often mistaken for fossil eggs, turtle shells, or bones, they are actually not fossils at all but a common geologic phenomenon in almost all types of sedimentary rock, including sandstones, shales, siltstones, and limestones.

Are concretions magnetic?

Iron sulfide concretions, such as the Kansas Pop rocks, consisting of either pyrite and marcasite, are nonmagnetic. On the other hand, iron sulfide concretions, which either are composed of or contain either pyrrhotite or smythite, will be magnetic to varying degrees.

What is an iron concretion?

Iron oxide concretions are formed from post depositional, paleogroundwater chemical interaction with iron minerals in porous sedimentary rocks. The concretions record a history of iron mobilization and precipitation caused by changes in pH, oxidation conditions, and activity of bacteria.

How long do concretions take to form?

Moeraki boulders: Gigantic concretions formed in mudstone on New Zealand’s Moeraki coast; about 50 million years old. The present research shows that even concretions of this size formed very rapidly, within several decades.

How do you break open a concretion?

The best method to open concretions is to alternately freeze and thaw them. Put the concretions in a bucket of water and allow them to soak for several days so the water thoroughly penetrates the concretion.

Are geodes concretions?

In sedimentary rocks, geodes may form by dissolving out cavities by ground water and re-depositing of minerals as crystals, which usually point towards the centre of the cavity. They may also form in pre-existing concretions or in spaces left by expansion of the rock under internal fluid pressure.

Is flint a concretion?

Chert Nodules Chert is micro- or cryptocrystalline silica (SiO₂; also known as chalcedony or flint) which forms under particular chemical conditions inside soft sediments. Like concretions, they form inside soft sediments, and when they weather out, their shapes can frequently look like fossils.

Do concretions have fossils?

Is a geode a concretion?

Geodes are spherical concretions with a hollow interior, ranging from 2 cm to 1 m in diameter. The outermost layer is generally composed of chalcedony, a brown microcrystalline variety of quartz. The interior contains a lining of inwardly projecting crystals, usually quartz of many varieties.

How do you know what rocks to crack open for fossils?

This is often easiest if you find a nodule with lines of weakness or cracks that you can split it along. It is also a good idea to look for signs that the rock contains a fossil before trying to break it, part of a fossil may be visible on the surface of the rock.

How do you open fossil concretions?

The best method to open concretions is to alternately freeze and thaw them. Put the concretions in a bucket of water and allow them to soak for several days so the water thoroughly penetrates the concretion. Then, place the bucket of concretions in a freezer.