Concretion

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 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.

Formation

There are two main theories to explain the formation of concretions. The first theory suggests that concretions are formed from mineral precipitation around some kind of nucleus while a second theory proposes that concretions are formed by the selective pressure dissolution of sediment, releasing the cementing agent that forms the concretion.

Types of Concretions

There are several types of concretions found in the field. These include:

• Septarian concretions: These are concretions containing angular cavities or cracks, called "septaria". The word comes from the Latin word septum; "partition", and refers to the cracks/separations in this kind of rock.

• Moqui Marbles: These are small, brownish-black balls composed of iron oxide and sandstone that can be found on the southern Utah desert.

• Thundereggs: A thunderegg (or thunder egg) is a nodule-like rock, similar to a filled geode, that is formed within rhyolitic volcanic ash layers.

See Also

• Sedimentary rock
• Mineral
• Soil
• Geode

References

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