Cave popcorn, or coralloids, are small nodes of

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

aragonite Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation fr ...

, or

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk ...

that form on surfaces in

cave Caves or caverns are natural voids under the Earth's Planetary surface, surface. Caves often form by the weathering of rock and often extend deep underground. Exogene caves are smaller openings that extend a relatively short distance undergrou ...

s, especially

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

caves. They are a common type of

speleothem A speleothem (; ) is a geological formation made by mineral deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depen ...

Appearance

The individual nodules of cave popcorn range in size from 5 to 20 mm and may be decorated by other speleothems, especially aragonite needles or

frostwork In geology, frostwork is a type of speleothem with acicular ("needle-like") growths almost always composed of aragonite (a polymorph of calcite) or calcite replaced aragonite.Hill, C. and Forti, P. (1997), ''Cave Minerals of the World'', Nationa ...

. The nodules tend to grow in clusters on bedrock or the sides of other speleothems. These clusters may terminate suddenly in either an upward or downward direction, forming a stratographic layer. When they terminate in a downward direction, they may appear as flat-bottomed formations known as trays. Individual nodes of popcorn can assume a variety of shapes from round to flattened ear-like or button-like shapes. The color of cave popcorn is usually white, but various other colors are possible depending on the composition.

Formation

Cave popcorn can form by precipitation. Water seeping through limestone walls or splashing onto them leaves deposits when CO₂ loss causes its minerals to precipitate. When formed in this way, the resultant nodules have the characteristics of small balls of

flowstone Flowstones are sheetlike deposits of calcite or other carbonate minerals, formed where water flows down the walls or along the floors of a cave. They are typically found in "solution caves", in limestone, where they are the most common speleothe ...

. Cave popcorn can also form by evaporation in which case it is chalky and white like edible

popcorn Popcorn (also called popped corn, popcorns, or pop-corn) is a variety of corn kernel which expands and puffs up when heated. The term also refers to the snack food produced by the expansion. It is one of the oldest snacks, with evidence of p ...

. In the right conditions, evaporative cave popcorn may grow on the windward side of the surface to which it is attached or appear on the edges of projecting surfaces.

On manmade structures (outside the cave environment)

Popcorn can also occur on concrete structures outside the cave environment; these are classified as

calthemite Calthemite is a secondary deposit, derived from concrete, Lime (material), lime, Mortar (masonry), mortar or other calcareous material outside the cave environment.Smith, G.K. (2016). "Calcite straw stalactites growing from concrete structures" ...

coralloids. Calthemite coralloids also occur in "artificial caves", such as mines, railways, or vehicle tunnels where there is a source of lime, mortar, or cement from which the calcium ions can be leached. Coralloids can form by a number of different methods in caves; however, the most common form on concrete is created when a hyperalkaline solution seeps from fine cracks. Due to solution evaporation, deposition of calcium carbonate occurs before any drop can form. The resulting coralloids are small and chalky with a cauliflower appearance.

Gallery

File:Alisadr Cave, Hamedan, Iran.JPG, Alisadr Cave, Hamedan, Iran File:Cave popcorn trays.jpg, Cave popcorn trays File:Tropfsteine.svg, Diagram of dripstone cave structures (cave popcorn labelled AH) File:Button cave popcorn.jpg, Button cave popcorn File:Calthemite coralloids.jpg, Calthemite coralloids under concrete, with

soda straw A soda straw (or simply straw) is a speleothem in the form of a hollow mineral cylinder (geometry), cylindrical tube. They are also known as tubular stalactites. Soda straws grow in places where water leaches slowly through cracks in rock, such ...

References

External links

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Kids' page on popcorn
Underground Adventures (archived)

{{Authority control Speleothems