Cave popcorn, or coralloids, are small nodes of

calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...

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

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate 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, blackboard or sidewalk chalk, and drywa ...

that form on surfaces in

cave A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word ''cave'' can refer to smaller openings such as sea ...

s, especially

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...

caves. They are a common type of

speleothem A speleothem (; ) is a geological formation 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, dependi ...

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 (cave formation) with acicular ("needle-like") growths almost always composed of aragonite (a polymorph of calcite) or calcite replaced aragonite. It is a variety of anthodite. Frostwork can also be ...

. 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 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 speleot ...

. 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 same names also refer to the foodstuff produced by the expansion. A popcorn kernel's strong hull contains the se ...

. 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, mortar or other calcareous material outside the cave environment.Smith, G.K. (2016). "Calcite straw stalactites growing from concrete structures", Cave and Karst Science 43(1), 4 ...

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 Z) 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 cylindrical tube. They are also known as tubular stalactites. Soda straws grow in places where water leaches slowly through cracks in rock, such as on the roofs of c ...

References

External links

The Virtual Cave's page on cave popcorn
{{Authority control Speleothems