DEFINITION OF SPELEOTHEM

Any secondary mineral deposit formed in a

cave by the action of groundwater. Most speleothems are made

of carbonate minerals such as calcite, aragonite, or dolomite,

but some are made of silicates and evaporites. Dripstone and

flowstone are the most common carbonate speleothems. Yellow,

brown, orange, tan, green, and red colors in dripstone

and flowstone are formed through staining by organic compounds,

oxides derived from overlying clays and soils, and

rarely by ionic substitution in the carbonate minerals. Dripstone

forms where water enters the cave through joints, bedding

planes, or other structures and degasses CO2 from water

droplets, forming a small ring of calcite before each drop

breaks free and falls into the cave. Each succeeding drop

deposits another small ring of calcite eventually forming a hollow

tube called a straw stalactite. Additional growth may

occur on the outside of the straw stalactite forming a wedgeshaped

hanging calcite deposit. Where the drops fall to the

cave floor below they deposit additional calcite forming a

mound-shaped structure known as a stalagmite. These have no

central canal but consist of a series of layers deposited one

over the other and typically are symmetric about a vertical

axis. Flowstone is a massive secondary carbonate deposit

formed by water that moves as sheet flows over cave walls and

floors. The water deposits layered and terraced carbonate with

complex and bizarre shapes, with shapes and patterns determined

by the flow rates of the water and the shape of the cave

walls, shelves, and floor. Draperies are layered deposits with

furled forms intermediate between dripstone and flowstone.

Less common types of speleothems include shields, massive

plate-like forms that protrude from cave walls. They are

fed by water that flows through a medial crack separating

two similar sides of the shield, with the crack typically parallel

to regional joints in the cave.

Some speleothems have erratic forms not controlled by

joints, walls, or other structures. Helictites are curved stalactite-

like forms with a central canal, anthodites are clusters of

radiating crystals such as aragonite and a variety of botryoidal

forms that resemble beads or corals. Moonmilk is a

wet powder or wet pasty mass of calcite, aragonite, or magnesium

carbonate minerals. Travertine forms speleothems in

some cave systems, where the waters are saturated in carbon

dioxide.

Evaporite minerals form deposits in some dry dusty

caves where the relative humidity drops to below 90 percent,

and the waters have dissolved anions. Gypsum is the most

common evaporite mineral found as a speleothem, with magnesium,

sodium, and strontium sulfates being less common.

Phosphates, nitrates, iron minerals, and even ice form

speleothems in other less common settings.

See also GROUNDWATER; KARST.

stalactite See SPELEOTHEM.

stalagmite See SPELEOTHEM.

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