Coquina

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For the small marine mollusc, see Donax variabilis.
Coquina outcropping on the beach at Washington Oaks State Gardens, Florida

Coquina (/kˈknə/; Spanish: "cockle.") is a sedimentary rock that is composed either wholly or almost entirely of the transported, abraded, and mechanically sorted fragments of the shells of either molluscs, trilobites, brachiopods, or other invertebrates. For a sediment to be considered to be a coquina, the average size of the particles composing it should be 2 mm or greater in size. Coquina can vary in hardness from poorly to moderately cemented. The term "coquina" is derived from the Spanish word for cockleshells or shellfish.[1][2] Incompletely consolidated and poorly cemented coquinas are considered grainstones in the Dunham classification system for carbonate sedimentary rocks.[3] Well-cemented coquinas are classified as biosparites according to the Folk classification of sedimentary rocks.[4]

Coquinas accumulate in high-energy marine and lacustrine environments where currents and waves result in the vigorous winnowing, abrasion, fracturing, and sorting of the shells, which compose them. As a result, they typically exhibit well-developed bedding or cross-bedding, close packing, and good orientation of the shell fragments composing them. The high-energy marine or lacustrine associated with coquinas include beaches, shallow submarine raised banks, swift tidal channels, and barrier bars.[4][5]

Composition and distribution[edit]

Coquina is mainly composed of the mineral calcite, often including some phosphate, in the form of seashells or coral. Coquinas dating from the Devonian period through to the much more recent Pleistocene are a common find all over the world, with the depositional requirements to form a coquina being a common thing in many marine facies.

History and use[edit]

Coquina from Florida.
Close-up of coquina from Florida. The scale bar is 10 mm.

Occasionally quarried or mined and used as a building stone in Florida for over 400 years, coquina forms the walls of the Castillo de San Marcos, Saint Augustine. The stone makes a very good material for forts, particularly those built during the period of heavy cannon use. Because of coquina's softness, cannonballs would sink into, rather than shatter or puncture, the walls of the Castillo de San Marcos.

In the past coquina was used for the construction of buildings in Denham, Western Australia, but quarrying is no longer permitted in the World Heritage Site.

When first quarried, coquina is extremely soft. This softness makes it very easy to remove from the quarry and cut into shape. However, the stone is also at first much too soft to be used for building. In order to be used as a building material, the stone is left out to dry for approximately one to three years, which causes the stone to harden into a usable, but still comparatively soft, form.

Coquina has also been used as a source of paving material. It is usually poorly cemented and easily breaks into component shell or coral fragments, which can be substituted for gravel or crushed harder rocks. Large pieces of coquina of unusual shape are sometimes used as landscape decoration.

Because coquina often includes a component of phosphate, it is sometimes mined for use as fertilizer.

Notable exposures of coquina[edit]

Coquina in architecture[edit]

See also[edit]

References[edit]

  1. ^ U.S. Bureau of Mines Staff (1996) Dictionary of Mining, Mineral, & Related Terms. Report SP-96-1, U.S. Department of Interior, U.S. Bureau of Mines, Washington, D.C.
  2. ^ Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, J.A., eds. (2005) Glossary of Geology (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. ISBN 0-922152-76-4
  3. ^ Dunham, R.J. (1962). Classification of carbonate rocks according to depositional texture. In Ham, W. E.. Classification of carbonate rocks. American Association of Petroleum Geologists Memoir. no. 1, pp. 108-121.
  4. ^ a b Folk, R.L. (1980) The Petrology of Sedimentary Rocks. Austin, Texas, Hemphill Publishing Company. 182 pp. ISBN 0-914696-14-9
  5. ^ Scholle, P.A., D.G. Bebout, and C.H. Moore (1983) Carbonate Depositional Environments. Memoir no. 33. Tulsa, Oklahoma, American Association of Petroleum Geologists. 708 pp. ISBN 978-0-89181-310-1

External links[edit]