Vaughan, T. W., 1919. Fossil corals from Central America, Cuba
and Porto Rico with an account of the American Tertiar y, Pleistocene and recent coral reefs. U.S. Natural History Museum Bulletin, 103, 189–524.
Vaughan, T. W., 1923. Coral reefs and submerged platforms. Proceedings of the 2nd Pan Pacific Congress, Australia, 2,
1128–1131.
Wharton, W. J. L., 1890. Coral Reefs, fossil and recent. Nature, 42,
172.
Wharton, W. J. L., 1897. Foundations of coral reefs. Nature, 55,
390–393.
Yabe, H., 1942. Problems of the coral reefs. Tokyo Imperial
University, Geological and Palaeontological Institute Reports,
39, 1–6.
Cross-references
Accommodation Space
Belize Barrier and Atoll Reefs
Bermuda
Great Barrier Reef Committee
Sea Level Change and Its Effect on Reef Growth
Subsidence Hypothesis of Reef Development
ARAGONITE
Sue J. McLaren
University of Leicester, Leicester, UK
Definition
Aragonite is a mineral comprising calcium carbonate
(CaCO 3 ), it is polymorphous to calcite, meaning that they
both contain the same main elements but have different
crystal structures. Aragonite forms orthorhombic crystals,
whereas calcite is trigonal. Aragonite is characterized not
only by pseudohexagonal crystals but also acicular habits,
often in the form of radiating groups of needles, which are
also common. Aragonite has an imperfect to poor cleavage, a subconchoidal fracture, and a hardness on Moh’s
scale of 3.5–4. Aragonite tends to be colorless or white,
and most are relatively pure, with strontium and lead,
acting as the most common substitutions for calcium.
Aragonite is highly soluble in cold dilute hydrochloric
acid, resulting in significant effervescence. Feigl’s solution stains aragonite black, whereas calcite remains
unaffected.
Aragonite most commonly precipitates in seawater and
so is found in marine sediments and forms the shells of
some marine organisms. The skeleton of scleractinian
corals are composite organo-minerals comprising organic
matrices and inorganic aragonite crystallites. Aragonite is
a meta-stable form of calcium carbonate and over time
undergoes transformation to the more stable calcite polymorph. Aragonite is more soluble than calcite in the meteoric environment and dissolution results in the partial or
total loss of the aragonite clasts, matrix and cements, often
leaving molds. These molds along with primary pores may
subsequently be infilled by allochthonous calcite cement.
Alternatively, neomorphism may occur by a concomitant
dissolution of aragonite and reprecipitation of autochthonous low-magnesian calcite cement (e.g., Rabier et al.,
2008). On land, aragonite can be found in dryland environments associated with evaporite minerals such as halite
and gypsum. Aragonite also forms speleothems in caves,
tufa, and travertine as well as sinter in hot springs.
Bibliography
Rabier, C., Anguy, Y., Cabioch, G., Genthon, P., 2008. Characterization of various stages of calcitization in Porites sp corals
from uplifted reefs – case studies from New Caledonia, Vanuatu
and Futuna (South-West Pacific). Sedimentary Geology, 211,
73–86.
Cross-references
Calcite
ATOLL ISLANDS (MOTU)
Roger McLean
University of New South Wales, Canberra, ACT, Australia
Synonyms
Cays; Gravel islands; Islets; Keys; Rubble islands; Shingle
islands
Definition
Atoll islands are morphologically coherent wave-built
accumulations of bioclastic sediment, located on atoll rims
or on patch reefs within atoll lagoons, and emergent at all
stages of the tide. They can be of any size and shape and
are characterized by low elevation (typically 2–4 m above
MSL), shallow alkaline soils, absence of surface water,
and a limited terrestrial biota. Atoll islands are composed
of skeletal sands and/or coral rubble sourced from the
adjacent reef or lagoon. On sea-level atolls, islands are
geologically very young having accumulated during the
mid-late Holocene. In spite of this short time frame, some
unconsolidated island sediments have become indurated
to form phosphate rock, cay sandstone, beach rock, and
beach conglomerate, which offer some stability to atoll
islands. Motu (n) is the word for any island or islet in many
Polynesian languages, including Maori, Tuvaluan, and
Paumotu, although the word has been given more specific
meanings in coral reef geomorphology.
Number, dimensions, and location of atoll islands
Atolls are annular reefs around a central lagoon. The reef
rim may be continuous, or broken by a few channels as
is common in the central Pacific, or have many shallow
passes or hoa as in French Polynesia, or deep passages
as in the Maldives. Rarely are rim reefs completely
ATOLL ISLANDS (MOTU)
47
and Porto Rico with an account of the American Tertiar y, Pleistocene and recent coral reefs. U.S. Natural History Museum Bulletin, 103, 189–524.
Vaughan, T. W., 1923. Coral reefs and submerged platforms. Proceedings of the 2nd Pan Pacific Congress, Australia, 2,
1128–1131.
Wharton, W. J. L., 1890. Coral Reefs, fossil and recent. Nature, 42,
172.
Wharton, W. J. L., 1897. Foundations of coral reefs. Nature, 55,
390–393.
Yabe, H., 1942. Problems of the coral reefs. Tokyo Imperial
University, Geological and Palaeontological Institute Reports,
39, 1–6.
Cross-references
Accommodation Space
Belize Barrier and Atoll Reefs
Bermuda
Great Barrier Reef Committee
Sea Level Change and Its Effect on Reef Growth
Subsidence Hypothesis of Reef Development
ARAGONITE
Sue J. McLaren
University of Leicester, Leicester, UK
Definition
Aragonite is a mineral comprising calcium carbonate
(CaCO 3 ), it is polymorphous to calcite, meaning that they
both contain the same main elements but have different
crystal structures. Aragonite forms orthorhombic crystals,
whereas calcite is trigonal. Aragonite is characterized not
only by pseudohexagonal crystals but also acicular habits,
often in the form of radiating groups of needles, which are
also common. Aragonite has an imperfect to poor cleavage, a subconchoidal fracture, and a hardness on Moh’s
scale of 3.5–4. Aragonite tends to be colorless or white,
and most are relatively pure, with strontium and lead,
acting as the most common substitutions for calcium.
Aragonite is highly soluble in cold dilute hydrochloric
acid, resulting in significant effervescence. Feigl’s solution stains aragonite black, whereas calcite remains
unaffected.
Aragonite most commonly precipitates in seawater and
so is found in marine sediments and forms the shells of
some marine organisms. The skeleton of scleractinian
corals are composite organo-minerals comprising organic
matrices and inorganic aragonite crystallites. Aragonite is
a meta-stable form of calcium carbonate and over time
undergoes transformation to the more stable calcite polymorph. Aragonite is more soluble than calcite in the meteoric environment and dissolution results in the partial or
total loss of the aragonite clasts, matrix and cements, often
leaving molds. These molds along with primary pores may
subsequently be infilled by allochthonous calcite cement.
Alternatively, neomorphism may occur by a concomitant
dissolution of aragonite and reprecipitation of autochthonous low-magnesian calcite cement (e.g., Rabier et al.,
2008). On land, aragonite can be found in dryland environments associated with evaporite minerals such as halite
and gypsum. Aragonite also forms speleothems in caves,
tufa, and travertine as well as sinter in hot springs.
Bibliography
Rabier, C., Anguy, Y., Cabioch, G., Genthon, P., 2008. Characterization of various stages of calcitization in Porites sp corals
from uplifted reefs – case studies from New Caledonia, Vanuatu
and Futuna (South-West Pacific). Sedimentary Geology, 211,
73–86.
Cross-references
Calcite
ATOLL ISLANDS (MOTU)
Roger McLean
University of New South Wales, Canberra, ACT, Australia
Synonyms
Cays; Gravel islands; Islets; Keys; Rubble islands; Shingle
islands
Definition
Atoll islands are morphologically coherent wave-built
accumulations of bioclastic sediment, located on atoll rims
or on patch reefs within atoll lagoons, and emergent at all
stages of the tide. They can be of any size and shape and
are characterized by low elevation (typically 2–4 m above
MSL), shallow alkaline soils, absence of surface water,
and a limited terrestrial biota. Atoll islands are composed
of skeletal sands and/or coral rubble sourced from the
adjacent reef or lagoon. On sea-level atolls, islands are
geologically very young having accumulated during the
mid-late Holocene. In spite of this short time frame, some
unconsolidated island sediments have become indurated
to form phosphate rock, cay sandstone, beach rock, and
beach conglomerate, which offer some stability to atoll
islands. Motu (n) is the word for any island or islet in many
Polynesian languages, including Maori, Tuvaluan, and
Paumotu, although the word has been given more specific
meanings in coral reef geomorphology.
Number, dimensions, and location of atoll islands
Atolls are annular reefs around a central lagoon. The reef
rim may be continuous, or broken by a few channels as
is common in the central Pacific, or have many shallow
passes or hoa as in French Polynesia, or deep passages
as in the Maldives. Rarely are rim reefs completely
ATOLL ISLANDS (MOTU)
47