The
crystal lattice of aragonite differs from that of calcite, resulting in a different crystal shape, an
orthorhombic crystal system with
acicular crystal.[5] Repeated
twinning results in pseudo-hexagonal forms. Aragonite may be columnar or fibrous, occasionally in branching
helictitic forms called flos-ferri ("flowers of iron") from their association with the
ores at the
Carinthian iron mines.[6]
Aragonite forms naturally in almost all
mollusk shells, and as the
calcareous endoskeleton of warm- and cold-water
corals (
Scleractinia). Several
serpulids have aragonitic tubes.[14] Because the mineral deposition in mollusk shells is strongly biologically controlled,[15] some crystal forms are distinctively different from those of inorganic aragonite.[16] In some mollusks, the entire shell is aragonite;[17] in others, aragonite forms only discrete parts of a bimineralic shell (aragonite plus calcite).[15] The nacreous layer of the aragonite
fossil shells of some extinct
ammonites forms an
iridescent material called
ammolite.[18]
Aragonite also forms in the ocean inorganic precipitates called marine cements (in the
sediment) or as free crystals (in the water column).[20][21]
Inorganic precipitation of aragonite in caves can occur in the form of
speleothems.[22] Aragonite is common in serpentinites where magnesium-rich pore solutions apparently inhibit calcite growth and promote aragonite precipitation.[23]
Aragonite is
metastable at the low pressures near the Earth's surface and is thus commonly replaced by calcite in fossils. Aragonite older than the
Carboniferous is essentially unknown.[24]
Aragonite can be synthesized by adding a
calcium chloride solution to a
sodium carbonate solution at temperatures above 60 °C (140 °F) or in water-ethanol mixtures at ambient temperatures.[25]
Physical properties
Aragonite is a
thermodynamically unstable phase of calcium carbonate at any pressure below about 3,000 bars (300,000 kPa) at any temperature.[26] Aragonite nonetheless frequently forms in near-surface environments at ambient temperatures. The weak
Van der Waals forces inside aragonite give an important contribution to both the crystallographic and elastic properties of this mineral.[27] The difference in stability between aragonite and calcite, as measured by the
Gibbs free energy of formation, is small, and effects of grain size and impurities can be important. The formation of aragonite at temperatures and pressures where calcite should be the stable polymorph may be an example of
Ostwald's step rule, where a less stable phase is the first to form.[28] The presence of
magnesium ions may inhibit calcite formation in favor of aragonite.[29] Once formed, aragonite tends to alter to
calcite on scales of 107 to 108 years.[30]
The mineral
vaterite, also known as μ-CaCO3, is another phase of calcium carbonate that is
metastable at ambient conditions typical of Earth's surface, and decomposes even more readily than aragonite.[31][32]
Uses
In
aquaria, aragonite is considered essential for the replication of reef conditions. Aragonite provides the materials necessary for much sea life and also keeps the pH of the water close to its natural level, to prevent the
dissolution of
biogeniccalcium carbonate.[33]
Aragonite has been successfully tested for the removal of pollutants like
zinc,
cobalt and
lead from contaminated wastewaters.[34]
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^Cairncross, B.; McCarthy, T. (2015). Understanding Minerals & Crystals. Cape Town: Struik Nature. p. 187.
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^Calvo, Miguel (2012). Minerales y Minas de España. Vol. V. Carbonatos y Nitratos. Madrid: Escuela Técnica Superior de Ingenieros de Minas de Madrid. Fundación Gómez Pardo. pp. 314–398.
ISBN978-84-95063-98-4.
^Gonzalez, Luis A.; Lohmann, Kyger C. (1988). "Controls on Mineralogy and Composition of Spelean Carbonates: Carlsbad Caverns, New Mexico". In James, Noel P.; Choquette, Philip W. (eds.). Paleokarst. New York: Springer-Verlag. pp. 81–101.
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^Balaz, Christine (2009). An Explorer's Guide: Utah. Vermont: The Countryman Press. p. 368.
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^Nesse, William D. (2000). Introduction to mineralogy. New York: Oxford University Press. pp. 336–337.
ISBN9780195106916.
^Boggs, Sam (2006). Principles of sedimentology and stratigraphy (4th ed.). Upper Saddle River, N.J.: Pearson Prentice Hall. pp. 161–164.
ISBN0131547283.
^
abBelcher, A. M.; Wu, X. H.; Christensen, R. J.; Hansma, P. K.; Stucky, G. D.; Morse, D. E. (May 1996). "Control of crystal phase switching and orientation by soluble mollusc-shell proteins". Nature. 381 (6577): 56–58.
Bibcode:
1996Natur.381...56B.
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10.1038/381056a0.
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^Chateigner, D.; Ouhenia, S.; Krauss, C.; Belkhir, M.; Morales, M. (February 2010). "Structural distortion of biogenic aragonite in strongly textured mollusc shell layers". Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms. 268 (3–4): 341–345.
Bibcode:
2010NIMPB.268..341C.
doi:
10.1016/j.nimb.2009.07.007.
^Loftus, Emma; Rogers, Keith; Lee-Thorp, Julia (November 2015). "A simple method to establish calcite:aragonite ratios in archaeological mollusc shells: CALCITE:ARAGONITE IN ARCHAEOLOGICAL SHELLS". Journal of Quaternary Science. 30 (8): 731–735.
doi:
10.1002/jqs.2819.
S2CID130591343.
^Runnegar, B. (1987). "Shell microstructures of Cambrian molluscs replicated by phosphate". Alcheringa: An Australasian Journal of Palaeontology. 9 (4): 245–257.
doi:
10.1080/03115518508618971.
^Sand, K.K., Rodriguez-Blanco, J.D., Makovicky, E., Benning, L.G. and Stipp, S. (2012) Crystallization of CaCO3 in water-ethanol mixtures: spherulitic growth, polymorph stabilization and morphology change. Crystal Growth & Design, 12, 842-853.
doi:
10.1021/cg2012342.
^Kamiya, Kanichi; Sakka, Sumio; Terada, Katsuyuki (November 1977). "Aragonite formation through precipitation of calcium carbonate monohydrate". Materials Research Bulletin. 12 (11): 1095–1102.
doi:
10.1016/0025-5408(77)90038-1.
^Orr, J. C., et al. (2005)
Anthropogenic ocean acidification over the 21st century and its impact on calcifying organisms. Nature 437: 681-686
^Köhler, S., Cubillas, et al. (2007) Removal of cadmium from wastewaters by aragonite shells and the influence of other divalent cations. Environmental Science and Technology, 41, 112-118.
doi:
10.1021/es060756j
^Kozic, Viljem; Hamler, Anton; Ban, Irena; Lipus, Lucija C. (October 2010). "Magnetic water treatment for scale control in heating and alkaline conditions". Desalination and Water Treatment. 22 (1–3): 65–71.
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