Thecrystal lattice of aragonite differs from that of calcite, resulting in a different crystal shape, anorthorhombic crystal system withacicular crystal.[5] Repeatedtwinning results in pseudo-hexagonal forms. Aragonite may be columnar or fibrous, occasionally in branchinghelictitic forms calledflos-ferri ("flowers of iron") from their association with theores at theCarinthian iron mines.[6]
Aragonite forms naturally in almost allmollusk shells, and as thecalcareous endoskeleton of warm- and cold-watercorals (Scleractinia). Severalserpulids 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 aragonitefossil shells of some extinctammonites forms aniridescent material calledammolite.[18]
Aragonite also forms in the ocean inorganic precipitates called marine cements (in thesediment) or as free crystals (in the water column).[22][23]Inorganic precipitation of aragonite in caves can occur in the form ofspeleothems.[24] Aragonite is common in serpentinites where magnesium-rich pore solutions apparently inhibit calcite growth and promote aragonite precipitation.[25]
Aragonite ismetastable at the low pressures near the Earth's surface and is thus commonly replaced by calcite in fossils. Aragonite older than theCarboniferous is essentially unknown.[26]
Aragonite can be synthesized by adding acalcium chloride solution to asodium carbonate solution at temperatures above 60 °C (140 °F) or in water-ethanol mixtures at ambient temperatures.[27]
Aragonite is athermodynamically unstable phase of calcium carbonate at any pressure below about 3,000 bars (300,000 kPa) at any temperature.[28] Aragonite nonetheless frequently forms in near-surface environments at ambient temperatures. The weakVan der Waals forces inside aragonite give an important contribution to both the crystallographic and elastic properties of this mineral.[29] The difference in stability between aragonite and calcite, as measured by theGibbs 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 ofOstwald's step rule, where a less stable phase is the first to form.[30] The presence ofmagnesium ions may inhibit calcite formation in favor of aragonite.[31] Once formed, aragonite tends to alter tocalcite on scales of 107 to 108 years.[32]
The mineralvaterite, also known as μ-CaCO3, is another phase of calcium carbonate that ismetastable at ambient conditions typical of Earth's surface, and decomposes even more readily than aragonite.[33][34]
Inaquaria, 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 thedissolution ofbiogeniccalcium carbonate.[35]
Aragonite has been successfully tested for the removal of pollutants likezinc,cobalt andlead from contaminated wastewaters.[36]
^abSinkankas, John (1964).Mineralogy for amateurs. Princeton, N.J.: Van Nostrand. pp. 371–372.ISBN0442276249.
^Cairncross, B.; McCarthy, T. (2015).Understanding Minerals & Crystals. Cape Town: Struik Nature. p. 187.ISBN978-1-43170-084-4.
^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.doi:10.1007/978-1-4612-3748-8.ISBN978-1-4612-3748-8.
^Balaz, Christine (2009).An Explorer's Guide: Utah. Vermont: The Countryman Press. p. 368.ISBN978-0-88150-738-6.
^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.doi:10.1038/381056a0.S2CID4285912.
^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
