Carbonate ooids on the surface of a limestone; Carmel Formation (Middle Jurassic) of southern Utah, USA. Largest is 1.0 mm in diameter.

Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3), and dolomite rock (also known as dolostone), which is composed of mineral dolomite (CaMg(CO3)2). They are usually classified based on texture and grain size.[1] Importantly, carbonate rocks can exist as metamorphic and igneous rocks, too. When recrystallized carbonate rocks are metamorphosed, marble is created. Rare igneous carbonate rocks even exist as intrusive carbonatites and, even rarer, there exists volcanic carbonate lava.

Carbonate rocks are also crucial components to understanding geologic history due to processes such as diagenesis in which carbonates undergo compositional changes based on kinetic effects.[2] The correlation between this compositional change and temperature can be exploited to reconstruct past climate as is done in paleoclimatology. Carbonate rocks can also be used for understanding various other systems as described below.

Limestone

Limestone is the most common carbonate rock[3] and is a sedimentary rock made of calcium carbonate with two main polymorphs: calcite and aragonite. While the chemical composition of these two minerals is the same, their physical properties differ significantly due to their different crystalline form. The most common form found in the seafloor is calcite, while aragonite is more found in biological organisms.[4]

Calcite

Calcite crystals from Irai, Brazil.

Calcite can be either dissolved by groundwater or precipitated by groundwater,[5] depending on several factors including the water temperature, pH, and dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility in which it becomes less soluble in water as the temperature increases. When conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together or it can fill fractures.

Aragonite

Compared to calcite, aragonite is less stable and more soluble,[6] and can thus be converted to calcite under certain conditions. In solution, magnesium ions can act as promoters of aragonite growth as they inhibit calcite precipitation.[7] Often this inhibited precipitation occurs in biology where organisms aim to precipitate calcium carbonate for their structural features such as for skeleton and shells.

Dolostone

The discovery of dolomite rock, or dolostone, was first published in 1791[8] and has been found across the Earth's crust from various different time periods.[9] Because the rock is made of calcium, magnesium, and carbonate ions, the mineral crystalline structure can be visualized similar to calcite and magnesite.[10] Due to this composition, the dolomite mineral present in dolostone can be classified by varying degree of calcium inclusion, and occasionally iron, too.[9]

Calcian dolomite

Calcium-rich dolomite, or calcian dolomite, is dolomite which has more calcium than magnesium in its mineral form. This is the most common form of dolomite found naturally and artificially from synthesis.[9] This dolomite, when formed in the oceans, can prove to be metastable.[9] The resultant structure of this mineral presents minimal differences from regular dolomite likely as a result of formation after initial crystal growth.[9]

Ferroan dolomite / ankerite

Iron-rich dolomite, or ferroan dolomite, is doloimite which contains significant trace levels of iron. Due to the similar ionic radii of iron(II) and magnesium, iron(II) can easily substitute magnesium to form ferroan dolomite; manganese can also substitute this atom. The result can be defined as ankerite. The exact delineation between which minerals are considered ferroan dolomite and which are ankerite is unclear. Ankerite with the "pure" CaFe(CO3)2 chemical formula has yet to be found in nature.[9]

Significance

Carbonate rocks are significant for both human understanding of Earth's atmospheric and geologic history, in addition to providing humans with significant resources for current civilizational endeavors such as concrete.

Limestone and concrete

Limestone is often used in concrete as powder due to its cheap cost. During the formation of concrete, however, breakdown of limestone releases carbon dioxide and contributes significantly to the greenhouse effect.[11] There is significant amount of research studying the ideal quantity of calcium carbonate (derived from limestone) in concrete and if other compounds can be used to provide the same economic and structural integrity benefits.[11]

Paleoclimatology from carbonate minerals

Many forms of paleoclimatology exist whereby carbonate rocks can be used to determine past climate. Corals and sediments are well-known proxies for these reconstructions. Corals are marine organisms with calcium carbonate skeletons (rocks) which grow specific to oceanic conditions at the time of growth. Diagenesis refers to the process whereby sediments are being converted to sedimentary rock.[12] This includes biological activity, erosion, and other chemical reactions. Due to the strong correlation between diagenesis and seawater temperature, coral skeletons can be used as proxies for understanding past climate effects.[13] Specifically, the ratio of Strontium to Calcium in the aragonite of coral skeleton can be used, alongside other proxies like oxygen isotopic ratios, to reconstruct climate variability when the coral was growing. This is because Strontium will sometimes substitute for Calcium in the calcium carbonate molecule depending on temperature effects.

Similar to the concept for using compositional changes in coral skeletons as proxies for climate conditions, compositional changes in marine sediments can be used for the same purpose (and more). The changes in trace metal ratios from carbonate minerals found here can be used to determine patterns from parent [carbonate] rocks, too.[14]

See also

References

  1. ^ Regnet, J. B.; David, C.; Robion, P.; Menéndez, B. (2019-05-01). "Microstructures and physical properties in carbonate rocks: A comprehensive review". Marine and Petroleum Geology. 103: 366–376. Bibcode:2019MarPG.103..366R. doi:10.1016/j.marpetgeo.2019.02.022. ISSN 0264-8172. S2CID 135198700.
  2. ^ Fantle, Matthew (2020). "The Role of Diagenesis in Shaping the Geochemistry of the Marine Carbonate Record". Annual Review of Earth and Planetary Sciences. 48: 549–583. Bibcode:2020AREPS..48..549F. doi:10.1146/annurev-earth-073019-060021. S2CID 219738220.
  3. ^ Boggs, Sam (2006). Principles of sedimentology and stratigraphy (4th ed.). Upper Saddle River, N.J. pp. 177, 181. ISBN 0-13-154728-3.((cite book)): CS1 maint: location missing publisher (link)
  4. ^ Sulpis, Olivier; Agrawal, Priyanka; Wolthers, Mariette; Munhoven, Guy; Walker, Matthew; Middelburg, Jack J. (2022-03-01). "Aragonite dissolution protects calcite at the seafloor". Nature Communications. 13 (1): 1104. Bibcode:2022NatCo..13.1104S. doi:10.1038/s41467-022-28711-z. ISSN 2041-1723. PMC 8888755. PMID 35232971.
  5. ^ Sanz, E.; Ayora, C.; Carrera, J.; Tephly, T. R. (6 May 2011). "Calcite dissolution by mixing waters: geochemical modeling and flow-through experiments". Geologica Acta. 9 (1): 67–77. doi:10.1344/105.000001652.
  6. ^ "Mystery solved: Why seashells' mineral forms differently in seawater". MIT News | Massachusetts Institute of Technology. 2 March 2015. Retrieved 2023-03-17.
  7. ^ Regnet, J. B.; David, C.; Robion, P.; Menéndez, B. (2019-05-01). "Microstructures and physical properties in carbonate rocks: A comprehensive review". Marine and Petroleum Geology. 103: 366–376. Bibcode:2019MarPG.103..366R. doi:10.1016/j.marpetgeo.2019.02.022. ISSN 0264-8172. S2CID 135198700.
  8. ^ Dolomieu, D.G.D. (1791). "Sur un de pierres très-peu effervescentes avec les acides of phosphorescentes par la collision". Journal de Physique. 39: 3–10.
  9. ^ a b c d e f Gregg, Jay M.; Bish, David L.; Kaczmarek, Stephen E.; Machel, Hans G. (October 2015). Hollis, Cathy (ed.). "Mineralogy, nucleation and growth of dolomite in the laboratory and sedimentary environment: A review". Sedimentology. 62 (6): 1749–1769. doi:10.1111/sed.12202. S2CID 130135125.
  10. ^ Petrash, Daniel A.; Bialik, Or M.; Bontognali, Tomaso R. R.; Vasconcelos, Crisógono; Roberts, Jennifer A.; McKenzie, Judith A.; Konhauser, Kurt O. (2017-08-01). "Microbially catalyzed dolomite formation: From near-surface to burial". Earth-Science Reviews. 171: 558–582. Bibcode:2017ESRv..171..558P. doi:10.1016/j.earscirev.2017.06.015. ISSN 0012-8252.
  11. ^ a b Wang, Dehui; Shi, Caijun; Farzadnia, Nima; Shi, Zhenguo; Jia, Huangfei (2018-12-20). "A review on effects of limestone powder on the properties of concrete". Construction and Building Materials. 192: 153–166. doi:10.1016/j.conbuildmat.2018.10.119. ISSN 0950-0618. S2CID 139571589.
  12. ^ "Diagenesis - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2023-03-17.
  13. ^ Watanabe, Tsuyoshi; Juillet-Leclerc, Anne; Cuif, Jean-Pierre; Rollion-Bard, Claire; Dauphin, Yannicke; Reynaud, Stéphanie (2007-01-01), Kawahata, H.; Awaya, Y. (eds.), "Chapter 10 Recent Advances in Coral Biomineralization with Implications for Paleo-Climatology: A Brief Overview", Elsevier Oceanography Series, Global Climate Change and Response of Carbon Cycle in the Equatorial Pacific and Indian Oceans and Adjacent Landmasses, Elsevier, vol. 73, pp. 239–495, doi:10.1016/S0422-9894(06)73010-0, hdl:2115/56427, ISBN 9780444529480, S2CID 54844318, retrieved 2023-03-17
  14. ^ Martinez-Ruiz, F.; Kastner, M.; Gallego-Torres, D.; Rodrigo-Gámiz, M.; Nieto-Moreno, V.; Ortega-Huertas, M. (2015-01-01). "Paleoclimate and paleoceanography over the past 20,000 yr in the Mediterranean Sea Basins as indicated by sediment elemental proxies". Quaternary Science Reviews. 107: 25–46. Bibcode:2015QSRv..107...25M. doi:10.1016/j.quascirev.2014.09.018. ISSN 0277-3791.