Carbonate rocks are a class ofsedimentary rocks composed primarily ofcarbonate minerals. The two major types arelimestone, which is composed ofcalcite oraragonite (different crystal forms of CaCO3), anddolomite rock (also known as dolostone), which is composed ofdolomite (CaMg(CO3)2). They are usuallyclassified on the basis of texture and grain size.[1] Importantly, carbonate rocks can exist as metamorphic and igneous rocks, too. When recrystallized carbonate rocks aremetamorphosed,marble is created. Rareigneous carbonate rocks even exist asintrusivecarbonatites and, even rarer, there existsvolcanic carbonatelava.
Carbonate rocks are also crucial components to understandinggeologic history due to processes such asdiagenesis in which carbonates undergo compositional changes based onkinetic effects.[2] Thecorrelation between this compositional change andtemperature can be exploited to reconstruct past climate as is done inpaleoclimatology. Carbonate rocks can also be used for understanding various other systems as described below.
Limestone is the most common carbonate rock[3] and is a sedimentary rock made ofcalcium carbonate with two mainpolymorphs: calcite and aragonite. While the chemical composition of these two minerals is the same, theirphysical properties differ significantly due to their differentcrystalline form. The most common form found in the seafloor is calcite, while aragonite is more found in biological organisms.[4]
Calcite can be eitherdissolved bygroundwater orprecipitated by groundwater,[5] depending on several factors including the watertemperature,pH, and dissolvedion concentrations. Calcite exhibits an unusual characteristic calledretrograde solubility in which it becomes lesssoluble 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.
Compared to calcite,aragonite is lessstable and moresoluble,[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 calciteprecipitation.[1] Often this inhibited precipitation occurs in biology where organisms aim to precipitate calcium carbonate for their structural features such as for skeleton andshells.
The discovery of dolomite rock, ordolostone, was first published in 1791[7] and has been found across theEarth's crust from various differenttime periods.[8] Because the rock is made ofcalcium,magnesium, andcarbonate ions, the mineral crystalline structure can be visualized similar to calcite andmagnesite.[9] Due to this composition, the dolomite mineral present in dolostone can be classified by varying degree of calcium inclusion, and occasionally iron, too.[8]
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 fromsynthesis.[8] This dolomite, when formed in the oceans, can prove to bemetastable.[8] The resultant structure of this mineral presents minimal differences from regular dolomite likely as a result of formation after initial crystal growth.[8]
Iron-rich dolomite, or ferroan dolomite, is dolomite which contains significant trace levels of iron. Due to the similarionic radii ofiron(II) andmagnesium, iron(II) can easily substitute magnesium to form ferroan dolomite;manganese can also substitute this atom. The result can be defined asankerite. The exact delineation between which minerals are considered ferroan dolomite and which are ankerite is unclear. Ankerite with the "pure" CaFe(CO3)2chemical formula has yet to be found innature.[8]
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 asconcrete.
Limestone is often used in concrete as powder due to its cheap cost. During the formation of concrete, however, breakdown of limestone releasescarbon dioxide and contributes significantly to thegreenhouse effect.[10] 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.[10]
Many forms ofpaleoclimatology exist whereby carbonate rocks can be used to determine past climate. Corals and sediments are well-knownproxies 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.[citation needed] This includes biological activity, erosion, and other chemical reactions. Due to the strongcorrelation between diagenesis andseawater temperature, coral skeletons can be used asproxies for understanding past climate effects.[11] Specifically, the ratio ofStrontium toCalcium in the aragonite of coral skeleton can be used, alongside other proxies likeoxygen isotopic ratios, to reconstruct climate variability when the coral was growing. This is becauseStrontium will sometimes substitute forCalcium in the calcium carbonate molecule depending on temperature effects.
Similar to the concept for using compositional changes in coral skeletons asproxies for climate conditions, compositional changes in marine sediments can be used for the same purpose (and more). The changes intrace metal ratios from carbonate minerals found here can be used to determine patterns fromparent [carbonate] rocks, too.[12]
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