Dolomitization

{{Short description|Geological process forming dolomite}} [[File:Bedded_dolomitized_limestone_close-up_view,_Tanay,_Rizal,_Philippines.jpg|thumb|Sedimentary layers of dolomitized limestone in an outcrop in the Philippines]] [[File:Dolomites_mountains_of_northern_Italy,_Sella_group.JPG|thumb|Mountains with dolomite rock formations in northern Italy]] '''Dolomitization''' is a geological process where magnesium ions replace calcium ions in the mineral calcite, resulting in the formation of dolomite.

Dolomitization conditions are present in Abu Dhabi, the Mediterranean Sea, and some Brazilian hypersaline lagoons (most notably Lagoa Vermelha Lagoon).<ref name=":0">{{Cite journal |last1=Chen |first1=Chao |last2=Zhong |first2=Hanting |last3=Wang |first3=Xinyu |last4=Ning |first4=Meng |last5=Wang |first5=Xia |last6=Ge |first6=Yuzhu |last7=Wang |first7=Han |last8=Tang |first8=Ruifeng |last9=Hou |first9=Mingcai |date=2023 |title=Thermodynamic and Kinetic Studies of Dolomite Formation: A Review |journal=Minerals |language=en |volume=13 |issue=12 |pages=1479 |doi=10.3390/min13121479 |doi-access=free |bibcode=2023Mine...13.1479C |issn=2075-163X}}</ref> The areas where dolomitization take place are limited, as modern seawater is less suited to dolomite formation. This is evident in the noticeable decrease in modern dolomite depositions compared to older depositions.<ref name=":0" /> Dolomitization involves substantial recrystallization which can be described by the following equation:<ref name=":0" />

:2 CaCO_3(calcite) + Mg²⁺ {{eqm}} CaMg(CO₃)_2(dolomite) + Ca²⁺

The conditions for dolomitization depend on several factors, including temperature, saturation state, Mg:Ca ratio, and the presence of inhibitors and microorganisms.<ref name=":0" />Microorganisms allow the precipitation of preliminary dolomite stages through certain metabolic pathways.<ref name=":2">{{Cite journal |last1=Petrash |first1=Daniel A. |last2=Bialik |first2=Or M. |last3=Bontognali |first3=Tomaso R. R. |last4=Vasconcelos |first4=Crisógono |last5=Roberts |first5=Jennifer A. |last6=McKenzie |first6=Judith A. |last7=Konhauser |first7=Kurt O. |date=2017 |title=Microbially catalyzed dolomite formation: From near-surface to burial |url=https://linkinghub.elsevier.com/retrieve/pii/S0012825217301885 |journal=Earth-Science Reviews |language=en |volume=171 |pages=558–582 |doi=10.1016/j.earscirev.2017.06.015|bibcode=2017ESRv..171..558P |url-access=subscription |hdl=11104/0297906 |hdl-access=free }}</ref> Dolomitization occurs in kinetic intermediate stages. First, a protodolomite is formed, then it is transformed to dolomite in a dissolution-recrystallization process.<ref name=":0" />

== Microbes and dolomitization == [[File:Microbial_mats,_Laguna_Negra.jpg|thumb|Microbial mats in Laguna Negra Lake, Argentina]] Microorganisms allow the precipitation of dolomite by raising alkalinity and increasing pH buffering through metabolic pathways.<ref name=":2" /> Higher alkalinity leads to the transformation of {{chem2|HCO3-}} to {{chem2|CO3(2-)}}, which allows for interruption of the magnesium hydrate to form dolomite.<ref name=":2" /> In most modern dolomitization sites, ‘microbial mats’ are present. Microbial mats are populations of microorganisms and their associated extracellular polymeric substances (EPS).<ref name=":2" /> EPS groups have been shown to be critical for the formation of stable nuclei in the early stages of dolomite formation, thus increasing dolomite saturation.<ref name=":2" />

There are several key metabolic pathways for dolomite formation. These include microbial sulfate reduction, aerobic heterotrophy, chemotrophic sulfide oxidation, and the coupling of methanogenesis and anaerobic methane oxidation.<ref name=":2" />

=== Microbial sulfate reduction === Microbial sulfate reduction reduces sulfate concentrations in seawaters, effectively removing one of the key inhibiting factors to dolomitization. This process also results in the production of sulfide ions, which promote Mg2+ dehydration.<ref name=":2" />

=== Aerobic heterotrophy === Aerobic heterotrophy increases alkalinity by using nitrogenized organic matter as an electron acceptor.<ref name=":2" />

=== Chemotrophic sulfate oxidation === In chemotrophic sulfide oxidation, organisms use sulfide for CO₂ fixation, leading to seawater conditions that are thermodynamically favorable for dolomite formation.<ref name=":2" />

=== Methanogenesis and anaerobic methane oxidation === Methanogenic archaea support dolomite formation through the coupling of methanogenesis and anaerobic methane oxidation. This coupling increases {{chem2|CO3(2-)}} concentration and sulfate reduction, leading to increased dolomite saturation.<ref name=":2" />

== Inhibitors to dolomitization ==

=== Mg²⁺ hydration === Mg²⁺ hydration inhibits dolomitization because in ambient conditions, the closeness of water molecules and magnesium ions reduces the ease at which Mg²⁺ enters the carbonate lattice.<ref name=":0" /> This can be partially overcome with the help of microorganisms. <ref name=":0" />

=== Sulfate presence === Presence of sulfate results in tight ion pairs of {{chem2|SO4(2-)}} and Mg²⁺, preventing magnesium from joining the dolomite lattice.<ref name=":0" /> It has been suggested that this effect is less strong at lower temperatures.<ref name=":0" />

=== Low carbonate activity === In modern seawater, carbonate ion concentration is lower compared to Mg²⁺ concentration.<ref name=":0" />

== The dolomite problem == The 'dolomite problem' refers to the fact that dolomite is difficult to precipitate experimentally in ambient conditions and that it precipitates at a starkly lower rate in the modern ocean compared to geologic history (despite modern seawater’s supersaturation of dolomite).<ref name=":0" /> Ordered dolomite can be produced in a lab at high temperatures.<ref name=":1">{{Cite journal |last1=Kim |first1=Joonsoo |last2=Kimura |first2=Yuki |last3=Puchala |first3=Brian |last4=Yamazaki |first4=Tomoya |last5=Becker |first5=Udo |last6=Sun |first6=Wenhao |date=2023-11-24 |title=Dissolution enables dolomite crystal growth near ambient conditions |url=https://www.science.org/doi/10.1126/science.adi3690 |journal=Science |language=en |volume=382 |issue=6673 |pages=915–920 |doi=10.1126/science.adi3690 |pmid=37995221 |bibcode=2023Sci...382..915K |issn=0036-8075|url-access=subscription }}</ref> Past laboratory experiments have sought to produce dolomite in ambient conditions, however the resulting products lacked cation ordering, indicating the product was not strictly dolomite, but disordered dolomite.<ref name=":0" /> A recent study claims to have produced an ordered dolomite by fluctuating between supersaturated and undersaturated states.<ref name=":1" /> In this proposed mechanism, initial dolomite precipitation results in a cation-disordered surface. Subsequent undersaturation dissolves the disordered crystals and allows for increased order when precipitated again.<ref name=":1" /> Modern dolomitization zones frequently experience alternating periods with and without rain, which in turn allows for fluctuation in saturation state.<ref name=":1" /> Saturation fluctuations can increase dolomite precipitation up to 7 orders of magnitude.<ref name=":1" />

==See also== * Diagenesis * Dolomite (rock)

==References== <references /> Category:Dolomite group Category:Calcite group Category:Geological processes Category:Inorganic reactions Category:Mineralogy Category:Dolomite (rock)