Editing Calcite (section)

== Formation processes ==
[[File:P-T Diagram for CaCO3.svg|left|frameless|400x400px]]
Calcite formation can proceed by several pathways, from the classical [[terrace ledge kink model]]<ref>{{cite journal |last1=De Yoreo |first1=J. J. |last2=Vekilov |first2=P. G. |title=Principles of crystal nucleation and growth |journal=Reviews in Mineralogy and Geochemistry |volume=54 |pages=57–93 |date=2003 |issue=1 |doi=10.2113/0540057 |bibcode=2003RvMG...54...57D |citeseerx=10.1.1.324.6362 }}</ref> to the crystallization of poorly ordered precursor phases like [[amorphous calcium carbonate]] (ACC) via an [[Ostwald ripening]] process, or via the agglomeration of nanocrystals.<ref>{{cite journal |last1=De Yoreo |first1=J. |last2=Gilbert |first2=P.U. |last3=Sommerdijk |first3=N. A. J. M. |last4=Penn |first4=R. L. |last5=Whitelam |first5=S. |last6=Joester |first6=D. |last7=Zhang |first7=H. |last8=Rimer |first8=J. D. |last9=Navrotsky |first9=A. |last10=Banfield |first10=J. F. |last11=Wallace |first11=A. F. |last12=Michel |first12=F. M. |last13=Meldrum |first13=F. C. |last14=Cölfen |first14=H. |last15=Dove |first15=P. M. |title=Crystallization by particle attachment in synthetic, biogenic, and geologic environments |journal=Science |date=2015 |volume=349 |issue=6247 |page=aaa6760 |doi=10.1126/science.aaa6760 |pmid=26228157|s2cid=14742194 |url=http://eprints.whiterose.ac.uk/88828/1/Jim%20Science%20Review%20Unformatted%205-8-15.pdf }}</ref>

The crystallization of ACC can occur in two stages. First, the ACC nanoparticles rapidly dehydrate and crystallize to form individual particles of [[vaterite]]. Second, the vaterite transforms to calcite via a [[Solvation|dissolution]] and [[Precipitation (chemistry)|reprecipitation]] mechanism, with the [[reaction rate]] controlled by the [[surface area]] of a calcite crystal.<ref name=Geb>{{cite journal |doi=10.1039/C3CS60451A|title=Pre-nucleation clusters as solute precursors in crystallisation |year=2014 |last1=Gebauer |first1=Denis |last2=Kellermeier |first2=Matthias |last3=Gale |first3=Julian D. |last4=Bergström |first4=Lennart |last5=Cölfen |first5=Helmut |journal=Chem. Soc. Rev. |volume=43 |issue=7 |pages=2348–2371 |pmid=24457316 |s2cid=585569 |hdl=20.500.11937/6133 |hdl-access=free }}</ref> The second stage of the reaction is approximately 10 times slower.

However, crystallization of calcite has been observed to be dependent on the starting [[pH]] and [[concentration]] of [[magnesium]] in solution. A neutral starting pH during mixing promotes the direct transformation of ACC into calcite without a vaterite intermediate. But when ACC forms in a solution with a [[Base (chemistry)|basic]] initial pH, the transformation to calcite occurs via [[Metastability|metastable]] vaterite, following the pathway outlined above.<ref name=Geb/> Magnesium has a noteworthy effect on both the stability of ACC and its transformation to crystalline CaCO<sub>3</sub>, resulting in the formation of calcite directly from ACC, as this ion destabilizes the structure of vaterite.

[[Epitaxy|Epitaxial]] overgrowths of calcite precipitated on [[Weathering|weathered]] [[Cleavage (crystal)|cleavage]] surfaces have morphologies that vary with the type of weathering the substrate experienced: growth on physically weathered surfaces has a shingled morphology due to Volmer-Weber growth, growth on chemically weathered surfaces has  characteristics of Stranski-Krastanov growth, and growth on pristine cleavage surfaces has characteristics of Frank - van der Merwe growth.<ref>{{Cite journal |last1=Acosta |first1=Marisa D. |last2=Olsen |first2=Ellen K. |last3=Pickerel |first3=Molly E. |date=2023-09-20 |title=Surface roughness and overgrowth dynamics: The effect of substrate micro-topography on calcite growth and Sr uptake |journal=Chemical Geology |language=en |volume=634 |pages=121585 |doi=10.1016/j.chemgeo.2023.121585 |issn=0009-2541|doi-access=free }}</ref> These differences are apparently due to the influence of surface roughness on layer coalescence dynamics.

Calcite may form in the subsurface in response to [[microorganism]] activity, such as [[sulfate]]-dependent [[anaerobic oxidation of methane]], where [[methane]] is [[Redox|oxidized]] and sulfate is [[Redox|reduced]], leading to precipitation of calcite and [[pyrite]] from the produced [[bicarbonate]] and [[sulfide]]. These processes can be traced by the specific [[carbon isotope]] composition of the calcites, which are extremely depleted in the [[Carbon-13|<sup>13</sup>C]] isotope, by as much as −125 per mil [[Δ13C#Reference standard|PDB]] (δ<sup>13</sup>C).<ref>{{cite journal |journal=Nature Communications |volume=6 |pages=7020 |year=2015 |title=Extreme <sup>13</sup>C depletion of carbonates formed during oxidation of biogenic methane in fractured granite |author=Drake, H. |author2=Astrom, M. E. |author3=Heim, C. |author4=Broman, C. |author5=Astrom, J. |author6=Whitehouse, M. |author7=Ivarsson, M. |author8=Siljestrom, S. |author9=Sjovall, P. |pmid=25948095 |pmc=4432592 |doi=10.1038/ncomms8020|bibcode = 2015NatCo...6.7020D }}</ref>