View source for Magnesite

{{short description|Type of mineral}}
{{Distinguish|magnetite}}{{Lead too short|date=November 2024}}{{Infobox mineral
| name             = Magnesite
| category         = [[Carbonate mineral]]
| boxwidth         = 
| boxbgcolor       = 
| image            = Magnesite-121892.jpg
| imagesize        = 260px
| caption          = Magnesite crystals from Brazil (11.4 × 9.2 × 3.6 cm)
| formula          = MgCO<sub>3</sub>
| IMAsymbol   = Mgs<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3 |pages=291–320|doi=10.1180/mgm.2021.43 |bibcode=2021MinM...85..291W |s2cid=235729616 |doi-access=free}}</ref>
| molweight        = 
| strunz           = 5.AB.05
| system           = [[Trigonal]]
| class            = Hexagonal scalenohedral ({{overline|3}}m)<br />[[H-M symbol]]: ({{overline|3}} 2/m)
| symmetry         = ''R''{{overline|3}}c
| color            = Colorless, white, pale yellow, pale brown, faintly pink, lilac-rose
| habit            = Usually massive, rarely as rhombohedrons or hexagonal prisms
| twinning         = 
| cleavage         = [10{{overline|1}}1] perfect
| fracture         = Conchoidal
| tenacity         = Brittle
| mohs             = 3.5–4.5
| luster           = Vitreous
| refractive       = n<sub>ω</sub>=1.508 – 1.510 n<sub>ε</sub>=1.700
| opticalprop      = Uniaxial (−)
| birefringence    = 0.191
| pleochroism      = 
| streak           = white
| gravity          = 3.0–3.2
| melt             = 
| fusibility       = infusible
| diagnostic       = 
| solubility       = Effervesces in hot HCl
| diaphaneity      = Transparent to translucent
| other            = May exhibit pale green to pale blue fluorescence and phosphorescence under UV; triboluminescent
| references       = <ref name="Handbook">http://rruff.geo.arizona.edu/doclib/hom/magnesite.pdf Handbook of Mineralogy</ref><ref name="Mindat">http://www.mindat.org/min-2482.html Mindat.org</ref><ref name="Webmin">http://webmineral.com/data/Magnesite.shtml Webmineral data</ref><ref name="Klein">Klein, Cornelis and Cornelius S. Hurlbut, Jr., ''Manual of Mineralogy,'' Wiley, 20th ed., p. 332 {{ISBN|0-471-80580-7}}</ref>
}}

'''Magnesite''' is a [[mineral]] with the chemical formula {{chem|[[magnesium|Mg]]|[[carbon|C]]|[[oxygen|O]]|3}} ([[magnesium carbonate]]). [[Iron]], [[manganese]], [[cobalt]], and [[nickel]] may occur as admixtures, but only in small amounts.

== Occurrence ==
Magnesite occurs as veins in and an alteration product of [[ultramafic rocks]], [[serpentinite]] and other magnesium rich rock types in both contact and regional [[metamorphic rock|metamorphic]] terrains. These magnesites are often [[cryptocrystalline]] and contain silica in the form of [[opal]] or [[chert]].

Magnesite is also present within the [[regolith]] above ultramafic rocks as a secondary carbonate within soil and [[subsoil]], where it is deposited as a consequence of dissolution of magnesium-bearing minerals by carbon dioxide in groundwaters.[[File:Two_types_of_magnesite.jpg|thumb|339x339px|Difference between cryptocrystalline and crystalline magnesite.]]

Crystalline and cryptocrystalline magnesites have very different mineral structures. While crystalline magnesite has a well developed crystal structure, the cryptocrystalline magnesite is amorphous- mostly aggregate of fine grains. 

== Formation ==
Magnesite can be formed via [[talc carbonate]] [[metasomatism]] of [[peridotite]] and other ultramafic rocks. Magnesite is formed via carbonation of [[olivine]] in the presence of water and carbon dioxide at elevated temperatures and high pressures typical of the [[Greenschist#Greenschist facies|greenschist facies]].

Magnesite can also be formed via the carbonation of magnesium [[Serpentine group|serpentine]] (lizardite) via the following [[Metamorphic reaction|reaction]]:
:2 Mg<sub>3</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub> + 3 CO<sub>2</sub> → Mg<sub>3</sub>Si<sub>4</sub>O<sub>10</sub>(OH)<sub>2</sub> + 3 MgCO<sub>3</sub> + 3 H<sub>2</sub>O

However, when performing this reaction in the laboratory, the trihydrated form of [[magnesium carbonate]] (nesquehonite) will form at room temperature.<ref>Leitmeier, H.(1916): Einige Bemerkungen über die Entstehung von Magnesit und Sideritlagerstätten, ''Mitteilungen der Geologischen Gesellschaft in Wien'', vol.9, pp. 159–166.</ref> This very observation led to the postulation of a "dehydration barrier" being involved in the low-temperature formation of anhydrous magnesium carbonate.<ref>Lippmann, F. (1973): Sedimentary carbonate minerals.  Springer Verlag, Berlin, 228 p.</ref> Laboratory experiments with [[formamide]], a liquid resembling water, have shown how no such dehydration barrier can be involved. The fundamental difficulty to nucleate anhydrous magnesium carbonate remains when using this non-aqueous solution. Not cation dehydration, but rather the spatial configuration of carbonate anions creates the barrier in the low-temperature nucleation of magnesite.<ref>Xu, J; Yan, C.; Zhang, F.; Konishi, H., Xu, H. & Teng, H. H. (2013): Testing the cation-hydration effect on the crystallization of Ca – Mg- CO3 systems.  Proc. Natl. Acad. Sci. US, vol.110 (44), pp.17750-17755.</ref>

Magnesite has been found in modern sediments, caves and soils. Its low-temperature (around {{convert|40|C|disp=sqbr}}) formation is known to require alternations between precipitation and dissolution intervals.<ref>Deelman, J.C. (1999): [http://www.jcdeelman.demon.nl/dolomite/files/NeuesJahrbuchMineral.pdf "Low-temperature nucleation of magnesite and dolomite"], ''Neues Jahrbuch für Mineralogie'', Monatshefte, pp. 289–302.</ref><ref>Alves dos Anjos et al. (2011): Synthesis of magnesite at low temperature.  Carbonates and Evaporites, vol.26, pp.213–215. [https://www.researchgate.net/publication/226426989_Synthesis_of_magnesite_at_low_temperature]</ref><ref>Hobbs, F. W. C. and Xu, H. (2020): Magnesite formation through temperature and pH cycling as a proxy for lagoon and playa environments.  Geochimica et Cosmochimica Acta, vol.269, pp.101–116.</ref> The low-temperature formation of magnesite might well be of significance toward large-scale [[carbon sequestration]].<ref>Oelkers, E. H.; Gislason, S. R. and Matter, J. (2008): Mineral carbonation of CO2.  Elements, vol.4, pp.333–337.</ref> A major step forward toward the industrial production of magnesite at atmospheric pressure and a temperature of 316 K was described by Vandeginste.<ref>V. Vandeginste (2021): Effect of pH cycling and zinc ions on calcium and magnesium carbonate formation in saline fluids at low temperature.  Minerals, vol.11, pp.723–734.</ref><ref>V. Vandeginste, V.; Snell, O.; Hall, M. R.; Steer, E. and Vandeginste, A. (2019): Acceleration of dolomitization by zinc in saline waters.  Nature Communications, vol.10, 1851.</ref> In those experiments small additions of [[hydrochloric acid]] alternated periodically with additions of sodium carbonate solution. New was also the very short duration of only a few hours for the alternating dissolution and precipitation cycles.

Magnesite was detected in [[meteorite]] [[ALH84001]] and on planet [[Mars]] itself. Magnesite was identified on Mars using [[infrared spectroscopy]] from satellite orbit.<ref>Ehlmann, B. L. et al. (2008): Orbital identification of carbonate-bearing rocks on Mars. Science, vol.322, no.5909, pp.1828–1832.</ref> Near [[Jezero (crater)|Jezero Crater]], Mg-carbonates have been detected and reported to have formed in lacustrine environment prevailing there.<ref>{{Cite journal|last1=Horgan|first1=Briony H.N.|last2=Anderson|first2=Ryan B.|last3=Dromart|first3=Gilles|last4=Amador|first4=Elena S.|last5=Rice|first5=Melissa S.|date=March 2020|title=The mineral diversity of Jezero crater: Evidence for possible lacustrine carbonates on Mars|journal=Icarus|volume=339|article-number=113526|doi=10.1016/j.icarus.2019.113526|bibcode=2020Icar..33913526H|issn=0019-1035|doi-access=free}}</ref> Controversy still exists over the temperature of formation of [[Carbonates on Mars|these carbonates]]. Low-temperature formation has been suggested for the magnesite from the Mars-derived ALH84001 meteorite.<ref>McSween Jr, H. Y and Harvey, R. P.(1998): An evaporation model for formation of carbonates in the ALH84001 Martian meteorite.  International Geology Review, vol.49, pp.774–783.</ref><ref>Warren, P. H. (1998): Petrologic evidence for low-temperature, possibly flood evaporitic origin of carbonates in the ALH84001 meteorite.  Journal of Geophysical Research, vol.103, no.E7, 16759-16773.</ref>

Magnesium-rich [[olivine]] ([[forsterite]]) favors production of magnesite from peridotite. Iron-rich olivine ([[fayalite]]) favors production of magnetite-magnesite-silica compositions.

Magnesite can also be formed by way of metasomatism in [[skarn]] deposits, in [[Dolomite (mineral)|dolomitic]] [[limestone]]s, associated with [[wollastonite]], [[periclase]], and [[talc]].

Resistant to high temperature and able to withstand high pressure, magnesite has been proposed to be one of the major carbonate bearing phase in Earth's mantle<ref>{{Cite journal|last1=Isshiki|first1=Maiko|last2=Irifune|first2=Tetsuo|last3=Hirose|first3=Kei|last4=Ono|first4=Shigeaki|last5=Ohishi|first5=Yasuo|last6=Watanuki|first6=Tetsu|last7=Nishibori|first7=Eiji|last8=Takata|first8=Masaki|last9=Sakata|first9=Makoto|date=January 2004|title=Stability of magnesite and its high-pressure form in the lowermost mantle|journal=Nature|volume=427|issue=6969|pages=60–63|doi=10.1038/nature02181|pmid=14702083|bibcode=2004Natur.427...60I|s2cid=4351925|issn=0028-0836}}</ref> and possible carriers for deep carbon reservoirs.<ref>{{Cite journal|last1=Marcondes|first1=M. L.|last2=Justo|first2=J. F.|last3=Assali|first3=L. V. C.|date=2016-09-23|title=Carbonates at high pressures: Possible carriers for deep carbon reservoirs in the Earth's lower mantle|url=https://link.aps.org/doi/10.1103/PhysRevB.94.104112|journal=Physical Review B|volume=94|issue=10|article-number=104112|doi=10.1103/PhysRevB.94.104112|bibcode=2016PhRvB..94j4112M|url-access=subscription}}</ref> For similar reason, it is found in metamorphosed peridotite rocks in Central Alps, Switzerland<ref>{{Cite journal|last1=Ferry|first1=John M.|last2=Rumble|first2=Douglas|last3=Wing|first3=Boswell A.|last4=Penniston-Dorland|first4=Sarah C.|date=2005-04-22|title=A New Interpretation of Centimetre-scale Variations in the Progress of Infiltration-driven Metamorphic Reactions: Case Study of Carbonated Metaperidotite, Val d'Efra, Central Alps, Switzerland|journal=Journal of Petrology|volume=46|issue=8|pages=1725–1746|doi=10.1093/petrology/egi034|issn=1460-2415|doi-access=}}</ref> and high pressure [[Eclogitization|eclogitic]] rocks from Tianshan, China.<ref>{{Cite journal|last1=Zhang|first1=Lifei|last2=Ellis|first2=David J.|last3=Williams|first3=Samantha|last4=Jiang|first4=Wenbo|date=July 2002|title=Ultra-high pressure metamorphism in western Tianshan, China: Part II. Evidence from magnesite in eclogite|journal=American Mineralogist|volume=87|issue=7|pages=861–866|doi=10.2138/am-2002-0708|bibcode=2002AmMin..87..861Z|s2cid=101814149|issn=0003-004X}}</ref>

Magnesite can also precipitate in lakes in presence of bacteria either as hydrous Mg-carbonates or magnesite.<ref>{{Cite journal|last1=Mavromatis|first1=Vasileios|last2=Pearce|first2=Christopher R.|last3=Shirokova|first3=Liudmila S.|last4=Bundeleva|first4=Irina A.|last5=Pokrovsky|first5=Oleg S.|last6=Benezeth|first6=Pascale|last7=Oelkers|first7=Eric H.|date=2012-01-01|title=Magnesium isotope fractionation during hydrous magnesium carbonate precipitation with and without cyanobacteria|url=http://www.sciencedirect.com/science/article/pii/S0016703711006065|journal=Geochimica et Cosmochimica Acta|language=en|volume=76|pages=161–174|doi=10.1016/j.gca.2011.10.019|bibcode=2012GeCoA..76..161M|s2cid=15405751 |issn=0016-7037|url-access=subscription}}</ref><ref>{{Cite journal|last1=Shirokova|first1=Liudmila S.|last2=Mavromatis|first2=Vasileios|last3=Bundeleva|first3=Irina A.|last4=Pokrovsky|first4=Oleg S.|last5=Bénézeth|first5=Pascale|last6=Gérard|first6=Emmanuelle|last7=Pearce|first7=Christopher R.|last8=Oelkers|first8=Eric H.|date=2013-01-01|title=Using Mg Isotopes to Trace Cyanobacterially Mediated Magnesium Carbonate Precipitation in Alkaline Lakes|journal=Aquatic Geochemistry|language=en|volume=19|issue=1|pages=1–24|doi=10.1007/s10498-012-9174-3|bibcode=2013AqGeo..19....1S |s2cid=129854388|issn=1573-1421}}</ref>

=== Isotopic evidence ===
[[File:Isotopic structure of CO2 and MgCO3.pdf|thumb|267x267px|Isotopic structure of CO<sub>2</sub> and MgCO<sub>3</sub> illustrating singly and doubly substituted species of CO<sub>2</sub>.]]
Clumped isotopes have been used in interpreting conditions of magnesite formation and the isotopic composition of the precipitating fluid. Within ultramafic complexes, magnesites are found within veins and [[stockwork]]s in [[cryptocrystalline]] form as well as within carbonated peridotite units in [[crystal]]line form. These cryptocrystalline forms are mostly variably weathered and yield low temperature of formation.<ref>{{Cite journal |last1=Quesnel |first1=Benoît |last2=Boulvais |first2=Philippe |last3=Gautier |first3=Pierre |last4=Cathelineau |first4=Michel |last5=John |first5=Cédric M. |last6=Dierick |first6=Malorie |last7=Agrinier |first7=Pierre |last8=Drouillet |first8=Maxime |date=June 2016 |title=Paired stable isotopes (O, C) and clumped isotope thermometry of magnesite and silica veins in the New Caledonia Peridotite Nappe |url=https://hal-insu.archives-ouvertes.fr/insu-01294413/file/quesnel-GCA-2016.pdf |journal=Geochimica et Cosmochimica Acta |volume=183 |pages=234–249 |bibcode=2016GeCoA.183..234Q |doi=10.1016/j.gca.2016.03.021 |issn=0016-7037 |hdl-access=free |hdl=10044/1/33108}}</ref> On the other hand, coarse magnesites yield very high temperature indicating [[Hydrothermal circulation|hydrothermal]] origin. It is speculated that coarse high temperature magnesites are formed from mantle derived fluids whereas cryptocrystalline ones are precipitated by circulating meteoric water, taking up carbon from dissolved inorganic carbon pool, [[soil carbon]] and affected by disequilibrium isotope effects.

Magnesites forming in [[lake]]s and [[Endorheic basin|playa]] settings are in general enriched in heavy isotopes of C and O because of evaporation and CO<sub>2</sub> degassing. This reflects in the clumped isotope derived temperature being very low. These are affected by [[pH]] effect, biological activity as well as [[kinetic isotope effect]] associated with degassing. Magnesite forms as surface moulds in such conditions but more generally occur as hydrous Mg-carbonates since their precipitation is kinetically favored. Most of the times, they derive C from DIC or nearby ultramafic complexes (e.g., Altin Playa, British Columbia, Canada<ref>{{Cite journal |last1=Power |first1=Ian M. |last2=Harrison |first2=Anna L. |last3=Dipple |first3=Gregory M. |last4=Wilson |first4=Siobhan A. |last5=Barker |first5=Shaun L.L. |last6=Fallon |first6=Stewart J. |date=June 2019 |title=Magnesite formation in playa environments near Atlin, British Columbia, Canada |journal=Geochimica et Cosmochimica Acta |volume=255 |pages=1–24 |bibcode=2019GeCoA.255....1P |doi=10.1016/j.gca.2019.04.008 |issn=0016-7037 |s2cid=146307705}}</ref>).

Magnesites in metamorphic rocks, on the other hand, indicate very high temperature of formation. Isotopic composition of parental fluid is also heavy- generally metamorphic fluids. This has been verified by fluid inclusion derived temperature as well as traditional O isotope thermometry involving co-precipitating quartz-magnesite.

Often, magnesite records lower clumped isotope temperature than associated dolomite, calcite.<ref>{{Cite journal |last1=Streit |first1=Elisabeth |last2=Kelemen |first2=Peter |last3=Eiler |first3=John |date=2012-06-17 |title=Coexisting serpentine and quartz from carbonate-bearing serpentinized peridotite in the Samail Ophiolite, Oman |journal=Contributions to Mineralogy and Petrology |volume=164 |issue=5 |pages=821–837 |bibcode=2012CoMP..164..821S |doi=10.1007/s00410-012-0775-z |issn=0010-7999 |s2cid=12595278}}</ref> The reason might be that calcite, dolomite form earlier at higher temperature (from mantle like fluids) which increases Mg/Ca ratio in the fluid sufficiently so as to precipitate magnesite. As this happens with increasing time, fluid cools, evolves by mixing with other fluids and when it forms magnesite, it decreases its temperature. So the presence of associated carbonates have a control on magnesite isotopic composition.

Origin of [[Carbonates on Mars|Martian carbonates]] can be deconvolved{{Clarify|date=October 2025}} with the application of clumped isotope. Source of the CO<sub>2</sub>, climatic-hydrologic conditions on Mars could be assessed from these rocks. Recent study has shown (implementing [[Clumped isotopes|clumped isotope thermometry]]) that carbonates in [[Allan Hills 84001|ALH84001]] indicate formation at low temperature evaporative condition from subsurface water and derivation of CO<sub>2</sub> from Martian atmosphere.<ref name="Halevy-2011">{{Cite journal |last1=Halevy |first1=Itay |last2=Fischer |first2=Woodward W. |last3=Eiler |first3=John M. |date=2011-10-11 |title=Carbonates in the Martian meteorite Allan Hills 84001 formed at 18 ± 4 °C in a near-surface aqueous environment |journal=Proceedings of the National Academy of Sciences |language=en |volume=108 |issue=41 |pages=16895–16899 |doi=10.1073/pnas.1109444108 |issn=0027-8424 |pmc=3193235 |pmid=21969543 |doi-access=free}}</ref>

== Uses ==

=== Refractory material ===
[[File:Dyed magnesite beads.jpg|thumb|upright|Polished and Dyed magnesite beads]]
[[File:Magnesite of Salem.jpg|thumb|Magnesite of [[Salem, Tamil Nadu|Salem]]]]
Similar to the production of lime, magnesite can be burned in the presence of charcoal to produce [[MgO]], which, in the form of a mineral, is known as [[periclase]]. Large quantities of magnesite are burnt to make [[magnesium oxide]]: an important [[refractory]] (heat-resistant) material used as a lining in [[blast furnace]]s, [[kiln]]s and [[incinerators]].

Calcination temperatures determine the reactivity of resulting oxide products and the classifications of light burnt and [[Roasting (metallurgy)|dead burnt]] refer to the surface area and resulting reactivity of the product (this is typically determined by an industry metric of the iodine number).

''''Light burnt'''<nowiki/>' product generally refers to calcination commencing at 450&nbsp;°C and proceeding to an upper limit of 900&nbsp;°C – which results in good surface area and reactivity.

Above 900&nbsp;°C, the material loses its reactive crystalline structure and reverts to the chemically inert ''''dead-burnt'''<nowiki/>' product- which is preferred for use in refractory materials such as furnace linings.

In fire assay, magnesite cupels can be used for [[cupellation]], as the magnesite cupel will resist the high temperatures involved.

=== Other uses ===
Magnesite can also be used as a binder in flooring material ([[magnesite screed]]).<ref>Information about [http://www.wcdeckwaterproofing.com/what-you-need-to-know-about-magnesite-floors/ magnesite flooring], West Coast Deck Water Proofing</ref> Furthermore, it is being used as a catalyst and filler in the production of [[synthetic rubber]] and in the preparation of magnesium chemicals and fertilizers.

Research is proceeding to evaluate the practicality of sequestering the [[greenhouse gas]] carbon dioxide in magnesite on a large scale.<ref>{{Cite web|title = Scientists find way to make mineral which can remove CO<sub>2</sub> from atmosphere|url = https://phys.org/news/2018-08-scientists-mineral-co2-atmosphere.html|website = phys.org/news|access-date = 2018-08-15}}</ref> This has focused on peridotites from [[ophiolite]]s (obducted mantle rocks on crust) where magnesite can be created by letting carbon dioxide react with these rocks. Some progress has been made in ophiolites from Oman.<ref>{{Cite journal|last1=Kelemen|first1=Peter B.|last2=Matter|first2=Juerg|last3=Streit|first3=Elisabeth E.|last4=Rudge|first4=John F.|last5=Curry|first5=William B.|last6=Blusztajn|first6=Jerzy|date=2011-05-30|title=Rates and Mechanisms of Mineral Carbonation in Peridotite: Natural Processes and Recipes for Enhanced, in situ CO<sub>2</sub> Capture and Storage|journal=Annual Review of Earth and Planetary Sciences|volume=39|issue=1|pages=545–576|doi=10.1146/annurev-earth-092010-152509|bibcode=2011AREPS..39..545K|issn=0084-6597}}</ref> But the major problem is that these artificial processes require sufficient porosity-permeability so that the fluids can flow but this is hardly the case in [[peridotite]]s.

=== Artworks ===
Magnesite can be cut, drilled, and polished to form beads that are used in jewelry-making. Magnesite beads can be dyed into a broad spectrum of bold colors, including a light blue color that mimics the appearance of [[turquoise]].

The Japanese-American artist [[Isamu Noguchi]] used magnesite as a sculptural material for some of his artworks.<ref>{{cite web |title=Ford Fountain for the New York World's Fair |url=https://archive.noguchi.org/Detail/artwork/194 |website=The Noguchi Museum |access-date=2022-01-02}}</ref>

== Occupational safety and health ==
People can be exposed to magnesite in the workplace by inhaling it, skin contact, and eye contact.

=== United States ===
The [[Occupational Safety and Health Administration]] (OSHA) has set the legal limit ([[permissible exposure limit]]) for magnesite exposure in the workplace as 15&nbsp;mg/m<sup>3</sup> total exposure and 5&nbsp;mg/m<sup>3</sup> respiratory exposure over an 8-hour workday. The [[National Institute for Occupational Safety and Health]] (NIOSH) has set a [[recommended exposure limit]] (REL) of 10&nbsp;mg/m<sup>3</sup> total exposure and 5&nbsp;mg/m<sup>3</sup> respiratory exposure over an 8-hour workday.<ref>{{Cite web|title = CDC – NIOSH Pocket Guide to Chemical Hazards – Magnesite|url = https://www.cdc.gov/niosh/npg/npgd0373.html|website = www.cdc.gov|access-date = 2015-11-19}}</ref>

== References ==
{{Reflist}}
* Smithsonian Rock and Gem {{ISBN|0-7566-0962-3}}

{{Ores}}
{{Authority control}}

[[Category:Magnesium minerals]]
[[Category:Carbonate minerals]]
[[Category:Calcite group]]
[[Category:Trigonal minerals]]
[[Category:Minerals in space group 167]]
[[Category:Luminescent minerals]]
[[Category:Evaporite]]