{{Short description|Hydrated hydroxy-carbonate mineral of magnesium}} {{Infobox mineral | name = Hydromagnesite | category = Carbonate mineral | boxwidth = | boxbgcolor = | image = Hydromagnesite balloon.jpg | imagesize = | caption = Hydromagnesite balloon in [[Jewel Cave National Monument|Jewel Cave]] | formula = {{chem2|Mg5(CO3)4(OH)2*4H2O}} | IMAsymbol = Hmgs<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 = 467.64 g/mol | strunz = 5.DA.05 | dana = 16b.07.01.01 | system = [[Monoclinic]] | class = Prismatic (2/m) <br/><small>(same [[H-M symbol]])</small> | symmetry = ''P2''<sub>1</sub>/c | color = Colorless, white | colour = | habit = Acicular and as encrustations; pseudo-orthorhombic | twinning = Polysynthetic lamellar on {100} | cleavage = {010} Perfect, {100} Distinct | fracture = Uneven | tenacity = Brittle | mohs = 3.5 | luster = Vitreous, silky, pearly, earthy | streak = White | diaphaneity = Transparent to translucent | gravity = 2.16–2.2 | density = | polish = | opticalprop = Biaxial (+) | refractive = n<sub>α</sub> = 1.523 n<sub>β</sub> = 1.527 n<sub>γ</sub> = 1.545 | birefringence = δ = 0.022 | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence= Fluorescent, short UV=green, long UV=bluish white. | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = | alteration = | references = <ref name=Handbook>[http://rruff.geo.arizona.edu/doclib/hom/hydromagnesite.pdf Handbook of Mineralogy]</ref><ref name=Webmin>[http://www.webmineral.com/data/Hydromagnesite.shtml Webmineral data]</ref><ref name=Mindat>[http://www.mindat.org/show.php?id=1979&ld=1#themap Mindat]</ref> }} '''Hydromagnesite''' is a hydrated [[magnesium]] [[carbonate mineral]] with the formula {{chem2|Mg5(CO3)4(OH)2*4H2O}}.

It generally occurs associated with the weathering products of magnesium containing minerals such as [[serpentine group|serpentine]] or [[brucite]]. It occurs as incrustations and vein or fracture fillings in [[ultramafic rock]]s and [[serpentinite]]s, and occurs in [[hydrothermal]]ly altered [[Dolomite (rock)|dolomite]] and [[marble]]. Hydromagnesite commonly appears in [[cave]]s as [[speleothem]]s and "[[moonmilk]]", deposited from water that has seeped through magnesium rich rocks. It is the most common cave carbonate after [[calcite]] and [[aragonite]].<ref name=Handbook/> The mineral thermally decomposes,<ref name=Rev1>{{cite journal|last=Hollingbery|first=LA|author2=Hull TR|title=The Thermal Decomposition of Huntite and Hydromagnesite - A Review|journal=Thermochimica Acta|year=2010|volume=509|issue=1–2|pages=1–11|url=http://clok.uclan.ac.uk/1139/|doi=10.1016/j.tca.2010.06.012}}</ref><ref name=Therm1>{{cite journal|last=Hollingbery|first=LA|author2=Hull TR|title=The Thermal Decomposition of Natural Mixtures of Huntite and Hydromagnesite|journal=Thermochimica Acta|year=2012|volume=528|pages=45–52|url=http://clok.uclan.ac.uk/3414|doi=10.1016/j.tca.2011.11.002|bibcode=2012TcAc..528...45H }}</ref> over a temperature range of approximately 220&nbsp;°C to 550&nbsp;°C, releasing water and carbon dioxide leaving a magnesium oxide residue.

Hydromagnesite was first described in 1836 for an occurrence in [[Hoboken, New Jersey]].<ref name=Webmin/>

[[Stromatolite]]s in an [[alkaline]] ([[pH]] greater than 9) freshwater lake ([[Lake Salda|Salda Gölü]]) in southern [[Turkey]] are made of hydromagnesite precipitated by [[diatom]]s and [[cyanobacteria]].<ref>{{cite journal |last1= Braithwaite |first1= C. |last2= Zedef |first2= Veysel |year= 1996 |title= Living hydromagnesite stromatolites from Turkey |journal= Sedimentary Geology |volume= 106 |issue= 3–4 |page= 309 |doi= 10.1016/S0037-0738(96)00073-5 |bibcode= 1996SedG..106..309B}}</ref>

[[File:HydromagnesiteSEM.jpg|thumbnail|left|SEM micrograph of hydromagnesite showing platy crystal morphology. Sample was collected from the hydromagnesite-magnesite playas near Atlin, British Columbia, Canada.<ref name="sciencedirect.com">{{cite journal |last1= Power |first1= I.M. |last2= Wilson |first2= S.A. |last3= Thom |first3= J.M. |last4= Dipple |first4= G.M. |last5= Gabites |first5= J.E. |last6= Southam |first6= G. |title= The hydromagnesite playas of Atlin, British Columbia, Canada: A biogeochemical model for CO<sub>2</sub> sequestration |journal= Chemical Geology |volume= 206 |issue= 3–4 |pages= 302–316 |doi= 10.1016/j.chemgeo.2009.01.012|bibcode= 2009ChGeo.260..286P |year= 2009 |s2cid= 128900805 }}</ref>]] Microbial deposition of hydromagnesite is also reported from [[Dry lake|playa]]s in [[British Columbia]].<ref>R. W. Renaut, ''Recent Mamgnesite-Hydromagnesite sedimesntation in Playa Basins of the Caribou Plateau'', {{cite web |url=http://www.em.gov.bc.ca/DL/GSBPubs/GeoFldWk/1990/279-288-renaut.pdf |title=Archived copy |access-date=2009-08-13 |url-status=dead |archive-url=https://web.archive.org/web/20041122123229/http://www.em.gov.bc.ca/DL/GSBPubs/GeoFldWk/1990/279-288-renaut.pdf |archive-date=2004-11-22 }} British Columbia Geologic Survey</ref> The hydromagnesite-magnesite playas near Atlin, British Columbia are some of the most studied deposits of hydromagnesite. These deposits have been characterized in the context of a biogeochemical model for [[CO2 sequestration|CO<sub>2</sub> sequestration]].<ref name="sciencedirect.com"/>

One of the largest deposits of hydromagnesite exists in Greece.<ref>{{cite journal|last=Georgiades|first=GN|title=Huntite-hydromagnesite production and applications|journal=Proceedings of the 12th Industrial Minerals Congress|year=1996|pages=57–60}}</ref> It consists of a natural mixture with [[huntite]]. Local people have used the white mineral as a source of material for whitewashing buildings for centuries. In the mid 20th century the minerals, ground to a fine powder, found use as a filler for rubber shoe soles. The locals used the granite mills designed for grinding wheat. Commercial exploitation of the minerals began in the late 70s and early 80s with the mineral being exported worldwide. The Greek deposit is still operated commercially, although the world's largest commercially operated reserves are in Turkey.

==Uses==

Its most common industrial use is as a mixture with [[huntite]] as a [[flame retardant]] or [[fire retardant]] additive for [[polymers]].<ref name=Rev2>{{cite journal|last=Hollingbery|first=LA|author2=Hull TR|title=The Fire Retardant Behaviour of Huntite and Hydromagnesite - A Review|journal=Polymer Degradation and Stability|year=2010|volume=95|issue=12|pages=2213–2225|url=http://clok.uclan.ac.uk/1432|doi=10.1016/j.polymdegradstab.2010.08.019}}</ref><ref name=Fire1>{{cite journal|last=Hollingbery|first=LA|author2=Hull TR|title=The Fire Retardant Effects Huntite in Natural Mixtures with Hydromagnesite|journal=Polymer Degradation and Stability|year=2012|volume=97|issue=4|pages=504–512|url=http://clok.uclan.ac.uk/3420/|doi=10.1016/j.polymdegradstab.2012.01.024}}</ref><ref name=Fire2>{{cite journal|last=Hull|first=TR|author2=Witlowski A |author3=Hollingbery LA |title=Fire Retardant Action of Mineral Fillers|journal=Polymer Degradation and Stability|year=2011|volume=96|issue=8|pages=1462–1469|url=http://clok.uclan.ac.uk/2963|doi=10.1016/j.polymdegradstab.2011.05.006|s2cid=96208830 }}</ref> Hydromagnesite decomposes endothermically,<ref name = Rev1 /><ref name = Therm1 /> giving off water and carbon dioxide, leaving a magnesium oxide residue. The initial decomposition begins at about 220&nbsp;°C making it ideal for use as a filler in polymers and giving it certain advantages over the most commonly used fire retardant, [[aluminium hydroxide]].<ref>Rothon. R., Particulate-Filled Polymer Composites, 2nd Edition, 2003</ref> Syntheic hydromagnesite is known as [[magnesium carbonate|light magnesium carbonate]] due to its low [[bulk density]].

==Thermal decomposition== Hydromagnesite thermally decomposes in three stages releasing water and carbon dioxide.<ref name=Rev1 /><ref name=Therm1 />

The first stage starting at about 220&nbsp;°C, is the release of the four molecules of water of crystallisation. This is followed at about 330&nbsp;°C by the decomposition of the hydroxide ion to a further molecule of water. Finally, at about 350&nbsp;°C carbon dioxide begins to be released. The release of the carbon dioxide can be further broken down into two stages depending on the rate of heating.<ref name=Therm1/>

==References==

{{Reflist}} {{Commons}} [[File:Hydromagnesite-Magnesite-200594.jpg|thumb|left| Radial sprays of glassy '''hydromagnesite''' needles are scattered on pastel-green [[magnesite]] (coloured by nickel impurities) with an unusual knobby/bubbly/drusy form. The specimen is from the Cedar Hill Quarry, [[Fulton Township, Lancaster County, Pennsylvania]]. Size: 7.3 x 5.5 x 3.1 cm.]]

[[Category:Magnesium minerals]] [[Category:Carbonate minerals]] [[Category:Hydroxide minerals]] [[Category:Tetrahydrate minerals]] [[Category:Cave minerals]] [[Category:Monoclinic minerals]] [[Category:Minerals in space group 14]] [[Category:Luminescent minerals]]