ChemicalSedimentaryRocks:CARBONATES

aquicksummary

AlessandroGrippo,Ph.D.

WhatareCarbonates?

•  Carbonaterocksarechemicalsedimentaryrockscomposedmainlyoronlybycarbonateminerals

•  CarbonatemineralsincludetheionCO32-intheir

composition

•  Thereareseveralcarbonatemineralsbutonlytwoarereallycommon:–  Calcite– Dolomite

Othercarbonates

•  Azurite•  Malachite•  Magnesite•  Rhodocrosite•  Siderite•  Cerussite•  Trona

CarbonateRocks

•  Carbonaterocksincludeallrocksformedbycarbonateminerals

•  Byfar,themostcommoncarbonaterocksare:– LimestoneCaCO3

•  Mostlycalcite•  Sometimesaragonite

– Dolostone(CaMg)[CO3]2•  Dolomite

Calcitevs.Aragonite•  Magnesiumanditsroleintheformationofcalcite

•  Calcitevs.aragonite–  Polymorphs!– Whydosomeorganismspreferaragonitetocalcite?

– Whatistheroleofplatetectonicsintheestablishmentof“calciteseas”vs.aragoniteseas”?

SchematiccrystalstructureoftheCaCO3polymorphsaragoniteandcalcite

From:https://www.researchgate.net/figure/Schematic-crystal-structure-of-the-CaCO-3-polymorphs-aragonite-and-calcite-Aragonite-a_fig1_305654328

Calcitevs.Dolomite

•  Calcitevs.dolomite– Wheredoesmagnesiumgo?

•  Relativeabundanceoflimestonesanddolostones–  Today:dolostonesarerare–  Geologicalrecord:dolostonesalmostascommonaslimestonesinshallow-waterdeposits

3-DstructureofcalciteanddolomiteBoth crystals consistof alternating sheetsof carbonate ionsbut incalcite carbonate sheets can only alternate with calcium ions. Incalcite, magnesium can disturb the calcium sheets, originatingimpurities. In dolomite instead, magnesium forms its ownseparated,alternatingsheets.

Modifiedfromhttp://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-50532013000200018

CarbonatechangesthroughGeologicTime

•  Mostdolomiteisnotaprimaryprecipitatebutformsasaslowalterationoforiginalcalcite

•  Oceanconditionsthatfavordolomitereplacementforcalciteare;–  Highsalinity–  HighpH–  LowCa2+/Mg2+ratio–  Hightemperature

Relativevariationofdolomite(andotherparameters)overgeologictime

TheDolomitesofItaly

TreCime(ThreePeaks)diLavaredoDolomitiregionofBellunoandBolzano,Italy

Dolomites as rocks were first investigated and described in northern Italy by theFrenchgeologistDeodatdeDolomieu.Themineral(dolomite),therock(dolostone),andthiswholegeologicalregion(Dolomiti)arenamedafterhim.

©AlessandroGrippo

Calcite•  Calciteisthemostcommoncarbonatemineralinnature•  Calcitecanformeitherinorganicallyorbecauseofthebiological

actionoforganisms

•  Equilibriumreactionofcalciteinwater: CaCO3+H2O+CO2!"Ca2++2HCO3

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•  AddingCO2tosolutiondissolvescalciteintoitscomponentions•  RemovingCO2fromsolutionwillcauseprecipitationofcalcite

•  HighpressureandlowtemperaturekeepCO2insolution•  LowpressureandhightemperatureremoveCO2fromsolution

TypesofLimestones

•  Inorganic–  formdirectlyinwaterbecauseofchangingchemicalequilibriuminsolution•  Travertine,Tufa,OoliticLimestone

•  Biochemical/Bioclastic–  formbecauseofthebiologicalactivityofanorganism(biochemical)andsuccessivemechanicalweatheringofremains(bioclastic)•  CoralReefsandStromatolites,FossiliferousLimestone,Coquina,Chalk,Micrite

Travertine

•  TravertineiscalciumcarbonatethatformsdirectlyinthephysicalenvironmentuponlossofCO2fromwater

•  Terrestrialenvironments:waterfall,springs,hotsprings,cavesandcaverns

Above:fossilleafinItaliantravertine,GettyCenter,LosAngeles,CABelow:bandedtravertine

SMCrockcollection,SantaMonica,CaBothimages©AlessandroGrippo

Travertineactivelyforming(andimmediatelycoveredbymoss)ataspring,creatingitsownwaterfall.Carbondioxideleavesthesolutionuponwaterleavingacloseenvironment(groundwater)andsprayingatthewaterfall

AnevaRiver,LabantediCastelD’Aiano,Bologna,Italy

Uponcomingtothesurface,hotwatercanquicklylosecarbondioxide,favoringtheprecipitationofcalciteTravertineterraces,YellowstoneNationalPark,Wyoming,USA

©AlessandroGrippo

Waterdrippingfromtheceilingofacavequicklylosescarbondioxidetotheatmosphere,favoringtheformationoftravertine.Inthiscase“dripstones”areexpressedasstalactiteshangingfromabove

MitchellCaverns,MojaveNationalPreserve,Kelso,California,USA

©AlessandroGrippo

Tufa

•  TufaisavarietyoftravertinethatformsinafewplayalakesoftheAmericanWest,wheredifferentwaterchemistriesmix

•  Apresentday-exampleisMonoLake

•  AnextinctsystemisatSearlesLake(Trona)

Above:MonoLake,LeeViningCaliforniaBelow:Tronapinnacles,atdrySearlesLake,Tron,California

©AlessandroGrippo

Oolites(ooliticlimestones)

•  Inorganiclimestoneformedbythecementationofsand-sizedooids

•  Ooidsaresmallspheresofcalciteprecipitatedinwarm,shallowseawater

•  Tidalcurrentsandsea-wavesmovegrains,particles,smallfossils,thatcoatthemselvesincalciumcarbonateupongrowingonatropicalcarbonateshelf

Whiteoolite

SMCrockcollection

©AlessandroGrippo

Blackoolite

SMCrockcollection

©AlessandroGrippo

Thinsectionofanoolite,

showingbothconcentricandradialgrowth

BiochemicalandBioclasticLimestones

•  Biochemicallimestonesarethoseproducedbytheactivityoforganisms– Mostlymarineorganisms– Alsofreshwaterorganisms

•  Bioclasticlimestonesarethoseformedbythecementationofbrokenshells,piecesofcoralreef,etc.

•  Coralreefs•  Fossiliferouslimestones

•  Coquina•  Chalk•  Micrite(andSparite)