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Petrology, geochemistry (Mineralogy)

CO2 geological storage: The environmental mineralogy perspective

Mine ralogie environnementale et stockage ge ologique de CO 2

Franc ois Guyot a ,*,b , Damien Daval a ,c ,d , Sebastien Dupraz a ,e , Isabelle Martinez a ,Bene dicte Menez a , Olivier Sissmann a ,ca E quipe de recherche technologique, stockage ge ologique de CO 2 , universite Paris-Diderot and Institut de Physique du Globe de Paris (IPGP), 1, rue Jussieu,75005 Paris, Franceb Institut de mine ralogie et de physique des milieux condense s (IMPMC), 4, place Jussieu, 75005 Paris, Francec UMR 8538 du CNRS, laboratoire de ge ologie, E cole normale supe rieure, 24, rue Lhomond, 75005 Paris, Franced Lawrence Berkeley National Laboratory, Earth Sciences Division, Berkeley, USAe BRGM, 3, avenue Claude Guillemin, 45000 Orle ans, France

1. Mineralogy of CO 2 geological storage

Before thinking of manipulatingcarbondioxide (CO 2 ) ata large scale (which mankind has done already by burning

large quantities of fossil fuels), it is useful to have acoherent view of the natural CO 2 geochemical cycle. Thiswill allow assessment of the extent of environmentalanthropogenic perturbation and identication of themineralogical mechanismsof interaction of carbon dioxidewith its surrounding. A general scheme of the CO 2 cyclewithout anthropogenic perturbation is given in Fig. 1 with

C. R. Geoscience 343 (2011) 246259

A R T I C L E I N F O

Article history:Received 9 November 2010Accepted after revision 17 December 2010

Written on invitation of theEditorial Board

Keywords:Carbon dioxideCarbonateGeological storageCarbonationDeep biosphereBiomineralization

Mots cle s :Dioxyde de carboneCarbonateStockage ge ologiqueCarbonatationBiopshe`re profondeBiomineralisation

A B S T R A C T

Geological storage of carbon dioxide (CO 2 ) is one of the options envisaged for mitigatingthe environmental consequences of anthropogenic CO 2 increases in the atmosphere. Thegeneral principle is to capture carbon dioxide at the exhaust of power plants and then toinject the compressed uid into deep geological formations. Before implementation overlarge scales, it is necessary to assess the efciency of the process and its environmentalconsequences. The goal of this paper is to discuss some environmental mineralogyresearch perspectives raised by CO 2 geological storage.

2011 Acade mie des sciences. Published by Elsevier Masson SAS. All rights reserved.

R E S U M E

Le stockage ge ologique du CO 2 constitue une des options envisage es pour faire face auxconsequences environnementales de laugmentation de la concentration atmosphe riqueen dioxyde de carbone. Le principe ge ne ral repose sur la capture du CO 2 en sortie descentrales e lectriques, suivie de son injection sous pression dans des formationsgeologiques souterraines. Avant une e ventuelle application a ` grande echelle, lefcacite dece proce de ainsi queses conse quences environnementales doivent e tre evaluees. Le butde cet article est de discuter quelques perspectives de recherche en mine ralogie

environnementale souleve es par le stockage ge ologique du CO 2 . 2011 Acade mie des sciences. Publie par Elsevier Masson SAS. Tous droits re serve s.

* Corresponding author.E-mail address: [email protected] (F. Guyot).

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1631-0713/$ see front matter 2011 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.doi: 10.1016/j.crte.2010.12.007
http://dx.doi.org/10.1016/j.crte.2010.12.007mailto:[email protected]://www.sciencedirect.com/science/journal/16310713http://dx.doi.org/10.1016/j.crte.2010.12.007http://dx.doi.org/10.1016/j.crte.2010.12.007http://www.sciencedirect.com/science/journal/16310713mailto:[email protected]://dx.doi.org/10.1016/j.crte.2010.12.007

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emphasis on the main processes where mineralogicalscience will be most useful. More detailed representationsof the carbon cycle can be found elsewhere (e.g., Gaillardetand Galy, 2008 ). In a simplied view, one considersgenerally that the ux of outgassing of CO 2 from the solidEarth by magmatic and metamorphic processes is of thesame order of magnitude as that of its export to the solidEarth in the form of carbonate minerals and of fossilorganic matter. The feedback mechanisms within thiscycle are still unclear and constitute the object of intenseresearch (e.g., Berner, 1991; Gaillardet and Galy, 2008 ).Two major non-mutually exclusive hypotheses for thesemechanisms are considered:

In the rst, the alteration rate of minerals is indirectlycorrelated to CO 2 contents of the atmosphere andhydrosphere (see a review in Kump et al., 2000 ) throughtemperature variations, erosion rates, role of vegetation,soil pressure of CO 2 , etc. The main chemical reactionsinvolved in the alteration of minerals and subsequentprecipitation of carbonates are summarized as dissolutionof silicates (reaction (1) ), precipitation of carbonates(reaction (2) ), and capture of CO 2 by chemical weathering(reaction (3) , which represents thesumof reactions (1) and(2) ):

MSiO3 H2 O 2CO2 ! M2 2HCO3 SiO2 (1)

M2 2HCO3 ! MCO3 H2 O CO2 (2)

MSiO3 CO2 ! MCO3 SiO2 (3)

In these chemical reactions, only silicates with divalentM cations such as Mg 2+ and Ca 2+ are represented, butdissolution reactions can also be written with monovalentcations (Na +, K+). However, for crystal chemical reasons,alkali cations are rarely involved in solid carbonateprecipitation and thus in the global export of CO

2 to the

solid Earth (e.g., Oelkers et al., 2008 ). Conversely, such

cations could be involved in reverse weathering oncontinental margins and are actually thought to potential-ly reduce the uptake of CO 2 from mineral weathering(Mackenzie and Kump, 1995 ).

Positive feedback between the thermodynamic activityof CO2 and silicate dissolution is not easy to demonstrate,and the governing mechanisms are not well established.For example, positive temperature-silicate dissolutioncorrelations have been demonstrated in the case of basalts(Dessert et al., 2003 , Gaillardet et al., 1999 ) and suggestedfor granites ( White and Blum, 1995 ). Of course, under-standing in detail the mineralogical aspects of silicatedissolution and of carbonate precipitation is an essentialcomponent required for understanding that regulation.

The second hypothesis involves photosynthesis bybacteria, photosynthetic unicellular eukaryotes and greenplants, as represented simplistically by reaction (4) , whichrepresents primary production via photosynthesis:

CO2 H2 O ! CH2 O O2 (4)

Oxygenic photosynthesis is enhanced as carbon dioxideincreases, either through temperature increase or because

of CO2 limitations of biomass in some ecosystems. Thereverse reaction

CH2 O O2 ! CO2 H2 O (5)

which represents respiration and combustion of biomass,is also enhanced. However, because some of the organicmatter unavoidably escapes as it is transformed intorefractory phases (the term refractory will be used here fororganic matter resistant to biological respiration andfermentation), the difference [ (4) (5) ], namely, the globalnet primary production, is positive. Then, either directly(by sedimentation in the ocean) or indirectly (by transportof soil refractory organic matter to the ocean and thensedimentation), some CO 2 is transferred to the solid Earth.

[

Fig. 1. Schematic representation of the CO 2 natural cycle in the mode of geochemical boxes. Arrows are indicative of uxes of the order of 10 13 moles of carbon per year. Weathering (alteration of minerals at the interface between solid Earth and uid envelopes) and net primary production (essentiallyphotosynthesis minus respiration on a global scale) are natural mechanisms of capture of CO 2 outgassed from the solid Earth by magmatic andmetamorphic processes. Processes in which mineralogical control is primordial are underlined.

Fig. 1. Repre sentation sche matique du cycle naturel de CO 2 sous forme de changes entre re servoirs ge ochimiques. Les e `ches repre sentent des ux delordre de 10 13 moles datomesde carboneparan. Lalte ration (re action chimiquedes mine raux a` linterfaceentre la Terre solide et lesenveloppesuides)etla production primaire nette (essentiellement le bilan global photosynthe `se-respiration) sont des me canismes naturels qui permettent de capturer le CO 2de gaze depuis linte rieurde la Terrepar magmatisme et me tamorphisme.Les processus danslesquelsla mine ralogie joue un ro le primordial sont souligne s.

F. Guyot et al. / C. R. Geoscience 343 (2011) 246259 247

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Demonstrating effective positive feedbacks between car-bon dioxide increase and net primary production, at leastin some ecosystems, and measuring them, is one of thegrand challenges in ecological sciences. The role of mineralogy is less obvious in this second hypothesis thanin the case of silicate dissolution and carbonate precipita-tion, but it is nonetheless important. Indeed, the questionof the full stoichiometriccoupling between photosynthesisand carbonate precipitation, as represented in reaction(6)

Ca2 2HCO3 ! CaCO3 CH2 O O2 (6)

requires biomineralization in coccolithophores, foraminif-era and corals to be studied and understood in detail.

Together, these two mechanismssilicate dissolution/carbonate precipitation and transfer of organic matter tothe solid Earthare considered to be of the same order of magnitude. Perturbations due to glacial/interglacial alter-nations are not included in this framework because theymostly concern carbon dioxide redistribution between theatmosphere and theocean. Theburning of fossil fuel is verydifferent because it modies the transfer of CO 2 from solidEarth to Earths uid envelopes. In that sense, it is almostuseless to discuss the environmental consequences of fossil fuel burning in light of climate/CO 2 correlations overthe last 500,000 years. Although very different, it would bemore appropriate to compare the CO 2 contributed by fossilfuel burning with that resulting from major volcanicevents that led to deposition of massive basaltic traps.Although to date, the levels of CO 2 contributed by the latterprocess are poorly constrained, anthropogenic fossil fuel(extracted from the solid Earth) burning, which isestimated to involve about 6 10 14 moles of carbon peryear, might possibly be even larger. In response to thatunusual perturbation, the Earth has initiated otheremergency mechanisms to regulate CO 2 levels.

First, because CO 2 is a highly soluble gas, most of thecarbon dioxide transferred from the solid Earth to Earthsuid envelopes should indeed end up in the ocean. This iswhat will eventually happen, but it will require acharacteristic absorption time of thousands of years.Quasi-immediate CO 2 uptake by surface waters on the timescale ofyearsaccounts forabout half ofthe 6 10 14 moles of carbon per year. Although absorption of the rest will takemuch longer, this mechanism contributes to a decrease inthe CO

2content of the atmosphere. As a means of enhancing

this mechanism, several projectshaveproposed injection of some CO 2 in the deep ocean, which would substantiallyshortenthe thousands of years required for theuptake of theremaining carbon dioxide (e.g., Adams and Caldeira, 2008 ,and references therein). Whether an abrupt increase indissolved CO 2 in different compartments of the ocean willhave an effecton pH andon marine ecosystems is,however,not well understood (e.g., Ricketts et al., 2009 ). In thatrespect,the highlypublicizedproblemsof theeffectof oceanacidication on corals, although highly complex and due tomultiple causes, are not very encouraging from theperspective of CO 2 ocean storage.

Second, because the rate of photosynthesis is on theorder of 10 16 moles of carbon per year (e.g., Garrels et al.,

1975 ), which is about 20 times faster than the anthropo-genic perturbation, one might think that a small relativeincrease ( 1%) in the rate of reaction (4) would be enoughto fully compensate for the additional emission of carbondioxide from the burning of fossil fuels. Thecommon belief that CO 2 , directly or indirectly (through temperatureincrease) favors the growth of green plants, adds credibili-ty to this possibility. This would indeed be the case if CO 2were a limiting factor of reaction (4) on a global scale. Thisbelief is not true, however, in most ecosystems. Otherfactors, such as available nitrogen and phosphorus, x theglobal amount of biomass. The response of plants toincreasing CO 2 levels is highly complex (e.g., Dukes et al.,2005; Field et al., 1995 ) but, to roughly summarize, itappears that natural photosynthetic processes can thusonly marginally absorb more CO 2 than what they do now an idea supported by recent experiments on the fumiga-tion of forest plots with increased atmospheric CO 2 bySchlesinger and Lichter (2001) . The ideas of enhancing netprimary production by planting trees andusingmore woodon a global scale areworth investigating, but they will haveto deal with the natural and anthropogenic cycles of nitrogen andphosphorus,whichwill affect their actualCO 2impact in a complex manner. Other ideas such as oceanfertilization for enhancing sedimentation of organic matter(e.g., Buesseler et al., 2004; Coale et al., 2004 ) imply verycomplex ecological interactions in the ocean and will thusprobably never be performed on a large scale. From anenvironmental perspective, the actual biomass enhance-ment, and thus CO 2 capture due to human fertilization, isdirectly related to eutrophication, a major environmentalproblem that results in the capture of about 1020% of the6 10 14 moles of carbon per year due to fossil fuel burning.However, such eutrophication will make the use of biomass enhancement for mitigating excess atmosphericcarbon dioxide challenging on a large scale. The possiblemassive use of biomass fuels in the coming decades isrelated to these questions.

We are thus left with several 10 14 moles of CO 2 per yearin excess of the non-anthropogenic, naturally regulatedcontribution that are presently and will continue for tensof years to be transferred from the solid Earth to Earthsuid envelopes. One might decide not to try to regulatethat new ux, noting that nobody knows what the effectsof that increase will be; some even argue that this new uxmight indeed be benecial, as S. Ahrrenius proposed over100 years ago: By the inuence of the increasingpercentage of carbonic acid in the atmosphere, we mayhope to enjoy ages with more equable and better climates,especially as regards the colder regions of the Earth, ageswhen theEarth will bring forth much more abundant cropsthan at present, for the benet of rapidly propagatingmankind ( Arrhenius, 1908 ). However, another possibilityis that the consequences of increased CO 2 levels in theatmosphere could be devastating,as suggestedby relation-ships between stable isotopes of carbon in the geologicalrecord and major biosphere crises (e.g., Schidlowski,1988 ); we thus should try to regulate the carbon dioxidelevels in the atmosphere. Based on current understanding,Pacala and Socolow (2004) have proposed that atmospher-ic increases of CO 2 concentration due to fossil fuel burning

F. Guyot et al. / C. R. Geoscience 343 (2011) 246259248

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should not exceed 500 ppm by volume. However, there isno strong rationale for 500 ppm rather than other values,and thus this level should be taken as a symbolic thresholdof about twice the pre-industrial level. Taking this intoaccount, as well as economic and population growthconsiderations, Pacala and Socolow (2004) have proposedthat 5 to 8 technologies should be used together formitigating this increase, with each technology reducingthe total release of CO 2 by 10 14 moles ( 4 Gigatons) peryear in order to be signicant.

Among those technologies, CO 2 capture and geologicalstorage is a possibility whose safety, feasibility, reliability,and costs need to be assessed. This technology is alreadyused on a mid-sized scale, in particular as CO 2 is injectedunderground to help oil and gas recoveries. Just to give aspecic example (one not motivated by recovery consid-erations but rather by national environmental regula-tions), CO 2 has been injected in a submarine aquifer by anhydrocarbon company offshore of Norway in the gas eldof Sleipner at a rate of 3000 tons of CO 2 per day for a totalinjection of 15 millions of tons of carbon dioxide (e.g.,Akervoll et al., 2009; Lindeberg et al., 2009; Torp and Gale,2004 , and references therein). If the global target for CO 2geological storage is to avoid the release of about200 Gtons of CO 2 in the next 50 years (10 14 moles of CO2 ( 4 Gigatons) per year during 50 years), we wouldneed a few thousand sites of that type. This amount of storage is a lot, but this level is not out of reach if thetechnology is well understood. In order for that generali-zation to be acceptable, however, one needs to know indetail what will be the fate and reactivity of carbon dioxideunderground, not only at the scale of years but also at ascale of thousands of years in order not to leave ourdescendants with unexpected adverse consequences. Fromthe environmental mineralogy perspective, this level of CO2 capture and storage requires the following goals,among others: (1) that we understand the reactivity of theCO2 injected within rock porosity with the differentminerals characteristic of each geological storage site;(2) that we are able to model the reaction rates of carbondioxide with minerals for different thermodynamicactivities of CO 2 in presence of possible accessory co-injected uids; (3) that we understand how differentelements will be mobilized in the uids as a result of theseinteractions. Achieving these goals requires both labora-tory studies anddetailed eldwork at actual injection sites.

In this context, it is important to understand how thenatural mechanisms by which our planet regulates CO 2 bytransfer of carbonates and refractory organic matter to thesolid Earth, will be operating at different CO 2 injectionsites. Of course, the mechanisms at work in the globalcarbon cycle will never be identical to those operating inengineering projects in the subsurface. The purpose of thispaper is rather to see how they may inspire betterknowledge of the processes at work at injection sites. Itis common knowledge that conversion of CO 2 into solidcarbonates or into organic matter, considered over thetime scale of years, will involve a very small fraction (a few%) of the CO2 injected since, as discussed above, they canonly regulate amounts of CO

2 about 50 times less than the

present day anthropogenic perturbation. This limitation

immediately raises two questions that will be examined intherest of that paper: (1) cansilicate dissolution/carbonateprecipitation reactions be accelerated in some specialinstances so that most of the injected CO 2 would be storednot as supercritical uid or as dissolved aqueous CO 2 butrather as stable solid carbonates; (2) can signicantconversion of CO 2 into refractory organic matter or solidcarbonates occur at carbon dioxide injection sites if specic conditions exist?

2. Mineralogical controls on carbonation kinetics

An important parameter for evaluating the amount of injected carbon dioxide that will be converted in a giventime into solid carbonates is the rate of reaction (3) relativeto the hydrodynamic parameters that characterize CO 2leakage to the surface or to other reservoirs. Becausereaction (3) is a combination of reactions (1) and (2) , it isimportant to determine which of the two steps iskinetically limiting. Data from Daval et al. (2009a) are

shown in Fig. 2. Modeling of the extent of carbonation of wollastonite (CaSiO 3 ) into calcite (CaCO 3 ) is not signi-cantly different with and without consideration of the

0

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Weissbart & Rimstidt (2000) + Shriraki & Brantley (1995)

Weissbart & Rimstidt (2000) + equilibrium/calcite

Golubev et al. (2005) + Shiraki & Brantley (1995)

Experimental data

Fig. 2. Normalised extent of CaSiO 3 wollastonite carbonation as afunction of time. The diamonds are the experimental data obtained byDaval et al. (2009a) . The solid curves are the result of kinetic modeling of wollastonite carbonation using published rate laws for dissolution of wollastonite and precipitation of calcite, respectively by Weissbart andRimstidt (2000); Golubev et al. (2005) and Shiraki and Brantley (1995) ;for details of the modelling strategy, see Daval et al. (2009a) . The dashedline corresponds to the same modelling using innitely rapid kinetics of calcite precipitation.

Fig. 2. Taux normalise de carbonatation de la wollastonite (CaSiO 3 ) enfonction du temps. Les losanges repre sentent les donne es experimentalesobtenues par Daval et al. (2009a) . Les courbes continues sont le re sultatdunmode `le utilisant lesdonne es publie es decine tiques de dissolution dela wollastonite par Weissbart et Rimstidt (2000) et Golubev et al. (2005) ,et de pre cipitation de calcite par Shiraki et Brantley (1995) ; les details dela modelisation sont de crits dans Daval et al. (2009a) . La courbe en

pointille s correspond a ` la me me mode lisation, mais en utilisant unecine tique instantane e (inniment rapide) de pre cipitation de la calcite.


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measured kinetics of precipitation of calcite instead of itsequilibrium formation (i.e. at innite rate). This observa-tion demonstrates that what limits the kinetics of carbonation in such laboratory experiments is silicatedissolution (reaction (1) ) and not the rate of carbonateformation (reaction (2) ). This result can condently beextended to all situations of calcite formation as a result of alteration of silicates at temperatures close to 100 8 C,because wollastonite is presumably the most reactive Ca-bearing silicate under these conditions (cf. Fig. 7 in Davalet al., 2009a ). At a temperature typical of the Earthssurface, where Si is incorporated in clays rather than inamorphous silica, this statement could be more question-able because the slow precipitation rate of these phasesmaintain aqueous concentrations close to equilibriumwith respect to silicate dissolution, such that the weather-ing of primary silicates could be ultimately controlled bythe rate of clay formation (see e.g., Maher et al., 2009 , Zhu,2009 for further details).

In cases where the carbonate formed is magnesite(MgCO 3 ), determination of the limiting step is not so clearanddeserves more research. For instance, recent dataon theprecipitation kinetics of magnesite by Saldi (2009) suggestthat at100 8 C, therateconstant of magnesite precipitation is6 order of magnitude below that of calcite precipitationunder similar conditions. However, the rate limiting step of thecarbonationof Mg-rich silicates could still be the silicatedissolutionreaction, at least below 120 8 C. Moreover, at lowtemperatures, the carbonate formed would not be magne-site indeed,but rather hydratedmagnesian carbonates suchas hydromagnesite: Mg 5 (CO3 )4 (OH) 2 .4H2 O (Hanchen et al.,2008; Saldi et al., 2009 , and references therein), with morerapid, yet unknown, precipitation rates.

A key issue for controlling conversion of CO 2 into solidcarbonates is thus the rate of dissolution of silicateminerals. The reaction rate data in Fig. 2 for CaSiO3(wollastonite) at a grain size of about 0.2 mm show thatfull transformation of the silicate into carbonate occursrapidly. This rapid transformation is due to two factors:wollastonite dissolves rapidly and Ca 2+ is a carbonate-

forming cation. In that respect, geological sites containinglarge amounts of wollastonite would be interesting forconverting injected CO 2 into calcite, although wollastoniteis not a common mineral. Other more common calcicminerals, e.g., anorthite, might also be favorable forcarbonation, and more generally, minerals whose compo-sition is dominated by magnesium and/or calcium wouldbe interesting targets. This is the main idea behind projectsinvolving CO 2 injection in basic and ultrabasic geologicalcontexts (e.g., Gislason et al., 2010; Kelemen and Matter,2008; Matter and Kelemen, 2009; McGrail et al., 2006;Schaef and McGrail, 2009; Schaef et al., 2010 ). The mainpremise of these strategies is that reaction (1) is an acid-basic neutralization of the mineral (base) by dissolvedcarbon dioxide (acid). Anorthite, CaAl 2 Si2 O8 , an importantcomponentof basalts, could be an interesting subject in theperspective of geological storage of CO 2 in basalts. In thisregard, a few recent studies have focused on water-CO 2 -anorthite interactions (e.g., Berg and Banwart, 2000;Regnault et al., 2005; Sorai et al., 2007 , Sorai and Sasaki,2010 ). Indeed, basalts, although representing only 5% of the silicate rocks at Earths surface contribute to more than30% of reaction (1) (Dessert et al., 2003 ). In laboratoryexperiments, however, some promising minerals showunexpected slow carbonation rates at temperatures below100 8 C. Experimental studies of the dissolution of olivine(forsterite Mg 2 SiO4 ), even with small grains of 0.2 mm incharacteristic sizes, have been highly disappointingbecause they yielded reaction rates being at least 4 ordersof magnitude slower than that predicted by numericalsimulation of the process (see Daval, 2009 and Daval et al.,2010b for details). Olivine, however, is one of the mostthermodynamically favorable substrates for carbonation(Fig. 3), and is also one of the fastest dissolving minerals(Fig. 4). Therefore, this should make it an excellentsubstrate for conversion of CO 2 into carbonates. Slowexperimental carbonation rates below 100 8 C emphasizecomplexities whose origins will be discussed below. Of course, it is possible to increase carbonation rate byincreasing temperature ( Fig. 5) or by decreasing grain size

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60 AlbiteGlaucophaneSerpentineMagnetite AnorthitePyroxeneOlivineWollastonite

G r0 [kJ/mol] data from Robie et al., 1995

Fig. 3. Standard Gibbs free energies for the carbonation reaction (3) of different silicate minerals. Obviously, minerals containing divalent carbonateforming cations are thermodynamically more prone to favor that reaction. Note that a favourable reaction is indicated by a negative standard Gibbs freeenergy.

Fig. 3. Enthalpies libres standardde carbonatation(3) de diffe rents silicates. Il appara t logiquement quela carbonatation de mine rauxqui contiennentdes

ions divalents formateurs de carbonates est thermodynamiquement favorise e. Note : une re action thermodynamiquement favorise e correspond a ` desvaleurs ne gatives de lenthalpie libre.


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(e.g., Dufaud et al., 2009 , Garcia et al., 2010 ). Althoughachieved in some cases, temperatures will seldom reach90 8 CatCO 2 geological storage sites. Whereas temperatureincrease is an efcient but often impractical solution,increasing reactive surface area is an interesting optionthat deserves further research. Injection of CO 2 underpressures higher than hydrostatic and lithostatic values atthe injection point, sufcient to create microcracks in therocks, might help to increase uid/rock contact surfacearea by creation of microfractures, and thus presumablyincrease reactive surface area as well. This main issue of coupled mineralogy and geomechanics thus needs to beinvestigated in detail (seeanalogousstudies in carbonationof concrete (e.g., Corvisier et al., 2010; Fabbri et al., 2009 )and limestone (e.g., Le Guen et al., 2007 )). It should benoted, however, that the issue of inducing fracturingunderground has proven difcult on a societal basis, atleast under populated areas ( Giardini, 2009 ). On the otherhand, ex situ carbonation (i.e. in a chemical plant at Earthssurface, see e.g., Gerdemann et al., 2007 ) of materialssusceptible to carbonation, with continuous grinding toreduce grain sizes, might be a promising approach (aFrench project supported by ANR CO 2 coordinated byFranc oise Bodenan at BRGM in Orle ans and by FlorentBourgeois at E cole de Chimie in Toulouse is an example of research efforts currently exploring this possibility). In thislatter case, a condition of success is to dispose of materialsfavorable to carbonation, which are already present andoccupying an important volume, and which cannot be

disposed of in other manners. Mine waste materials wouldbe interesting targets (see Wilson et al., 2006 ), especially if mining occurred in basic and ultrabasic rocks. Finally, evenin the continental crust, ferromagnesian minerals such asbiotite could be interesting targets.

Even if thesurface area problem could be solved, thanksto large porosities or to microfracturing, the effectivedissolution rates of olivine and many other minerals willbe in most cases still far too slow to make the rate of mineral carbonation competitive with the rate of hydro-dynamic ow of CO 2 out of the system. We thus have tounderstand whyolivine, as a typical example, converts intocarbonates so slowly in comparison to wollastonite,especially in light of the observation that the far-from-equilibrium dissolution rates of olivine reported in theliterature are only oneorder of magnitudeslower than thatof wollastonite ( Fig. 4). The answer is found in the detailedexamination of the mineral surfaces that underwentdissolution in batch experiments at similar conditions(90 8 C and pCO 2 = 250 bar, duration: 2 days for wollaston-ite; 45 days for olivine) ( Fig. 6). Whereas the silica layersshown in these pictures look different, they may originatefrom similar interfacial, nm-sized precursors. For bothexperiments, the bulk solution was found to be saturatedwith respect to SiO 2 (am), such that the interfacial Si-richlayers usually observed to form at far-from-equilibriumconditions in mixed-ow reactor experiments for theseminerals (see e.g., Casey et al., 1993 for wollastonite;Pokrovsky and Schott, 2000a,b for olivine) were stabilized.

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AnthophylliteChlorite

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Lizardite

TalcWollastonite

T = 90C

Fig.4. Dissolution rates of magnesian silicatesas a functionof pH extrapolatedat 90 8 C. Forthe sake of comparison, thedissolution rate of wollastonitewas

also mentioned in this graph. Data used for these calculations:Fig. 4. Vitesses de dissolution des silicates de magne sium en fonction du pH, extrapole es a 90 8 C. A titre de comparaison, la vitesse de dissolution de lawollastonite a e galement e te reporte e. Donnees utilise es pour le calcul :

Forsterite Rosso and Rimstidt, 2000Enstatite Oelkers and Schott, 2001Diopside Knauss et al., 1993Lizardite Hellmann et al., 2008Chlorite Lowson et al., 2007Talc Saldi et al., 2007Anthophyllite Chen and Brantley, 1998Wollastonite Weissbart and Rimstidt, 2000


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For wollastonite, it is striking that the silica-rich amor-phous layer kept growing during the carbonation processwhile its external interface stopped dissolving. Thus, thegrowth of the silica layer at the expense of wollastonitewas possible because this coating did not prevent the uidfrom reaching the pristine interface of wollastonite, whichthus could keep on dissolving. Signicant porosity of thesilica layer in the case of wollastonite ( Fig. 7, see also

further images anddetails in Daval et al., 2009b ) was foundand actually allows the reaction to proceed by transport of solutes through the layer, whereas this transport wasseverely limited for forsterite. In this latter case, the thinsilica layer was non-porous, blocking dissolution and thuscarbonation of forsterite.

The origin of such differences is so far unknown, andcurrent research in ourgroup is directed at ascertaining the

[

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Fig. 5. Yield of the carbonation reaction (3) of forsterite Mg 2 SiO4 (modied after Gerdemann et al., 2007 ) after one hour of reaction with grain sizes of less

than 75 mm. Notice the optimum of 50% at T = 1858

C and the very low yield at 908

C. Conditions above 908

C are generally considered as difcult for CO 2geological storage.

Fig. 5. Taux de carbonatation (3) de la forste rite Mg 2 SiO4 (gure modie e dapre`s Gerdemann et al., 2007 ), apre s une heure de re action sur des grains detaille infe rieure a ` 75 mm. Notez loptimum de 50% a ` T = 185 8 C et le tre`s faible rendement a ` 90 8 C. Des tempe ratures de passant 90 8 C sont souventconsidere es comme complique es pour le stockage ge ologique de CO 2 .

[

Fig. 6. A. Transmission electron microscopy image of the surface of olivine during dissolution process in a batch experiment of olivine carbonation at 90 8 C(after quench recovery and preparation by focused ion beam). A thin compact silica layer is observed at the surface of the mineral. B. Observation byscanningelectron microscopy in a batchexperimentof wollastonitecarbonationat 90 8 C asshownin Fig.2 . Thesilicalayeris obviouslymuch thicker than inthe case of olivine but as seen in Fig. 2, it did not prevent reaction to proceed. Note the three order of magnitude difference in scale bar between A and B.

Fig. 6. A. Image de microscopie e lectronique en transmission de la surface dune olivine en cours de dissolution dans une expe rience de carbonatation, enmilieu ferme a 90 8 C (apre s trempe, re cuperation et pre paration par faisceaudions focalise s). Unene couchecompacte de siliceamorpheest observe e a lasurface du mine ral. B. Observation par microscopie e lectronique a ` balayage dans une expe rience de carbonatation de wollastonite en milieu ferme a 90 8 C,

telle que montre e dans la Fig. 2. La couche de silice est bien plus e paisse que dans le cas de lolivine, mais comme on le constate dans la Fig. 2, elle na pasempeche la carbonatation de se produiremassivement et rapidement. Noter la diffe rence de trois ordresde grandeur entre lesbarres de chelleen A eten B.


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origin of these surprising differences. Dissolution of nuclear waste glasses, where minor elements present inthe initial material canstrongly change thedissolution rateof the primary phase by modifying the structure of thesecondary silica layer (see the case of Zr in Cailleteau et al.,2008 ), may be related to the dissolution process of silicateminerals. However, one major difference in the dissolutionprocess of nuclear waste glasses and olivine is the muchthinner silica layer on olivine, which makes its investiga-tion highly challenging. Recent analytical developments inthe surface analysis of minerals could help to resolve thisproblem (e.g., Daval et al., 2009b; Davis et al., 2009;Hellmann et al., 2003 ). Interesting studies could also bedone on synthetic basaltic glasses with different minorelements in order to test the conjecture that alterationrates can be strongly modied by minor elements. In allcases, the issue will be to recognize, both in the laboratoryand in the eld, silica layers on different minerals and todetermine how the chemistry of the surrounding uid ismanipulating their structure and properties, and in netheir alteration rates.

Finally, even if both grain size issues and the problem of passivating layers are solved, another fundamental dif-culty might be encountered. Dissolution rate laws areusually measured under far from equilibrium conditionsin practice, under an active uid ow which is constantlyrenewing the water/solid interface (the so-called mixed-

ow reactor set-ups, see, e.g., Posey-Dowty et al. (1986) fora technicaldescription). Thisexperimental design mightbevery different from the hydrodynamic reality at CO 2injection sites where the ow will be on hydrological timescales rather than those typical of laboratory ow rates.Supporting this idea, a few eld studies showed that uid-rock interactions could occur over a broad range of degreesof undersaturation with respect to feldspars and othermajor rock forming minerals (e.g., Kampman et al., 2009;White et al., 2001; White et al., 2002 ). A consequence of having little renewal of the uid in contact with minerals isthat their dissolution rate might indeed be much slowerthan that measured under far from equilibrium conditions.

Overcoming this difference is one of the interests inperforming batch experiments, which are obviously lessprecise than chemostats, but which allowintegration of thereal effect of different conditions of departures fromequilibrium (see e.g., Daval et al., 2010c , for quantitativedetermination of kinetic parameters in batch reactors). Inany case, accurate knowledge of the relations betweenmineral dissolution rate ( R) and the Gibbs free energy of reaction ( DGr ) is necessary to foresee the long-termevolution of uids and rocks after CO 2 injection. The mostwidespread equation in geochemical codes that links R toDGr was established in the framework of transition-statetheory ( Eyring, 1935a,b; Lasaga, 1981 ), and states that R isroughly independent of DGr over a wide free energy rangefar-from-equilibrium (for instance, at 90 8 C, the dissolutionrate remains 95% of the far-from-equilibrium rate as long asDGr < 10kJmol 1 ), whereas it decreases linearly over avery narrow range of DGr values as equilibrium isapproached. The simplest TST equation, that is used inmost of the long-term simulation attempts of CO 2 seques-tration (e.g., Gherardi et al., 2007; Knauss et al., 2005; Xuet al., 2004, 2005 ) fails, however, to explain either sharpdrops of dissolution rate close-to-equilibrium (see thecomparison of the close-to-equilibrium data on feldsparsweathering from Kampman et al. (2009) to the far-from-equilibrium dissolution rate of labradorite), or a decrease of mineral dissolution rate under conditions usually consid-ered as to represent far-from-equilibrium conditions (e.g.,Beig and Luttge, 2006; Berger et al., 2002; Hellmann andTisserand,2006; Hellmann et al., 2010 ; or Taylor et al., 2000for feldspars; Dixit andCarroll, 2007 or Davalet al., 2010a forpyroxenes; Cama et al., 2000 for clays). In that respect, theimplementation in geochemical modeling of empiricalkinetic rate laws that take into account the actual rate-afnity relations measured in recent studies was thusproposed to help bridge the gap between laboratorymeasurements and eld estimations of weathering, at leastpartly (e.g., Daval et al., 2010a; Ganor et al., 2007; Maheret al., 2009; Zhu, 2009 ). Such results emphasize the need toincrease our efforts to determine experimentally the effectofuidcomposition on thedissolution rates of minerals, andeventually understand why the simplest TST expressionfails to link far-from-equilibrium to close-to-equilibriumdissolution rates.

To date, the mechanistic reasons for these incorrectpredictions of TST in the case of some minerals are notclear, and intensely debated. The existing explanationsand/or alternative models can be divided into 4 main

[

Fig. 7. Transmission electron microscopy image showing the highporosity at the nanometer scale of the silica rich amorphous phasethat developedon theCaSiO 3 wollastonite substrate. Notethat irradiationdamage under the electron beam produces signicantly differentmicrostructures in that gel phase. Calcite precipitates probablynucleating on or in the layer are also visible.

Fig. 7. Image de microscopie e lectronique en transmission montrantlimportante porosite a nano-echelle de la silice amorphe de veloppee surle substrat de wollastonite CaSiO 3 . Note: ona teste que lirradiation par lefaisceaude lectronsproduisait desde fauts distincts de cette porosite dansle gel de silice. On observe e galement des pre cipites de calcite ayantnuclee sur ou dans la couche de silice.


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categories: (1) the actual interfacial phase which controlsthe dissolution of the primary silicates has an equilibriumconstant different from that of the underlying phase (e.g.,Bourcier et al., 1990; Grambow and Muller, 2001 ), suchthat the application of TST to the primary silicate may bemeaningless; (2) dissolution is controlled by the sponta-neous nucleation of etch pits at crystal defects ( Lasaga andLuttge, 2003 ); (3) the silicate dissolution rate is controlledby the concentration of a precursor surface complex, and R DGr relations more sophisticated than TST can be derivedwithin the framework of TST ( Oelkers et al., 1994; Oelkers,2001 ); (4) the abovementioned discrepancies may not beascribed to DGr directly, but to a combination of differentprocesses such as the surface restructuring dynamics of primary silicates, diffusion of solutes across passivationbarriers and others ( Casey, 2008 ). Although reviewing indetails all of the proposed mechanisms is beyond the scopeof the present paper, the two rst models will be brieydescribed because they heavily rely on mineralogicalcontrols. Moreover, note that a thorough review of the 3 rd

category not discussed hereafter can be found in Schottet al. (2009) .

The rst idea is that the dissolution rate of primaryphases would drop because the uid would reach equilibri-um not with theoriginal mineralbut with secondary phasescoating the surface, potentially including silica layers thatcontain minor elements, which would determine the solid/uidexchanges(seefor e.g., Grambowand Muller (2001) forglassesand Oelkers(2001) forminerals; a theoreticalreviewof some of these models is also available in Gin et al., 2008 ).Detailed knowledge of the structures and compositions of these secondary phases, as well as their observation in theeld, will thus be another key for identifying appropriateeld sites for efcient conversion of CO 2 into solidcarbonates. Although such a model may be consistent withour own observations on olivine, note that wollastonitecarbonationcannot be explained in thisframework becausethis mineral keeps on dissolving whereas saturation isreached with respect to SiO 2 (am) which coats its surface(Daval et al., 2010b ). Thus, although interesting and asemphasized above, thismodel would needfurther develop-ment to be consistent for all silicates.

The second idea was proposed by Lasaga et al. in theearly 1990s (e.g., Burch et al., 1993; Nagy et al., 1991;Taylor et al., 2000 ), and further explored and developed byLuttge et al. (e.g., Arvidson and Luttge, 2010; Beig andLuttge, 2006; Lasaga and Luttge, 2001 ; 2003; Luttge, 2006 ).Basically, the observation of a dramatic drop in dissolutionrate under conditions classically considered to be far-from-equilibrium ( DGr

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possibly why carbonate-forming organisms are in dangerwhen atmospheric carbon dioxide increases by virtue of aLe Chatelier principle on reaction (7) or simply becausetheir carbonate components might eventually dissolve.However, because one of the two carbon atoms present atthe beginning of reaction (7) ends up in solid carbonates,which, if sedimentation conditions are favorable, consti-tute a very stable storage of CO 2 in the solid Earth, theprocess subtracts carbon dioxide from the atmosphere/hydrosphere. This is somewhat of a paradox because theformation of carbonates by life liberates some CO 2 but it isalso the mechanism by which carbon dioxide is convertedinto a stable solid form that is separated from thehydrosphere/atmosphere system. The liberation of CO 2in reaction (7) is strongly decreased by coupling with theorganic biological cycle as shown in reaction (6) . Thedegree of coupling between reactions (7) and (4) on thelarge scale is one of the most difcult global geochemicalmass balances to measure, but it is believed to be high. Onemight argue that organisms such as mussels, oysters, andsnails obviously have no direct relationship with photo-synthesis, which is true, but the amount of carbonateprecipitation theyrepresent isverysmallwhencompared tococcolithophores, foraminiferas,andcorals,which eitherarephotosynthetic (coccolithophores) or often coupled tophotosynthetic symbionts (foramniferas and corals). Itshould be noted that even in case of perfect coupling of reactions (7) and (4) , and if reaction (6) fully describescarbonate biomineralization, an increase of CO 2 concentra-tion (i.e. ocean acidication) will still be adverse tocarbonate biomineralizing organisms if dissolutionof CaCO 3occurs. Such a scenario would damage the carbonate shellsand skeletons and favor an ecological transition (a majorbiological crisis and possible extinction event) toward non-carbonate producing organisms such as diatoms.

How can these concepts be transferred to the subterra-nean context of a CO 2 injection site? Not easily, obviously.Of course, photosynthesis will not occur deep in thesubsurface. Butother mechanisms can lead deep biospheremicroorganisms to induce carbonate precipitation. Al-though biomineralization of carbonates has not yet beendocumented in the subterranean biosphere, one microor-ganism, Sporosarcina pasteurii , also often calledBacillus pasteurii , isolated from wastewaters, has provento be very efcient for carbonate biomineralization underunderground conditions ( Ferris et al., 2004; Mitchell andFerris, 2005 ). This microorganism performs urea degrada-tion, causing a pH increase which favors the precipitationof carbonates, according to the following urea hydrolysis(8) and carbonate precipitation (9) reactions:

H2 N2 CO 2H2 O ! 2NH 4 CO3 2 (8)

Ca CO3 2 ! CaCO3 (9)

Several laboratory experiments ( Dupraz et al., 2009a,b )have shown that injection of urea and of B. pasteurii in asynthetic aquifer water (inspired by the Paris Basin Doggeraquifer) leads to an effective pH increase and to theformation of CaCO 3 (Fig. 8). Of course, this has nooperational value for CO

2 injection and should only be

considered as a useful model since urea degradation

produces its own inorganic carbon, as shown by reaction(8) . The next step in the context of possible application tocarbon dioxide geological storage would be to ndsubterranean species with metabolisms related to thatof B. pasteurii , able to degrade organic matter intoammonia and into partially or totally denitrogenizedorganic molecules according to a reaction of the type:

R-NH2 H2 O ! NH3 R-OH (10)

In that case, the hydrolysis of the amine would beassociated with a pH increase, allowing conversion of injected CO 2 into carbonate resulting in calcite precipita-tion. This process would imply that there is sufcient pre-existing organic matter in the injection reservoir or to co-inject wastewaters with the carbon dioxide, which can beenvisaged since CO 2 injection will never occur in potentialwater resource aquifers.

A second issue is the interaction of injected CO 2 with theproduction of organicmatter discussed in therstsectionof this paper.At theEarths surface,someCO 2 is converted intorefractory organic matter, escaping the almost fully closed(to about 999 per mil) cycle of photosynthesis andrespiration, thus constituting a second stable formof carbondioxide storage (the so called kerogen reservoir, which hasabout onefth of the mass of solid carbonates, a proportionthat has been maintained over most of the Earths history,e.g., Schidlowski, 1988 ). Can similar processes occurunderground at CO 2 injection sites? As already mentioned,no photosynthesis occurs in the absence of light; however,some other modes of primary production are adapted tounderground conditions. Among those actually documen-ted, methanogenesisis probablythemost important ( Fig.9 ).Other metabolisms also exist (e.g., acetogenesis) that alsomostly rely on H 2 naturally present in the subsurface. SomeCO2 might thus bepumpedby thermodynamically favorablereactions with autochthonous or co-injected H 2 . The fate of the organic matter thus synthesized (in rst instancemethane and acetic acid) from CO 2 and H 2 in the deepsubsurfaceisabsolutelyunknownbecausethe knowledge of the biological cycle of organic molecules in the deepsubsurface is only in its infancy. It is thus hard to speculatewhether these processes could operateat CO 2 injection sitesfor a durable conversion of carbon dioxide into stableorganic matter in response to large CO 2 overconcentrations.Also, the extent of biomass that could be stimulated bycarbon dioxide injection and would thus constitute anotherform of CO 2 storage is unknown. Geotechnical engineerscontending with the development of massive biolms indeep geothermal and oil elds installations know thatbiomassformationis notalwaysnegligibleandthatbiocidesneed to be injected foravoiding these types of undergroundblooms (e.g., Fardeau et al., 2009 , and references therein).The consequences of CO 2 injection on autochthonous or co-injected microorganisms are thus largely unknown andneed to be studied as a high priority. Indeed, methanogen-esis, although considered by some as a way to backtrans-form CO 2 into a useful CH 4 resource in former gas reservoirs,could also be seen as a very adverse effect of the interactionof injected carbon dioxide with the deep biosphere. This isbecause methane has a stronger greenhouse gas potential


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than carbon dioxide, and even small leakages would becounterproductive with respect to greenhouse gas balance.Incubation experiments with cores drilled from the deepsubsurface with special care to maintain the original (verysmall) biomass and to avoid contaminations are probablythe most appropriate way to progress in this so far verypoorly constrained research area. One point of knowledgewe have in this ocean of unknowns is provided by themicrobiology of oil elds ( Ollivier and Magot, 2005 ) and bythat of deep aquifers (e.g., Fardeau et al., 2009 ). We knowfrom these studies that sulfate-reducing bacteria areprominent in these media ( Fardeau et al., 2009; Ollivierand Magot, 2005 ). In ocean waters, methanogenesis andsulfate reduction are indeed sometimes intimately related;microbial consortia (or looser connections) of methano-trophic and sulfate-reducing organisms have been evi-denced (e.g., Boetius et al., 2000 ), which transform methaneaccording to the following reaction:

CH4 SO4 2 ! CO3 2 H2 S 2H2 O (11)

In this process, carbonate ions are produced whicheventually lead to the formation of calcium carbonates(e.g., Aloisi et al., 2000 ). The whole biological transforma-

tion of CO 2 can thus be represented by the followingreaction:

Ca2 CO2 4H 2 SO4 2 ! CaCO3 H2 S 3H2 O (12)

Whether such metabolic phenomena can occur under-ground, nobody knows. A potential subsurface biologicalprocess based on this reaction is shown in Fig. 9. It wouldstore injected carbon dioxide as solid carbonates but

would, however, be a potential problem because of thegeneration of H 2 S (a corrosive and strong greenhouse gasthat could escape unless used by other microbes as itoccurs on the ocean oor). Such a biological phenomenonis related to well-documented so-called souring in oilreservoirs ( Ollivier and Magot, 2005 ). In some geologicalenvironments poor in sulfates or where iron-reducingmetabolism would have been favored (e.g., in the presenceof Geobacter or Shewanella species), the problem might besolved and could lead to durable conversion of the injectedCO2 into solid carbonates, FeCO 3 siderite in that case,according to the following reaction (see Fig. 9):

8CO2 4H 2 8FeOOH ! 8FeCO3 8H 2 O (13)

[

Fig. 8. Examples of vateriteand aragoniteCaCO 3 carbonates (SEMimages)formedas a result of thebiological activityof B. pasteurii inoculated withureain asynthetic Dogger aquifer water. Vaterite appears as spheres in which bacterial imprints are visible. Aragonite has a brous microstructure.

Fig. 8. Exemples de carbonates CaCO 3 (images de microscopie e lectronique a ` balayage) re sultant de lactivite biologique de B. pasteurii inoculee, avec delure e, dans une eau synthe tique de laquife `re du Dogger. La vate rite se pre sente sous forme de sphe `res dans lesquelles des empreintes de cellulesbacteriennes sont visibles. Laragonite a une microstructure breuse.


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Such microbial processes and environments need to beactively searched for in the deep subsurface. Interestingly,it is worth noting that some siderite and ankeriteprecipitation are expected at some sites such as Frio ( Xuet al., 2004 )far surpassing storage of injected CO 2 incalcite. The role of ankerite has also been emphasized inMcGrail et al., 2006 . Identication of possible biominerali-zation processes requires eld investigations of themineralogy of the new carbonates formed from theinjected carbon dioxide.

4. Conclusions

The purpose of this paper has been to show that bycareful examination of the natural processes of carbondioxide transfer from Earths uid envelopes to the solidEarth (i.e. precipitation of solid carbonates and formation of refractory organic matter), one might nd innovativeprocesses in the eld of CO 2 geological storage. We haveemphasized two important environmental mineralogy

aspects: (1)detailed understandingof thesurfaceof silicatesduring their dissolution is a key to discovering andimplementing rapid underground or ex situ carbonationof minerals; (2) understanding of carbonate biomineraliza-tion by deep biosphere microorganisms opens possibilitiesfor subterranean biological conversion of carbon dioxideinto stable carbonates. However, the deep biosphere alsohas the potential to complicate CO 2 geological storage bygenerating some strong greenhouse gases such as CH 4 andH2 S. Subterranean microorganisms, therefore, need to bebetter mapped and understood. In any case, comparison of laboratory (bio)mineralogy studies with eld observations(atCO 2 injection pilot sites, in enhancedoil andgasrecovery

systems, and in natural CO 2 elds) is a formidableopportunity (and challenge) for environmental mineralogy.

Acknowledgements

Parts of this research have been supported throughfunding by ADEME, Total, Schlumberger, and ANR (Agencenationale de le Recherche programmes CO 2 carbonatationand CARMEX). The authors are grateful to Pr. Kate Maherfor her insightful comments on this manuscript.

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