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The Canadian MineralogistVol.47,pp.1245-1255(2009)DOI:10.3749/canmin.47.5.1245

THE ORTHORHOMBIC STRUCTURE OF CaCO3, SrCO3, PbCO3 AND BaCO3: LINEAR STRUCTURAL TRENDS

SytleM.ANtAO§

Department of Geoscience, University of Calgary, Calgary, Alberta T2N 1N4, Canada

IShMAelhASSAN

Department of Chemistry, University of the West Indies, Mona, Kingston 7, Jamaica

AbStrAct

Thecrystalstructuresoffourisostructuralorthorhombiccarbonates,CaCO3(aragonite),SrCO3(strontianite),PbCO3(cerus-site),andBaCO3(witherite),wereobtainedbyRietveldrefinementsusingdataacquiredbysynchrotron high-resolution powderX-raydiffraction(HRPXRD).ForBaCO3,powderneutron-diffractiondatawereobtainedandrefinedbytheRietveldmethod.Foraragonite,wealsocarriedoutarefinementofthestructurebysingle-crystalX-raydiffraction.ThesecarbonatesbelongtothespacegroupPmcn,withZ=4.TheCO3groupisslightlynon-planar,andthetwoindependentC–Odistancesareslightlydifferent.TheCO3groupbecomesmoresymmetricalandlessaplanarfromCaCO3toBaCO3(M2+

radii:Ca<Sr<Pb<Ba).TheCaCO3structureis,therefore,themostdistorted,whereastheBaCO3structureistheleastdistorted.Severallinearstructuraltrendsareobservedinplotsofselectedparametersasafunctionoftheunit-cellvolume,V.Theseparametersareradiiofthenine-coordinatedM2+ cations, the unit-cell axes, the average<M–O>and<C–O>distances, average<O–C–O>angle, andaplanarity.TheselineartrendsaretheresultoftheeffectivesizeofthedivalentionicradiusoftheMcationsthatarecoordinatedtonineoxygenatoms.ThegeometricalfeaturesoftheCO3groupcanbeobtainedreliablyonlybyusingneutron-diffractiondata,especiallyinthepresenceofotherheavyatoms.

Keywords:CaCO3,aragonite,SrCO3,strontianite,PbCO3,cerussite,BaCO3,witherite,Rietveldrefinements,synchrotronhigh-resolutionpowderX-raydiffraction(HRPXRD),neutrondiffraction,crystalstructure.

SOMMAIre

Nousavonsaffinélastructurecristallinedequatrecarbonatesorthorhombiquesisostructuraux,CaCO3(aragonite),SrCO3(strontianite),PbCO3(cérusite),etBaCO3(witherite),parlaméthodedeRietveldenutilisantlesdonnéesacquisespardiffractionXàhauterésolutionavecrayonnementsynchrotron.PourleBaCO3,nousavonsutilisédesdonnéesobtenuespardiffractionneutronique,aussiaffinéespar laméthodedeRietveld.Pour l’aragonite,nousavonsaffiné lastructurepardiffractionXsurmonocristal.CescarbonatesrépondentaugroupespatialPmcn,avecZ=4.LegroupeCO3estlégèrementnonplanaire,etlesdeuxdistancesC–Oindépendantesdiffèrentlégèrement.LegroupeCO3devientplussymétriqueetmoinsaplanaireenallantdeCaCO3àBaCO3(M2+

rayon:Ca<Sr<Pb<Ba).LastructuredeCaCO3seraitdonclaplusdifforme,tandisquelastructuredeBaCO3seraitlamoinsdifforme.Plusieurstendancesstructuralessontévidentesdanslestracésdeparamètreschoisisenfonctionduvolumedelamailleélémentaire,V.ParmicesparamètressontlerayondescationsM2+àcoordinenceneuf,lesdimensionslelongdesaxesdelamailleélémentaire,lesdistances<M–O>et<C–O>moyennes,l’angle<O–C–O>moyen,etl’aplanarité.CestracéslinéairessontlerésultatdeladimensioneffectivedurayonioniqueducationMbivalentcoordonnéauxneufatomesd’oxygène.OnnepeutévaluerlesaspectsgéométriquesdugroupeCO3qu’aveclesdonnéesendiffractionneutronique,surtoutenprésencedesatomeslourdsdanslastructure.

(TraduitparlaRédaction)

Mots-clés:CaCO3,aragonite,SrCO3,strontianite,PbCO3,cérusite,BaCO3,withérite,affinementsdeRietveld,diffractionXàhauterésolutionavecrayonnementsynchrotron,diffractionneutronique,structurecristalline.

§ E-mail address:[email protected]

1246 thecANAdIANMINerAlOgISt

Bowen1971,Jarosch&Heger1986,Bevanet al.2002,Caspiet al.2005,Pokroyet al.2007).Pilatiet al.(1998)carriedout lattice-dynamicstudiesonseveralcarbon-ates.ThestructuresofaragonitewererefinedinspacegroupPmcn, except for one structure refined in thetriclinicsystem,spacegroupP1,byBevanet al.(2002).

Thecrystalstructureofstrontianitewasdeterminedby Zachariasen (1928), and refined byDeVilliers(1971)andJarosch&Heger(1988),whereasthecrystalstructureofwitheritewasdeterminedbyColby&LaCoste(1933)andrefinedbyothers(DeVilliers1971,Holletal.2000).

Studiesoftheisostructuralorthorhombiccarbonatephases are important in understanding the transitionsequencesinCaCO3andmayprovideinsightsintothebehaviorofcarboninthemantle(e.g.,Berg1986,Antao et al.2004).BecauseoftheimportanceofCO2seques-trationandpossibleorder–disorderofcationsandCO3groups,wehaverecentlyexaminedthecrystalstructuresofseveralcarbonates(Antao et al.2004,2008a,2009,Antao&Hassan2007).Inaddition,resultsofseveralhigh-pressurestudiesareavailableforCaCO3,BaCO3andSrCO3(Lin&Liu1997,Holl et al.2000,Santillán&Williams2004,Ono2007).

experIMeNtAlMethOdS

Sample characterization

OnesampleofaragoniteusedinthisstudyisfromTuscany,Italy,andoccursascolorlessneedle-likecrys-talsthatoccurinacavity.Thechemicalanalysisofthisaragonite (using small crystalsencapsulated inepoxyresin, polished, andcarboncoated)wasdoneusingaCamecaCamebaxelectronmicroprobe(EMP)usingtheoperatingprogramMBX(copyrightbyCarlHenderson,UniversityofMichigan);thecorrectionwasdoneusingCameca’s PAP program.The analytical conditionswere 15 kV, andwith a beam current of 9.6 nA.Adiffusebeamofelectronswasusedfortheanalysistoavoidvolatilization.Mineralswere used as standards[e.g., dolomite (CaKa,MgKa), siderite (FeKa), andrhodonite(MnKa)].Typicaloxideweightpercentagesresulting from theEMPanalysesaregiven (Table1).The chemical formula obtained for this aragonite isCa0.996Mg0.001Sr0.003CO3,which is close to the idealformula,CaCO3.TheaverageamountofSr, inatomsper formulaunit (apfu), is 0.005,withminimumandmaximumvaluesrangingfrom0to0.007apfu.

TheotheraragonitesampleisfromCuenca,Spain;it occurs as a large (several cm in size) euhedralcrystalhexagonalinshape.Thisaragonitesamplewasanalyzedinasimilarmanner(Table1),andtheformulaisCa0.985Sr0.014CO3.TheaverageamountofSris0.014atoms per formula unit (apfu), withminimum andmaximumvalues ranging from0.012 to 0.015apfu.ThisamountofSrisaboutthreetimesmorethanthatinthesamplefromItaly.

INtrOductION

Thenaturallyoccurringisostructuralorthorhombiccarbonatesarearagonite(CaCO3),strontianite(SrCO3),cerussite(PbCO3),andwitherite(BaCO3), listedwithincreasingsizeoftheM2+cation.Inaddition,isostruc-turalrare-earthandradiumcarbonates(YbCO3,EuCO3,SmCO3, andRaCO3) have been synthesized (Speer1983).Linear structural trends are expected to occurin theseseries; theircrystalstructureswereexaminedpreviously,butonlygeneraltrendswereobserved(DeVilliers 1971,Chevrier et al. 1992).TheM2+ cationsshould have some effect on the slightly aplanarCO3groupandonthetwoindependentbutslightlydifferentC–O distances, althoughCO3 groups are generallyconsidered to be rigid.WithX-ray diffraction, it isdifficulttoaccuratelylocatelightatomssuchasCinthepresenceofheavyatomssuchasPb(includingproblemsfromabsorption). For this reason, neutron-diffractionexperimentswereperformedonaragonite,strontianite,and cerussite (Chevrier et al. 1992, Jarosch&Heger1986,1988),butnotonwitherite.Theapparentlackoflineartrendstodateseemstoarisefromaninappropriatechoiceofastructuralparameteragainstwhichtoplotsuitablestructuralvariables.

Although the structure of aragonite iswell estab-lishedandhasbeenacceptedforalongtime,thefind-ingsofBevanet al.(2002)onasamplethatcontainsasmallamountofSratomwasunexpected.Theyrefinedthestructureofatwinnedcrystalofaragonitefromanunknown locality in space groupP1, and cast somedoubtaboutthestructuresofaragonitethatwererefinedinspacegroupPmcn.Thepresentstructureofaragonitewasrefinedinordertoresolvesomeofthiscontroversy.Refinementdatafortwoaragonitesamplesfromlocali-tiesdifferentthanthosesampledbyotherinvestigatorswerealsoobtained.

Thepurposeof thisstudy is toexamine thestruc-tural trends in the orthorhombic carbonatesCaCO3,SrCO3, PbCO3, andBaCO3 usingRietveld structurerefinements and synchrotron high-resolution powderX-ray-diffraction(HRPXRD)data.WealsostudiedbyneutrondiffractionthestructureofBaCO3.Inaddition,werefinedthestructureofonearagonitesampleusingasingle-crystalapproach.Theresultsfromthepresentstudygavelinearstructuraltrendsacrosstheseries.

bAckgrOuNdINfOrMAtION

Aragonite, the most common orthorhombiccarbonate,occursastheinorganicconstituentofmanyinvertebrate skeletons and sediments derived fromthem.Italsooccursasaprimaryphaseinhigh-pressuremetamorphicrocks.ThecrystalstructureofaragonitewasdeterminedbyBragg(1924)andWyckoff(1925).Subsequently,severalrefinementswerecarriedout(DalNegro&Ungaretti1971,DeVilliers1971,Dickens&

OrthOrhOMbIcStructureOfcacO3,SrcO3,pbcO3ANdbacO3 1247

The othermaterials (PbCO3,BaCO3 andSrCO3)wereobtainedashigh-purity(99.999%)reagent-gradepowders andwere assumed to be pure.The formulaMCO3wasusedinthestructurerefinementsbelow.

Single-crystal X-ray-diffraction study of aragonite from Tuscany, Italy

Asingle-crystal fragment from the Italian samplewasselectedunderabinocularmicroscopeforstructurerefinement using aBruker P4 automated four-circlesingle-crystalX-ray diffractometerwith a SMART–CCDdetector and graphite-monochromatizedMoKaradiationgeneratedat50kVand30mA.Werecordedour datawith an exposure time of 30 s per frame,withadetectordistanceof5.016cm.Fivesectionsofframeswerecollectedwithastepwidthof0.30°inw.All datawere integrated usingSAINt (Bruker 2000),and empirical absorption correctionswere performedusingSAdAbS (Sheldrick 2001).The datawere alsocorrected for Lorentz, polarization, and background

effects.Theunit-cell parameterswere calculated andrefinedfromalltheintensitieswithI>15s(I).However,thecellparametersusedinthesingle-crystalstructurerefinementwere thoseobtained fromRietveld refine-ment using synchrotronHRPXRDdata because thisapproachprovidesmoreaccuratecellparameters(seebelow).The structure refinementwas doneusing theShelxtl97 suite of programs (Sheldrick 1997).Therefinementswere carried out by varying the param-etersinthefollowingsequence:scale,atompositions,isotropic, and anisotropic displacement parameters.The cell parameters and other information regardingdata collection, criteria for observed reflections, andrefinement are given inTables 2a and2b.The spacegroupPmcnwasusedinthisstudy,andnoreflectionswereobservedthatviolatethisspacegroup.Finally,allvariableswererefinedsimultaneouslytoanRfactorof0.0277.TheatompositionsandisotropicdisplacementparametersaregiveninTable3a.Anisotropicdisplace-mentparametersaregiveninTable3b.Selectedbonddistances and angles are given inTable 4.Table 5containsobservedandcalculatedstructure-factors;itisavailablefromtheDepositoryofUnpublishedDataontheMACwebsite[documentAragoniteCM47_1245].

Synchrotron high-resolution powder X-ray diffraction(HRPXRD)

Fivesamples(syntheticSrCO3,PbCO3,BaCO3andthe two natural aragonite samples)were studied byHRPXRD.Theexperimentswereperformedatbeam-line11–BM,AdvancedPhotonSource(APS),ArgonneNationalLaboratory(ANL).Eachsamplewascrushed(forabout5minutes)toafinepowderusinganagatemortarandpestle.Thesampleswereloadedintokaptoncapillariesandrotatedduringtheexperimentatarateof 90 rotations per second.The datawere collected


to amaximum 2u of about 40°with a step size of0.0005°andasteptimeof0.2sperstep.TheHRPXRDtraceswerecollectedwithtwelvesilicon(111)crystalanalyzers that increasedetector efficiency, reduce theangularrangetobescanned,andallowrapidacquisitionofdata.AsiliconandaluminaNISTstandard(ratioof1/3Si:2/3Al2O3)wasusedtocalibratetheinstrumentand to determine and refine thewavelength used intheexperiment.Additionaldetailsoftheexperimentalset-uparegivenelsewhere(Antao et al.2008b,Lee et al.2008,Wang et al.2008).

Neutron powder-diffraction refinement of BaCO3

Time-of-flightneutron-diffractionmeasurementsonBaCO3wereperformedusingtheSpecialEnvironmentPowderDiffractometer (SEPD)of the IntensePulsedNeutronSource(IPNS)atANL.About8gofsample

waspackedintoavanadiumcan;datawerecollectedatambientconditionsoveraperiodofthreehours.

Rietveld structure refinements

To extract structural parameters, themeasuredHRPXRDandneutronprofileswere analyzedby theRietveldmethod (Rietveld 1969), as implemented intheGSASprogram(Larson&VonDreele2000),andusing theEXPGUI interface (Toby2001). Scatteringcurvesforneutralatomswereusedinallrefinements.Thestartingcoordinatesofallatoms, thecellparam-eters,andthespacegroup,Pmcn,weretakenfromDeVilliers(1971).ThebackgroundwasmodeledusingaChebyschev polynomial.The reflection-peak profileswerefittedusingtype-3profileintheGSASprogram.Full-matrixleast-squaresrefinementswerecarriedoutbyvarying theparameters in the followingsequence:a scale factor, cell parameters, atomcoordinates, andisotropic displacement parameters.Toward the endof the refinement, all theparameterswereallowed tovary simultaneously, and the refinement proceededto convergence.TheHRPXRD traces are shown inFigures1and2.NoreflectionsviolatingspacegroupPmcnwereobservedinthisstudy,whichisincontrasttotheworkonadifferentsampleofaragonite(Bevanet al.2002).TheneutrondiffractionpatternforBaCO3isshowninFigure3.

The cell parameters and theRietveld refinementstatistics are listed inTable 2a.Atompositions andisotropic displacement parameters are given inTable3a.BonddistancesandanglesaregiveninTable4.


reSultSANddIScuSSION

Structure of orthorhombic CaCO3, SrCO3, PbCO3, and BaCO3

ThephasesCaCO3,SrCO3,PbCO3,andBaCO3areisostructural,andhavethearagonite-typestructure(Fig.4).Layersofnine-coordinatedM2+(M:Ca,Sr,Pb,orBa)cationsareparallel to(001),andtheatomsoccurapproximately in the positions of hexagonal close-packing.This is in contrastwith the deformed cubicclose-packedarrangementofCa2+cationsincalcite,andgivesrisetothepseudo-hexagonalsymmetryintheseorthorhombic compounds. SuccessiveCO3

2– groupsalongc point alternately to the+b and–b directions(Fig. 4).Although theM2+ cations are nearly in ahexagonalarray,thearrangementoftheCO3

2–groupslowers the symmetry toorthorhombic, and theCO3

2–groupsareslightlyaplanar(Table4).

Linear structural trends

Theunit-cellparametersfortheisostructuralcarbon-ates are in excellent agreementwith those obtainedpreviously (e.g.,DeVilliers 1971, Jarosch&Heger

1986, 1988, Lucas et al. 1999, Holl et al. 2000).However, cell parameters obtained in this study aresignificantlymoreaccurate,astheywereobtainedfromHRPXRDdata(Table2a).Ingeneral,thehighqualityof theHRPXRD trace (including both statistics andangular resolution) allows the unit-cell parameters tobedeterminedaccuratelyandreliably.

UsingaHRPXRDtechniquesimilartothatusedinthepresentstudy,thestructureofanaragonitesamplefromSefrou,Moroccowas obtained byCaspiet al.(2005).Theircellparameters(seefootnotetoTable2)are in excellent agreement (within 2 3 10–4Å)withthoseforoursamplefromSpain,anddeviateslightlyfromthoseforthesamplefromItaly(within1 3 10–3Å).Thesesmalldifferences incellparametersamongaragonitesamplesarisefromdifferentcontentsoftraceelements, in particular Sr; the larger cell volume isassociatedwith larger amounts ofSr.The amount ofSr(apfu)inthesamplefromSpainisaboutthreetimesmorethanthatfromItaly(Table1).OtherstudieshavealsoshownthatSrisimportantinaragonite(Caspiet al.2005,Finch&Allison2007).

Strontium andMg atoms in calcite and aragoniteare used asproxies of temperature in paleoenviron-mental reconstructions. Finch&Allison (2007) usedX-ray absorptionfine structure (XAFS) to show thatSr substitutes forCa in aragonite and causes a small(2%)dilation,whereasSrsubstitutionforCaincalcitecausesa6.5%dilation.

AlthoughBevanet al.(2002)usedP1andmerohe-draltwinningtoproduceastructuralmodelforarago-nite,theirresultscannotbegeneralizedtoallsamplesof aragonite. In particular, their sample of unknownorigincontainsasmallamountofSr(about1atom%),whereasoursamplescontainminorimpurities,asisthecaseformostsamplesofaragonite,andgiverisetotheidealformulaCaCO3(seeDeeret al.1992).Caspiet al.


(2005)didnotobserveanyreflectionsviolatingPmcnsymmetry,asinourresultsforbothsingle-crystalandpowder samples. In powderX-ray diffraction, twin-ningeffectscanbeeliminated.ThelackofanysplitorunindexedreflectionpeaksinourdataindicatesthatasymmetrylowerthanPmcnisunlikelyforend-memberaragonite.This isalsosupportedby thesingle-crystaldataforouraragonitesamplefromItaly.

The radii of the nine-coordinatedM2+ cations(Shannon1976)andthea,b,cunit-cellparametersareplottedagainst thevolume,V (Fig.5).Data from theliteraturearealsoincludedinthegraphs(seecaptionto

fIg.1. ComparisonoftheHRPXRDtracesforaragonite,CaCO3,from(a)Tuscany,Italy,and (b)Cuenca,Spain, togetherwith the calculated (continuous line) andobserved(crosses)profiles.Thedifferencecurve(Iobs–Icalc)isshownatthebottom.Theshortverticallinesindicatethepositionsofallowedreflections.Inbothcases,theintensitiesbeyond20°2uarescaledbyafactorof320.TheFWHMin(a)ismorethanthatin(b).

fIg. 2. Comparison of theHRPXRD traces for SrCO3,PbCO3,andBaCO3,togetherwiththecalculated(continu-ous line) andobserved (crosses)profiles.Thedifferencecurve(Iobs–Icalc)isshownatthebottom.Theshortverti-callinesindicatethepositionsofallowedreflections.Notethechangeinintensityscaleathigh2u,andthefactorsaregiven as315,etc.TheFWHMofSrCO3 is the lowest,whereasthatofPbCO3isthehighest.


fIg.3. TheneutrondiffractiontraceforBaCO3,togetherwiththecalculated(continuousline)andobserved(crosses)profiles.Thedifferencecurve(Iobs–Icalc)isshownatthebottom.Theshortverticallinesindicatethepositionsofallowedreflections.

fIg.4. ProjectionoftheorthorhombicCaCO3structureshowingtheapproximatehexago-nalclose-packingoftheOatomsandthecoordinationofaCaatomtonineOatomsofsixdifferentCO3groups.TheO2atomsareunlabeled.


Fig.5).LineartrendsareobservedwithaR2valueof1,itsidealvalueforanexcellentfit(insertsinFig.5).

The average <M–O> distances are also plottedagainstV(Fig.6).Withallthedataincluded,alineartrendisobserved.ThepresentdataalsoshowthattheM–Opolyhedra become less distortedwith increaseinthesizeofM2+cations,asindicatedbyDeVilliers(1971).Thatis,theindividualM–Odistancesareclosertothemean<M–O>distanceinBaCO3thaninCaCO3(Table4).

The excellent linear fits of the above structuralparametersshowthatreliablecelland<M–O>distancescan easily be obtainedwith eitherX-ray diffractionor neutron diffraction, using either single-crystal orpowder samples (Figs. 5, 6).This is not the case forthegeometryoftheCO3groupbecauseofthelightCatominthepresenceofheavyatomsinthearagonite-typestructure.

The geometrical features of the CO3 group areplottedagainstV (Figs.7a,b, c).Only thedata fromneutron refinements are shown. If other datawereplotted(notshown),therewouldbeconsiderablescatter,indicatingthattheCatompositionisnotlocatedaccu-ratelywithX-raydiffraction.ThestructuralparametersoftheCO3groupinPbCO3areshown(asxsymbolsingraphs),buttheydonotfallonthetrendlines.Thosepointswere not included in the computation of thelineartrendlines.Itshouldbenotedthatthe<Pb–O>

distancedoes fallon theexpected trend line (Fig.6),so it appears that theC positionmay not have beenaccuratelydeterminedinPbCO3.

fIg.5. Theradiiofthenine-coordinatedM2+cationsandthea,b,cunit-cellparametersversusvolume,V.Theyincreaselinearly.Dataaretakenfromthisstudyandfromtheliterature(Caspi et al.2005,Chevrier et al.1992,DalNegro&Ungaretti1971,DeVilliers1971,Jarosch&Heger1986,1988,Pokroy et al.2007).

fIg. 6. Linear increase in average <M–O> distancewithvolume,V.Dataaretakenfromthisstudyandthelitera-ture (seecaption toFig.5).Theequationof the straightline and the goodness-of-fit (R2) are given as inserts inFigures6and7.


Theaverage<C–O>distanceand<O–C–O>angleincreaselinearlywithV,whereastheaplanarityoftheCO3 group decreases (Fig. 7).The aplanarity of theCO3 group is defined by the distance of theC atomfromtheplaneformedbythethreeOatoms.ThisCO3groupthusbecomesmoresymmetrical(C–Odistancesnearlyequal)andlessaplanaringoingfromaragonitetowitherite.Therefore, in this isostructural series ofmaterials,thearagonitestructureisthemostdistorted,

whereasthewitheritestructureistheleastdistortedintermsofthegeometryoftheCO3groupandthe[MO9]polyhedron.Thestructuralparametersofthisseriesofphases are influenced and correlatedwith size of theMcations.

AckNOwledgeMeNtS

WethankD.J.M.Bevan,S.Ness,oneanonymousreferee, theAssociate Editor, R.L. Flemming, andEditor, R.F.Martin for useful comments.We alsothankD.H.Lindsley andR.Marr for their helpwiththeEMPanalysis.TheHRPXRDdatawerecollectedattheX-rayOperationsandResearchbeamline11–BM,Advanced Photon Source (APS),ArgonneNationalLaboratory (ANL).We thank J.S.Fieramosca for hishelpwiththeneutron-diffractionmeasurements,whichwereperformedusingtheSpecialEnvironmentPowderDiffractometer(SEPD),IntensePulsedNeutronSource(IPNS),ANL.UseoftheAPSandIPNSwassupportedbytheU.S.DepartmentofEnergy,OfficeofScience,OfficeofBasicEnergySciences, underContractNo.DE–AC02–06CH11357. SMAacknowledges supportfromaUniversityofCalgarygrant,aDiscoverygrantRT735238fromtheNaturalSciencesandEngineeringResearchCouncilofCanada,andanAlbertaIngenuityNewFacultyaward.

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Received February 13, 2009, revised manuscript accepted October 6, 2009.