Raman Spectroscopy for Geological Materials Analysis

Natural rocks composing the Earth are complex. They consist of an aggregate of one or more minerals. Each mineral can be defined by its chemical composition and its crystalline structure and sometimes can also contain fluid inclusions. Geologists need a powerful characterization technique to get detailed information on the rock formation history. Raman spectroscopy is extremely information-rich (chemical identification, characterization of molecular structures, effects of bonding, environment and stress on a sample). With its non-destructive properties and high spatial resolution (< 1 m), it is thus a tool of choice for geological studies.

Raman Spectroscopy specificities

The Raman effect is highly sensitive to even slightdifferences in chemical structures. Even if not allvibrations are observable with Raman spectroscopy(dependingonthesymmetryofthemoleculeorcrystal),sufficient information can be obtained to discriminatedifferentstructuralgroupsorphasesrelatedtothesamemineralclass,likesilicatesorcarbonates.

Sometimes a high spectral resolution is needed toresolve closely spaced peaks. This is the case whenspecificpressureandtemperatureconditionsareappliedto a mineral using for example a diamond anvil cell(DAC)torecreatewhatcanhappendeep intheearth.Indeedspectraldiscriminationwillbe thekey factor todetectsmallpeakshifts inducedbyhighpressureandtemperatureconditions.

Systemsofferingmultiplelaserexcitationscanefficientlycounterthepossiblefluorescenceofgeologicalsamples.Coupledtoconfocalmicroscopy,Ramanspectroscopyprovides information with high spatial resolution (< 1m).

On-site analysis can also be undertaken with mobileunitspossiblybyusingremotesamplingaccessories.

Ramanspectral librariesarealsoagreathelp foreasyidentificationofminerals,relyingonsupplierslibraries[1]orondatabasescreatedbytheend-user.

Chemical selectivity

Silicate minerals

Thesilicateminerals,oftendesignatedsimplybythetermsilicates,representthemostimportantclassofrocksfor-mingthecrustandmantleoftheEarth.Asilicateisacom-poundwhose skeleton is essentially composed of siliconandoxygen tetrahedraandwithadditionalelementssuchasaluminum (aluminosilicates),magnesium, iron,calcium,potassium,etc.Theyareclassifiedbasedonthemode in

Olivine is one of the most common minerals of theigneousrockstype (formedbymagma). Itbelongstothenesosilicates (or orthosilicates) subclass characterized byanisolatedtetrahedra[SiO4]

4.ItsRamanspectrumshowsintensestretchingmodesinthe800-1000cm-1regionandbendingmodesbetween300and650cm-1.(Figure2)

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Figure 1: Different tetrahedral arrangements

Raman SpectroscopyRA23

GeologyApplica

tion Note

whichthesilicategroupscoalesce.TheRamanspectraofthedifferentclassesofsilicatemineralsproviderapididen-tificationoftheparticularclass.Figure1indicatessomeofthearrangementsoffusedtetrahedrathatarepossible.

Figure 2: Raman spectrum of olivine

The term inosilicates is used for chain silicates. Theyare divided in two groups: pyroxenes and amphiboles.PyroxenescontainsinglechainsofSiO4groups.Likejadeite,theirspectrapresentanintenseRamanbandbetween650and700cm-1[2].Incontrasttothesinglechainstructureofpyroxenes,amphibolescontaindoublechainsoftetrahedra.One representative amphibolemineral is actinolite and itsvarietynephrite.Bothjadeiteandnephritearerecognizedasthegemstonejade(Figure3).

Feldsparsandzeolitesbelongtothetectosilicatesgroups,also known as three-dimensional framework silicates.Feldspars can have three end-membered elements:potassium (orthoclase), sodium (albite) and calcium(anorthite).Avariation inpeakpositions,especially for thepeaksnear500and280cm-1canbeusedtodiscriminatebetweenthem.Becauseoftheirporousproperties,zeolitesarerarelypureandnumeroustypesofzeoliteframeworksexist.Anatrolitespectrumisgivenforexample(Figure4).Themostintensebandat532cm-1correspondstothebreathingmodeofthefour-memberedalumino-silicatering[3].

Figure 3: Nephrite and jadeite Raman spectra

2

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-between1080and1100cm-1thisbandisspecifictothecarbonateionCO3

2-

AtestsamplecontainingdifferentspeciesofcarbonateshasbeenfullyimagedusingtheDuoScanoptioninmacro-map-pingmode.Averagespectra inpixelsof200m2havebeencollectedonasurfaceofabout1cm2 in lessthananhour.The lowfrequencyregionhasbeenstudiedandsixdifferentspecieshavebeeneasilydetected.Theirdistributionandassociatedspectraaregiveninthefigure5a.Anothermap in traditional point-by-pointmode has beencarried out on a small area containing only two dolomitecrystals. The high spectral resolution of the LabRAM HRhasenableddetectionof thesmall frequencyshiftexistingbetweenthetwocarbonates, thegreenonehavingtracesofFe(Figure5b).

Figure 4: Natrolite and orthoclase Raman spectra

Carbonates

Like silicates, carbonates areminerals that are present inabundanceonthesurfaceoftheEarth.Theyarecharacte-rizedbythepresenceofacarbonateionCO3

2-andRamanspectraareslightlydifferentdependingonthedivalentca-tion(Ca2+,Fe2+,etc)coordinatedtothecarbonateion.Fre-quencyshiftscanbeobservedintwospectralareas:-between120and450cm-1thisregionindicatescrystal-lographicstructures

Figure 5: a- Raman imaging of different types of carbonates crystals using the Duo-San option and their associated spectra

b- High spectral resolution map of two dolomite crystals based on the small spectral shift existing between the two crystals spectra.

Silicate and carbonate classes contain numerous species,each of them having characteristic Raman bands. Ramanspectroscopy is thus an ideal technique toquickly identifythem.

Fluid inclusions and confocality

Mineralsoftencontainfluidsorsolidswhichhavebeentrap-pedduringtheformationofthecrystalandthuscanprovideinformationabouttheconditionsexistingduringtheminerali-ziation(composition,temperature,pressure).Liquidorgasbubblesarecalledfluidinclusions.Theirsizerangesfromlessthan amicrometer to several hundredmicrometers in dia-meter.ARamansystemcoupledtoaconfocalmicroscopeis required toobtainhighspatial resolutionand toanalyseinclusionslocatedbelowthesurfacewithoutdestruction.

Principle of confocality

Inaconfocalmicroscope,anadjustablepinhole isoptical-ly conjugated to the sample. Thus, all out-of-focus radia-tionscanberejectedfromthecollectedRamansignal.Forexample,ifthepinholeisclosedenough,asmallembeddedinclusionsignalisefficientlydiscriminatedfromthematrixsi-gnal.Thefigure6illustratesthisprinciple.Inconfocalmode,thesignalofthematrixislessvisiblethaninthenon-confocalmode.

Point measurements

Raman analysis of a 20 m fluid inclusion embedded inaquartzmatrixwascarriedouton theLabRAMHR.Theinclusion is a two phase system, where the gas phaseconsistingofamixtureofCO2(1285cm-1and1388cm-1),CH4 (2914 cm

-1), N2 (2331 cm-1) and H2S (2611 cm

-1) inenclosedintheliquidphase(3200-3700cm-1)(Figure7)

Figure 6: Comparaison of the confocal and non confocal mode for the analysis of a fluid inclusion

Figure 7: Raman spectra obtained in different areas of the fluid inclusion

Figure 8: Raman image of a fluid inclusion

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Use of a corrected objective

Fluid inclusions are embedded in a transparent or semi-transparent matrix with an index of refraction higher than1.4. Formatrix having an index of refraction close of thisglass, theuseofadryobjectivewithcoverslipcorrectioncanbeasolutiontominimizethedegradationofthelateraland axial resolutions impliedby thedrastic changeof theindex.[4]AtestwasperformedonaCO2bubbleembedded100mdeepinaquartzmatrix(Figure9).TheRamanpeaksof theCO2 (1285and1388cm-1)aremuchstrongerwhenusingthecorrectedobjectivebecausethecorrectionsprovidebettercollectionefficiencyandfocusontheentranceslitofthespectrograph.

Because of their size and location inminerals, fluid inclu-sionscanbehardtoanalyze.Withitshighspatialresolution,Ramanspectroscopyisveryusefultodeterminethecompo-sitionoftheembeddedfluids.Imagingoffluidinclusionsisalsopossibleandtheuseofcorrectedobjectivecanhelptoobtainhighersignals.

Conclusion

Ramanspectroscopyisapracticalexplorationtooltostudygeologicalmaterials.This technique isa rapidand reliableway to confirm, for example, that a geological sample isreallyajadegemstone,orthatitscompositionisdolomite.Chemical informationcanbeobtainedwithoutanyextrac-tionprocedureorsamplepreparation.Thus,theanalysiscanbeachievedinsituandifnecessarydirectlyatthegeologicalsite.RamanimagingisalsousefultostudyheterogeneoussamplesprovidedbythevariousfieldsofapplicationofEarthScience:mineralogy,gemmology,petrology,geoarcheology,paleontology,planetologyorvolcanology.Bythemeanofthisvibrationaltechnique,thecomplexhis-toryoftheEarthcanbebetterunderstood.

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Imaging

Thehighspatialresolutionofthesystem,betterthan1m,enablesonetoimagethecomponentsofthefluidinclusionpresentesintheexampleaboveandsohighlightthedistri-butionofthedifferentphases(Figure8).

Figure 9: Raman spectra obtained in different areas of the fluid inclusion

References

[1]SpectralID(3.02)ThermoGalactic(HoribaJobinYvonDatabaseofRamanspectra)[2]HandbookofVibrationalSpectroscopy,vol.4,Ed.J.M.ChalmersandP.R.Griffiths,Wiley,p3180[3] Raman spectra and lattice-dynamical calculations ofnatrolite, S.V. Goryainov, M.B. Smirnoov, Eur.J.Mineral.,2001,13,507-519[4] Useof amicroscopeobjectivecorrected for acoverglasstoimproveconfocalspatialresolutioninsideasamplewithafiniteindexofrefraction,F.Adar,C.Naudin,A.WhitleyandR.Bodnar,AppliedSpectr