Structural signatures of medium-range order in annealed laboratory silicates

15 Pages·2008·0.67 MB·English

by  Thompson S. P.

A&A484,251–265(2008) Astronomy
DOI:10.1051/0004-6361:20078675 &
(cid:1)c ESO2008 Astrophysics
Structural signatures of medium-range order in annealed
laboratory silicates
S.P.Thompson
DiamondLightSource,DiamondHouse,HarwellScienceandInnovationCampus,Didcot,OxfordshireOX110DE,GB
e-mail:[email protected]
Received14September2007/Accepted18March2008
ABSTRACT
Context.Silicate dust grains exist in a wide range of astronomical environments and understanding the effect of these on grain
structureisofgreatinterest,particularlytheeffectofthermalannealingonamorphoussilicatesasapossibleroutetotheformation
of crystallinegrains. Although laboratory simulations havelargely focussed on IR-spectroscopic measurements, since these relate
directlytoobservationaldata,laboratorysynchrotrontechniquessuchasX-rayabsorptionspectroscopy(XAS)andX-rayscattering
arebecoming increasinglyroutineintheanalysisof recoveredmaterials.Withtheincreasingprospect of performingastronomical
XASobservations,thereismuchtobegainedfromapplyingthesetechniquestolaboratoryanalogues.
Aims.Diagnosticmarkersformedium-rangeorder andtheeffectofthermalannealingontheseinamorphoussilicatesofdiffering
MgcontentarecharacterisedusingsynchrotronX-raymethods.
Methods. Threesyntheticamorphoussilicateswithhigh,mediumandlowMg:Siratioswereannealedatvaryingtemperaturesup
to1300K.X-rayAbsorptionNearEdgeStructure(XANES)spectraattheSiandMgK-shellabsorptionedgesandX-rayscattering
patterns for low values of theX-ray scattering wavevector wererecorded for these along withcomparative data for commercially
producedamorphousSiO andmineralsamplesofforsterite,enstatiteandquartz.
2
Results. XANESfeaturesduetoshort-andmedium-rangestructureareidentifiedatbothMgandSiedgesandanewtemperature
dependentbehaviourobservedinthemedium-rangestructuresurroundingMgandSi.Basedonchangestothemorphologicaldetails
oftheXANESspectra,themedium-rangestructurechangesbetweenanenstatite-likeandforsterite-likecoordinationwithincreasing
temperature and appears to correlate directly with Mg content. Low wavevector X-ray scattering features were also found to be
diagnosticofthetypeofmedium-rangestructuralordering.However,thesefeaturesdependonwhethertherelativearrangementof
clustersofmedium-rangestructureexhibitsemi-periodicorderingoverthelonger-range,whichcanvarywithannealing.
Keywords.methods:laboratory–X-rays:ISM–ISM:dust,extinction–stars:circumstellarmatter–comets:general
1. Introduction silicates exhibit strong absorption resonances. However, it was
quicklyrealisedthatIRspectroscopyinisolationcannotreveal
Ever since the Infrared Space Observatory (ISO) revealed the thefullstoryunderlyinggrainpropertiesasthelimitedstructural
presenceofcrystallinesilicatedustinawiderangeofastronom- informationreturneddoesnotallowforuniquesolutionsorcon-
ical settings (e.g. Waters et al. 1996; Tielens et al. 1998) there straintstobefound(seereviewbyColangelietal.2003;ordis-
hasbeenmuchinterestinthethermalprocessingofamorphous cussionbyNuthetal.2002).Secondarycharacterisationbylab-
silicategrainsasapossibleroutetotheirformation.PriortoISO, oratorytechniquessuchasEDX,diffraction,SEM,TEMetc.of
silicates were believed to form as disordered, or amorphous, propertiesnotdirectlyobservablebyastronomershavethusbeen
grains in the outflows of O-rich stars at various stages of their required in order to constrain the spectroscopic measurements,
evolution (e.g. Dorschner 1993). As such, crystalline silicates butdonotalwayseasilytranslateintoobservationalpredictions.
posesignificantproblems.Forexample,crystallinesilicate fea- Comparativemeasurementsusingsecondarytechniquesoncos-
turesinstarswithhighmass-lossrateschallengeourunderstand- mic grains from meteorites and collected interplanetary dust
ingof thenucleationandsubsequentevolutionofcircumstellar particles (IDPs) has offered a useful channel for validating re-
matter(Kemperetal.2001),whilesimilarfeaturesinthespectra sults from analogue materials, though in both instances some
ofcertaincomets(Bregmanetal.1987;Campins&Ryan1989; degree of selection via secondaryprocessing/alterationof pris-
Crovisieretal.1996;Russell&Lynch1996;Hayward&Hanner tine grainswithin parentbodiesis likely to have occurredwith
1997), young pre-main sequence stars (Waelkens et al. 1996) only the most refractory components surviving through to the
anddebrisdisksystems(Knackeetal.1993;Fajardo-Acosta& meteorite and IDPpopulations.The recentsuccessful returnof
Knacke1995)similarlychallengeourunderstandingofthepre- whatarelikelyto bepristinegrainsbySTARDUST (Brownlee
vailingconditionsandprocessesthatoccurduringtheformation et al. 2006; Ishii et al. 2008) along with the development of
ofplanetarysystems. micro-handlingandcharacterisingtechniquesnowoffersadirect
To place physical constraints on both models and obser- routeto ground-truthvalidationofmeasurementsmadeon lab-
vations it is necessary to investigate the behaviour of silicate oratoryanaloguesusingtechniquesotherthanIRspectroscopy.
analogue materials through laboratory investigation. Early ef- Indeed,IRtechniquesarelikelytobelessprominentintheanal-
forts were primarily aimed at reproducing astronomical obser- ysisofSTARDUSTsamplesduetothepotentiallysmallparticle
vations, particularly in the region near 10 and 20 µm where sizes (∼5 µm) and the aerogelenvironmentthey are embedded
ArticlepublishedbyEDPSciences
252 S.P.Thompson:Medium-rangeorderinannealedsilicate
in.IRmicrospectrometryoverthe2.5–20µmregionwheresili- onlylimitedobservationalresultshavesofarbeenobtained.Lee
catesshowtheirstrongestabsorptionfeaturesislikelytobecom- etal.(2001,2002)claimedtohaveobservedstrongmodulations
plicated by the strong responseof aerogelin this regiondueto neartheSiK-edgealonglinesofsighttoseveralblackholesys-
its SiO content, though Keller & Flynn (2003) have obtained tems, though poor statistics and the possibility of the modula-
2
some mineralogical information from measurements made be- tions being instrumental in origin make the identification with
yond20µmonaerogelembeddedparticles.Ramanmicrospec- dust grains questionable. Petric et al. (2005) identified oscilla-
troscopymayalsobesimilarlylimitedbystrongaerogelfluores- tionsinthespectrumofthemicroquasarsourceGRS1915+105
cence,thoughagainthetechniquehassuccessfullybeenusedon also near the absorption edge energy of Si, while Ueda et al.
largergrains(Burchelletal.2004). (2005)reportedobservationsoffinestructureneartheSiK-edge
attributedbythemtosilicatedustinthegalacticISMalonglines
X-ray scattering and X-ray absorptionspectroscopy (XAS)
ofsighttothreehighlyabsorbedX-raybinaries.Thesefirststeps
are two non-destructive laboratory techniques for probing and
towardsrealising astronomicalXAS, along with the possibility
characterising material structure which, following the develop-
of collecting laboratory data from recovered grains now mean
ment of high intensity tuneable synchrotron sources, have be-
that XAS is set to become a primary channel for gathering in-
come routinely available in materials based disciplines. While
formation about the nature of cosmic dust. Laboratory simula-
X-rayscatteringhasoftenbeenusedtodeterminewhetherastro-
tion is likely to play as pivotal a role here as it has in the IR,
physicalanaloguesare amorphousor crystalline, the use of X-
for although XAS is well understood from a theoretical point
ray absorption spectroscopy has so far been limited (Colangeli
ofview,calculationsfromfirstprinciples,particularlyinthelow
et al. 2003). XAS involves measuring the small oscillations in
energyXANESregion,areverydetailed,involvealargenumber
the X-ray absorption coefficient near the absorption edge of a
ofvariableparameters,arecomputerintensiveandhighlymodel
givenatomicelementthatarisefromthescatteringoftheejected
dependentwiththeconsequencethatmuchXANESworkisstill
photoelectron by surrounding atoms and provides information
empiricallydriven.
about their localised arrangement. The post-edge XAS spec-
XANES features in silicates originate from structural ar-
trum is typically divided into two main regionswith respectto
rangementsextendingouttoradialdistancesof∼10Å(Wuetal.
theejectedphotoelectronmomentum,k :thelow-energyregion
e
(k <∼ 4Å−1),whichproducestheX-rayabsorptionnearedge 1998; Mottana et al. 1999) and are thereforeof particular rele-
e vancetothestudyofastrophysicalsilicatessinceitisthevibra-
structure (XANES) and the high-energyregion (k >∼ 4Å−1),
e tions of functional units on this scale of tetrahedral and inter-
which produces extended X-ray absorption fine structure
tetrahedralbondingthatgiverisetomuchoftheircharacteristic
(EXAFS).Thelatterhigh-energyregionisdominatedbysingle
IRspectralfeatures(Hofmiester1997;Jägeretal.1998).Indeed,
scatteringeventsfromimmediateneighbouringatoms,whereas theopticalpropertiesofamorphoussilicatesinboththeUV/VIS
the low-energy region is dominated by multiple scattering in- and IR regions arise from the presence of different structures,
volvingmoredistantneighbours.
each with varying optical constants, that are themselves deter-
Withthedevelopmentofmicro-focussingandsamplescan- minedbyshort-andmedium-rangeorder(Jägeretal.2003).
ningtechnologiesatsynchrotronsources,XAShasbeenapplied In this paper the evolution, with annealing temperature,
to individualIDPs (e.g.Flynn etal. 2003, 2004;Brennanetal. of medium-range structure is investigated using synchrotron
2005) and to cosmic grains embeddedin meteorites (Bernhard X-rays. The local environmentssurroundingMg and Si are in-
etal.2006).Indeed,X-raytechniquesingeneralarelikelytobe vestigatedviaXANESmeasurementsattheMgandSiK-edges
importantinthefutureanalysisofSTARDUSTandsimilarlyre- and features attributable to short- and medium-range ordering
covered samples since aerogel is highly transparent to X-rays, are identified. A new thermal behaviour is observed, relating
duetoitslowdensityresultinginlongpenetrationdepths(Bish to changes in the medium-rangestructure characterised by the
et al. 2001). Preliminary investigationsin this direction are al- XANESsignalmorphology.TheresultsofX-rayscatteringmea-
readybeingreported(Flynnetal.2006;Gainsforthetal.2007; surements, conducted using low values of the X-ray scattering
Grossemyetal.2007a,b;Matrajtetal.2007).Meanwhile,spec- wavevectorarealsopresented.Underthiscondition,featuresin
ulation on the possibility of performingastronomicalXAS ob- thescatteringpatternsareidentifiedwiththepseudo-periodicar-
servationsforcondensedmatteralongthelinesofsighttosuit- rangement of different types of medium-range structural clus-
able X-raysourceshashad a longhistory(Martin1970;Evans tersandtheevolutionofthesearrangementswithtemperatureis
1986; Woo 1995; Woo et al. 1997; Forrey et al. 1998; Lee & investigated.
Ravel2005).AlthoughtheearliestdetectionofX-rayabsorption
featuresassociatedwiththeISMweremadebySchattenberg&
Canizares(1986),itwasnotuntilthelaunchoftheCHANDRA 2. Theory
X-ray observatory that these speculations became a realistic
2.1.Orderinginthesolidphase
possibility (Woo 1995; Woo et al. 1997; Forrey et al. 1998).
Although most discussions on astronomical XAS have largely Unlikecrystallinematerials,wherestructureischaracterizedby
concentrated on the EXAFS region where calculations using long-rangeperiodicorder(i.e.bycorrelationsbetweentheposi-
model compounds are relatively straight forward, the low en- tionsofatomssituatedasfarawayaspossiblefromeachother),
ergy XANES region represents a better observational prospect amorphousornon-crystallinematerialswerehistoricallyclassi-
sincecertainfeaturesinthisregiontendtobestronger.Theam- fied by the absence of long-rangeorder. However,once the re-
plitudeof EXAFSoscillationsbothdecreasein inversepropor- quirementforlong-rangeorderis removedwhatremainsis not
tiontothephotoelectronmomentum,k ,andaresimultaneously a total disorder, but a certain limited order termed short-range
e
damped by the action of a term involving the finite lifetime of order,definedbytheinteratomiccorrelationsinthefirstcoordi-
the photoelectron such that at increasingly higher k its mean nationspheressurroundinganarbitraryatom(i.e.uptothemaxi-
e
freepathshortensandittravelsshorterdistancesbeforebeingre- mumdistanceoverwhichatomicforcescanact).However,many
absorbed.TheseeffectscombinetomaketheXANESsignalfor non-crystallinematerialswerefoundtoexhibitadegreeofstruc-
mostmaterialsmorepronouncedthantheEXAFSone.However, turalorderthatextendedovermuchlargerdistancesand,onthis
S.P.Thompson:Medium-rangeorderinannealedsilicate 253
basis, a new concept of medium-range (or intermediate-range) deGroot2001).XASspectra(e.g.Fig.8)generallycontainfour
orderwasintroduced(Philips1979;Lucovski&Galeener1980). distinctregions(seereviewbyStern1978;orAgarwal1991):
Orderonthisscalederivesfromstructuralcorrelationstypically
– theabsorptionedgestep;
in the range of ∼5 to 20 Å and is in excess of what would be
– a large intense feature (termed the white line), sometimes
expectedforanidealcontinuousrandomnetworkcharacterized
presentattheabsorptionedgepeak;
byarandomdihedralangledistribution. – a post-edgeXANESregionextendingfrom∼10eVupto40
The forces linking atoms are the same in both crystalline
or50eVbeyondtheabsorptionedge;
and non-crystalline solids and, differences in formation condi-
– apost-edgeEXAFSregionextendingfromtheXANESregion
tions aside, the extended three dimensional structures of non- outtoa∼few×100eV.
crystalline solids are producedin much the same way as those
incrystallinematerials,butwithouttherequirementsofperiod- FinestructureintheXANES/EXAFSregionsarisesfromtransi-
icity and symmetrythatcharacterizethe crystallinestate. Non- tionsoftheejectedinner-shellphotoelectronfromboundtoun-
crystallinematerialsarethusbydefinitiondisorderedinsomere- boundcontinuumstates,withthedifferencebetweentheincident
spectoftheirstructure.Unfortunately,order/disordercannotbe X-rayenergyandtheelectron’sbindingenergybeingconserved
preciselydefinedfornon-crystallinematerialsandusuallyrests as kinetic energy. The photoelectron undergoes elastic and in-
on determining what degree of order can be identified experi- elastic scattering from the surroundingelectron density associ-
mentally at different length scales. Four generic scales can be atedwithneighbouringatoms,causinginterferenceeffectsinits
identified(Wright1994): wavefunction.For N0 absorbingatoms per unit volume,irradi-
ated by X-rays of circular frequency ω, the absorption coeffi-
1. thebasicrepeatingstructuralunit; cient,µ,canbedescribedbythedipoleapproximation,
2. theinterconnectionofthesetoformanetwork; ω
3. thenetworktopology; µ= N0π2e2c |(cid:3)f |z|i(cid:4)|2 ρ(Ef). (1)
4. long-rangedensityfluctuations.
Here z is a scalar potential proportional to distance and ρ(E )
f
isthedensityoffinalstatesfortheunboundphotoelectron.The
Thefirsttworepresentshort-rangestructure,whilethethirdand
electron’sinitialboundstateis|i(cid:4)anditsfinalstate|f(cid:4),thelatter
fourthdescribemedium-andlong-rangestructurerespectively.
beingthesumofasphericaloutgoingwaveplusabackscattered
Short-rangestructureencompassestheinteratomicregionof
wave due to surrounding atoms. These combine constructively
nearest neighboursand first coordination shells, generally rep-
anddestructivelysothatµdoesnotvarysmoothlywithenergy,
resented by a structural building block with a radial distance
butdisplayscomplexoscillatoryfinestructure.
typically out to ∼2 Å. In silicates this unit is the Si-O tetrahe-
When the excess photoelectron energy is below ∼10 eV,
dron and is ubiquitous, varying little from one silicate to an-
inelastic scattering from valence electrons at binding energies
other (crystalline or otherwise). Consequently, at this level sil-
less than 10 eV is significant and the mean free path shortens
icates are highly ordered. However, the parameters governing
(∼fewÅ),scatteringisthenfromnearestneighbouratomsonly.
the way these units link togetherare highly variable (Mozzi&
However, elastic scattering mainly occurs from electrons with
Warren1969)anditiswiththeinterconnectionoftetrahedra,at
much greater binding energies(which do not normally partici-
theboundarybetweentheshort-andmedium-range,thatdisor-
pate in chemicalbonding)and beyond10 eV pastthe edgethe
dercanbegin.Medium-rangeordergenerallyextendsfrom∼2Å
photoelectronmeanfreepathbecomeslongandscatteringfrom
out to a radialdistance of ∼10–20Å (Wright1990). Structural
distantneighboursisstrong,withmultiplescatteringeventsfor
features such as rings or chains occur over this range. Long-
small and large bond angle configurations making significant
rangeordercompriseslargerperiodicandsymmetricgroupings
contributions to the underlying scattering from pairs of atoms.
oftheseclusteringelementsoverlengthsgreaterthan∼10–20Å
Consequently, the XANES region is sensitive to the structural
andorderingatthislevelequatestocrystallinity.
symmetryofthelocalenvironmentandhencethebondingstate
Although medium-range order is believed to play an im- oftheabsorberaswellaspaircorrelations.Inone-electronmul-
portant role in determining many of the physical-mechanical tiple scattering theory (Lee & Pendry1975; Natoli et al. 1980;
properties of solid disordered materials (Elliot 1983), both its Natoli & Benfatto 1986; Natoli et al. 1990) XANES peak pro-
physicalnatureandinfluenceonsuchpropertiesisstillhowever files and intensities are related to the number of possible scat-
poorly understood.Indeed, the relationshipbetween short- and tering pathways accessible for a given photoelectron energy.
medium-range order varies between material systems. For ex- Hence XANES are affectedby site symmetry,numberof near-
ample in covalent systems where the short-range order is well estneighbouratoms,bondanglesanddistances(Bianconietal.
definedandisstrictlydependentontheconstraintsofchemical 1982), while peak energiesare related to interatomic distances
bonding,orderingoverthemedium-rangearisesasadirectcon- andhencebondproperties.
sequenceofshort-rangeorder.However,incertainmetallicsys- XANES end when the photoelectron mean free path again
tems (e.g.amorphoustransition metal-metalloidalloy systems) decreases and multiple scattering events become weak. This
where non-directionalbondingprevails, the existence of short- happenswhen the de Broglie wavelength of the photoelectron,
range order is believed to be a direct consequence of the exis- λ = hp−1 (which decreases with increasing excesskinetic en-
e
tenceofawelldefinedmedium-rangeorder(Ossi2003). ergy), becomes comparable to the nearest neighbour distance.
TheXASsignalisthendominatedbythesinglescatteringevents
oftheEXAFSregion.
2.2.X-rayabsorptionnearedgestructure(XANES)
XAS is an element-specific technique sensitive to short- and 2.3.X-rayscatteringandthelow-k regime
medium-range structure. Light elements (e.g. Si, Mg or O),
with K (or L) absorption edges in the 1–2 keV range re- Away from any absorptionedges, the scattered X-ray intensity
quire measurement in high vacuum (see Wong et al. 1994; I(k),inelectronunits,fromamaterialwithNatomsisdescribed
254 S.P.Thompson:Medium-rangeorderinannealedsilicate
by(Elliot1983) thecaseofafullyconnectedSiO network,forexample,define
2
a single medium-range order generating element to be a clus-
I(k) = B(k)+N(cid:3)f2(cid:4)
(cid:1) terofthreetetrahedrasharingacommonOatomatonevertex.
∞
sinkr Thisarrangementcanberepresentedbyaspherecentredonone
+N(cid:3)f(cid:4)2 4πr2[ρ(r)−ρ ] dr (2)
0 kr Siatom,whosesurfacetouchesthecentresoftheothertwotetra-
0
hedra.Thus,r (cid:6) 2rSi−O andfeaturesinthescatteringpatternat
wXh-rearyewthaevemveacgtonritukdfeor(ia.eb.ewamaveonfuwmabveerle),ngkt,hoλfitnhceidsecnatttaetritnhge valuesof k ≤ kc(= 7.725/2rSi−O) can be attributedto medium-
rangeorder.Tetrahedralclusteringshouldthusbevisibleforval-
sampleatangleθisgivenby uesofkbelow∼2.5Å−1(e.g.seeFig.18).Thislimit,whichde-
k =4πλ−1sinθ. (3) fines the low-k regime,is howeveronlyapproximatesince it is
forthelimitingcaseofacompletelyrandomassemblage.
The X-ray scattering wavevector is the difference between the A well documented property of systems showing single
incomingandscatteredX-raybeamsanddefinesawavelength- FSDPfeatures(Wrightetal.1985,1991;Elliot1992;Ossi2003)
independent reciprocal measurement space. B(k) is a back- is that when plotted as a functionof the reducedparameter kr,
ground term accounting for air scatter and diffuse incoherent theFSDPpeakvalueislocatedat
scatteringand f istheatomiccontributiontoI(k),whichinpoly-
atomic systems is a weighted sum of contributions from each kr (cid:6)2.5, (9)
element,suchthat
whichallowstheshort-rangebondlengthofthemedium-range
(cid:3)f(cid:4)2 =(Σixifi)2 (4) order generating cluster to be determined from the measured
data.
and
An additional length can also be determined by analogy
(cid:3)f2(cid:4)=Σx f2. (5) with crystalline Bragg diffraction,where peak positionis char-
i i i
acteristicofthedistancebetweenlatticeplanes.Thepositionof
The quantity within the integral in Eq. (2) is the contribution theFSDPcanberelatedtoareal-spacedistance,D,by
fromthereal-spacevariation(duetothemoleculararrangement)
ofelectrondensity,ρ(r),relativetotheaveragemacroscopicbulk 2π
D= · (10)
density, ρ0. After correctingfor backgroundscatter, the atomic k
contribution to the measured intensity can be subtracted out to
Which can be interpreted as the semi-orderedinter-cluster dis-
givethesocalledstructurefactor,S(k),
tance(seediscussioninSect.4.3).
(cid:2) (cid:3)
I(k)N−1−(cid:3)f2(cid:4)
S(k)=k (6)
(cid:3)f(cid:4)2
3. Experimentaldetails
which contains only the contribution from the molecular ar-
3.1.Samplemanufacture
rangement,i.e.
(cid:1)
∞ Cosmicsilicateanalogueshavebeenmanufacturedbyavariety
S(k)= 4πr[ρ(r)−ρ0]sinkrdr. (7) ofmethods(seereviewbyColangelietal.2003;anddiscussion
0 by Nuth et al. 2002). Most have focussed on producing amor-
At low values of k many non-crystalline materials, particu- phousparticlesandonlyafew(e.g.Hallenbecketal.1998;Nuth
larly network formers such as silicates, exhibit a sharp peak etal.2002)attempttosimulate,toanydegree,theastrophysical
in S(k). This feature, referred to as the first sharp diffraction conditionsofgrainformation.Forthepresentworkamorphous
peak (FSDP), or pre-peak, has long been recognised as a sig- silicates were manufactured using three variations of a sol-gel
nature of medium-range order (Elliot 1991) since it does not processduetoSabatier(1950)andDay(1974),whichhasbeen
correlate with features in the atomic radial distribution func- described in detail elsewhere (Thompson et al. 2007). Three
tion(Greavesetal.1979).Althoughnouniversaltheorydescrib- silicates (hereafter, following Day 1976, denoted Product I, II
ingFSDPfeaturesindifferentmaterialsexists(Ossi2003),itis andIIIrespectively)wereproducedaccordingto:
knownthattheyarerelatedtotherelativearrangementofregions ProductI:
thatarethemselvesorderedoverthemediumrange.Inthissense,
therelationshipbetweentheFSDPandmedium-rangestructure Na2SiO3+MgCl2+H2O→MgSiO3 (11)
iswellestablished(Gaskell2001). +2NaCl+H O.
2
The rangein k overwhichFSDP featuresare observedcan
beapproximatedbyrewritingthenon-backgroundpartofEq.(2) ProductII:
in the Debye form, which describes the scattering contribution
Na SiO +2MgCl +H O+NaOH→MgSiO (12)
fromarandomarrangement, 4 4 2 2 3
+4NaCl+Mg(OH) +NaOH+H O.
sinkr 2 2
I(k)=ΣNΣNf f nm (8)
n m n m kr ProductIII:
nm
where fn and fm are the atomic scattering contributions from Na4SiO4+2MgCl2 →Mg2SiO4+4NaCl+H2O. (13)
atoms n and m and r is their separation distance. The term
nm
sinkr/kr has its first kr > 0 maximumat kr (cid:6) 7.725 and thus In reaction (12) NaOH maintains the sodium orthosilicate sol-
the limiting magnitude of the scattering vector, kc = 7.725/r, ubility while inhibitingthe precipitationof brucite(Mg(OH)2).
is determinedbytheshort-rangeorderbondlength.Insilicates Once produced, the silicates were ground by hand to produce
theSi-ObondlengthistypicallyrSi−O ∼ 1.6Åandwecouldin fine-grainedpowders.
S.P.Thompson:Medium-rangeorderinannealedsilicate 255
Fig.1. WideangleX-raydiffractionpatternsshowing development of Fig.2. Wide angle X-ray diffraction patterns for annealed samples of
crystallinityin annealed Product I silicate.Patternsinthisand subse- theProductIIandIIIsilicates.Theabsenceofsharpdiffractionpeaks
quentfigureshavebeenoffsetinthey-axisdirectionforclarity. indicatesanabsenceoflong-rangecrystallinestructure.
Batches of each ground product were then placed in a Pt- Table1.AverageelementalratiosforProductI,IIandIIIsilicatesde-
coated Mo crucible and annealed under vacuum in a RF in- terminedfromEDXmeasurements(Thompsonetal.2007).
duction furnace (Tang et al. 1998) for ∼16 h, at the follow-
ing temperatures:900 K, 1000K, 1200 K, 1300K. At the end Mg:Si Si:O
of each run the furnace power was switched off and the sam- ProductI 1.08:1 1:4.94
ProductII 0.52:1 1:2.89
ple allowed to cool. Since the furnace body and RF coil are
ProductIII 0.40:1 1:2.82
water cooled, the rate of temperature decrease is of the order
of ∼300 ◦C per minute from high temperature and effectively
quenches the structure. The long-range structure (amorphous
(Thompson et al. 2007) and are consistent, for example, with
v. crystalline) of each annealed sample was then characterised
estimates of Mg:Si ratios for Comet Halley dust (Jessberger
using wide angle X-ray synchrotron diffraction (station 2.3,
et al. 1988; Lawler et al. 1989). The average Mg:Si ratios for
DaresburyLaboratorySynchrotronRadiationSource(SRS)).At
ProductsIandIIaregenerallyconsistentwiththeenstatiteend-
1000K (Fig.1)ProductIcrystallizesintoa wellformedphase
product predicted by reactions (11) and (12). However reac-
identifiedasforsterite(Thompson&Tang2001).At1300Kad-
tion (13) has failed to produce the expected stoichimoetry and
ditional diffraction features attributable to crystalline enstatite
asilicatewithaverylowMgcontenthasresulted.Nevertheless,
havealsobeenfoundtoform(Thompsonetal.2007).Figure2
alongwiththe crystallineforsterite,enstatite andquartzminer-
showspowderdiffractionpatternsforProductsIIandIII,bothof
alsandcommercialamorphoussilicaalsousedinthisstudy,the
whichremainamorphousthroughouttheentireannealingrange,
three silicates form partof a series of silicates with decreasing
althoughproductIIdoesshowachangeinthemorphologyofits
Mg:Sicontent.
powderpatternwhenannealed.
While it is recognised that the method of production does
notattempttosimulatetheformationmechanismofsilicatesin 3.2.Measurement
spaceitdoesoffercertainadvantages.Inparticular,theproduc-
3.2.1. XANES
tion process is easily scalable and can yield large quantities of
material allowing multiple multi-technique experiments to be XANESspectrawerecollectedattheDaresburyLaboratorySRS
performed. Making comparisons between published measure- using station 3.4 (MacDowell et al. 1988; Roper et al. 1992),
ments of analogue samples produced by different methods has which is a XAS station covering the range 0.8 to 3.5 keV. At
been a long-standingissue (Jägeret al. 1994, 2003; Nuth etal. theseenergiesabsorptionbyairisstrongandmeasurementsare
2002).Theresultspresentedherehoweverhaveahighdegreeof performed in high vacuum (typically 1.7 × 10−7 Torr). Each
paritywith previouslyreportedinvestigations(Thompsonetal. sample powder was mixed with a small amount of high purity
1996, 2002a,b, 2003, 2007; Thompson & Tang 2001). Sol-gel graphite powder and mounted on flat-faced aluminium holders
basedsilicateshavealsobeenusedasanaloguesforcircumstel- to which double-sided conducting tape had been applied. The
larsilicatesinAGBstarsbyJägeretal.(2003)whofoundthegel tapedholderwaspressedintothe sample/graphitemixandany
methodproduceshydrogenatedmaterialswithchemicalproper- non-adhering powder removed by gentle tapping. The holders
tiessimilartothoseexpectedofcosmicsilicates.Thehydrogena- were then mounted in a spring-loaded conducting rack within
tionofthesamplesusedinthecurrentworkhasbeendiscussed the vacuum chamber, such that the angle of incidence of the
previously(Thompsonetal.2003). synchrotronbeamwas∼45◦ inthedirectionoftheX-raybeam
Average Mg:Si and Si:O ratios for the three silicates andperpendiculartoitintheverticaldirection.Sampleprepara-
are given in Table 1, based on previous EDX analysis tionandmountingwereperformedundercleanroomconditions
256 S.P.Thompson:Medium-rangeorderinannealedsilicate
toavoidcontamination(particularlyfromNaClinhumansweat
since Na EXAFS oscillations could overlap the Mg XANES).
DuringloadingthesamplechamberwaspurgedwithN gas.The
2
energyrangescoveringtheSiandMgK-shellabsorptionedges
were scanned using the beamline’s high vacuum water-cooled
double-crystalmonochromatorlocatedup-streamofthesample
chamber. In this configuration a constant position monochro-
matic beam was achieved at each energy step by rotating the
top crystal and both rotating and translating the bottom one,
ensuring the number of absorbers in the beam for each spec-
trum remained constant. Spectra were recorded by total elec-
tronyield(Kordesch&Hoffman1984),measureddirectlyfrom
the sample holder/rack assembly via a Kiethley 427 current
amplifier. Incident beam intensity was monitored via a post-
monochromatorfine-gaugetungstenmeshandwasusedtonor-
malisethespectratothetimedecayofthesynchrotronbeam.
3.2.2. low-k scattering
Scattering measurements were performed using Station 2.3 at
the SRS. Located 15 m from a 1.2 T dipole bending mag-
net, the 2.3 instrument is a concentric two-circle (θ-2θ) high- Fig.3.MeasuredMgK-edgeXANESspectraforforsteriteandenstatite
mineralsamples,withthemainfeatureslabelledAtoF.
resolution angle-scanning device (Cernik et al. 1990; Collins
et al. 1992), based on a parallel-beam optics design (Parrish
et al. 1986). It receives X-rays in the range 0.5–2.5 Å, with
monochromaticbeambeingselectedviaawatercooleddouble-
bounce Si(111) channel-cut single crystal. This delivers a
high-intensity parallel beam to the sample stage located at the
centre of the instrument’s two circles. Automatic wavelength
calibration (Laundy et al. 2003) was performed using a high
purity Si powder standard (National Institute of Standards and
Technology NBS SRM640c). The powdered silicate samples
weremountedinflat-platemodeonasinglecrystalSiwafercut
alongaforbiddencrystallographicdirectiontosuppressdiffrac-
tion contributions from the wafer itself. For the present work,
the scattered beam was passed through an evacuated parallel-
foils collimatingassembly (Roberts& Tang 1998) mountedon
the2θ-circlearmanddetectedusinganenhanceddynamicrange
scintillationcounter.Thedetectoracceptanceangleusingthisar-
rangementis0.065◦2θ,whilethe2θ-circlemotorsteppingreso-
lutionwassetto0.01◦.AnX-raywavelengthof1.499795Å(cal-
ibrated)was selected, to give goodresolutionof the low-k fea-
tureswithareasonablyhighX-rayphotonfluxtominimisedata
collection times. Individual atomic scattering factors, f were
i
calculated by the procedure described by Waasmaier & Kirfel
(1995)andtheweightings, x werebasedonTable1.Theback- Fig.4.MeasuredMgK-edgeXANESspectraforunprocessedsamples
i
ofthehigh-,medium-andlow-MgProductI,IIandIIIsilicates.Feature
ground contribution due to Compton scattering was calculated
labelsasperFig.3.
using the procedureof Smith et al. (1975) to whicha relativis-
tic correction was applied (Korsounskiet al. 2003). Air scatter
wasassumednegligibleduetotheveryshortsample-to-foilsdis-
tance.
intensities in forsterite immediately suggest the sites occupied
byMgarelessstructuredthanthoseofenstatite.
4. Results Comparingspectraforthethreeunprocessedamorphoussil-
icates (Fig.4), thesame A–Ffeaturescanbe identifiedin each
4.1.MgK-edgeXANES
spectrum, with ProductsI and II exhibitingmorphologiessim-
Figure3showsMgK-edgeXANESspectrameasuredforcrys- ilar to enstatite, albeit with weakened intensities. Of the two,
tallinemineralsamplesofenstatiteandforsterite,withthemain ProductI’sfeaturesaretheweakest.Surprisinglyperhaps,given
features labelled A to F. While most features are present in itslowMg:Sicontent,themorphologyofProductIII’sspectrum
both spectra, in the forsterite spectrum the intensities of fea- isbettermatchedbyforsterite,wherefeatureCappearsonlyas
turesB–FrelativetoAareweakerandtheABCcomplexappears a very weak shoulder and features D, E and F are also weak.
moreasymmetric.Sincepeakintensityisrelatedtothenumber Feature A in all three amorphous samples is less pronounced
of available multiplescattering pathwaysinvolvingthe absorb- thanineitherofthecrystallinespectra.Overalltheenvironment
ingatomanditsfirst-andhigher-orderneighbours,thereduced occupied by Mg in the amorphous samples appears similar to
S.P.Thompson:Medium-rangeorderinannealedsilicate 257
Fig.5.ThermalevolutionofthemeasuredMgK-edgeXANESspectra Fig.6.ThermalevolutionofmeasuredMgK-edgeXANESspectrafor
forannealedsamplesofthehigh-MgcontentProductIsilicate.Feature annealedsamplesofthemediumMg-contentProductIIsilicate.Feature
labelsasperFig.3. labelsasperFig.3.
the crystalline environments but with reduced structural order,
asevidencedbythereductioninpeakintensities.
ConsideringnowtheannealedspectraforProductI(Fig.5),
althoughinitsunprocessedstatetheMgenvironmentresembles
crystallineenstatite,annealingat900Kproducesaspectrumbet-
termatchedbyforsterite.FeatureDhasreducedinintensity,A
has become stronger and better defined, while C has become
a broad weak shoulder to an asymmetric ABC complex. The
1000 K spectrumalso exhibitsa similar forsterite morphology.
However,at1200KthistrendappearstoreverseandtheXANES
oncemorebegintoresembleenstatite,withfeature’sCandDin-
creasinginstrength.At1300K,thesymmetricABCcomplexof
enstatite is clearly visible (note:forthis particularsample only
a small quantity of material survived the annealing and prepa-
ration stage, as evidenced by the small pre- to post-edge step,
whichexaggeratesthestrengthoffeaturesA,BandCrelativeto
thepost-edgeregion).
The XANES spectra forProductI clearly exhibitan evolu-
tion in the local structural environment surrounding Mg from
enstatite-liketoforsterite-likeandbacktoanenstatite-likewith
Fig.7. Thermal evolution of measured Mg K-edge XANES for an-
increasingtemperature.
nealedsamplesofthelowMg-content ProductIIIsilicate.Featurela-
Product II exhibits a similar evolution (Fig. 6). Feature A
belsasperFig.3.
becomes more defined, while C to F become progressively
weaker and the ABC complexbecomesincreasinglyasymmet-
ric. However unlike Product I which has the highest Mg:Si All three silicates show Mg K-edge XANES morpholo-
ratio and changed to a forsterite-like spectrum at 900 K, the gies that correspond well with those of crystalline enstatite
ProductIIspectrumassumesamoreforsterite-likemorphology and forsterite and in turn match closely other published spec-
atthe highertemperatureof1000K. Furthermore,forthe tem- traforthesetwominerals(Wuetal.1996;Mottanaetal.1999;
peraturesstudied,ProductIIdoesnotrevertbacktoanenstatite- Cabaretetal.1998).Inaddition,thesemorphologiescorrespond
likemorphology.At1300KfeaturesCandDdoshowsignsof closely to those of other materials where Mg is known to be
increasingintensityandmaybesuggestiveofareversalathigher in6-foldoctahedralcoordinationwitheitherOorOH,e.g.talc
temperature. and brucite (Wong et al. 1994); Ca-pyroxenes, (Mottana et al.
For the silicate with the lowest Mg:Si ratio, Product III 1999) and various olivines (Mottana et al. 1996). The number
(Fig. 7), the initial morphology appears forsterite-like and al- of features (which reflects the number of scattering pathways)
thoughsome minorchangesoccurwith temperature(notablya is less than the number measured for periclase (MgO) which
strengtheningofA,weakeningofDandanincreaseintheABC is widely acknowledgedas a modelcompoundwhere Mg is in
asymmetry),theMgenvironmentremainsessentiallyforsterite- regularoctahedralcoordination(Rowenetal.1993;Wongetal.
likeovertheentireannealingrange. 1994; Ildefonse et al. 1995; Wu et al. 1996). In particular the
258 S.P.Thompson:Medium-rangeorderinannealedsilicate
ABC triplet feature, with varying relative intensities, near the
absorptionedgeisacommonfeature.Differencesintherelative
intensitiesofA,BandCwereattributedbyWongetal.(1994)
to variationsinthenumberofcoordinatingOH ligandsinvari-
ous materials.In brucite (Mg(OH) ) for example,where Mg2+
2
isoctahedrallycoordinatedbyOHgroups,octahedrashareadja-
cent edgesto formlayer structures,givinga XANES spectrum
(Yoshidaetal.1995;Aritanietal.2000)verysimilartoenstatite.
AlthoughsilicatesalsocontainSi-OtetrahedrawithH,OorOH
attheirverticesthesimilarityoftheXANESforbruciteanden-
statite suggestssilicate MgK-edgeXANESfeaturesare dueto
theexistenceofalargerstructurecontainingorderedoctahedral
Mgextendingwellbeyondthefirstcoordinationshell.
Model calculations for Ca-bearing crystalline pyroxeneus-
ing atomic clusters of increasing size built around a cen-
tral Mg atom show the Mg K-edge XANES to contain both
short- and medium-range contributions (Mottana et al. 1999).
Typically,a2Åradiuscluster(7-atoms,1Mgand6O)givesa
spectrum principallyconsisting of a strong un-splitfeature, in-
termediate of A and B in Fig. 3. Progressively increasing the
cluster radius to 4.97 Å (50 atoms) produces a large feature
Fig.8. Calculated Mg K-edge XANES for clusters of radius of 2 Å
with the A, B & C fine structure seen in Figs. 3 to 7. The
centred on an Mg atom and based on crystalline Clino-Enstatite,
C feature, that differentiates between crystalline enstatite and
OrthorhombicEnstatiteandForsteritestructures,showingcontribution
forsteriteonlyappearsincalculatedspectrawhentheclusterra- ofshort-rangeordertoXANESmorphology.
diusisgreaterthan∼5–6Å.Duetotheclosesimilaritybetween
enstatite and brucite XANES it seems reasonable to associate
theCpeakwiththepresenceoflargeorderedstructuresinvolv-
ingMgoctahedra,suggestingthereforethattheobservedchange
in morphologyfromenstatite to forsteriteand back to enstatite
observed for Product I (Fig. 5) and in part for Product II are
the result of changes in the ordering of Mg-O octahedra over
medium-range distances within large clusters, most likely due
to the mobility of Mg during annealing. Since the three sili-
cates contain progressively less Mg the Mg in product III, un-
like Products I & II, is sufficiently dilute such that large or-
deredstructuresinvolvingoccupiedMg-Ooctahedradonotform
withinthetemperaturesandtimescalesused.
TotestthedependenceofXANESonmedium-rangeorder-
ing,theoreticalMgK-edgeXANESwerecalculatedforclusters
basedonidealforsterite,rhombohedralenstatiteandmonoclinic
enstatitestructures(i.e.withoutstaticorthermaldisorder)using
the FDMNES code (Joly 2001). These calculations reproduce
only µ and do not contain the pre- and post-edge background
signals, nor the step function across the edge. Figure 8 shows
calculated spectra for a 2 Å radius cluster, while Fig. 9 shows
spectrafor5Åclusters.Fromtheseitisclearthatasinglefea-
turedevoidoffinestructurelocatedjustpasttheabsorptionedge Fig.9.CalculatedMgK-edgeXANESfor5Åradiusclusterscentred
arisesfromtheinteractionoftheejectedMgphotoelectronwith onanMgatomforcrystallineClino-Enstatite,OrthorhombicEnstatite
and Forsterite structures showing effect of increasing ordered cluster
the six nearest O atoms and relates to the ordering of the first
sizeonXANESmorphology. Orderedmedium-rangestructureresults
short-range co-ordinationshell of O around Mg, while the de-
informationoffinestructurefeatures.
velopmentoffeaturesA,B&Cin thelargerclusteraredueto
interactionsofthephotoelectronatlargerdistancesandmarkthe
presenceofmedium-rangestructuralorderingextendingbeyond
the first short-range order shell. Differences in the relative in- atverysimilarenergies,butinProductsIIandIIIshowsashift
tensities of the fine structure features for the different silicates to lower energy with annealing temperature. Product I shows
at different stages of annealing are thus due to differences in an initial shift to higher energy at 900 K, but then follows the
their medium-range ordering rather than short-range coordina- same trend towards lower energy. The average Mg-O distance
tion.Similarly,theunprocessed(i.e.asmanufactured)ProductsI forforsterite(∼2.1098Å)isslightlysmallerthanfororthorhom-
andIIexhibitmorestructuredmedium-rangeMgenvironments bicenstatite(∼2.1782Å)andtheseshiftsmaybeattributableto
thanProductIII. ashorteningoftheaverageMg-Odistance,sincetheenergiesof
The position of the first strong post-edgefeature in the an- post-edgefeatures,∆E,whenmeasuredrelativetotheenergyof
nealedsilicateshoweveralsoexhibitssometemperaturedepen- theabsorptionedge,scalewithinter-atomicdistance,r,accord-
dency.Forenstatiteandforsteriteitspeak(featureB)islocated ingto∆E =const.×r−2 (Natoli1983).
S.P.Thompson:Medium-rangeorderinannealedsilicate 259
Fig.10.SiK-edgeXANESspectrameasuredforamorphousandcrys- Fig.11.SiK-edgeXANESspectrameasuredforforsteriteandenstatite
talline(Quartz)SiO ,withthemainfeatureslabelledGtoK. mineralsamples,withfeaturelabelsasperFig.10.
2
4.2.SiK-edgeXANES
Figures 10–12 show measured Si K-edge XANES for amor-
phousandcrystallineSiO (quartz),forsteriteandenstatitemin-
2
eral samples and the three unprocessed silicates respectively.
CleardifferencesexistbetweencrystallineandamorphousSiO
2
andagaincanbeusedtodescribethemorphologiesoftheother
silicate spectra. Crystalline SiO exhibitsfive main featuresla-
2
belledG–K,plusapossibleshoulderonthehighenergysideof
featureK.AmorphousSiO showsonlyfourofthese:G,K,plus
2
weak H and very weak I features. Crystalline enstatite shows
strong G, I and K features plus a very weak J feature, while
forsteriteshowsstrongG andKfeaturesplusveryweakHand
Ifeatures.TheunprocessedProductIandIIsilicatesalsoshow
strongG andK features,plusintermediateH andI features.In
Product III the intermediate features are extremely weak. The
morphologiesofProductsIandIIappeartofallsomewherebe-
tween the crystalline forsterite and enstatite spectra, while the
morphologyofProductIIIappearsmuchclosertoforsterite.
Considering now the Si K-edge spectra for the annealed
ProductI(Fig.13),at900KfeaturesHandIhavediminished,
such that the spectrum now resembles forsterite. At 1000 K Fig.12.Meaured SiK-edgeXANESspectraforunprocessed samples
the intermediate features disappear. However at 1200 K fea- ofthehigh-,medium-andlow-MgProductsI,IIandIII.Featurelabels
asperFig.10.
tureIreappearsasastrongfeaturesimilartotheenstatitepattern
and remains so up to 1300 K. The behaviourof the Si K-edge
XANESthusfollowtheMgK-edgeinchangingfromenstatite-
like to forsterite-like and back to enstatite-like. However the andamorphoussilicatesexhibitverysimilarK-edgeXANES(Li
presence of feature H as a weak shouldersuggests some Si re- etal.1993,1995;Wuetal.1998;Poeetal.1997,2004;Bender
mainsinforsterite-likeco-ordination.TheProductIISiK-edge 2002; Henderson & Fleet 1997;Henderson 1995; Farges et al.
XANES(Fig.14)showasimilarevolution,GandKarepresent 1999;Thompsonetal. 1996;Gilbertetal. 2003).Typically,all
throughout,whileHandIdecreasewithincreasingtemperature. showthestrongwhitelinefeatureGandabroadhumpfeatureK
As with the Mg K-edge XANES the Si K-edge XANES fea- seen in Fig. 10. Only the relative intensities of the intermedi-
tures do not recoverat higher temperature.Finally, Product III ateH,I,&Jfeaturestendtoshowanyvariationwithcomposi-
(Fig.15)againshowsverylittleevolutionwithtemperature,fea- tionorstateandareusuallystrongerincrystallinesamples.On
turesHandIareabsentintheannealedspectraleavingGandK firstinspectionthiscouldbetakentoimplythattheSi-Otetrahe-
as the only featurespresentat each temperature.The morphol- dra in amorphoussamplesarethemselvesdisordered.However
ogyoftheannealedProductIIIspectrashowlessstructurethan incrystallineconfigurationsthetetrahedraareslightlydistorted,
eitherforsteriteoramorphousSiO . while in amorphous configurationsthey tend to be more regu-
2
ThetetrahedralcoordinationofSibyfourOatomsisacom- lar (Henderson1995; Hendersonetal. 1995). Accordingto the
monfeatureofallsilicatesandasaconsequencebothcrystalline molecular orbital model of Li et al. (1993), the intermediate
260 S.P.Thompson:Medium-rangeorderinannealedsilicate
Fig.13.EvolutionofmeasuredSiK-edgeXANESspectrawithanneal- Fig.15.EvolutionofmeasuredSiK-edgeXANESspectrawithanneal-
ingtemperatureforthehigh-MgProductIsilicate.Featurelabelsasper ingtemperatureforsamplesofthelow-MgProductIII.Featurelabels
Fig.10. asperFig.10.
Fig.14. Evolution of measured Si K-edge XANES spectra with an- Fig.16. Calculated Si K-edge XANES for the first Si-O coordina-
nealingtemperatureforsamplesofthemedium-MgProductII.Feature tion shell for Clino-Enstatite, Orthorhombic Enstatite and Forsterite
labelsasperFig.10. structures, showing contribution of short-range order to XANES
morphology.
features are due to spin-forbiddentransitions of the Si 1s elec-
trons to empty 3d states. The asymmetry caused by the slight
bymodelcalculationsfor1.6and5Åclusterradiiforforsterite
distortion of the tetrahedra in the crystalline phase results in a
andenstatite,asshowninFigs.16and17.
mixingofunoccupied3plikefeaturesintothe3dstatestopro-
ducesymmetryforbiddenshaperesonances.Inamorphouscon-
figurationsthereductionintetrahedraldistortioncausesthepho- 4.3.low-k scattering
toelectron to sample a more symmetric environment, reducing
themixingofstatesandhencetheintensitiesoftheintermediate Figure18showsthelow-kscatteringpatternforamorphousSiO
2
features.Theabsenceoflong-rangeorderthereforeimprovesthe whileFig.19comparesthelow-kscatteringfortheunprocessed
silicate’sshort-rangeorder.ThepresenceoftheintermediateH– and 900 K Product I silicate (at higher annealing temperatures
Jfeaturesarethusclearindicatorsofstructuralorderextending the Product I pattern showed crystalline Bragg diffraction fea-
beyondthe first Si-O co-ordinationshell and is consistentwith tures which either obscure or destroy the FSDP signal). The
modelcalculationsforα-quartz(Wuetal.1998)andamorphous ProductIpatternsdiffermarkedlyfromamorphousSiO inthat
2
SiO (Chaboyetal.1995;Levelutetal.2001).Thisisconfirmed multiplefeaturesoccur(labelled(1)to(3)inthefigure).Aclear
2