The role of very fine particle sizes in the reflectance spectroscopy of plagioclase-bearing mixtures: New understanding for the interpretation of the finest sizes of the lunar regolith

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2017

Publication Year

2020-09-16T11:46:43Z

Acceptance in OA@INAF

The role of very fine particle sizes in the reflectance spectroscopy of

plagioclase-bearing mixtures: New understanding for the interpretation of the finest

sizes of the lunar regolith

Title

GIOVANNA, SERVENTI; CARLI, CRISTIAN

Authors

10.1016/j.icarus.2017.04.018

DOI

http://hdl.handle.net/20.500.12386/27418

Handle

ICARUS

Journal

293 Number

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ContentslistsavailableatScienceDirect

Icarus

journalhomepage:www.elsevier.com/locate/icarus

The

role

of

very

ﬁne

particle

sizes

in

the

reﬂectance

spectroscopy

of

plagioclase-bearing

mixtures:

New

understanding

for

the

interpretation

of

the

ﬁnest

sizes

of

the

lunar

regolith

Giovanna

Serventi

a,∗

_,

_Cristian

_Carli

b

a Department of Chemistry, Life Sciences and Environmental Sustainability, University of Parma, Viale delle Scienze 157/A, Parma 43124, Italy b IAPS-Inaf, Viale Fosso del Cavaliere Tor Vergata, Roma, 00133 Italy

a

r

t

i

c

l

e

i

n

f

o

Article history:

Received 16 November 2016 Revised 6 April 2017 Accepted 18 April 2017 Available online 21 April 2017

a

b

s

t

r

a

c

t

Thelunarsurfaceconsistsofaregolithlayerthatcoverstheunderlyingbedrocks,andisgenerally char-acterizedbyparticulates<1cm.Lunarsoilistheﬁnefractionoftheregolith,andisgenerallybetween 60 and80μm.Sizes<10μm,accountingfor ca.5–20%ofthe soil,wererecognized and petrologically classiﬁed.

Thecoarsestsizesoftheregolitharechemicallyandmineralogicallysimilar,whiletheﬁnestfractions aremorefeldspathic,probablyduetoeasierfracturingofplagioclasethanmaﬁcminerals.

Duetothemorefeldspathicnatureoftheveryfinelunarsoils,inthispaper,wequantitatively inves-tigatetheinfluenceofveryfine(<10μm)plagioclaseontheabsorptionbandsofmaficmineralsusing theModifiedGaussianModel.Weconsideredtwoplagioclaseswithdifferentironcontentandtwomafic end-members(1)56%orthopyroxeneand44%clinopyroxene,and(2)30%orthopyroxeneand70%olivine. Wealsocomparedourresultswiththedeconvolutionofthesamemixturesatcoarsersizes.Ourresults mainlyshowthat:

(1)finesizesactprincipallyonreflectanceandonspectralcontrast(withtheformerincreasingand thelatterdecreasing);(2)veryfineplagioclasehasablueslopeintheNearInfraredandveryshallow 1250nmbanddepth,closetozero;(3)consequently,theplagioclasebandisalwaysshallowerthanmafic bands;(4)inmixtureswitholivine,thecompositebandcenter alwaysshowsthetypicalolivinevalue, differentlyfromcoarser mixtures;and (5)mafic materials haveablueslopeinthe ShortWavelength InfraredRegion,amoreV-shaped1μmpyroxeneabsorptionandthe1μmmaficbandcentersareshifted byca.40nmvs.coarsesizes,reflectingadifferentweightwithinthecrystalfieldabsorptionofthemafic componentinveryfinesize.Wealsoevidencedthatacoarseplagioclasecouldbeoverestimated,while averyfineonecouldbeunderestimatedifcomparedwiththe63–125μmsize.

1. Introduction

Veryfinesizesdominatemanyplanetarysurfacesandtheir re-golith, e.g., Moon,Marsand Mercury.Different particle size frac-tionsintheregolith(suchassoilanddust)affecttheoptical prop-erties of the surfacein different ways.For this reason, very fine particles and their effects on thereflectance spectraof the most commonplanetarymineralshavetobeinvestigatedindetailto ob-tain correctinformationaboutthemineralogical composition and thesurfacetexture.

∗ _{Corresponding author.}

E-mail addresses: giovanna.serventi@unipr.it (G. Serventi), cristian.carli@iaps.inaf.it (C. Carli).

Inparticular,thelunar surfaceconsistsofa regolithlayerthat coverstheunderlyingbedrocks,withtheexceptionofsteep-sided craterwalls,central peaksandlavachannels(McKayetal.,1991), asshownbythe lunarlandingsandobservations.Due tothe ab-sence of atmosphere on the Moon, the lunar regolith is the re-sultofdifferentprocesses,e.g.,theimpactofmeteoroidsand bom-bardments of protons fromthe sun andthe stars, andis gener-ally considered to be characterized by material lower than 1cm insize (McKay etal., 1991). Theﬁne fractionof theregolith, de-rivingfrommechanicaldisintegrationoflunar rocks,both basaltic andanorthositic, constitutesthelunarsoil.The averagelunarsoil sizeisgenerallybetween60and80μm(McKayetal.,1991).Lunar dustconsistsofevenﬁnermaterialthan lunarsoils(ca.<50μm). Furthermore,sizes<10μm,whichcompriseca.5–20%ofthe soil,

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158 G. Serventi, C. Carli / Icarus 293 (2017) 157–171

havebeenrecognizedandpetrologicallyclassiﬁed(Lauletal.,1978, 1979,1980).

Silicate minerals, such as orthopyroxene (OPX), clinopyroxene (CPX), olivine (OL) and plagioclase (PL), are the most important constituents of the lunar surface, associated with oxides (e.g., Papike et al., 1991), and can be spectrally identiﬁed on the ba-sis oftheir absorption bands. While theiron-richer maﬁc miner-alshave always been easily detected (e.g.,Tompkins andPieters, 1999; Spudis et al., 1984), only recently, and thanks to the im-provementsinspectrometersonboardduringthelastmissions,the absorptionduetothelowamountofFe2+_in_PL_could_be_detected

(e.g.,Ohtakeetal.,2009;Cheeketal.,2012a,b;Krameretal.,2013), permittingnew evaluationof its modalabundance and composi-tion.

Whilecoarsesizesoftheregolitharechemicallyand mineralog-icallyvery similar, the ﬁnestfractions, <10μm,are different and more feldspathic. This may be due to simple comminution pro-cessesandeasierfracturing ofPLthan tomaﬁc minerals(Laulet al.,1978,1979,1980;Devineetal.,1982).

Accordingtothemorefeldspathicnatureoftheveryfinelunar soils,inthispaperwepresentresultsforasetofPL-bearing mix-turesanalyzedatthe <10μmparticlesize, alsoproposing a com-parison with coarser mixtures analyzed by Serventi et al. (2013, 2015),toinvestigatetheeffectsofveryfinesizesonthereflectance spectroscopyoflunar-likeminerals.

2. Background

2.1.PLandPL-bearingmixtures

Onlyinthelastdecade,PLandPL-bearingmixtureshavebeen studiedindetail,sincetheimprovementsintermsofspectral res-olutionofthespectrometersonboardinlunarmissionspermitted toclearlyrecognizethePLabsorptionbandatca.1250nmdueto Fe2+_transition_in_its_crystal_structure₍_Adams_and_Goulland,_1978;

Burns,1993).

Cheek et al. (2011) demonstrated that in a set of synthetic An85 PL with different iron content, the 1250nm band

deep-ens as the iron content increases, up to a maximum value (ca. 0.4wt.% FeO) after which the band depth remains quite constant.

Serventietal.(2013,2015) showed that PLcan be easily recog-nizedinmixtureswithpyroxenes(PX)formodalabundancehigher than70%,whileinmixtureswithOLitcanbe maskedduetothe narrowspectralrange,inwhichbothOLandPLabsorb,thus cre-atingacomplex,composite(COMP)absorption.Moreover,spectral parameters,Gaussian modelingandHapkemodelingshow thatPL canspectrallyinﬂuencetheabsorptionofmaﬁcminerals(Serventi etal.,2013,2015;Carlietal.,2014).

CheekandPieters(2014) analyzed PL-richmixtureswith vary-ingcontent andcomposition ofOL, PX andveryhigh-Mg spinels and demonstrated that PL can signiﬁcantly contribute to re-ﬂectancespectraifstronglyabsorbingmineralsarepresentinlow abundances,particularlyinmixtureswithPX.

Serventi et al. (2016) concluded that, in PL-dominated mix-tures: (1) PL can be an important contributor to reﬂectance spectroscopy also affecting maﬁc mineral absorption bands (as also demonstrated by Cheek and Pieters, 2014); (2) PL absorbs in the 1250nm spectral region but iron-rich PL also affects the longest wavelengths (1600–1800nm), as also suggested by Pieters (1986) and Hiroi et al. (2012); and (3) PX are easily recognizable in mixtures with PL, even at very low PX con-centration (∼1%), while OL, if less than 5%, can be masked by iron-richPL.

2.2. Particlesize

Generally, variation in particle size affects albedo, reﬂectance andspectral contrast; inparticular, asthe particle size decreases albedoincreases,whilespectralcontrast,meantasthestrengthof absorptions, is reduceddue to the decrease in the mean optical pathlengthofreﬂectedlight(Adams,1968;Pieters,1983).Onthe other hand,band centers andband widths are almostunaffected by differentparticle sizes(Nash andConel, 1974), orshow shifts that fallinthe spectralresolution oftheinstruments(Serventiet al.,2013).

Crown andPieters (1987) evaluated the reﬂectance spectra of mixtures composed of different modal abundance of labradorite (An80 PL) and enstatite (Mg-OPX) at different particle sizes. The

authorsshowedthat,inthespectraofbothend-membersand mix-tures, by reducing the size reflectance increasesand thespectral contrastdecreases.In addition,they pointedout that theamount of PL detectable inmixtures withmafic minerals is particle size dependable:morePLisrequiredinfinermixtures.

Mustard and Hays (1997) investigated the effects of fine par-ticles that are approximatelythe samesize asthe wavelengthof light on reflectance spectra of OL and quartz (in the 0.3–25μm spectral range).Inparticular,spectraexhibit a dropinreflectance withthefinestsizes. Looking attheirFig. 4,inthe300–2500nm range, as the size becomes finer, OL albedo increases while the spectral contrast decreases. Furthermore,the slope in the NIR is less red as size decreases. On the contrary, looking at their Fig. 5,quartz’salbedodoesnotchangelinearlywiththe particlesize; however,thebroadabsorptionatca.1000–3000nmdisappearsat veryfinesizes.

Furthermore,thetransparencyfeatures showseveralimportant changes asparticlesize decreases:thespectral contrastincreases then decreases, the position of the maximum reﬂectance of the transparencyfeaturesshiftssystematicallytoshorterwavelengths, andthesymmetryofthefeatureschanges.

CooperandMustard(1999),analyzingMartiananalogminerals (e.g., montmorillonite and palagonite), concluded that extremely fineparticlesizesgeneratesignificant decreasesinband strength. In particular, the <5μm fractions showed vibrational absorption bands that were half the strength of the absorptions from the coarser size fractions. They also concluded that this reduction is duetochangesintheratioofscatteringtoabsorptionalongwith changesinparticlesize,withimportantimplicationsforthe spec-troscopic determination ofthe composition ofthe extremely fine Martiandust.

Craig et al. (2007) evidenced that, as particle size increases, theoverallreﬂectanceofOL,OPXandbasaltsdecreases,whilethe bandsdeepen to reach a maximumbetween45 and250μmand thendecrease.Particlesizecoarseningcanalsoproduceband sat-urationeffectsthatcomplicatethedeterminationoftheabsorption bandcenter.

Serventi et al.(2013, 2015, 2016) showed that different parti-cle sizes (125–250μm; 63–125μm; 36–63μm) of PX, OLand PL spectra do not affect the band center (shifts of few nanometers fall within theinstrumental resolution) butonlythe band depth, whichdecreasesastheparticlesizedecreases.

3. Experimentalprocedure:samplepreparationandanalytical methods

3.1. End-memberpreparationandcharacteristics

Separate end-member minerals were obtained from samples belongingtotheStillwaterComplexlayeredintrusion.Thesamples were accurately investigated under thin section to evidence the rock mineralassociation andto reduce alteredsamples as much

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Table 1

Mineral abundances in the starting samples. OPX = orthopyroxene; CPX = clinopyroxene; OL = olivine; PL = plagioclase; Op = opaque minerals; Zois = zoisite. Table also shows the applied amperage (Amp.), the minerals removed and the ﬁnal samples obtained with the Frantz Isodynamic Magnetic Separator. Chemistry and iron abundance of the new samples are also reported.

Lithology Mineral abundance Amp. Minerals removed New samples Chemistry FeO wt.% OPX % CPX % OL % PL % Op % FeO wt. %

Ultramaﬁc rock 47 – 19.7 5.1 28.3 FeO wt. % 0.3 Op CPX + OPX + OL(E3) OPX(28.2%) En 82 -Wo 4 9.03

1.4 PL 3.8

CPX(3.4%) En 45 -Wo 46 14.65 OL(68.4%) Fo 84

Anorthosite 0.8 4.3 – 94.2 – FeO wt. % 1.4 CPX + OPX + Zois. PL(PL3) PL3 An 80 0.36 Gabbronorite 22.1 17.3 – 60.6 – FeO wt. % 1.4 None CPX + OPX(E1) CPX(43.9%) En 45 -Wo 46 5.72

PL(PL2) 0.5

PL2 An 80 13.45 OPX(56.1%) En 77 -Wo 2

aspossible(Carlietal.,2009).Fromamongthedifferentsamples, we selectedananorthosite,agabbronoriteandanultramaﬁcrock (for the composition, please refer to Table 1). The chemistry of therock-formingmineralswasdeterminedbyelectronmicroprobe analyseswithaCAMECASX50(EMP)atthemicroprobelaboratory ofC.N.R.-IGG(ConsiglioNazionale delleRicerche,Geosciences and EarthResources),Padua,Italy.

The selected rock sampleswere ﬁrst crushed to a coarse size class_<2.00mm,inordertopreservetheoriginalrockcomposition inpowderedsamples.Thepowderswerethenquarteredandeach fractionwasgroundtosmallersizeclasses.Inparticular,wehave consideredtwoparticle-sizes,<250μmand<125μm.Eachpowder particle-size class was then quartered again andhalf of the ma-terialwasthen sievedintotwo sizeranges: 63–125μmand125– 250μm. Thesize fractionswere produced bydry sieving;we de-cidednottoperformwetsievingtoavoid:(1)possiblealterationor contamination;(2)leaching,whichcansettlethesizedistribution ofoursamples,differentlyfromwhatexpectedinatmosphereless planetaryconditions.

From these samples,two PLs with different iron content and two different maﬁc, multimineralic compositions were separated using aFrantz Isodynamic MagneticSeparator attheDepartment ofChemistry, LifeSciences andEnvironmentalSustainability, Uni-versity of Parma, Italy. Table 1 shows the applied amperage, re-movedmineralsandnewsamplesobtainedaftermagnetic separa-tion.Thepurityoftheend-memberswasdetermined withoptical means(X-raydiffractionwasruled outsinceitisnotaccurate for phaseabundancelessthan5%).

The selected PL phases include a medium-iron (PL2) and an iron-rich (PL3) PL (both An80, with 0.36 and 0.5wt. % FeO,

re-spectively), whilethe Fe,Mg (maﬁc)compositions consist oftwo distinct mineral assemblages, one PX-bearing (E1), and one OL-richandPX-poor(E3).TheFe,Mgphaseswerealsoanalyzedwith Mössbauer spectroscopy, in order to have a better comprehen-sionoftheironoxidationstate(forfurtherdetails,pleasereferto Serventi etal., 2013).TheﬁttingofMössbauerresultsshowedthe absenceofadditionalphases.

3.2. Mixturepreparation

Mixtures of Fe, Mg (E1, E3) and PL (PL2, PL3) end-members were prepared at two different particle sizes (63–125 and 125– 250μm). Foreach particlesize,contents rangingbetween20 and 90wt.%ofeachPLend-memberwereaddedtoeachFe,Mg end-member.

Foreachoftheresultingmixtures,wecalculatedthevol.FeO% duetoPLphasewithrespecttothemixture.ThevolumetricFeO% wascalculatedastheFeOwt.%contentinPLmultipliedbymodal

PL(%)ineachmixture.Alltheresultspresentedinthisworkhave beenplottedvs.thevol.FeOinPL.

Startingfromthe63–125μmmixturesandend-members,very finesizes(<10μm) were producedusing amicronizer atthe De-partmentofGeosciences,Padua,Italy.Eachpowder,withthe addi-tionofwatertopreventtheheatingofthesample,wasmicronized for5min;theveryfinepowderpluswaterwasmixedwithethanol andkeptinstoveforca.2weeks,untilallthewater,andethanol, evaporated. Then, the very fine powderswere weighed to check thatnomaterialwaslostduringthemechanicalgrinding. 3.3.Spectralmeasurements

Bidirectionalreﬂectancespectraofeachmixturewereacquired at room temperature and normal atmospheric pressure with a Field-ProSpectrometer®_mounted_on_a_goniometer_(Istituto_di

As-troﬁsica ePlanetologia Spaziali, Inaf-IAPS, Rome,Italy). The spec-tra were acquiredwitha spectral resolutionof∼3nm inthe VIS and of ∼10–12nm in the NIR and spectral sampling of 1nm,

withi₌30° ande₌0° The sourceusedwasaQTH (Quartz

Tung-sten Halogen) lamp and the spotilluminated had an area of ca. 0.5cm2_. _The _calibration _of _the _spectrometer _was _performed_with

Spectralon® _optical _standard _(registered _trademark _of _Labsphere,

Inc.).

Tenspectrawereacquiredforeachpowder,emptyingand reﬁll-ingthecupeachtime,andthenonlythemeanspectrumwas con-sideredforsubsequentanalyses.Inordertoavoidproblemsdueto apressedsurfacewitha“slab-likebehavior” ortocoarserpowder roughnessdueto the aggregationof such ﬁneparticles, powders weregentlyshakenandpressedinthesampleholder.

Fig.1 shows the end-member spectra;mixture spectra are plot-tedinFigs.2 and3,whilethecontinuum-removedspectraof end-membersandmixturesarereportedinFig.4.Asacomparison, re-ﬂectancespectraofcoarsersizes(63–125μmand125–250μm)are showninFigs.2 and3.

3.4.MGMdeconvolution

Each spectrum was analyzed applying the Modified Gaussian Model(MGM)algorithm(Sunshineetal.,1990).Inshort,we con-sidered a fixed continuum tangent to the spectrum reflectance maximaasmorereliableinthedescriptionofthespectrumshape andformodelingthestrength oftheabsorption bands(approach alreadyfollowedbyClenetetal.,2011 andServentietal.,2015).

First, we deconvolved the end-members, maﬁc and PL, as mineralogicallyunknown end-members, assigningGaussiansonly whereeitheran absorptionband oraband asymmetrywas visu-allyperceptible(e.g.,900nmbandinE1isasymmetrictowardsthe NearInfrared– NIR-region).Then,wedeconvolvedthemixtures:

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Fig. 1. End-member reflectance spectra at three different particles sizes, 125–250 μm (black), 63–125 μm (dark gray) and < 10 μm (light gray). Generally, reducing the particle sizes the albedo increases and the spectral contrast is reduced. (a, b) Very fine PL are characterized by featureless spectra with a blue slope; (c) very fine E1 shows a 900 nm band shifted towards shorter wavelengths and more asymmetric towards the NIR with respect to the coarse sizes; and (d) the fine E3 complex 1050 nm band is characterized by three multiple absorptions, but the absorption centered at ca. 1250 nm seems shallower with respect to the coarse sizes and is evident the appearance of an absorption at 20 0 0 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 2

In the table are reported a list of Gaussians used in the end-member deconvolution, the wavelength position and the mineralogical interpretation.

Gaussian Position (nm) Interpretation G1 900–950 Fe 2 + in OPX M2 site G2 10 0 0–110 0 Fe 2 + in CPX M2 site G3 1850–20 0 0 Fe 2 + in OPX M2 site/ chromite G4 220 0–230 0 Fe 2 + in CPX M2 site/ Al-OH alterations G5 840 Adjustment Gaussian

G6 850–950 Fe 2+_{in OL M1 site} G7 10 0 0–110 0 Fe 2+_{in OL M2 site} G8 1200–1250 Fe 2 + in OL M1 site

G9 120 0–130 0 COMP band (Fe 2 + in OL M1 site + Fe 2 + in PL) G10 1250–1300 Fe 2 + in PL

G11 160 0–180 0 PL asymmetry/PL alteration

1) ﬁxing band center and band width assigned to the maﬁc end-member absorption bands, with the exception of 1800– 2000nmabsorptionbandwidththatbecamevisuallynarrower asPLincreased;

2) leavingthe maﬁc band depthfree, whichdependson mineral abundance ina mixture (e.g.Sunshine andPieters, 1993; Ser-venti et al., 2015), and Gaussian parameters assigned to the 1250nm absorption band (for further details, interested read-ersarereferredtoServentietal.,2015).

Avisualcheckoftheresidualsafterthefittingevidenceda pat-tern in the 700–1100nm spectral range of the E1-bearing mix-turesnot detected atcoarser sizes. Forthis reason, startingfrom valuesobtainedfromthefit withthe proceduredescribedabove, we recomputed MGM leaving the mafic parameters free (see Section4.3).

A number was assigned to each Gaussian, as described in Table2.

4. Results

4.1. End-memberandmixturereﬂectancespectroscopy

Acomparisonbetweentheend-membersisplottedinFig.1.In particular,consideringthe veryﬁnesize,(1)PLspectra(Fig.1a,b) are characterized by a blueslope inthe NIR andare almost fea-tureless; (2)E1900nmband becomesV-shaped (Fig.1c)andthe band asymmetry towards the NIR is emphasized. This asymme-tryseems tobecorrelated toashiftofthe900nmband towards shorterwavelengths,asifOPX becameevenmoredominantthan CPXasthesizedecreased;(3)theE3complex1050nm band(Fig. 1d)ischaracterizedbythreemultipleabsorptionsduetotheFe2+

transitions in OL M1 and M2 sites, but the absorption centered atca.1250nm seems shallowerwithrespect tocoarsersizes;(4) maﬁc assemblages (Fig. 1c,d)show amore marked blueslope in the short wavelengthinfrared region (SWIR)with respect to the coarsestsize,and(5)E3(Fig.1d)showstheappearanceofan evi-dentabsorptionat2000nm.

BothE1andE3indicatemoreinfluencingOPXreflectance prop-ertiesthanother maficphasessuchasOL,CPX.However,therock samplefromwhichE3wasmagneticallyseparatedcontains2%of chromite. Eveniftheresidual chromiteinoursampleafter sepa-ration isvery low (<<0.5%), chromite ischaracterized by a deep absorptionatca.2000(e.g.Cloutisetal.,2004),and,thus,we can-notcompletelyruleoutchromiteasacontributortothe2000nm absorption.

PLend-membersweremagneticallyseparatedfromrocks con-taining PX. The separated PL was investigated by optical micro-scope analysis with no detection ofresidual PX, but we have to considerthat PXinclusionscould stillbe present.Forthisreason, the900nmbandshowedbyPLspectrainFig.1acouldbe indica-tive of residual PX. Furthermore, Fig. 1b reveals that a ﬁne size emphasizes the effects of residual PX; the 900nm band due to Fe2+_in_PX_becomes_more_absorbing_as_size_decreases_(see_also_the

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Fig. 2. Reflectance spectra of E1-PL mixtures at three different particle sizes. (a, b) E1 + PL2 and E1 + PL3, < 10 μm; (c, d) E1 + PL2 and E1 + PL3, 63–125 μm; and (e, f) E1 + PL2 and E1 + PL3, 125–250 μm. Generally, a fine particle size produces spectra with higher reflectance and reduced spectral contrast compared to the coarsest sizes.

Figs.2 and3 showthataﬁneparticlesize(Fig.2a,band3a,b) increasesreﬂectanceandreducesspectralcontrastcomparedwith thecoarsestsizes(Fig.2c–fand3c–f).

Considering the63–125 and 125–250μm sizes(Figs. 2c–f and 3c–f), ahigher PLmodalabundance entailsa systematicincrease in reflectance, whereas, in the <10μm size, some spectra devi-ate from this behavior, such as the 70%PL3-E3 and the PL end-members(seeFigs.2a,band3a,b).AsexplainedinSection3, work-ing withsuch fine sizesmeans dealingwithproblemscaused by toopressedsurfacesortopowderroughness.Forthisreason, spec-tra acquiredon thesamesamplehavevariablereflectance values, with themean spectrum showinga standard deviation up toca. 6%, which can explain the reflectance behavior that isnot linear withtheincreaseinPLmodalabundance.

PLinveryﬁneE1-bearingmixturesresultsinaweaker absorp-tion band than for coarse sizes and is revealed by a ﬂattening in the 1200nm spectral region for80% PL-mixture (Fig. 4a,b). A weak absorptionband can berecognized onlyformorethan 90% PL (light-greyspectrain Fig.4a,b), butalways shallower than PX absorptionbands.

Serventietal.(2013) demonstratedthatthespectralparameters describingtheCOMPband,duetothesimultaneousabsorptionsof OLandPL,giveinformationontherelativeminerals’modal abun-dance. In particular, considering coarse E3-bearing mixtures, the

COMP band center shifts fromtypical OLvalues (1050nm)to PL values(1250nm)forPLcontent>70%(seetheirFig.13b).However, investigatingthe<10μmsize,thecentershiftingisnotdetectable (Fig.4c,d);thisisconsistentwiththefeaturelessnatureofPL.

4.2.End-memberdeconvolution

End-membersweredeconvolvedvia MGM(see Fig.5aand6a; toshow thegoodnessoftheﬁt,RMSisplotted).E1residual(Fig. 5a,h)showsasinusoidalpatterninthe700–1100nm;comparing ourresiduals to those by Klima etal. (2008),which showed the samepatternand thesame RMSasours, we decided not toadd additionalGaussians.

Acomparisonwiththedeconvolutionofcoarsersizes(seeTable 3)demonstratesthatthe900–1000nmabsorptionsareﬁttedwith the same number of Gaussians, with the only exception of E1, whereanadjustmentGaussian(G5)centeredatca.840nmhasnot beenused(inaccordancewithKlimaetal.,2007).Despitethe end-member residual seems to indicate a sinusoidal pattern, the co-variancematrixdoesnotallowmoreGaussianstobeadded. More-over,asimilarresidualpatternispresentalsoinKlimaetal.(2007, 2011).Onthecontrary,G5isrequiredforcoarsersizeasdiscussed inServentietal.(2015,2016) and in accordancewithSunshine and Pieters(1993).

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16 2 G. Serv enti, C. Carli / Icarus 293 (20 17) 15 7 – 17 1 Table 3

In table are reported the spectral parameters obtained by MGM deconvolution of mafic end-members, at the 63–125 μm (on the left), 125–250 μm (on the central column) and < 10 μm (on the right). Center and width are expressed in nanometers, while depth as the logarithm of the reflectance. RMS represents the Root Mean Square error: the lower the RMS the better the fit. To show the goodness of the fit, for band center, the error calculated at 95% level of confidence was reported.

E1

63–125 μm 125–250 μm < 10 μm

Center Width Depth RMS Center Width Depth RMS Center Width Depth RMS

G5 843 ± 9 110 –0.400 837 ± 6 113 –0.370 G1 934 ± 9 159 –0.929 949 ± 30 180 –0.895 903 ± 10 154 –0.252 G2 1028 ± 20 199 –0.535 1058 ± 30 191 –0.371 988 ± 10 220 –0.217 G3 1851 ± 11 496 –0.766 1813 ± 15 489 –0.631 1894 ± 10 509 –0.194 G4 2311 ± 50 730 –0.431 2202 ± 50 678 –0.586 2321 ± 40 138 –0.025 0.018 0.020 0.06 E3 63–125 μm 125–250 μm < 10 μm

G6 861 ± 7 210 –0.464 842 ± 2 193 –0.483 881 ± 10 163 –0.063

G7 1028 ± 6 204 –0.562 1009 ± 2 218 –0.722 1036 ± 8 241 –0.139

G8 1221 ± 3 400 –0.655 1221 ± 2 408 –0.789 1262 ± 10 296 –0.08

G3 1963 ± 2 442 –0.080

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Fig. 3. Reflectance spectra of E3-PL mixtures at three different particle sizes. (a, b) E3 + PL2 and E3 + PL3, < 10 μm; (c, d) E3 + PL2 and E3 + PL3, 63–125 μm; and (e, f) E3 + PL2 and E3 + PL3, 125–250 μm. Generally, a fine particle size produces spectra with higher reflectance and reduced spectral contrast compared to the coarsest sizes.

To summarize, Table 3 shows that reducing the particle size (i.e.,from250to10μm)ofE1:

1) Asmentionedabove,G5isnotneeded;

2) Bands are shallower, above all G1 andG3, and the Gaussians describing the different absorption bands are very similar in termsof band depth,thus reﬂecting thegeneralspectral con-trast;

3) G1 and G2 centers shift towards shorter wavelengths (more than40nm);

4) The1850nm absorptionisﬁttedwithonlyoneGaussian (G3); and

5) Bandwidthsdonotshowsigniﬁcantchanges.

ConsideringE3, a reduced sizeproduces shallowerbands, and a narrower G8 with respect to coarse ones; G6-G8 centers shift towards longerwavelengths,butwithreducedshiftscomparedto E1.

G7isthedeepestoneandthisisnotconsistentwiththeresults bySunshineandPieters(1998),whereG8isthedeepest.However, Buz and Ehlmann (2014) demonstrated that extreme size values mayaffecttheOLspectralparameters,which,consequently, devi-atefromthetrendfound bySunshineandPieters(1998).The au-thorsshowedthat, atveryﬁnesizes(<45μm),G7 isthe deepest Gaussian, also inaccordance withresults found by Burns (1993).

However, considering E3(Fig. 6a), G6 is slightlydeeper than the resultsbyBuzandEhlmann (2014),buthereweare dealingwith evenﬁner sizes. Furthermore,theOPX componentmay inﬂuence G6depth.

ThelongestwavelengthsofE3aredeconvolvedwithaGaussian at1963nm, which canbe tentatively assignedeitherto PXor to chromite(Fig.6a;Table3).

Table 4 reports the PL end-member spectral parameters af-ter MGMdeconvolution forthe differentparticle sizes. MGMﬁts are also shown in Fig. 5g,n. In particular, reducing the size, the 1250nm absorptioncan still beconsidered, evenif, fortheﬁnest size, the band depth isclose to zero. The band center shifts to-wardslongerwavelengthsbothforPL2andPL3,evenifwithshifts oflessthan10nm.

Table4 showsthat acoarsePL2isdeeperthantheiron-richer PL3(alsorefertoServentietal.,2015,2016);however,the<10μm PL3isdeeper thanPL2. Thiscan be relatedtotheresidual PXin PL2 (after magnetic separation is estimated to be less than 1%), which,atveryfinesizes,becomesspectrallyimportant,thus influ-encingand reducing the PL band depth. Furthermore,the resid-ual PX produces a 900nm absorption band that is only slightly less deep than the PL absorption band (0.015vs. 0.026 -LnRefl, also see Fig. 3a). The 900nm band depth in PL3, not detectable at the coarser sizes, accounts for 30% of the <10μm PL band

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16 4 G. Serv enti, C. Carli / Icarus 293 (20 17) 15 7 – 17 1 Table 4

In table are reported the spectral parameters obtained by MGM deconvolution of mafic end-members, at the 63–125 μm (on the left), 125–250 μm (on the central column) and < 10 μm (on the right). Center and width are expressed in nanometers, while depth as the logarithm of the reflectance. RMS represents the Root Mean Square error: the lower the RMS the better the fit. To show the goodness of the fit, for band center, the error calculated at 95% level of confidence was reported.

PL2

63–125 μm 125–250 μm < 10 μm

G10 1271 ± 1 505 −-0.2439 1282 ± 1 481 −0.3919 1286 ± 6 408 −0.026 G11 1837 ± 6 443 −0.04704 1822 ± 6 441 −0.06778 1944 ± 4 250 −0.016 G1 909 ± 2 122 −0.0395 881 ± 1 54 −0.0097 907 ± 3 129 −0.015 0.005 0.009 0.003 PL3 63–125 μm 125–250 μm < 10 μm

G10 1298 ± 2 438 −0.2356 1295 ± 2 431 −0.3598 1303 ± 5 432 −0.028

G11 1734 ± 12 591 −0.06649 1711 ± 13 636 −0.1066 1957 ± 6 325 −0.016

G1 941 ± 4 205 −0.01

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Fig. 4. End-member and mixture continuum-removed reflectance spectra. Generally, increasing the PL content in mixtures, the mafic absorptions are reduced, while the PL band behavior depends on the mafic end-member. (a, b) E1-PL2 and E1-Pl3 mixtures, respectively. PL produces an absorption bands only for very high PL content, more than 80%; (c, d) E3-PL2 and E3-Pl3, respectively. The COMP band center shifts from typical OL (1050 nm) values to PL (1250 nm) values is not detectable/noticeable.

depth(seeTable4),furtherevidenceofthefeaturelessbehaviorof ﬁnePL.

Whileintheliterature(e.g.,Pieters,1996;Hiroi,2012;Serventi etal.,2015,2016)G11hasbeententativelyattributedtoPL,inour work, at the _<10μm size G11 is shallower,shifts towards longer wavelengths(1940–1950nm)andisnarrowervs. thecoarsesizes. Consequently, in this case, G11 probably adjusts the vibrational processes related to the OH− alterations of the natural PL here considered.

4.3. Mixturedeconvolution

Mixtures were deconvolved followingthe procedure described in Section 3. Fitting results, as well as RMS, are shown in Figs. 5 and6.

PLband slightlydeepensin E1mixtures (trianglesinFig. 7a), whileCOMPbanddecreasesinE3mixtures(starsinFig.7a).InFig. 7b, PLand COMP band centershifts towards longer wavelengths asthevol.FeO%inPLincreases.BothPL(trianglesinFig.7c)and COMP (starsinFig.7c)band widenwithincreasingvol.FeO% val-uesinmixtures.

Fig.8aandbshowthatincreasingthevol.FeO%inPL,the Gaus-sians describing the mafic absorptionsbecome shallower. Fig. 8a displaysindetailthebehaviordescribedinSection4.1,withmafic absorption bandsdeeper in60%PL2-E1 andin 80%PL2-E1 thanin 50%PL2-E1 and 70% Pl2-E1, respectively. However, we report the error barfor band depth,calculated inthe 95% confidence inter-val, and it is clear that 50%PL2-E1 has a much larger error bar than60%PL2-E1(±0.05LnRefl vs.±0.015LnRefl).Fig.7bshowsthat

thedepthofGaussiansdescribingmaﬁcabsorptionsinE3-bearing mixturesdecreasesmorelinearlythaninE1-mixtures.

As stated in Section 3.4, the mafic parameters, with the ex-ceptionofdepth, hasbeenkept fixed inthe firstiteration.Based ontheresiduals,we decidedtoleavethoseparametersfree.Their variationshave been reported in Fig. 8b,c and in Fig. 8e, for E1 andE3-bearing mixtures,respectively. Fig. 8bshowsthat G1 and G2movetowardslongerwavelengthsasPLmodalabundances in-creases;however,G1variationsfor<90%PLandG2variationsfor <80% PL are less 10nm, respectively, and, so within the instru-mental spectral resolution. On the contrary, G1 andG2 shift to-wards longer wavelengths of12nm (90%PL), andof 20nm (80% PL)and40nm(90%PL),respectively.Thesevalues,inparticularfor G2,cannotbejustifiedbythespectral resolution,butmustbe re-latedto thepresence ofPL, whichis accommodatednot onlyby G10 andG11. Fig.8creports that G3 becomesnarrower with in-creasingPLmodalabundances,whileG2variesslightlyonlyforPL over80%.G3variationscanberelatedtoPL.

Regarding E3-bearing mixtures (see Fig. 8e), only G3 width varies:G3becomesnarrowerasthePLcontentincreases(withno differencesbetweenPL2 andPL3-bearing mixtures, asthe values lie on the bisector line), with substantial variations formixtures withmorethan70%PL.

Fig. 9 shows a comparison for the spectral parameters after MGMdeconvolutionforthethreedifferentparticlesizes:<10μm (pink and violet symbols, for PL2 andPL3-bearing mixtures, re-spectively), 63–125μm (light-blue and blue symbols forPL2 and PL3-bearing mixtures, respectively) and 125–250μm (yellow and redsymbolsforPL2andPL3-bearingmixtures,respectively).As

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al-166 G. Serventi, C. Carli / Icarus 293 (2017) 157–171

Fig. 5. MGM deconvolution and residuals of E1-bearing mixtures. RMS values are consistent with Klima et al. (2011) . (a–g) E1-PL2 mixtures; (h–n) E1-PL3 mixtures. Continuum-removed reﬂectance is expressed as –LnReﬂ.

readystatedin Section 4.2,the principal difference regardsband depth; Fig. 9a andd evidence that PL andCOMP band are shal-lowerthanin coarsersizes,withdepthcloseto zero.In addition, theband widthin the ﬁneparticle size is strongly reduced(Fig. 9c,f),particularlyinE3-bearingmixtures.

5. Discussions

Inthispaperweinvestigatedtheeffectsofa veryfineparticle sizeonthereflectancespectroscopyofPL,maficend-membersand PL-bearingmixtures.

As already statedby Adams (1968) and Pieters (1983), a fine particlesizeincreases reflectanceanddecreases spectral contrast. MustardandHayes(1997) alsoevidencedthat,asthesizebecomes finer,OLbecomesbrighterbutshallower,andbluerintheNIR(see theirFig.4A).

In thiswork, we alsoshowedthat the spectraof veryfine PL become featureless, with a bluer slope than a coarse PL in the IR, and that the difference in reflectance between mixtures and end-membersisreduced.Consequently,theincreaseingeneral re-flectance as the PL abundance increases, documented at coarser sizes,may notbe always respected. Thisbehavior, aswell as the bluerslopetowardsIR,similartowhathappensinbulksurfacesof rocksspectra(asalsopostulatedbyHarloff andArnold,2001, Pom-pilioetal.,2007;Carli,2009),clearlydemonstratesthataveryfine sizecouldcomplicate theinterpretationofareflectancespectrum

(e.g.,slopesimilarforveryﬁnesizeandslabofrocks),aswell as theevaluationoftheminerals’modalabundance.

Cheek and Pieters (2014) and Serventi et al. (2015, 2016) demonstrated that, considering PL-dominated mixtures (PL>70/80%), Fe2+_-bearing _PL _can _{signiﬁcantly} _contribute _to

re-flectancespectra,alsoaffectingmaficmineralabsorptionbands.In fact,inPX-bearingmixtures,thePLabsorptionbandisevendeeper thanPXbandsforveryhighPLcontent,while,inOL-bearing mix-tures,theCOMPbandpositionshiftstowardsthetypicalPLcenter values (1250–1300nm) for high PLcontent. All these effects are stronger in the 125–250μm size than in 63–125μm one, thus supportingthatacoarsePLisspectroscopicallymoreactivethana fine one. On thecontrary, this work evidences the unpredictable effectsofaveryfinesizeonreflectancespectroscopy.Forexample, the band depth of a very finePL is very shallow, with Gaussian depth almost close to zero, and the COMP band center always showsthetypicalOLvalues,evenforPL>90%.Thistestifiesthata reducedsizecanmaskthecontributionofPLonreflectance spec-tra, with a consequent and possible PL underestimation, which, obviously, leads to mistakes in the interpretation of reflectance spectrafromaSolarSystembody’ssurface.

Furthermore,wesupposethatthe1800nmabsorptiondetected atcoarse sizesmaybe not foundat sucha ﬁne size.Inthe very ﬁnePLconsideredinthiswork,G11isnarrowerandshiftstowards longerwavelengths(1940–1950nm)vs.thecoarsesizesand, there-fore,probablyrelatedtotheOH−alterationoftheterrestrialPL.

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Fig. 6. MGM deconvolution and residuals of E3-bearing mixtures. RMS values are consistent with Klima et al. (2011) . (a–g) E1-PL2 mixtures; (h–n) E1-PL3 mixtures. Continuum-removed reﬂectance is expressed as –LnReﬂ.

Here, we introduce an indexcalled Band DepthRatio (B.D.R.) calculatedasfollow:

B.D.R.

(

E1

)

= B.D.G10

B.D.G10+B.D.G1+B.D.G2 (1)

B.D.R.

(

E3

)

= _B B.D.G9

.D.G9+B.D.G6+B.D.G7 (2)

whereB.D.isthebanddepthofeachGaussian.

ThisindexreturnstherelativeintensityofPLandCOMPbands withrespect toall theGaussiansdescribing the1000nm absorp-tion.

We calculated B.D.R. (E1, E3) for <10, and 63–125 and 125– 250μm(Serventietal.,2015)sizes.Resultsareplottedvs.the63– 125μmsize(Fig.10);thebisectorindicatestheidealconditionsin whichrelativedepthsdonotchangewiththesize.However, sym-bolsdonotlieonthebisector:125–250μmvalues(bluesymbols) are in general slightlyabove the line,while _<10μm values (ma-gentasymbols)areundertheline.Thisdemonstratesthat, consid-eringthe63–125μmsizeasareference,coarsePLcanbe slightly overestimated, and very fine PL could be easily underestimated. Moreover, as thePL abundance increases,also the difference be-tween B.D.R. generally increases for <10μm and for 63–125μm. Thespectralpropertiesofmaficend-membersareaffectedbyvery finesizes, too.RegardingE1,the900nmabsorption bandismore V-shaped and more asymmetric towards the NIR vs. the coarse

sizesandshiftedtowardsshorterwavelengths,thussimulating dif-ferentmineralcontentand/ordifferentmineralchemistry(shorter wavelengthscorrespondtoMg-richerandCa-poorerPX).Thevery ﬁneE1deconvolutiondoesnot requireG5, whichis,on the con-trary, neededfor coarser sizes(see also Pieters et al., 1993; Ser-ventietal.,2013).

Onthe other hand,the fineE3 end-membershows two main differencesifcomparedtocoarsersizes.TheG6-G8relativedepths are not consistent with the trend proposed by Sunshine et al. (1998),confirmingthe unpredictableeffects ofvery fine sizes;in fact,BuzandEhlmann(2014) demonstratedthatextremesizes af-fectthespectralparametersofOL.Furthermore,thefineE3longest wavelengthsshow absorption not detected atcoarser sizes.Since E3ischaracterizedby70%ofOLand30%ofOPX,itseems plausi-blethat,atvery finesizes,thepresenceofOPX,notdetectableat coarsersizes, could become animportant spectroscopic contribu-tor.

The spectral properties seen in E1 (band center shifting), to-gether withthebehavior seenforE3 inthelongest wavelengths, seemtoevidencethatOPXspectrallydominatesatsuchaﬁnesize withrespecttoother,maﬁcphases,whichpresentaloweramount oftotaliron.However,E3bandcanberelatedtoresidualchromite aftermagneticseparation.

Comparing 63–125μm and 125–250μm sizes, Gaussians shift lessthan10nm.The FieldSpecspectral resolutionvariesbetween

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Fig. 7. The ﬁgure shows the spectral parameter variations in mixtures composed with E1/E3 and PL2,3 after MGM deconvolution. In particular, with increasing the volumetric FeO% in PL: (a) PL band deepens (triangles), while the COMP band depth decreases (stars); (b) PL band center (triangles) and COMP band center (stars) shift towards longer wavelengths (triangles); and (c) both PL (triangles) and COMP band (stars) widen.

2and12nm:thus,thebandcentershiftingfallswithinthe instru-mentalresolution.Onthecontrary,maﬁcbandcentersfor<10μm powdersareshiftedofca.40nmwithrespecttothecoarsersizes; thisis becausewe are dealingwithsizes that are approximately the same size as the wavelength of light and this could lead to unexpectedbehaviorsofthespectralparameters.

Fig.11 reports spectraof lunar soils(from the LunarSoil Char-acterization Consortium, Pieters et al., 1993, 2000) at three dif-ferentfinetovery fineparticlesizes(20–45μmblue spectra,10– 20μm red spectra and _<10μm black spectra) acquired from the Apollo missions 14 and 16. In particular, samples #67,481 and #67,461arericherinPLthan samples#14,141 and#62,231. Gen-erally,asthesizedecreases,reflectanceincreases.Asexpected, ab-sorptionbandsareshallowerand,inparticular,the1250nmweak bandbecomesalmostfeaturelessin<10μmspectra.Furthermore, Fig. 11 emphasizes the behavior of the _<10μm fractions: spec-tral contrast is extremely reduced if compared to the 10–20μm size.Anyway, we have totake intoaccount that spectra oflunar samples are affected by the presence of npFe° dueto the space weathering,whichimpliesared slopeandloweralbedo than ex-pectedforPL-richfinepowders.Nevertheless,thespectrafrom lu-narsoilsareinaccordancewitha relativeincrease inalbedoand astrongreductioninabsorptionsreducing thesize andanalmost featurelessspectrumfor<10μmsize,losinginformationaboutPL absorption.

Generally,themainfeaturethatdiscriminatestheveryfinesize isthehighalbedo,correlatedwithdecreasingabsorptionintensity. However,duetothespaceweathering,thisfeaturemaynotbeso easilyrecognizedonlunarspectra.Thelowspectralcontrastcould beevidence ofvery fine size aswell;however,a fine size isnot

theonlyreasontoexplainreducecontrasts(e.g.,spaceweathering, shockeffects).

6. ConclusionandimplicationsfortheMoon

The Moonregolith, generally<1cminsize, comprises the lu-nar soil, which represents the finest fractionof the regolith and derivesfrommechanicaldisintegrationoflunarrocks,bothbasaltic andanorthositic,andisgenerallybetween60and80μm.Veryfine sizes(_<10μm)wererecognizedandthoughttoaccountfor5–20% ofthelunarregolith;PListhedominantphaseofthisfineregolith (Lauletal.,1978,1979,1980).

In thiswork, we demonstratedthat a very smallsize implies higheralbedo anda decreaseinspectral contrast,particularlyfor thePLspectrathatbecomealmostfeatureless.Ontheotherhand, we showed that the iron-richestmineral in the considered mix-tures, OPX, seems to spectrally dominate other mafic and coge-neticminerals,CPXandOL,attheveryfinesize(e.g.,the900nm band shifts towards shorter wavelengths inE1 and the 2000nm reflectancedecreasesinE3).

Thishasimportantimplicationsforthelunarregolith,wherePL isabundant.Forexample,thepresenceofPLspectrawitha well-definedanddeep1250nmabsorptiongivesindicationsnotonlyon thecrystallinityofthemineralbutalsoontheparticulatesizethat cannotbetoofineintheareaswerePLabsorptionisdetected.On theotherhand,featurelessPLspectracanleadtoanerroneous in-terpretation ofthePLabundance and/or chemistry, whichcan be underestimated.Theseresultsevidencethatmoredetailed investi-gationsonsuchfinesizesshouldbepursuedtobetterunderstand howtoretrievecompositionbymodelingremotesensingdata.

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Fig. 8. The ﬁgure shows the spectral parameter variations in mixtures composed with E1/E3 and PL2,3 after MGM deconvolution. In particular, with increasing the volumetric FeO% in PL: (a, d) the maﬁc band depth decreases both in E1 and in E3-bearing mixtures. Fig. 8a also shows the vertical error bars for the 50–80% PL2-E1 mixtures; (b) G1 and G2 move towards longer wavelengths with increasing PL modal abundances. G1 variations and G2 variations for < 80% PL are less than 10 nm and, so, explainable by the instrument spectral resolution; G2 shifts for 80% and 90% PL can be related to the presence of PL; (c) G3 become narrower with increasing PL modal abundances, while G2 varies only slightly; only G2 variations for more than 70% PL2 can be related to PL, while other variations are, again, explainable with the instrument spectral resolution; and (e) G3 becomes narrower with increasing the PL content (with no differences between PL2 and PL3-bearing mixtures, as the values lay on the bisector line), but variations are substantial only for more than 80% PL.

Fig. 9. The figure shows a comparison for the spectral parameters after MGM deconvolution for the three different particle sizes: < 10 μm (violet symbols), 63–125 μm (pink symbols) and 125–250 μm (fuchsia symbols). (a, d) PL (in E1-bearing mixtures) and COMP (in E3-bearing mixtures) band depths are very reduced compared to the coarse sizes and close to zero; (b, e) the PL band center does not show great variations in E1-bearing mixtures, while in E3-PL2 mixtures the COMP band center is shifted towards longer wavelengths. On the contrary, is shifted towards shorter wavelength in E3-PL3 mixtures; and (c, f) the band width in the fine particle size is strongly reduced, particularly in the E3-bearing mixtures. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 10. Band Depth Ratio (B.D.R.) for the 125–250 μm and < 10 μm sizes vs. the 63–125 μm size. (a) E1-bearing mixtures; (b) E3-bearing mixtures. The bisector indicates the ideal conditions that the relative depths do not change with the size; however, 125–250 μm values (blue symbols) stay above the line, while < 10 μm values (magenta symbols) are under the line. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

Fig. 11. Spectra of lunar soils (from the Lunar Soil Characterization Consortium, from Pieters et al. (1993, 20 0 0) at three different, very fine particle sizes (20–45 μm blue spectra, 10–20 μm red spectra and < 10 μm black spectra). Samples #67,481 and #67,461 are more enriched in PL than samples #14,141 and #62,231.Generally, reducing the size the reflectance increases, absorption bands are shallower and, in particular, the 1250 nm band becomes almost featureless in < 10 μm spectra. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Acknowledgment

Spectroscopic measurements were carried out at Inaf-IAPS-IstitutoNazionalediAstroﬁsica,Roma.EMPAanalysesandpowder micronizationhavebeenperformedatDipartimentodiGeoscienze, Padua, Italy. The authors are grateful to prof. Maria Sgavetti for her thoughtful review that greatly improved the quality of the

manuscript. The authors are also grateful to two anonymous re-viewersfortheirstimulatingcommentsandsuggestions.

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