323

The CanadianM ineralogist Vol. 33,W.3%-332 (1995)

OUANTITATIVEELECTRON.MICROPROBE ANALYSIS OFALKALI SILICATE GLASSES: A REVIEWAND USERGUIDE

JOHN G. SPRAY Departrnentof Geology,University of NewBrw"rwick Fredericton, New Brw"ywick E3B 5A3

DAVID A. RAE ElectronMicroscopy Unit, University of NewBrmswicb Fredtricton, New Brunswick E3B 6El

ABSTRA T

The mobilization of alkali metalsin alkali silicateglasses and certain minerals during electron-microprobeanalysis is a result of beam-inducedheating and charging effects within the sample.The following procedwesare recommendedin order to minimize theseeffects. (1) Totat beam-powershould be lessthan 100pW to reduceheating within the inadiated volume; this is best achievedby decreasingthe beamcurrent and by using a cryogenicstage, though the latter may be impracticalfor routine analysis.(2) Samplecharging can be minimized by usrnglower beam-power,but higher voltagesmay be requiredto displace accumulatingelectons to deeperlevels within the sample.(3) Heating and charging effects can be reducedsignificantly if defocusedbeam or raster scan-modesare used. (4) Sampleconductivity can be improved by applying double carbon coats, by coating both sides of the sample,and by using conductive slide-mounts(i.e., copper instead of glass). (5) Count-times shouldbe shorGn€dto minimize the rate and sat€a16f alksli migration, while maintainingthe statisticalvalidity of the alata- (6) Conectionsfor alkali migration shouldbe doneprior to ZAF corrections.The low power requirementsand high efficiency of the Si(Li) detector clearly favor energy-dispersion@DS) over wavelength-dispenionspectroqetry (WDS) for the quantitativeanalysis of alkali silicate glasses.

Keywords: alkali silicate glass, electron-microprobeanalysis, energy-dispersionspectrometry, wavelength-dispersion spec- trometry, alkali-metalmobility, space-chargelayer, electromigration.

SoraMens

Ia mobilisationdes alcalins dans les verressilicatds d alcalinset danscertains min6raux pendant une analyse i la microsonde 6lectroniqueest le resultat du r6chauffementd'un 6chantillon par le faisceauet de I'accumulationdes chargesintemes. k protocolesuivant sert i minimiser ceseffets. (1) La puissancetotale du faisceaudevrait Otreinf6rieure i 100 pW afin de rdduire la chaleurdu volume irradi6; on doit donc diminuer le courantdu faisceauet utiliser un porte4chantillon cryog6nique,quolque cette demidremesure pourrait s'av6rerincommode dans le contexted'analyses routinibres. (2) L'accumulationdes chargesest minimis6een r6duisantla puissancedu faisceau,quoiqu'un potentiel plus 6lev6 pourrait s'av6rerndcessaire pour deplacerlqs 6lectons accumul6svers un niveau plus profond dans l'6chantil1on.(3) Les effets dus au chauffementet I I'accumulation descharges sont sensiblementr6duils en utilisant un faisceaud6focalis6 ou un modede pr6lbvementdes donndespar balayage. (4) Ia conductivitdde l'6chantillonpeut 6tre arn6lior6een appliquantune double couchede carboneou une couchedes deux c6tdsde ldchantillon, et en adoptantune lame i conductivit6plus 6lev6e,faite par exemplede cuiwe au lieu de verre. (5) Le tempsde comptagedevftdt 0[e raccourciafin de minimiser le taux et la port6ede la migration dss alsalins,tout en maif,tenant la validitd statistiquedes donn6es.(6) ks correctionsvisant d compenserpour la migration des alcalinsdevrait Otreeffectudes avant la correction pour le nombre atomique, I'absorptionet la fluorescence.Il est dvident que les faibles exigeancesen puissanceet I'efficacit6 61ev6ed'un d6tecteurSi(Li) favorisentune analysequantitative de verressiliceux riches en alcalinspar dispersiond'6nergie plut6t que par dispersionde longueursd'onde. (Iraduit par la R6daction)

Mots-cl6s:verre siliceux e akalins, analysepar microsonde6lectonique, dispersiond'6nergie, dispersion de longueursd'onde, mobilit6 des sls:lins, couchesurcharg6e, 6lectomigration.

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Ixrnopuctrox

Silicate glassesare an important group of materials, od@ of inlerest to both the Earth scientist and indusry. For naturally occurring aluminosilicateglasses, much ONdlm rclcadc ald Plt(fistone sulEr@led can be learned about the geological conditions of Tadylyle their forrnation if accurateand precise compositional P€le's bah/tem Glassy pyrelasdc marertal data can be obtained. In commercial applications,it may be necessaryto analyze synthetic glasses on Plaretary regotitbs (eg" m) lryacr-indued a routine basis for purposesof quality conhol. Much Meteorite shock veirs (supercooledor diaplesdc) ofthe work on natural glassesrequires the analysisof Tekdtes Dt plscdtes (eg, narkelydte) microscopicvolumes: zonesof intercumulusmaterial Ishatolisrlter in igneousrocks, fhin rinds of glass on pillow lavas, and veins of friction-generated"melto' (pseudo- Pmilotaclylyte ficdo-tndrced tachylyte) generatedalong slip surfaces.Many run Furion 6usts on mstootitgs dd sspe@led products of .the experimental petrologist comprise small volumes of synthesizedglass (e.g., Beard & Meanict minerals (e.& 8[adtc, iradiated Lofgren 1991). The electron nr,icroprobe(Castaing nonsdtg, thodl€, drcon) 1951) is an ideal tool with which to analyze these materials, but it is widely recognizedthat the alkali metals (particularly Na) can diffrrse out of or into the t ds Foduced by UglinhC stdks in mnd (fulgurtte) analyzedvolume (e.9., Sweatman& Long 1969, Goldsteiner aL 1992,Reed 1993). This effect is depen- dent on a number of factors, including the SiO2 and SvNmsnc GI-A,ssEs H2O contents of the analyzed material, and the dissipationof power aroundthe excitationvolume. As Syntheticinorganic glasses(fable 2) can be classi- a consequence,it is easyto collect incorrect analytical fied into four groups.Network-moffied SiO2 glasses data and accurateanalysis can be difftcult. This work are the mo$t important in terms of commercial reviews the situation regarding the electron-micro- applications@oremus L973, Zarzyck 1991).Halide, probe analysisof silicate glasses,presents models for chalcogenideand metallic glassesare relatively recent the causeof ionic mobility, and suggestsprocedures for the minimization of beam-inducedlosses and gains in alkaii metals. TAII.B 1 MAN SY}IIIBTIC INOBCAMC (IASSES

Nerunar, Gussns TYpc C€mpm€nl(o Many natural glasses are of volcanic origin Odde glas sio, Chedel redstasce" Clable 1). The composition of a quenchedigneous (usallywfth thermal sho{& rgistae product can be informative; basic glassescan be less notwort-modlien) (e.g; borosiltere - pyr*) fractionatedthan associatedcrystalline rocks, and so &cr. As dngle componeDlddeq maglmus Plo! the{g ore aons!ily of indicate more about source (e,g., Natland Geq sdenfnc bfrest 1991). Glassescan also be diagnosticof deformation processes at high and ultrahigh stain-rates, such as Ealtdo glas6 BeFr ZrF& AIF9 B;@[ed opdcal seismic slip (e.9., with the formation of pseudo- I{authdde)Fe tmsndldm ltroperdg impact ZnFlPbFb Bafr tachytyte: Spray 1987, 1993) and ."1eef1s 7-frtet. (e.9., with the generationof tektites: Koeberl 1986). (Lo" potEDrlal The high-speed atmospheric entry of meteors can opd€l fibr6) also induce glassy fusion-induced crusts on their surfaces due to Chalcogeltde Group VI elsm€ofi Idared opdel tmndsdm air-projectile frictional heating glas combhed vith and electtcal mitc&ing (McSween 1987). Diaplectic glass is produced by Grcups IV and V the passageof a shock wave through a rc& (e.g., elgmsl8 Bischoff & Stiiffler 1992), with the ransformarion

in the solid state without melting. Certain MetalUcglas Melal-retalloidr Very lw magrctic lmos, metamict minerals (Table 1) are rendered glassy M€tal-relal Hgh nec.hadol strmglh ud by o-particle-induced structural hardness,ldgh c.h€dcal damage during mnodon redstance md the decay of unstable isotopes (e.g., of U, Th). radladon redr&ace However, none of these minerals contain significant alkalis. e metallold: e.C. B, C' Si B Ce

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developments,and have various advanced-technology lower beam-current requirement of the ED applications.Network modifiers in the SiO, glasses specffometermeans that EDS is the better methodfor iuclude Mg, 84 Ca, Li, K and Na. Qompositional determiningthe major-elementcomposition of glasses delerminationsof manufachred silicate glass at the (bearing in mind that EDS power requirementsare microscopic scale are not usually a routine require- largely dependenton the distanceof the detectorfrom ment. This is because the amounts and types of the area analyzedand the geomefiryof the collimator ingredientsare predeterminedat the designstage, and and silicon detector). so ttre composition is known and controlled. Furthermore,the product is normally homogeneous. TrrEEFFFcT or Er.rcrnou BoNmapoNmvr However,electon-microprobe analysis can be usedfor oN ALKALTSrrcars Gressss rapid quality-conhol and for assessmentof composi- tional homogeneity.In theseinstancesn the imFortance How can mobilization of alkali metals be recog- of Na as a network-modiSing cation can lead to nized?(l) Alkali-metal count-ratemay shift at constant problemswith analyticalaccunrcy. voltage and current. (2) Loss may be indicatedby low analytical totals; however, if the glass contains H or EDS vm.sus WDS ANeryss halogens,low overall totals can be due to HrO content (e.9.,Stolper 1982) or halogenloss (e.9., Starmer et al. X-rays emitted from the samFle surface can be 1993). Gains are more difficult to detect becausethe collected by either an energy-dispersionspectrometer analyticaltotal may not be noticeablyhigh. Gains can @DS) or wavelength-dispersionspectometers (WDS). occur where extended count-times, higher beam- The fust commercial electron microprobe that currents, or thinner conductive coatings are used, appearedin 1958 deployedonly WDS spectometers, especiallyif the glass has a low thermal conductivity and it was not until the lale 1960sthat the solirl-state @orom & Hanneman 1967). However, nlkali-metal lithium-drifted silicon [i.e., Si(Li)] detector was gainsoccur less commonly than alkali-pe1al lesgss. developed to eventually facilitate quantitative EDS If loss has occurred,a map of Na disfibution viill analysis@iugerald et aL 1968,Reed 1993).The two show a sodium'hole'that approximatesthe excitation t)?es of specftometerdiffer in a number ef important diameter of the elecfton beam (Fig. 1a). A line scan respects(Table 3) that are pertinent to the analysisof acrosssuch a hole has a pit-like shape@ig. lb). The alkali silicate glasses. The WDS spectometer has rate of Na loss varies with time. An initially steep higher resolution and better detection-limits (10 ppm declinein count rate is typically followed by a reduced for optimum operatingconditions) than doesthe EDS but relativd steady-statecount-rate. With loss of Na specfrometer.However, the detector efficieucy for from the excitation volume, the count rates of the WDS is significantly poorer than that of bDS. lsmaining elements increase. This occurs because Consequently,whereas botl types of spectrometercan the electronmicroprobe measures the massfraction of be operatedat comparablevoltage, the WDS requires elementspresent in ttre interaction volume, so that a higher beam-currentsto achieve an adequa0eX-ray loss of one or more elementsis compensatedby an count-rate (Table 3). The higher power required for apparentgain in those This is illustrated in WDS results in higher temperatures and greater Figure lc, where the relative decreasein Na counts chargingwithin the sample,especially where a focused over time is mirrored by a relative increase in Si beamis used,and this promotesdiffirsion of the alkali counts.Varsbneya et al. (1966) documenteda similar metalswithin the silicate glass. effect in a KrO-SrO-SiO2 glass owing to K loss. If a The advent of improved EDS systemsmeans that gain has occurred,a map of Na distibution will show fully quantitative major-element analysis is now a Na build-up (Frg. 1d). A line scan across such a routine (e.9., Statham& Nashashibi1988). Thus, the build-up has a mound-like shape @9. 1e). If Na increaseswithin the analyzedvolume, fhe counl rates of the remaining elementsdecrease, such that there is TABI.B 3. CSARAC'IERFTICS OF M6 VRSUS WDII an apparentreduction in their content(Fig. 10. Boror ATiIALITICAL TECMtrQI'A & Haoneman (1967) showed that this occurs for K2O-SiO2and Na2G-FeO-SiO2glasses under cerlain wDs operatingconditions. Steady-stateconditions eventually lend to prevail for alkali-metalloss, although lpical voltage range ftV) 15.30 ur-30 for long-durationcount- Vtable surert nnge (rA) 0r5-5 s-50 times (several minutes), initial lossesmay be super- Power rarge (pW) 3.75- 100 75 - 1000 sededby gains. If alkali-metal gains occur on initial Resolution (eV)

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(d)

{nm /||lm

(b) (e)

*KL asl-h I

(c) (D I I

1.0 1.0

l(s) rt(8) (8)

Ha. 1. (a) Part plan view of a polished thin section of an alkali silicate glass showing Na distribution. Note electron-beam-induced"hole" where Na is absent.(b) Section '\rell". line-scanfor Na for samesample as above(cts = counts).Note presenceof Na (c) Time-dependence(t) of X-ray intensities(Q normalizedto zero time for Na and Si. Note Na loss and Si gain. (d),Planview as for (a) above,but note zoneofNa increase. 'lnound". (e) Sectionline-scan for Na for sane sample(d) as above.Note Na (f) Time- dependenceas for (c) opposite,but note overall Na gain and Si loss, as well as initial reversalin tends.

The degreeof elkali-metal migration is also depen- decreasein Na2Ocontent with successiveanalysis with dent on the mode of analysis. Point (focused-beam) defocusedand rasterscan-modes. analysis results in greater migration, whereas defo- The next slep is to ty to understandhow alkali- cused beam and raster scanning have less extreme metal migration occurs, so that the problem can be effects.Table 4 showsthe resultsof analyzingthe same avoidedor minimized. point or area four times using poing defocusedand raster scan-modeson basaltic glass VG2 (Jarosewich Ceusrs oF MoBIurY or Alr.lu ME"rALs 1975).The meanNa2O value from the point analysisis DuRn{cEr.rcrnoN Bommomlr the lowest and this method shows a more consistent depletion in Na for successiveanalysis of the same A number of mechanismshave been proposedto spot.Both defocusedand raster-scan modes produce an accountfor alkali-metal loss and gain during elecfron acceptable mean result, which is higher tlan the bombardmentof alkali silicate glasses.These include averagepoint analysisand closerto that of the quoted sample volatilization (Baird & Tnnger 1966) and internationalstandard value. Furthermore.there is no electromigration (e.g., Graham et al. 1984, Cazaux

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TABIT 4. PS ANALYSES.OFMSAL]IC GI.AIISSTAT{DAID VE order 1000'C or may Fm. Nr uS!NGDIFFBENT BEAM-MODEI of more arisefor focusedbeams (1 pm in diameler)in uncoatedsamples. Analyzing a s,ampleof low thermal conductivify (1.e.,a glass) in beam mode an uncoatedstate can lead to beam-inducedsurface damage,manifest as pits and craters,some of which foo$ed defocrsed raster developelevated rims @orom & tla:meman 1967).It (D - 1p-) @ - z0 pn) 1- r,oOOpm) a coating medium is used (usually carbon for geo- logical materials),the temperaturerise can be reduced I L70 274 ?^71 by as much as an order of magnitude(1e., down to 2 2.47 2.80 253 approximately100'C). Vassamillet & Caldwell(1969) 3 22l 273 269 equaled the energy dissipation of the beam to an 4 2i6 z@ LP equilibrium temperaturefor the periphery of the ira- Mean 245 L72 2.69 diatedvolume given by Standardvalue 2.62 T-t= Cr,I2t*r) (t)

rOperatingconditioos: 15 kY 2J nA. where T is the rise in lemperatureat the periphery of the inadiated volume, \ is the ambient temperature of the sample,W is power in watts, ft is the thermal conductivity of the sample(W m-l oc-l), and r is the 1986). Autefage & Couderc (1980) suggestedthat radius of irradiated area in meters.For a K"O-SiO" alkali-metal mobilization occurs in two stages: (1) glass,Vassamillet & Caldwell (1969) foundthat T" thermally activatedbreaking of Na-O bondsand initial (the oitical lemperaturefor K mobilization) is approx- ionic diffrrsion, and (2) elecfiomigration of free Na+ imately equalto 85oC,and is likely to be lessthan this ions to the zone of electron build-up (space-charge for Na becauseof its greaterionic mobility. layer). Figure 2 showslemperature increase as afunction of power and beam radius using a thermal conductivity oc-l Beam-induc ed thermal ffi as ft) value of 1 W m-l appropriatefor silicate glasses(Ashby & Jones1986, p. 151).Position 3 indi- Cast?ing(1951) and Friskney & Haworrh (1967) catesa value for focused-beamoperation suitable for have calculated that for operating condifions of WDS analysis (1 pra at 300 pW). Position z also 25-30 kV and 100 nA, temperatureincreases in the correspondsto the proposedT" of Vassamillet&

.001 .01 0.1 1 10 f (rrm) Ftc.2. Electron-beampower rersas beamradius andresultant temperature increases atthe periphery of the irradiated volume using the equation of Vassamillet & Caldwell (1969). Point 3 conespondsto t)?ical WDS conditions of analysis (see Table 4), as well as the temperatureof onset of diffusion ef slkali metals in silicate glasses (T" * 85'C). Defocusedbeam radii shownshaded. Power requirementsfor WDS and EDS analysisare indicatedon right.

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Caldwell (1969), which shows that those values elemental form (inducing a color change known as o'electron parallel to the isotherms and to the left of z would browning"), or to bond to new oxygen ions. facilitate alkali-metal migration e85'C). Note that A concomitantdecrease in Na X-ray intensities was typical valuesof r for a defocused$sam (i.e., yrelding noted, and the analyzedvolume becarnedepleted in analyzedareas of 10-20 pm diameter)result in T being this metal. Movement of Na ions down to the charge much lessthan T", suchthat bond disruptionand diffrr- layer results in oxygen being releasedfrom the upper sion are avertedfor both EDS and WDS operation. part of the sample where, on losing elecftonsto the Overall, it can be seenthat the likelihood of mobi- elecfodeoit escapesfrom the surfaceas O2 @g. 4). liz.ing Na is significantly reducedwith EDS compared The Lineweaver mechanism has been widely to WDS. In fact, for WDS analysis,it seemsdifficult accepted,but it is not applicable in all situations. not to mobiliz" 116ealkali metalsin silicate glassesif a Borom & Hanneman(1961 found that under certain focused beam is used. Use of low power with WDS conditions,an increasein alkali-metalX-ray inlensities necessitateslong count-times,which are also counlgr- can occur, with a correspondingdecrease in the inten- productive. sities for the remainingelements. This is characteristic of more intense beams and thinner conductive The sparc-ch,argelayer coatings,especially where less stablevarieties of glass are being analyzed.They atfibuted this to increased An important cotrcapt in our uuderstanding of thermal effectsand to destruction(vaporization) of the electromigrationis that of the oospace-chargelayet'', conductivecoating. This allows electronsto accumu- which is purportedto developwhere the elecfrongain late at the samplesurface, such that the mobile Na ions of the sample exceeds tle electron dissipation. are athacted toward the more negatively charged 1yl"1p1 alkali-metalions arelost from or gainedin the surface. inadiated volume dependson the size and location of Both the Lineweaver (1963) and Borom & the space-chargelayer within the sample. Hanneman (1967) models can be reconciled if the Lineweaver(1963), in his study of NarO-SiO2glass space-chargelayer can move within the sample.Estour cathode-raytubes, showed that oxygenis releasedfrom (1971) showed that the rate of decay in the X-ray glass surfaces{uring electronrastering, and proposed countsfor Na could be minimized if higher accelerat- that insident elecftonsaccumulate at somefin:ite depth ing voltagesare used (i.e.,>20kY). This was attributed within the sampleto forrn a oonegative-chargeregion" to the accumulationof elecfronsat deeperlevels in (Ftg. 3). Owing to this electon build-up, Na ions the sampleowing to their greaterkinetic energy,such decouple from their oxygen (unbridged) ions and that the near-surfaceNa ions are effiectivelyshielded migrateto the negativezone, to be eitherneuftalized to from the relocated more remote space-chmgelayer.

electron beampit o2 I I f" c" + t I *{f Na" Na* -Na* { / /**

Flc. 3. Cross-sectionof polished thin section of an alkali silicate glass showing the formation of a "space-chargelayet'' comprising accumulated electrons within the sample.Na-O bonds are broken, then Na cationsmigrate to the negativechmge layer, where they are neutrlized.

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(a) (b) (c) O2 Or

@ *^ r Sl Q brldglngoxygen O nonbrldglngorygen

Flc. 4. Linelveaver's model for alkali-me€l loss: (a) prebombardmentstructure of NarO-SiO2 glass (simFlified); (b) Na-O bond breaking,downward migrarion of Na* and upwardmovement of O! and (c) final (immediatelyposibombmdment) modified stucture, with neutalized Na atoms and 02 evolving from the sample surface. Modified after Lineweaver(1963).

Howeveroa higher voltage sipificantly reducesX-ray $fDS). By doing so,he increasedlemperatures, and so generation,such that Estour (1971) had to increasethe Na lossrecurred. Goodherv & Gulley (1974)confirmed probe current(to 20 nA) in order to generatesuffrcient Estour'sfindings and showedthat probe currentstwo X-rays for the less efficient crystal spectrometer ordersof magnitudesmaller could be used with EDS, andNamobility couldbe virtually eliminated.Thebest results were obtained at 30 kV and 0,2 nA (Fig. 5). 100 Goodhew (1975) subsequentlyshowed that if each analysiswere carriedout at a freshpoint on the sample, the Na decayrale could be reducedbelow 0.5Voof the counl rate. This would constitute only l-2Vo loss in a total count over a few hundredseconds. o o Compo sitional depenlenc e of allali -metal rnigration

The susceptibility of glassesto beam-induced mobilization ef alkali metals is also dependenton the chemical composition of the glass as well as on the analytical operating conditions. Highly siliceous glasses show high susceptibility to alkali mobility, whereasmorc basic Qow-SiO) material doesnot. The forrner are desipated the so-called unstable glasses 01234567 1991) which, geologically, would include Tlmslnmlnut€ Q.ar.zyck:- rhyolitic glass. The stable glassesinclude those of Ftc. 5. Decay of NaKo count rate during EDS analysisfor a variety of acceleratingvoltages at a beam cunent of mafic composition.The reasonfor the variable alkali- 0.2 nA (after Goodhew& Gulley 1974).laad Io are the metal response to electron bombardment remains X-ray intensities (counts) normalize

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of polymerization involves a more open network of mizesatfraction of alkalis from highel lsysls within the tetahedra that allows diffrrsion of alkali metal cations irradiated volume. If beam power is to be kept low, to occur with greaterease. Low polymerizationresults even lower currentswill have to be used and, again, in a more dsnselypacked sfucture ofthe glass,through this favors EDS over WDS. It should be noted that which it is more difficult for the alkali metals to ZAF correctionsmay be in error at high kV. diffrrse. Depolymerizationis causedby, for example, (3) Initial tempera.tareof the sample: the beam- the presenceof Fe and Mg breaking the SiO2 chains induced rise in temperatureCI) within the irradiated into shorterunits. As well as effectively blocking the volume may be greatly reduced by the ambient movement of alkali ions, these network-modifying temperature(t") of tle sample[see equation (1)]. If a elementsincrease electrical and thermal conductivities cold stagecooled by a cryo-unit is used, about -140o and hencereduce thermal and charging effects. Most can be attained at tle stage. However, analysis geologically important glasses are aluminosilicates, would become extremely time-consuming.Using a where the presenceof Al also acts to reduce alkali conductive sample-mount(1e., copper) also helps to volatility. The compositional dependenceof alkali improve heatremoval from the atalyzedarea. mobility must be borne in mind when selecting (4) Beam.mode: rf.point analysisis not requird use operating conditions. tligh-SiO2, high-alkali-metal, defocused(WDS or EDS) or raster scan-modes(EDS low-transition-metalglasses are particular diffrcult to only). Suchscan modes involve a larger beam-area,so aralyze quantitatively. that the power per unit area and hence temperature increaseis reducedsipificantly. Rasteringis difftcult Mwnnznqc Axeu MrcnenoN rN Gr.AssFs with WDS because of more precise geometrical requirements,but high-speedvdde-area analysis tech- [a e1d6p19 minimizg alkali migration,it is necessary niques have been developedwhereby the sample is to minimize both thermal effects and the developmenl moved physically beneath a static lsam (a.9., Ono of the space-chargelayer. These two effects are et al. L985). related. The energy required to break the Na(K)-O (5) Conduaivlry: with insulating materials,problems bond is related to the temperaturegenerated during of low thermaland elecfiical conductivitiesof samples power dissipation.Alkali migrationis dependenton the can lead to adversedevelopment of the space-charge size of the space-chargelayer, which is controlled by layer. Th:is situation can be improved by thicker beampower and by the conductivitiesof both coating (double) carboncoats and by coatingboth sidesofthe and sample.The location of the space-chargelayer can sample. heparation of ulfrathin samFlesglued onto be confrolledby varying the beamvoltage and the total conducting (e.9., copper) mounts also may help to count-t'me. alleviate development of the space-chargelayer. Finally, it is noted that alkali migration is not However,too thick a coatingcan causemore problems resfrictedto glassymaterials. Negative charging of the than it alleviatesbecause it increasesabsorption ofboth samplesurface and concomitantNa and K fluctuations incoming electronsand outgoing X-rays andso reduces have been noted amongstother minerals,in sepiolite the observedX-ray intensities.Furthermore, to obtaina @utt & Vigers 1977), autunite (Grahamet aI. 1984), consistentthickness of the coating, it is important to feldspars (Ribbe & Smith 1966), feldspathoids ensurethat the standardsarc coatedat the sametime as @rousseet al. 1969),jadeite and scapolite(Autefage the samplesto be analyzedor, if coatedseparately, by 1980). In all cases,the minerals were analyzed by use of thicknessmonitor (Greaves1970). WDS, and maximum mobility occurred when a (6) Coant-time:because of the time-dependentnature focusedbeam (i.e., approximately 1 pm. in diameter) of alkali mobility, reductionof the count-timecan help wasused. to reducemigration. A careful balancemust be struck The following guidelines will help to optimize between shorter count-times and accumulation of analytical conditions for the quantitative analysis of adequate counts. Reduced coutrt-times are more alkali silicateglasses: feasiblewith EDS becauseof the greaterefficiency of (l) Power the temperature rise in the irradiated Si(-i) detectors. If only two WD (or even three) volume within the sample is stongly controlled by spgcfrometersare available, the samplemust be irra- beampower. Reduction in kV is limited by the require- diatedfor a long period to collect sufficient countsfor mentfor 2-3 x overvoltageratio (e.9.,Fe 6.4 keV x 2.5 9-10 elements.Even if the sampleis analyzedfor Na = 16 kV). Reducing the cunent will reduce thermal firsr the concomitantincrease in other elementscon- effects, but this also lowers the intensity of X rays tinuesto occur through the remainingcount-time. generated, such that WDS analysis becomes im- Q) For EDS analysis: use a "thin window" if avail- practical. Reduction of the beam .current is more able,or operatein windowlessmode. This allows more feasiblewith EDS. countsto enterthe detectorand so reducescount-time. Q) Relocation of the space-chargelayen the space- However, windowlessmode risks permanentdamage chargelayer may be moved deeperia fte sampleb] to the detectorif the probe vacuumfails. higher acceleratingvoltages (i.e.,30 kV). This mini- (8) For WDS analysis: owing to the easewith which

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Flc. 6. Data-processingoptions to corect for migration of alkali metals. Performing pre-ZAF correctionsfor all elementsin order to rectify beam-inducedion mobility effectsis the only proper solution.

the alkali metals are mobilized in the WDS mode at AurEFec4 F. (1980): Variationsde la teneuren sodiumet en routine operating conditions, analyzefor Na and K potassiumdans des mindraux au coursde leur analyseb la Mindral. 103, 48-53. first so that their loss or gain is minimized. Note that microsonde6lectronique. Ball, this doesnot reducethe obverseeffect (Figs. lc, f) on & Couomc" J.-J. (1980): Etude du m6canismede other elements(e.9., Roberts et al. 1,990). la migration du sodium et du potassiumau cours de leur (9) Data corrections: Correct all elementsfor bearn- analysed la microsondedlectronique, Bull. Mindral.103, induced ion-mobiJity effects prior ta ZAF correction 623-629. Gtg. 6). Collect decay or enrichment curves for all elementssimultaneously. Na loss may be estimatedby Bano, A.Ii & Zmlcr& D.H. (1966): Use of the soft X-ray for exfrapolatingthe count-rateback to zero time (Nielsen specftogaph and the electron-probemicroanalyzer determinationof elementscarbon through iron in minerals & Sigurdsson1981). Some workers correct for alkali androcls. Adv.X-Ray Ana|.9,487-503. loss or gain by adjusting the Na nd K values ajler on-line ZAF processing,but such an approachfails to BEARD,J.S. & Iorcnnr, G.E. (1991): Dehydration melting compensatefor the apparentincrease (or decrease)in and water-saturatedmelting of basaltic and andesitic the remaining (unmobilized)elements and leadsto the greenstonesand amphibolitesat 1,3, atd6.9leb. J. PetroL compoundingof analyticalerror. 32,365401. (I0) Intemartonal standards: use intemational glass (1992): metamorphism standards(e.9., Myers et al. 1976, Abbey 1983, Brscrrorn,A. & Sr0mrm, D. Shock as a fundamentalprocess in the evolution of planetary N.I.S.T. 1994)to calibrateand to test for alkali migra- bodies: information from meteorites.Eun J, Mineral. 4. tion and checkoverall results. 707-755.

AcKr.IowLDGH\,IB,ns Bonou, M.P. & HaNNI$4AN,R.E. (1967): l,ocal conpo- sitional changesin alkali-silicate glassesduring elechon This work was funded tbrough NSERC operating microprobeanalysis. J. Appl. Phys. 38, 240624{7. grantsto J.G. Spray and NSERC infrasfiucture grants in support of UNB's Electron Mcroscopy Unit. We BRoussE,R., Vensr, J. & Bzouano, H. (1969): Iron in the golup. thank Frank Hawthorne, Robert Martin, Mati minerals of the sodalite Contrib. Mfuiral. PetroL ?2. 169-184. Raudseppand Peter Roeder for commenting on an earlier draft of the manuscript. Burr, C.R.M. & Vtcms, R.B.W. (1977): Rapid increaseof sodiumcount ratesduring the electronmicroprobe analy- sis of sepiolite.The role of zeolitic walsr. X-ray SpectronL Rmnnwcrs 6.1,4+148.

Annrv, S. (1983):Studies in "staadardsamples" of silicate CAsrANc, R. (1951):Applicationdes sondzs4lcctroniques d rocksand minerals 1969-1982. Geol, Surv. Can, Pap. une mlthoile il'ambse ponctuzlle chimiquc et cristallo- 83.15. graphique,Thbse, Universit6 de Paris,Paris, France.

AsmY,M.F. & Jonrs,D.R.H. (1986): Engincertng Maerials. Cezsvx, J. (1986): Someconsiderations on the electric field 2. An Introductianto Microstntctares,Processing and induced in insulatorsby electronbombardment J. Appl. Design, Pergmon hess, Odor4 U.K. Phys.59, 14l8-1430.

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