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. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/2/323/3446687/323.pdf by guest on 26 September 2021 3V1 TT{E CANADIAN MINERALOGIST 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 Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/2/323/3446687/323.pdf by guest on 26 September 2021 TTIE ELECTRON-MICROPROBEANALYS$ OF GLASS 325 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.