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LETTER Analysis of Oxygen with the Electron Microprobe: Applications To American Mineralogist, Volume 77, pages453-457, 1992 LETTER Analysis of oxygen with the electron microprobe: Applications to hydrated glass and minerals W. P. N.qsH Department of Geology and Geophysics,University of Utah, Salr Lake City, Utah 84112-1183,U.S.A. Ansrru.cr The development of layered synthetic dispersion elementsmakes it possible to analyze routinely for O with the electron microprobe. The precision for O in silicate glassis 0.60lo of the amount present,comparable to that for Si. Optimum results occur when standards similar in composition to unknowns are used and care is taken in standard and sample preparation. The procedure is particularly advantageousin the analysis of hydrous glass and minerals in which conventional microprobe analysesyield totals of less than 1000/o becauseO attached to H remains unmeasured.The direct determination of O provides a measnreof the quality of the analysis and yields an indirect estimate of the HrO content of the sample by comparison of measuredwith stoichiometric O concentrations. INrnonucrroN Onsracr,rs ro rHE ANALysrs oF LrcHT ELEMENTS The analysis by electron microprobe of light elements The problems encounteredin the analysis of light ele- (Z < 9) such as O presentsnumerous problems resulting ments are of two varieties: those resulting from the phys- from the low cross sectionsfor ionization, high absorp- ics of X-ray generation and detection, and those associ- tion ofthe low-energy X-rays, and spectral interferences ated with preparation of the sample and standard. Lrght by higher order X-ray lines of heavier elements. O has elementshave very low cross sectionsfor ionization that rarely been analyzed directly despite its abundance, and result in a low yield of X-rays, and theselow-energy X-rays its concentration is traditionally calculated by stoichi- are readily absorbedin the sample. Mass absorption co- ometry with measuredcations. This method neglectsO efrcients are often large, and if there are significant dif- associatedwith cations not measured,notably H in HrO- ferences in composition between standards and un- bearing materials, and other light elements such as Li, knowns, the mass absorption coefficientsmust be known Be, B, C, and N. BecauseO that may be associatedwith with high accuracy. Moreover, although O X-rays may theseelements is not determined,analytical totals are less be generatedat considerabledepth (micrometers) in the than l00o/0.Uncertainties in valence may also result in sample, the emission volume is much shallower because inaccurate totals when O is calculated by stoichiometry. of high absorption. Thus, the O signal may be strongly Under these circumstances,it is difrcult to assessthe influenced by surficial and near-surfaceconditions. quality of an analysis, although the calculation of the In the past, the problem of low X-ray yield was exac- structural formula for a mineral may confirm whether or erbated by inefficient diffraction of characteristic X-rays not the analysisis acceptable.In the caseofnatural glass, to the detector. The situation has been significantly im- no such recourseexists, and low totals are generally ac- proved by the development of layered synthetic disper- cepted and attributed to the presenceof HrO or the loss sion elements.These X-ray reflectorsconsist of alternat- of Na caused by interaction of the electron beam with ing layers of heavy and light elementsthat act as diffracting the sample. crystals,with d values determined by the thicknessof the The recentdevelopment of layeredsynthetic dispersion light element layer. The advantageof these X-ray reflec- elements(LSDE), often referred to as multilayer diffract- tors in the analysis of light elementshas been described ing crystals, has made O and other light elements mea- by severalauthors (Nicolosi et al., 1986; Love and Scott, surable,even in minor concentrations.Although it is now 1987;Armstrong, 1988;Kawabe et al., 1988;Bastin and possible to analyze for O routinely in minerals, the de- Heijligers,l99l; McGee et al., l99l). Not only do these termination of O is particularly advantageousin the anal- reflectors provide greater peak intensities, they also sup- ysis of hydrous minerals and glassesbecause, first, the presshigher order interfering X-ray lines, such as AlKa(IIf analytical total provides a direct measure of the quality on the OKa peak and AlKa(IV) on the N peak, which are of the analysis, and, second, HrO contents can be esti- presentif a lead stearatediffracting crystal is used.Count- mated from the differencein measuredand stoichiomet- ing rates with the LSDE crystalsare sufficiently high that rically calculatedO contents. the statistical precision for O is similar to heavier ele- ooo3404x/92l0304-o453$02.00 453 454 NASH: ANALYSIS OF O IN MINERALS AND GLASS ments conventionally analyzed with the electron micro- used for the remaining elements.Kaersutite was used as probe. Minor shifts in position and shapeof the OKa line the O. Si. and Al standard for the analysisof hornblende' occur with LSDE, as describedby Armstrong (1988)and and muscovite was used as the O standard for illite. Sam- Bastinand Heijligers(1989). In the applicaiiondescribed ples and unknowns were coatedwith C simultaneouslyto here, the problems of mass absorption and shifts in peak assurean equivalent thicknessofthe C coat. During anal- position and shape are minimized by the traditional ysis, repetitive measurementswere made on synthetic method of using standardssimilar in composition to the AlrO. to determine if O were present in amounts greater unknowns. than stoichiometric becauseof adsorbedH'O on the sam- In order to obtain accurate and reproducible micro- ples; no excessO was detected. probe results, care must also be taken with sample prep- H concentrations in melt inclusions were determined aration. The standardsand unknowns must be well pol- by secondaryion mass spectroscopywith a CamecaIMS ished and clean, have the same thickness of C coating, 4f ion microprobe at the University of Edinburgh (cf. and have surfacesperpendicular to the electron beam. Hervig et al., 1989,for discussionof SIMS measurements These proceduresare particularly critical for O because of HrO in melt inclusions). HrO concentrations of glass the signal is derived from very near the surface, and C separateswere determined manometrically on the H ex- has a high massabsorption coefficientfor OKa radiation. traction line at the University of Utah. Goldsteinet al. (1991)describe the effectsofsurficial ox- idation of metallic samples.Although this is not a prob- Rnsur-rs lem for silicate glassesand minerals, it could be a serious The O contents of 18 mineral standards,determined problem in the analysis of low concentrations of O in using synthetic AlrO, for O calibration, are illustrated in metals, sulfides, etc. In addition, adsorption of H.O by Figure l. Nominal O contents were calculated assuming the samplescould be problematic and should be evalu- stoichiometry of the mineral standards.The correlation ated. Finally, errors of up to l0loper degreeof inclination between measured and nominal O contents is generally are produced by samplesthat are inclined to the primary good. However, hematite and magnetite have measured electron beam. O contentsthat are low by l0loabsolute; these are the two data points at lowest O contents in Figure 1. This dis- AN.q.LvttcAl- coNDITIoNS crepancyis attributed to reduced peak intensities due to Analyseswere performed on a CamecaSX-50 electron peak broadening associatedwith differential energeticsof microprobe equipped with four wavelength-dispersive the bonding of Fe oxides compared with that of alumina spectrometers. The X-ray reflector, manufactured by or silicates. This emphasizes the need to correct for Ovonic Synthetic Materials Company (OVOMX OV- changesin peak shape,if present (cf. Bastin and Heijli- 060a) is a 60-A WSi multilayer deposited on a single- gers, 1989), or, alternatively, to utilize standardssimilar crystal substrateof Si (100). O and F were measuredon to unknowns in composition. a spectrometer with this crystal. The remaining spec- Conventional analysesofhydrated silicic glassesyield trometers were equipped with TAP, PET, and LiF ana- low analytical totals, which are attributed to HrO and lyzing crystals.Gas-flow proportional countersusing P-10 alkali mobility under the electron beam. Table I presents gas (900/oAr, l0o/omethane) were used on all spectrom- data for four silicic glasses.The first is the averageof 130 eters.Analyses were made at l5 keV acceleratingvoltage, analysesperformed over a period of 2 yt on MM3, the a beam current of 25 nA, and a beam diameter varying high-silica obsidian standard.The standard deviation for from 5 to 25 pm. O was measured at the beginning of O represents a reproducibility of 0.60/oof the amount eachanalysis; counting time was 20 s. A similar counting present, slightly better in this case than the error on Si time was used for standardization so that any effects of (0.80/0,relative). The conventional analysis in terms of C contamination in the samplechamber would be similar oxidesis shown for comparison. for the standard and the unknowns. Background inten- The second data set is for glass shards from the Lava sitieswere measuredon both sidesof the analytical peaks. Creek B tufl erupted from the region of Yellowstone Na- Concentrationswere calculatedfrom relative peak inten- tional Park 0.6 Ma. A common practice is to assumethat sities using the Qbz) algorithm of Pouchou and Pichoir the difference between the analytical total and 1000/ois ( 199l), utilizing massabsorption coefficientsfrom Henke causedby the presenceof unanalyzed HrO and to nor- et al. (1982). On the natural glass standard containing malize the data to 1000/0,producing an analysis of the 49.8 wto/oO, these conditions yielded a peak to back- "anhydrous" glass.The procedure has merit only if the ground ratio of 41, comparable to that for Al (50). analysis is of good quality, Na has not been lost during Synthetic AlrO, was used as a standard for reconnais- analysis,and there has not been an exchangeofHrO for sancemeasurements of O in a variety of mineral stan- other elementssuch as alkalis during hydration.
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