American Mineralogist, Volume 85, pages 1784–1794, 20000003-004X/00/1112–1784$05.00 1784INTRODUCTIONElectron microscopy has several advantages over conven-tional light microscopy, most notably better spatial resolutionand greater depth of field (Lloyd and Hall 1981; Trimby andPrior 1999), and high-resolution topographic, structural, andcompositional images from scanning electron microscopes arenow widely used in the earth sciences. The principle of scan-ning electron microscopy is simple—electrons from a thermi-onic or field-emission cathode are accelerated to form a beamthat is rastered across the sample surface. Signals arising frominteractions between the beam and sample are collected, andan image reconstructed that reflects variations in signal strength.The bombardment of geological materials with a beam of elec-trons, however, will generally result in electrical charging andspecimen damage unless steps are taken to reduce chargebuildup. The most common method used to alleviate samplecharging involves coating the specimen surface with a thin (50nm) conductive layer of C, Au, or Pt. Conductive coats canreduce signal/noise, compromise weak or low-energy signals,and/or obscure important emissions (e.g., AuMα X-ray emis-sion from a gold coat may conceal NbLα signals in niobates).A conductive coat also prevents the direct imaging of conduc-tivity contrasts in the underlying substrate. In uncoated samples,differences in the dielectric properties generate charge contraststhat may be used to detect compositional variations, structuralcontrasts, and enhanced conductivity pathways within and be-tween minerals. Despite an abundance of materials scienceliterature linking electron trapping and charging in semicon-ductors and ceramics with defects of geological significance(e.g., trace element impurities, lattice defects, twin planes)(Doehne 1998; Galvin and Griffin 1999; Gluszak et al. 1999;Gong et al. 1993; Oh et al. 1993), little attention has been paidto the phenomenon of charge contrast in geological samples.In this paper, we show that charging contrasts within singlecrystals and among mineral phases can provide important in-formation regarding growth, compositional variation, micro-structure, and alteration in a variety of geological specimens.We image several minerals and compare the informationfrom charge contrast with other electron imaging tech-niques such as backscattered electron (BSE) imaging andcathodoluminescence (CL). The samples presented includecathodoluminescent phases (zircon and quartz) and non-lumi-Charge contrast imaging of geological materials in the environmental scanning electronmicroscopeGORDON R. WATT,1,*,† BRENDAN J. GRIFFIN,2 AND PETER D. KINNY11Tectonics Special Research Centre, School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth 6845, WesternAustralia, Australia2Centre for Microscopy and Microanalysis, The University of Western Australia, Nedlands, Western Australia, 6907, AustraliaABSTRACTThe environmental scanning electron microscope (ESEM) allows high-resolution, high-magnifi-cation imaging of conductivity differences in uncoated geological samples. Under normal ESEMoperating conditions, negative charge buildup at the sample surface (from bombardment by theelectron beam) is prevented by the presence of a gas (usually water vapor) in the sample chamber.Backscattered and secondary electrons from the sample ionize this chamber gas, and the resultantpositively charged gaseous ions migrate toward the negatively charged sample. When chamber gaspressures lower than approximately 250 Pa are used, however, charging of the sample can occurbecause insufficient charge balancing positively charged gaseous ions are produced. Charge implan-tation in the sample alters secondary electron emission, and, because intracrystalline conductivitycontrasts occur in response to variations in defect density, secondary electron images reflect compo-sitional variations and/or microstructural features. These secondary electron images are referred toas charge contrast images (CCI). To demonstrate potential geological applications of CCI, we presentimages of growth zones, microfractures, differential diffusion domains, pleochroic haloes, and relictfluid pathways from zircon (strongly luminescent), quartz (weakly luminescent), and biotite andcordierite (non-luminescent). CCI detect defects in a similar way to cathodoluminescence (CL), buthave a higher resolution because the CCI signal is composed of secondary electrons that are gener-ated from a much smaller interaction volume than photons utilized in CL. CCI imaging also can beapplied to a wider variety of geological samples than CL, because electronic charge trapping is notrestricted to wide-band gap electronic configurations. One of the most important potential applica-tions of the CCI technique may lie in the direct imaging of relict fluid pathways in rocks that haveexperienced metasomatism or alteration.* E-mail: [email protected]† Current Address: Department of Geology & Petroleum Ge-ology, King's College, University of Aberdeen, Aberdeen, AB243UE, U.K.WATT ET AL.: CHARGE CONTRAST IMAGING OF GEOLOGICAL MATERIALS 1785nescent phases (cordierite and biotite). These minerals werechosen to illustrate various intra- and inter-grain charge con-trast features and demonstrate potential applications for charge-contrast imaging—a new high-resolution, non-destructivepetrographic tool for earth scientists.ENVIRONMENTAL SCANNING ELECTRON MICROSCOPYThe environmental scanning electron microscope (ESEM)allows the observation of uncoated samples at moderate to lowvacuums (Danilatos 1993; Griffin 1997a). In the ESEM, theprimary electron beam and backscattered and secondary elec-trons from the sample interact with gas molecules in the speci-men chamber to produce positively charged gaseous ions andan amplified “cascade” of secondary electrons that are acceler-ated toward a positively biased gaseous secondary electrondetector (GSED) (Danilatos 1993). Once the gas is ionized, itmigrates toward the sample where it acts as a charge-neutraliz-ing agent. The ESEM is normally operated at high enough gaspressures to produce sufficient positive gaseous ions to neu-tralize charge at the sample surface and, consequently, mostESEM images show no charging. At lower gas pressures, how-ever, it is possible to implant a limited amount of charge in anuncoated sample (Doehne 1998; Griffin 1997). These
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