Michael W. Davidson - Microscope · PDF fileJOURNAL OF GEOLOGICAL EDUCATION, 1991, V. 39, p. 403 2 illumination, differential interfer-ence contrast, and Rheinberg

JOURNAL OF GEOLOGICAL EDUCATION, 1991, V. 39, p. 403 1

ABSTRACTMicroscopy plays a paramount

role in the examination and identifi-cation of geological petrographicthin sections of rocks and minerals.A necessary responsibility ofmicroscopy is to capture the imagesseen in the microscope ontophotographic film to obtain “hardcopy” for research records. In thisarticle we describe the conversionof a standard biological brightfieldmicroscope for examination ofgeological petrographic thinsections and characterize, in detail,the use of both black and white andcolor photomicrography in thegeological sciences. Severalillustrative examples on the use oftransmitted and reflected polarizedlight microscopy to solve geologi-cal problems are presented.

INTRODUCTIONThe biological and medical

sciences have, for many years,relied heavily on visible lightmicroscopy to solve problemsrelating to the gross morphologicalfeatures of specimens as well as aquantitative tool for recordingspecific optical features and data.In this respect, the microscope hasproven invaluable in countlessinvestigations into the mysteries oflife. Geologists have also em-ployed light microscopy to examinepetrographic thin sections of rocksand minerals found on earth and onthe moon, and even meteorites fromspace. These geological studieshave led to a detailed understand-ing of the origin and compositionof our planet and its nearestneighbor.

In recent years, many new glassalloys have been discovered andlens fabrication technology hasshown some dramatic improve-ments (Delly, 1988). Also, thedevelopment of advanced anti-reflection lens coatings has led tothe production of new microscopesthat are optically superior to thosemade in the past. To furtherenhance the capabilities of modernmicroscopes, manufacturers arenow using photomultipliers tomonitor light intensity for photo-micrography. These photomulti-pliers are coupled to microproces-sor-controlled exposure monitorsthat determine correct exposuretimes based on user settings forvarious photographic film charac-teristics. Advanced opticaltechniques such as cross-polarized

Michael W. DavidsonInstitute of Molecular Biophysics

Center for Materials Research and Technology (MARTECH)National High Magnetic Field Laboratory (NHMFL)

Supercomputer Computations Research Institute (SCRI)Florida State University, Tallahassee, Florida 32306

Telephone: 850-644-0542 Fax: 850-644-8920

Gary E. LofgrenPlanetary Materials Branch

Solar System Exploration DivisionCode SN2

NASA Johnson Space CenterHouston, Texas 77058

Telephone: 713-483-6187 Fax: 713-483-2696


illumination, differential interfer-ence contrast, and Rheinbergillumination have become standardfeatures on many modern micro-scopes. As improvements in thequality of visible light microscopeshave developed, so have theapplications. Microscopy is rapidlybecoming a (main) staple in thescientific equipment inventory insuch diverse disciplines as chemis-try, engineering, geology, physics,materials sciences and even thesemiconductor and computerindustries. New materials that havecome under the scrutiny of lightmicroscopy include birefringentliquid crystalline systems (Saeva,1979), high temperature ceramicsuperconductor thin-films(Feldman, 1986), synthetic chon-drules in the study of chondriticmeteorites (Lofgren, 1989), ceramicsynthetic perovskites (Hazen,1988), and integrated circuitsurfaces (Davidson, 1991a;Silverman, 1987). The nature ofthese materials requires, in someinstances, examination by reflected(episcopic) illumination thatdeparts from the classical transmit-ted (diascopic) illuminationtechniques that have been so welldeveloped in the biological andgeological sciences.

Transmitted light microscopy isa mature discipline in which a widespectrum of illumination techniquesare available for contrast enhance-ment. For instance, living cells intissue culture can be observeddirectly at amazingly high resolu-tion using phase and differentialinterference contrast techniques. Athigh magnification, cellular internalfeatures are clearly resolvableusing these methods. In contrast,reflected light microscopy is justemerging as an important tool forinvestigating surface features in thematerials and geological sciences.A significant commercial boost toreflected light microscopy hascome from the semiconductorindustry which relies heavily onthis technique to capture micro-scopic details on integrated circuit

wafers during the manufacturingprocess. The need for contrast-enhancement methods in reflectedlight microscopy has led to thedevelopment of reflected lightdifferential interference contrastobjectives that rival, in quality,their counterparts on the transmit-ted light microscope.

Photomicrography has enjoyed arecent escalation in popularity due,in part, to the introduction of newtransparency films with improvedemulsions that respond correctly tothe color spectrum generated bytungsten-halide light bulbs found inmost microscopes. Improved glassalloys and optical coatings, asdescribed above, have enabledmanufacturers to produce micro-scope objectives that will producecrisp images of better quality thanhas been previously possible.These developments, coupled to thewidespread availability of micro-processor-assisted internal micro-scope exposure monitors havecollectively been directly respon-sible for the increased activity inthe field of visible light photomi-crography. The growing interest inphotomicrography is partiallyevidenced by the number ofphotomicrograph contests heldeach year both on a domestic andinternational basis (Davidson,1990a). Geological and materialsscience samples continually winprizes in these contests dedicated tothe advancement of art in science.Another indicator of the popularityof photomicrography is the increas-ing number of scientific tradejournals and arbitrated scientificperiodicals that feature photomicro-graphs on the front covers as aneye-catcher (For a list of periodi-cals that, in the past, have usedphotomicrographs on the cover, seeDavidson, 1990b).

Computer-assisted imageanalysis and scanning confocalmicroscopy are surfacing asimportant techniques, however theirhigh cost will prohibit their entryinto the educational arena in theforeseeable future. While these

techniques lie beyond the scope ofthis article, the interested reader isreferred to excellent reviews onthese exciting subjects (Inoue, 1986and Pawley, 1989).

MICROSCOPECONFIGURATION

The majority of microscopesfound in educational institutionsare of the transmitted light varietycommonly used in biologicallaboratories. At a relatively lowcost, these microscopes can beconverted for use in the materialsand geological sciences as well(Davidson 1991b; Brice and Lint,1987; Singh, 1983; Hasson, 1975).

Most microscope light sourcesemit visible light that vibrates as awave in all directions perpendicularto the optical axis of the micro-scope. When used in this fashion,the microscope is said to be operat-ing in the brightfield mode. Visiblelight consists of an electric fieldvector component and a magneticfield vector component that aremutually perpendicular to eachother and both are perpendicular tothe direction of propagation of thelight beam. The electric fieldinteracts strongly with materials, soits direction and amplitude are usedto describe the light. At anyparticular instant in time, theamplitude and direction of theelectric field can be utilized todefine a point on a plane that isperpendicular to the direction of thelight beam. To an observer lookingtoward the light source, this pointwould trace out a curve as the fieldvaried. If the curve is simple andrepetitive, the light is said to bepolarized and the form of the curvedefines the state of polarization. Ifthe curve is irregular and chaotic,the light is unpolarized. When theelectric field oscillates in ampli-tude, but has a fixed direction, thecurve traced out is a straight lineand the field remains in one planeas illustrated in Figure 1. This iscalled the plane of polarization(Wahlstrom, 1979) and the light isdefined as linear or plane polarized.


Figure 1. Schematic illustration of the electric field vector showing severalpossible states of plane polarized light where the vector remains in one plane.

Figure 2. Parallel light beams and a single ray in a birefringent sample. Thedotted curve is an indication of where the intensity of the light is at a maxi-mum. The electric-field vector E is perpendicular to the beam direction P.

In practice, polarized light can beproduced by passing the beam througha specially prepared crystal of themineral calcite (Iceland Spar) orthrough a Polaroid (r) polarizingelement. Placing a polarizing elementinto the light path restricts the passageof light, much as an optical slit, tothose waves that propagate in thevector plane of the polarizing element.This action reduces the amount oftransmitted light to approximately30% of the emitted value. A commonmicroscopy technique, especially ingeological petrography, involves theuse of crossed polarizers where twoindividual polarizing elements areinserted into the light path with theirvector propagation planes crossed at a90º angle with respect to one another.When a sample is placed in the lightpath between crossed polarizers, theonly light emitted will be that which isrefracted by the sample so that it canpass unimpeded through the secondpolarizer. In classical nomenclature,the first polarizer has been termed thepolarizer while the second polarizer istermed the analyzer.

Samples that are visible betweencrossed polarizers are said to haveoptical birefringence. This means thatthese samples have a refractive indexthat depends on the direction of theelectric field vector in the light. Manydifferent kinds of samples are birefrin-gent including liquid crystallinesystems, single crystals, polycrystal-line polymers, and minerals found inpetrographic thin sections of rocks,among others. Light waves passingthrough a birefringent material areusually split into two perpendicularlyplane polarized waves polarized in theprincipal directions. The material hastwo different refractive indices, n

1 and

n2, for the two plane polarized waves.

As they pass through the materialthere is a relative retardation andphase shift and the polarized lightpassing through a birefringent samplewill usually have its polarization statechanged. A complicating factor is thatin birefringent materials, the electricdisplacement vector D is usually notparallel to the electric field vector E asis depicted in Figure 2. The vector E


excellent textbooks on this subjectand a short review published byGarlick (1990).

Any brightfield microscope caneasily be converted for use withpolarizing elements. Twopolarizers will be needed to convertthe microscope for use with polar-ized light. The polarizer respon-sible for polarizing the light emittedfrom the microscope light source isplaced either directly on the fieldlens or can be taped (with electricaltape) to the substage condenser(refer to Figure 3 for a detailedmicroscope description). Onmicroscopes with an internallydirected lamp, it is sufficient toplace a polarizer onto the field lensas is illustrated in Figure 3. Thispolarizer should be large enough tocover the lens completely. Lowcost polarizers designed for cameralenses are sufficient for thispurpose. But, if an external lightsource is reflected into the substagecondenser through a mirror, itbecomes necessary to tape apolarizer onto the bottom of thecondenser (Figure 3). Manyscience supply houses and distribu-tors offer an excellent selection ofpolarizing materials at somewhatlow cost. When purchasing polariz-ing elements, remember to selectmaterials that are as close to aneutral grey as possible. Avoidmaterials that are amber or green incolor. These off-color materialswill cause color shifts that must becorrected for in color photomicrog-raphy.

The second polarizer (which istermed the analyzer, as describedabove) is inserted inside the bodyof the microscope between the mainbody tube and the eyepiece tube.There is usually a lens mountcovering the top main body tubeand the analyzer can be taped orsimply rested directly on thismount. Because of the restrictedspace within the main body tube,the analyzer is limited in physicalsize to 1-3 cm in diameter. Ananalyzer of this size can be ob-tained by cutting a small section

Figure 3. Schematic diagram of a light microscope outfittedfor photomicrography.

is perpendicular to the light wavebut the vector D is tangentical tothe wavefront illustrated in Figure2. Therefore, one must exercisecaution in describing the “directionof light” in discussions of birefrin-gence. Often, birefringent samplesdisplay a bright spectrum of colorswhen observed through a polarizingmicroscope. These beautiful colorsare not the ones we generallyequate with those observed inordinary daylight, but rather theyare interference colors formed in adefinite sequence according to thepath difference of the light beamthrough the birefringent material. Asimilar spectrum of colors are seenon soap bubbles or on a thin film ofoil on water. The interferencecolors are in a character sequencecalled Newton’s series (Delly,1988). In the case of birefringentmaterials, the colors are formed in

orders that must be interpretedaccording to material thickness andbirefringence (n

2 - n

1).

There are two classes of birefrin-gent materials, termed uniaxial andbiaxial, which have one and twooptical axes, respectively. Abirefringent material appears to beoptically isotropic when planepolarized light passes through italong an optical axis. If therefractive index of a uniaxialmaterial exists at a maximum whenthe plane of polarization containsthe optical axis it is said to possesspositive birefringence. However, ifthe refractive index is at a minimumunder these conditions, then thematerial has negative birefringence.The application of birefringenceprinciples to the examination ofpetrographic thin sections has beendescribed by many authors and thereader is referred to the many


from a sheet of polarizing materialor by purchasing a small polarizerfrom a dealer. The analyzer andupper main body lens should bethoroughly cleaned to eliminatedust and fingerprints beforereassembly of the microscope.After both polarizers are in place,activate the microscope lightsource, place a specimen on thestage, and view directly into theoculars (eyepieces). Next, bring theimage into focus and rotate thepolarizer on the microscope baseuntil the viewfield becomes verydark (maximum extinction). At thispoint, the polarization direction isperpendicular between the polarizerand analyzer and you then havecrossed polarizers as describedabove. Some microscope manufac-turers offer a low budget polariza-tion kit ($150.00 to $300.00) that iseasily user-installed. It is advisableto contact your microscope’sdistributor or manufacturer con-cerning the availability of theseitems if your budget is healthy. Anexcellent review on the assessment

of microscopes for petrographicuses has been provided byMathison (1990). O’Brien (1978)has developed a very inexpensivemicroscope setup designed forintroductory geology students, andHasson (1975) has described theinstallation of polarizers on aKodak Carousel slide projector toproject low magnification polarizedimages of geological thin sections.

Quantification of opticalbirefringence can be performed byadding compensators, also knownas tint, retardation, and ë plates,between the sample and the ana-lyzer. These plates are fabricatedfrom slabs of birefringent material(such as quartz, mica, and gypsum)that are cut in specially orientedcrystallographic directions. In thismanner, the direction of the fast andslow vibration components areknown with respect to the axis ofthe crossed polarizers. These platesare usually inserted into a slot inthe microscope near the analyzer sothat the slow axis of the plate issituated at a 45 angle to the polar-

izer. The plates will retard the lightby either a fixed or variable, butknown, amount. The colors seen inthe microscope when a compensa-tor is inserted into the light pathwill be changed depending onwhether the sample is in an additiveor subtractive position with respectto the compensator. The compensa-tors are sometimes useful to aid inphotomicrography of very weaklybirefringence samples, due to achange in background color fromblack to gray or magenta to red,depending on the wavelengthretardation of the compensatorplate.

Among the useful methods ofenhancing sample contrast are theinterference methods in whichcontrast augmentation is similar tothat observed in phase contrastmicroscopy with the elimination ofedge “halo” effects. Differentialinterference contrast microscopyrequires expensive objectivesequipped with a special set ofprisms termed Wollaston prismsthat are optically matched to eachobjective (Howard, 1990). Arelatively low-cost substitute forthe interference contrast effectgenerated by the Wollaston prismsinvolves adapting a polarized lightmicroscope with Savart plates(Delly, 1988) serving in place of thepolarizers. Savart plates are flatplates of a suitable quartz crystalcut and polished to produce thenecessary divided interferencebeams of light in order to generatethe three-dimensional effect ofinterference contrast. Savart platesare commonly available fromoptical supply houses.

Another method of contrastenhancement was developed bymicroscopist Julius Rheinberg in1896 (Strange, 1983). Rheinbergillumination is often referred to asoptical staining, and lends anexciting spectrum of color andhighlights to conventional micros-copy techniques. It is especiallyuseful for amorphous and unstainedsamples which lack sufficient detailfor successful photomicrography.

Figure 4. Filter configuration for Rheinberg illumination. The central andannular filters can be coupled together with transparent tape and placed at thebottom of the substage condenser.


Rheinberg illumination is similar toconventional darkfield illuminationexcept that the central opaque lightstop at the base of the substagecondenser (Figure 3) is substitutedfor a colored central stop as isillustrated in Figure 4. A sampleviewed with the colored centralstop will appear white on a back-ground the color of the stop.Addition of a colored transparentannular filter will add color to thesample (Figure 4). A variety ofmaterials can be employed forconstructing the colored stops andannular rings. Colored celluloseacetate or gelatin filters are prob-ably the most convenient, and manygraphics supply houses sell, orsometimes give away, small samplebooks of these materials. Byexperimenting with a variety ofdifferent colored stops and annularfilters, a wide spectrum of differentimages can be obtained.

The renovations describedabove apply only to transmittedlight microscopy where polarizedvisible light passes directly throughthe sample. In reflected lightmicroscopy, the light beam isreflected from the surface of thesample and scattered into themicroscope objective. It is not astraightforward process to convert abiological transmitted light micro-scope for use with either polarized

or interference contrast reflectedillumination methods. To avoidinvesting in expensive reflectedlight attachments, oblique illumina-tion from an external light sourcecan be substituted to produce thereflected light effect. A high-intensity light source such as a fiberoptics lamp (available for under$200.00) provides an excellentsubstitute. To achieve a semi-polarized light effect, the analyzercan be left in place and polarizersattached (by tape) to the externallight source. Before final attach-ment, the polarizers should berotated as described above toproduce maximum extinction.Unpolarized room light, that willunavoidably be reflected into theobjective, will slightly diminish thetotal amount of light extinction.This method of microscopy isparticular well suited for examina-tion of integrated circuits and otherspecimens that possess intricatesurface detail.

Common biological stereo-binocular (dissecting) microscopesare also useful for reflected lightexamination of materials scienceand geological rock and mineralsamples. These microscopes areparticularly useful for photomicro-graphy of small rocks and crystals.Stereo microscopes are often usefulin the brightfield mode, but can also

be operated with polarized light toobtain details of the surface structureof birefringent materials. To adapt astereo microscope for polarized light,tape a large polarizer (such as amodel designed to mount onto acamera lens) onto the lower portionof the main body tube. Insure thatthe polarizer completely covers thelens mounted in this lower portion ofthe tube. Next, adapt a polarizer tothe external light source. If a fiberoptics illumination device is used,simply tape the polarizer over theend of the fiber outlet tube. Fiberoptics sources can be obtained thathave a beam-splitter to divide theoutput light into two tubes. Thesesources are ideal for illumination ofsamples with reflected light becausea sample can be illuminated fromseveral directions to eliminate ashadow effect. In this case, be sureto cover both fiber outlet tubes with apolarizer. Place a mirror on the stageto reflect light directly into theobjective and rotate the polarizers onthe fiber outlet tubes until maximumextinction is reached. It is usuallyeasier to block the light from onetube and adjust the other for maxi-mum extinction, repeating theprocess again for the blocked tube.

Attaching a camera to the micro-scope is the last step. Microscopeviewing heads come in three variet-ies: monocular (one eyepiece),binocular (two eyepieces), andtrinocular (two eyepieces and aphotography tube). A camera can beadapted to each of these viewingheads. Commercially availableaftermarket camera adapters usuallyare attached to one of the viewingtubes with a thumbscrew and ad-justed to be parfocal with the eye-pieces by sliding the adapter up ordown on the viewing tube. A simplecamera back will be sufficient forphotomicrography because thecamera is required only to store,expose, and advance the film. Themicroscope itself acts as a cameralens. After a camera back has beenadapted to a microscope, one shouldnot rely on exposure values com-puted by in-camera exposure moni-

Figure 5. Transmitted light photomicrograph of a 0.02 inch-thick wafer cut froma single crystal of lanthanum aluminate (LaAlO3). Stairstep twinning, veryevident in this photomicrograph, interferes with confluent thin-film formation ofepitaxially deposited high temperature superconducting ceramics.


tors. It is best to bracket exposuresto get a handle on exposure times asis discussed in the section onphotomicrography.

For optimal photomicrography,it is very important to insure thatyour microscope is aligned toproduce an even illuminationacross the viewfield. Informationon microscope alignment is avail-able in owners manuals or intextbooks dealing with microscopyand photomicrography (See Delly,1988 for a particularly gooddiscussion on this subject).

PHOTOMICROGRAPHYRecording images seen in the

microscope onto photographic filmallows scientists to produce a “hardcopy” for research records. In aneducational environment, classicalphotography assignments can becoupled with science microscopystudies to provide amultidisciplinary program inphotomicrography.

Photomicrography encompassesthe techniques of both black andwhite and color photography.

Black and white film processing issubstantially lower in cost thatcolor film, if processing is done in-house. Unfortunately, manycommercial film processors nolonger offer black and whiteprocessing services or chargeexorbitant amounts for this service.If budget restrictions force theexclusive use of black and whitephotomicrography, it is advisable toinvest in darkroom equipment sostudents and research assistants candevelop and print their own photo-micrographs.

A green gelatin or interferencefilter should be inserted into themicroscope lightpath between thelight source and the first polarizer(see Figure 3) for black and whitephotomicrography. With built-inillumination, the filter can beplaced on the field lens prior to theplacing of the polarizer. Onmicroscopes that use an externallight source, the filter can be tapedbelow the polarizer on the substagecondenser.

We recommend the use of KodakTechnical pan film (or it’s equiva-

lent) with HC-110 developer forcrisp images with excellent contrastand resolution. This film providesintricate detail imaging and reducesthe continuous tonal tendencies ofthe popular black and white films.Printing can be done on KodakPolycontrast (or again, it’s equiva-lent) paper with Dektol developer.After processing a roll of film,carefully cut the negatives intosections of 5 frames each and storein specially made polyethylenestorage sheets. Contact prints canbe made by placing a sheet ofnegatives directly onto an 8" x 10"piece of Polycontrast paper andexposing for a few seconds with theenlarger lens aperture opened to thewidest f-stop. Contact sheets are anideal way of cataloging data andthey provide a compact method forstoring or sorting through manyimages. When an enlargement isneeded, simply remove the appro-priate negative strip from theprotective sheet and print with anenlarger.

In some instances black andwhite transparencies are needed for

Figure 6. Reflected differential interference contrast photomicrograph of the ab plane surface of the high temperaturesuperconducting ceramic, Bi

2Sr

2CaCu

2O

x. This material is formed in thin platelets that can be easily cleaved with a

piece of tape.


projection during lectures and atseminars. When these are desired,it will be necessary to use a direct-positive reversal processing kit toprepare the transparencies. Com-mercial reversal kits are available,although a much simpler methodwas described by Wisman and hiscolleagues several years ago(Wisman, et al, 1976).

Color photomicrography isconsiderably more complicatedthan black and white photomicrog-raphy because color film emulsionsare color balanced for a particularspectrum of light (Davidson andRill, 1989). The term color tem-perature refers to the wavelengthspectrum emitted by a particularlight source. For instance, filmsintended to be used outside inordinary daylight or under fluores-cent lighting are balanced duringmanufacture for a color temperatureof 5500ºK while films made forindoor tungsten lightbulb use arebalanced for a color temperatureof 3200ºK.

The majority of microscopes usea tungsten-halide bulb as a lightsource. These types of bulbs emit awavelength spectrum centered inthe 3200ºK color temperatureregion. Therefore, films colorbalanced for this type of illumina-tion will produce the best results.Using daylight balanced filmsunder tungsten illumination willshift all color tones towards adecidedly yellow cast. Likewise,using tungsten balanced filmsunder daylight illumination willshift color tones towards a bluercast. All major film manufacturershave one or several 3200ºK filmsavailable in 35mm transparencyformat. Transparency film ispreferable to color negative film forseveral reasons. Most importantly,all color negative films are colorbalanced for 5500ºK and must bemanipulated during printing toavoid the yellow cast mentionedabove. Most photo processors cannot or will not produce satisfactoryresults with photomicrographs on

color negative film. Also, thecontrast and color saturation intransparency film cannot beequaled by color negative film.Finally, color transparencies areeasier to label, store, and catalogand they can be projected atseminars.

With a 20 to 100 watt tungsten-halide bulb in your microscope,exposure times are usually veryshort and allow the use of slowfilms such as Ektachrome 50 orFujichrome 64T. Using slow filmsreduces the grain size in photomi-crographs and leads to higherquality enlargements. If tungstenbalanced films are not available, aKodak 80A or equivalent filter canbe inserted into the lightpathbetween the light source and thefirst polarizer to allow the use ofdaylight balanced films withminimal color shift. If this filter isused, exposure times must beincreased 1-3 f-stops to allow for areduction in light intensity. Re-cently, Polaroid introduced a slow

Figure 7. Transmitted light photomicrograph of a thin section of quenched synthetic chondrule displayingcrystals typical of porphyritic textures. The final product contains glass or a fine grained materialenclosing the equilibrium phenocrysts.


speed (ISO 40) color transparencyfilm, designated Polachrome HC,with high contrast that is ideal forphotomicrography. This filmproduces superb contrast and colorsaturation and can be user-pro-cessed in only 2 minutes with a lowcost Polaroid processor.

When photographing newsamples or after making changes tothe microscope (such as installationof polarizers), the new exposurecharacteristics should be deter-mined on a test roll of film. Bracketseveral exposures of the sameviewfield at least one and prefer-ably two f-steps over and underprevious exposure times. This willassure at least one or several goodexposures and will yield exposuretime information useful for photo-micrography of future samples.

The best film, in our opinion, isFujichrome 64T, a highly color-saturated E-6 transparency filmwith excellent contrast. Recently, anew emulsion of this film wasintroduced that is designed to allowpush processing with very littlereduction in image quality. Pushprocessing is a method developedto increase contrast (inherently lowin photomicrographs) and colorsaturation. This is done by under-exposing the film 1 to 2 f-steps andincreasing the process time in thefirst developer during the E-6process.

The complicated E-6 processcan be done in-house if the properequipment is available. There are 7steps to this process and thetemperature must be carefullycontrolled to a constant 38.3º C ±0.1º C. This is difficult to dowithout having a constant-tempera-ture water bath dedicated to thetask. A cheaper and easier alterna-tive to the E-6 process is marketedby Kodak in the form of a “hobbypack” that contains the essential E-6 chemicals combined and pack-aged into 4 individual solutions.The hobby pack is much easier touse and has a far wider temperaturelatitude, although by following the

temperature restrictions of the E-6process, optimal results are ob-tained. We have found that identi-cal, and in some cases superiorresults can be produced in thelaboratory with this simplifiedprocess if a few rules are followed.The first developer is the mostcritical part of the process and it isabsolutely essential that thetemperature be held to within thespecified limits. Contrast can beincreased by addition of 3.0 gramsof phenidone (commerciallyavailable at photography chemicaldistributors) per liter of firstdeveloper. The additional contrastobtained from addition of thephenidone dramatically helps theimages to reproduce what is seen inthe microscope oculars. In manyinstances, the processed film haseither an overall yellowish-green ormagenta cast after drying. This canbe eliminated by adjusting the pHof the color developer. The opti-mum pH given by Kodak is 11.1,but by increasing the pH by 0.2-0.7units any extra magenta can besubtracted or by reducing the pH bythe same amount, the yellowish-green cast can be eliminated. Thesehobby kits can be used to develop10 rolls of film with 24 exposuresin the magazines. As the solutionsage, extended processing times arenecessary to achieve the desiredeffects. Details on the time in-creases are given in an instructionbooklet supplied with the kit. It isprobably better to add 1-2 minutesto the color developer time givenand add about 5 minutes to thebleach/fixer time to insure adequatebleaching of unwanted dyes.

By extending the time in the firstdeveloper by approximately 30%,you can push the process 1 f-step.This is very tricky and it is advis-able to experiment with non-valuable film during the initialsetup for push processing. Toomuch time in the first developerwill definitely lead to an increase ingrain size and the transparencieswill lose a significant portion of

their overall density. Films that arepush processed should be underex-posed by one or several f-steps. Itis a good idea to bracket a set ofphotographs in the same viewfieldto determine the precise number off-steps to underexpose the film.Push processing will increasecontrast and also color saturation atthe cost of a small sacrifice in D-max, the overall film density. Toomuch underexposure will leave theprocessed transparencies very darkand dense. By carefully monitoringa bracket composed of exposures atevery half or third of an f-step, theoptimum underexposure can bedetermined.

For publication, it is sometimesnecessary to obtain color printsmade from photomicrographtransparencies. This can be donethrough commercial processors at areasonable cost, however thequality is generally poor. Ilfordproduces a direct positive paper,called Cibachrome, which can beprocessed at room temperature inan inexpensive roller drum. Al-though it is rather expensive,Cibachrome produces prints thathave superb contrast and colorsaturation. The process has onlythree steps: a developer, a bleach,and a fixer, and can be completed in15 minutes. The roller drums canhandle 8 x 10, 11 x 14, and 16 x 20inch prints. We usually purchaseonly the 11 x 14 inch paper and cuteach piece into 4 smaller 51/2 x 7sections. Four of the smaller printscan be exposed and loaded into an11 x 14 inch roller drum andprocessed simultaneously. Thesesmaller prints are ideal for publica-tion purposes.

It is important for any photomi-crographer to develop goodphotographic skills. The high costof commercial photography can beavoided if all film magazineloading and processing is done inthe laboratory. By paying attentionto all of the details, good photomi-crographs can become a matter ofroutine.


PHOTOMICROGRAPHY INGEOLOGY

In an effort to demonstrate theapplication of photomicrography tosolve problems in the geologicalsciences, we will now describethree applications that utilizemicroscopy. The first applicationdeals with the subject of hightemperature superconductivity froma mineralogical point of view. Thesecond involves investigations onthe structure of the most commonmeteorites and the origins of thesolar system, while the thirdexample derives from samplesrecovered from the Lunar surfaceby Apollo astronauts.

PerovskitesPerovskites are a large family of

crystalline ceramics that derivetheir name from a specific mineralknown as perovskite (Hazen, 1988).They are the most abundant miner-als on earth and have been ofcontinuing interest to geologists forthe clues they hold to the planet’s

history. The parent material,perovskite, was first described inthe 1830’s by the geologist GustavRose, who named it after thefamous Russian mineralogist CountLev Aleksevich von Perovski. Thegeneral formula for perovskites isABX

3, which consists of a cubic

lattice made from three distinctchemical elements present in theratios 1:1:3. The A and B atoms aremetallic cations and the X atoms arenon-metallic anions (usuallyoxygen). The A cation is usuallythe larger of the metal ions and liesin the center of the cubic lattice,with the smaller B cation occupyingeach of the 8 corners of the lattice.The non-metallic X anions populatethe midpoints of the cube’s 12edges. The number of materialshaving this elemental ratio isnumerous and many of the mineralsdo not fit ideally into the patterndisplayed the mineral perovskite,which is also known as calciumtitanate CaTiO

3. In this mineral the

larger calcium ion occupies the

center of the cubic lattice, while thesmaller titanium ions occupy thecorners of the lattice with theoxygen anions manning the edgecenters.

Many different ions can fill thecenter of the perovskite cubiclattice to produce the hundreds ofideal and modified perovskites nowknown. Barium, potassium and therare-earth elements, cerium throughlutetium (atomic numbers 58through 71), make up the two dozenor so ions that fall into the Acategory. An amazing 50 differentelements, almost half of the peri-odic table, are known to adopt Bsites in these ubitiquous ceramics.While oxygen is the most commonanion to take the X positions, othernon-metallic elements such as thehalogens can also form perovskites.

The properties of perovskitesare as diverse as their chemicalformulas. The ideal perovskites areelectrical insulators with all of theatomic sites filled with strong ionicbonding that represents a high

Figure 8. Transmitted light photomicrograph of a thin section of a synthetic chondrule displaying multipleplate dendrites. In these samples it is difficult to restrict nucleation to a single event and, subsequently, themultiple dendritic texture is commonly observed.


resistance to electron flow. Thesestrong bonds make perovskitesrock-like, and difficult to deformand melt. By substituting differentsized ions into the lattice positions,a wide spectrum of properties canbe obtained from this class ofceramic minerals. Barium titanate(BiTiO

3) is perhaps the best known

ferroelectric ceramic currently incommercial use. This material isoften incorporated in capacitors tomomentarily store electricalcharges for later use in electricalcircuits. The barium ion is muchlarger than calcium and this causesa distortion of the cubic lattice thatallows the off-centered cations tostore and release electrical energyquite efficiently. As strongerelectrical fields are applied to thismaterial, more cations are ener-gized and displaced and the morestrongly polarized the crystalbecomes. The off-center cationsalso contribute to a property knownas piezoelectricity. When thetitanium atoms are shifted by anexternal electrical field, the wholecrystal changes shape. Thismechanical deformation of thebarium titanate crystal generates anelectrical field and allows theconstruction of electrical transduc-ers that convert mechanical energyinto electrical energy or visa versa.

Barium lead oxide (BaPbO3) is a

black metal-like conductor thatprovides an excellent example ofthe changes that can result fromsubstitutions in the A and B posi-tions. By replacing the lead withincreasing amounts of bismuth, acontinuous series of compositionalvariants can be produced frombarium lead oxide to bariumbismuth oxide. As the compositionapproaches BaPb

0.8Bi

0.2O

3, the

ceramic mineral becomes a semi-conductor. Unlike either of theparent compounds, some of theseintermediate mixtures becomesuperconducting when they arecooled almost to absolute zero.

A commercially important classof perovskites is known collec-tively as PZT. This term encom-

passes lead titanate (PbTiO3) and

the continuous series generatedwhen zirconium is substituted fortitanium until the parent leadzirconate (PbZrO

3) is formed. All

known PZT ceramics so far devel-oped exhibit a remarkably strongpiezoelectric effect and are foundin a wide assortment of devicesincluding loudspeaker buzzers,electrical relays, pressure gaugesand scanning tunneling microscopeheads. The varying compositionsof PZT ceramics allows the crystalsto vibrate at distinct frequencies inresponse to an electrical field.Several television designers haveutilized this effect to design PZTperovskites as filters to reduceunwanted noise and mask undesir-able frequencies.

Another compositional variationin perovskites probably controls thestructure and properties of theearth’s interior. Magnesium-ironsilicate (Mg,Fe)SiO

3, which has

been intensively studied by anumber of geophysicists (Glazer,1972; Hazen, 1988; Yagi et al,1978), assumes the perovskitestructure at the high pressure ofseveral hundred atmospheres.Studies conducted on the earth’supper-mantle silicate materials suchas garnets, olivines, spinels, andpyroxenes suggest that at the highpressures that dominate the deepinterior, these silicates becometransformed and combine withother minerals to form rocks thatare primarily composed ofperovskites.

Currently, the most intenselystudied perovskites are those thatsuperconduct at liquid nitrogentemperatures. Superconductingperovskites were first discoveredby IBM researchers Bednorz andMüller (1986) who were examiningthe electrical properties of a familyof materials in the Ba-La-Cu-Osystem. They found a material ofuncertain composition,Ba

xLa

5xCu

5O

5(3-y) that lost all resis-

tance to electrical current flow anddisplayed the Meissner effect, thatof excluding all magnetic fields, at

the astonishingly high temperatureof 35ºK. A flurry of activity fol-lowed in the field and severalmonths later a yttrium containingperovskite (YBa

2Cu

3O

7-x) was

reported that could superconduct attemperatures slightly higher than90ºK (Wu et al, 1987). This materialhas received a considerable amountof attention in the literature (re-viewed by Engler, 1987, and Hazen,1988). Studies of thin films of theyttrium superconductor indicate thatit is capable of handling fairly highcurrents in films only a few micronsthick. Figure 5 is a polarizedtransmitted light micrograph of a<100> oriented polished wafer cutfrom a synthetic single crystal of therelated perovskite, lanthanumaluminate (LaAlO

3). This material is

a fairly good lattice match for thethin film deposition of the yttriumsuperconductor by epitaxial meth-ods such as Molecular BeamEpitaxy (MBE), Metal-OrganicChemical Vapor Deposition(MOCVD), and laser ablation. Theprimary or growth polysynthetictwins decorated by a staircaseoptical polarization effect, quiteevident in this photomicrograph, isprobably due to orientation rever-sals across twin planes by thesmaller domains that make up theelongate twin individuals. Thisstaircase effect interferes withconfluent thin film formation andresearch is currently underway tofind an economical related materialthat is devoid of these defects.

The highest Tc superconductingceramic discovered to date isBi

2Sr

2CaCu

2O

x, which superconducts

at 125ºK (Khurana, 1988). Figure 6illustrates a reflected light photomi-crograph of the ab plane surface of asingle crystal platelet of thissuperconducting ceramic. Thecrystals are easily cleaved with asection of Scotch tape to obtainsamples for chemical and physicalstudies.

Between 1986 and 1988 thecritical temperature for supercon-ductivity in perovskite ceramics wasraised by more than 100ºK, but in


the past three years only severaldegrees have been added to thisremarkable elevation. Research isfast and furious in this field, andwhile investigators are still synthe-sizing new ceramic perovskites,they are also directing their atten-tion to practical applications for thesuperconducting ceramics thatalready exist.

Synthetic ChondrulesMeteorites hit the earth with an

estimated rate of three to four a day(Marvin, 1986), although the planetis very large and is covered by somuch water that very few of thesefalls are ever witnessed and re-corded. Throughout the chronicledhistory of our planet, meteorite fallsand impacts have been observed.Geologists and Astrogeophysicistsstudy meteorite composition andproperties to learn more about theorigin and evolution of the solarsystem. Meteorites derive from avariety of sources including theasteroids and the moon, as well assome that were formed during thebirth of the solar system. For thepurposes of classification, meteor-ites can be roughly divided intofour categories. Over 90% ofrecovered meteorites are classifiedas stones (Wood, 1968). Thesestones consist of two classes. TheChrondites are the most commonstones comprising approximately85% of all meteorites recovered.Achrondrites are the other principalstone meteorite constituting about7% of the total number of meteor-ites recovered. The other principalmeteorites are the more rare stony-irons (approximately 1.5%) and theirons (approximately 6%). Thisdiscussion will be limited, however,to the stony Chondrite meteoriteclass.

Gustav Rose, the renownedgeologist who coined the mineral-ogical term perovskite, catalogedthe meteorite collection of theUniversity of Berlin in 1864 andnamed the largest class of stonymeteorites Chondrites based on theGreek word chondros, meaning

“grain of seed”, as a reference tothe tiny rounded bodies thatcharacterize these stones (Wood,1968). The small spherical inclu-sions have now been commonlyreferred to as Chondrules.

Chondrules are small spheres (.1to 10mm in diameter) which are themajor constituent of chondriticmeteorites. Chondrites are consid-ered samples of primitive solarsystem materials. If we can under-stand how chondrules form, we willhave an important clue to the earlyhistory of our solar system. Mostchondrules have an igneous texturewhich forms by crystal growth(usually rapid) from a supercooledmelt. Such textures are commonlydescribed as porphyritic (large,equant crystals in a fine grainedmatrix), barred (dendrites com-prised of parallel thin blades orplates), or radiating (sprays of finefibers).

The models proposed forformation of chondrules can bedivided into two groups (McSween,1977). In one group of models,chondrules form by melting andsubsequent crystallization ofpreexisting, largely crystallinematerial from the solar nebula. Theprimary differences between thesemodels are the kinds of materialswhich are melted and the nature ofthe sources of heat for the melting.In the other group of models,chondrules form by condensationof liquids from the solar nebula gaswhich then crystallize upon cool-ing. Variations between thesemodels result from differences inthe condensation sequence of theminerals and melts and the tempera-tures of nucleation.

One means of testing models ofchondrule formation is to determinethe conditions necessary to dupli-cate these textures by experimen-tally crystallizing chondrule meltsin the laboratory. Efforts to repro-duce the textures of chondrulesexperimentally have been success-ful only when we began to under-stand the important role thatheterogeneous nucleation plays in

the development of igneous rocktextures. Unless heterogeneousnuclei are present in the chondrulemelt, porphyritic textures will notbe produced. The dendritic orradiating textures will form instead.

The treatment of heterogeneousnucleation follows the modeldeveloped by Turnbull (1950) toexplain many of the characteristicsof heterogeneous nucleation. Thismodel was applied to heteroge-neous nucleation in basalticsystems by Lofgren (1983). Simplystated, the model says that in anysteady-state melt at a given tem-perature there is a characteristicdistribution of embryos. Theembryo is crystalline materialwhich is smaller than the criticalsize necessary to be a stablenucleus and cause nucleation. It isa subcritical-sized potentialheterogeneous nucleus. Embryosexist whether stable,supercritically-sized nuclei arepresent or not. If a melt is suffi-ciently superheated, embryos canbe eliminated. Nucleation wouldthen require a surface, presumablythe container and the barrier tonucleation would be much higherthan in the case where embryoswere present. Qualitatively, suchnucleation would resemble homo-geneous nucleation; but, if asurface is available, the energybarrier would be much lower thanfor homogeneous nucleation.Glasses would form from chondrulemelts most readily if they aresuperheated, thus destroying theembryos and increasing the barrierto nucleation. Lower meltingtemperatures would allow embryosto be retained. These can then growupon cooling and become nuclei.Embryos also can become nucleiwithout changing size, because thesize at which an embryo becomes anucleus depends upon the degree ofsupercooling in the melt. Thus, anincrease in the degree of supercool-ing can cause an embryo to becomea nucleus and nucleation to occur.

If relict crystals are present inthe melt at the initiation of cooling,


the more equilibrium-like crystalstypical of porphyritic textures areformed. When such experimentsare quenched, the final productcontains glass or fine grainedmaterial, often dendritic, enclosingthe equilibrium phenocrysts. Anexample of this texture produced inexperiments is shown in Figure 7.Equant, well formed crystals ofolivine are set in a glassy matrixwith a few dendrites present. In thenatural prophyritic chondrule theglass has usually crystallized to avery fine grained material. Ingeneral, the size of the phenocrystsdecreases and their number in-creases as the temperature at whichthe crystalline starting materialmelted is lowered and thus thenumber of nuclei increases. Therange of conditions that control thedevelopment of porphyritic texturehas been studied as a function ofvariations in the number, distribu-tion, and kinds of heterogeneousnuclei (Lofgren and Russell, 1986;Lofgren, 1989). The transition fromporphyritic texture to radial orbarred (dendritic) texture for meltsof constant composition has beenstudied as a function of the pres-ence or absence of heterogeneousnuclei and cooling rate. Suchvariations in texture within a singlemelt have already been demon-strated for melts of lunar andterrestrial basalt composition(Lofgren, 1980, 1983; Grove andBeatty, 1980).

The “classic” barred olivinetexture is a single plate dendrite(Donaldson, 1976) which shares theentire chondrule with the remainingglass or subsequent crystallizationproducts. Olivine rimming thechondrule is often in opticalcontinuity with the dendrite andthus is part of the plate dendrite.Because this texture is so striking,barred olivine (BO) chondrules arewell known even to people outsidethe field of meteorites. Whenchondrules are discussed, a photo-micrograph of a barred olivinetexture is usually chosen as one of afew or even the only example. It is

not surprising that considerableeffort has been expended under-standing its origin. Barred olivinetextures comprise only a fewpercent of melt-textured chon-drules, usually less than 5%(Gooding and Keil, 1981). The“classic” barred texture representsonly 10% of the type 3 ordinarychondrite BO chondrules. Bycareful study, Weisberg (1987)determined that the multiple platedendrite is a much more commonthat the single dendrite. Mostinvestigators propose that BOchondrules form from melt dropletsthat crystallize rapidly uponcooling.

Attempts to duplicate BOtextures experimentally showedthat it is difficult to produce the“classic” single dendrite chondrule;conversely, multiple plate dendritesare observed commonly in experi-mental charges (Lofgren andLanier, 1990). It turns out to bevery difficult to restrict nucleationto a single event. An example of abarred dendrite is shown in Figure8. Each dendrite is a series ofparallel plates connected in thethird dimension with respect to theplane of the thin section. Thedendrite forms when nuclei areeliminated from the melt and onlyembryos remain. If the melt iscooled rapidly and does notcrystallize, it becomes supercooledand embryos eventually becomestable nuclei. When an olivinenucleus begins to grow, it willbecome a dendrite if the supercool-ing is sufficiently high.

These experiments clearlydemonstrate the crystalline materialmust be present in the solar nebulawhen the chondrules form andsuggests that they did not form bydirect condensation from vapors inthe solar nebula. Individualcrystals most likely formed first andthese were remelted in clusters toform the chondrules. An interestingfact that has come out of thesestudies is that the rate at which themelt droplets cool is not critical.They can cool at nearly the same

rate and produce either the porphy-ritic texture if nuclei are presentwhen cooling is initiated, or formdendrites (barred) chondrules ifonly embryos are present. Theimportant factor is how hot thedroplets become before they beginto cool and thus whether they retainany crystalline precursor materialto act as nuclei or whether nucleihave to form from embryos. If themelt droplets are heated hot enoughthat even the embryos are elimi-nated, the droplets usually do notcrystallize when cooled and formglass chondrules. Glass chondrulesare rare and this places an uppertemperature limit to which the meltdroplets are heated which isapproximately 1650ºC. A minimummelting temperataure of 1550ºC isdictated by the minimum amount ofmelting required to produce theobserved textures. It is still notclear, however, what heat sourceprovides such conditions (Wood,1988). A popular model is heatingdue to viscous drag on particles asthey move through dense parts ofthe solar nebula as proposed byWood (1984).

Chemical analysis of chondrites(Wasson, 1974) indicates that thereis a variety in their compositionleading us to believe that they arenot all derived from a commonsource. Most chondrites arecomposed primarily of olivine,feldspar, orthopyroxene, withseveral metals including kamaciteand taenite. Continuing studies onthe chemical and physical nature ofchondrites and their formation isproviding insight into the history ofthe solar system.

Lunar MineralogyBetween 1969 and 1972 the

National Aeronautics and SpaceAdministration (NASA) success-fully landed 12 Apollo astronautson the lunar surface. The astro-nauts who visited the Moon care-fully collected 2, 196 documentedsamples of lunar soils and rocksweighing a total of 382 kilograms(843 pounds) during approximately


80 hours of exploration. It isimportant to note that these sampleswere gathered from a harsh lunarenvironment that included wildlyfluctuating temperatures in analmost complete vacuum, poten-tially dangerous solar irradiation,and the uncertainty of return toEarth due to equipment failure.

Geologists hoped that explora-tion of the lunar surface wouldestablish its composition, internalstructure, geological history, andevolution. In this respect it wasthought that the Moon would serve

as a model for the early history ofthe Earth and the rest of the solarsystem. Yet, from the Apollomissions, it has been determinedthat the Moon is far more complexthan previously thought. TheEarth’s only satellite, which wasformed approximately 4.6 billionyears ago along with the rest of theplanets, has partially melted anddifferentiated during in its history.The first melting occurred veryshortly after initial formation of theMoon and apparently resulted in awidespread molten ocean that

cooled to form a thick crust. Whilethe satellite was cooling to form asolid body, several sections of theinterior remelted, presumably dueto radioactive isotopic decay, toform lava flows on the side facingthe Earth.

The lunar surface can be roughlydivided into two domains. About20% of the Moon’s surface iscovered by dark lunar mares (latinfor seas) or lowland regions. At alow sun angle, the lunar maresdisplay wrinkle ridges and flowfronts indicating that they are filledwith frozen liquid. Before theApollo missions, it was thought thatthe mares were relatively youngbecause they are so sparselycratered (Meyer, 1987). The Apolloastronauts recovered a largecollection of samples from themares which, to the surprise ofmany geologists, gave evidencethat the maria are extremely oldwith ages ranging from 3.1 to 3.8billion years. The mares are filledwith basaltic lava similar to thelavas found here on Earth. Marebasalts are volcanic lavas rich iniron and titanium oxide mineralsthat formed when molten rock fromthe interior of the Moon surfacedand cooled. For chemical purposes,the mare basalts can be divided intotwo groups (Meyer, 1987). Anolder high titanium group with agesranging from 3.5 to 3.8 billion yearswas collected from the MareTranquillitatis and Taurus-Littrowmare regions by Apollo 11 and 17astronauts. A younger low titaniumgroup of basalts with ages rangingfrom 3.1 to 3.4 billion years wasrecovered from the OceanusProcellarum and Mare Embriumregions by the Apollo 12 and 15astronauts. These age differencesand the wide variety of differentmare chemistries, where thetitanium oxide composition variesfrom 1 to 13 percent, indicates thatthe mare basalts could not havebeen generated from one commonsource region or common parentalmagma through different degrees ofpartial melting. Figure 9 is a low

Figure 9. Transmitted polarized light photomicrograph of a lunar basaltic lavasample collected from the Oceanus Procellarum mare region by Apollo 12astronauts. This sample is 3.15 billion years old and gave the first indicationsthat Granite exists on the moon.


magnification, polarized transmit-ted light photomicrograph of apolished thin section cut from asample collected from the OceanusProcellarum mare region by Apollo12 astronauts. Studies conductedon this sample (Dungan and Brown,1977) indicate that this is a lowtitanium, medium-grained, olivinebasalt that possesses an exception-ally high magnesium content.Large pyroxene oikocrysts sur-round numerous rounded olivinecrystallites, and patches ofglomerophric olivine are present in

some areas. The sample is com-posed primarily of the mineralsolivine, plagioclase and pyroxenewith a trace amount of iron andtitanium oxides. Chemically, therock is about 42% silicon dioxide,22% ferrous oxide, with the restbeing mainly magnesium, calcium,and aluminum oxides.

Surrounding the mares are thelighter-colored hilly and mountain-ous highlands. The rugged high-lands are covered with countlesssmall craters formed when meteor-ites struck the surface early in the

history of the Moon. The lunarmares also possess craters, althoughnot as many because the rate atwhich meteorites hit the Moon hasdecreased with time. Most of therocks found in the highlands arebreccias, composed of lithifiedaggregates of clastic debris andmelt produced by meteorite bom-bardment of the lunar surface.Most of the breccias were recov-ered by the Apollo 14 and 16missions at the Fra Mauro andDescartes plains regions in thelunar highlands. These samplesproved to be very old with agesranging from 3.9 to 4.0 billionyears. Lunar breccias have a widevariety of matrix textures rangingfrom fragmental to vitric to crystal-line. Figure 10 is a low magnifica-tion polarized transmitted lightphotomicrograph of an impact-meltbreccia that was collected from thelunar highlands during the Apollo16 mission. As can be seen in themicrograph, it is a very dense andcoherent sample with a distinctivepoikilitic texture. This sample hasbeen studied in detail by Albee andcoworkers (Albee et al, 1973), andhas been radioactivity dated atabout 3.93 billions years.

The lunar surface is covered bya layer of (disorganized) debriswhich has been termed the lunarregolith (Meyer, 1987). Thethickness of this regolith variesfrom about 5 meters in the mareregions to approximately 10 metersin the lunar highlands. A majorityof the regolith is composed of a finegray soil with a density of about 1.5g/cm3, however the regolith alsocontains breccias and fragmentsfrom the local bedrock. Figure 11is a transmitted polarized lightphotomicrograph of a lunar regolithbreccia recovered part way up theslope of the Apennine front byApollo 15 astronauts. The sample,studied in detail by Simon and hiscolleagues (Simon et al, 1986) andhas been found to contain a widevariety of glass, mineral, and lithicfragments encapsulated into abrown glass matrix.

Figure 10. Transmitted polarized light photomicrograph of a lunar impact brecciasample collected from the Descartes highland plains region by Apollo 16 astro-nauts. This sample is 3.8 to 4.2 billion years old and gave evidence that the lunarhighland plains were formed by meteorite impacts.


Most of the minerals found inthe Moon rocks had already beenexamined and characterized duringroutine geological investigations ofrocks found here on the Earth. Theexception was three minerals thatcould only have been producedunder the extreme conditions foundon the Moon. A new mineral rich intitanium and iron oxides was namedarmalcolite after Neil Armstrong,Edwin Aldrin, and Michael Collins,crew members of Apollo 11, thefirst mission to successfully land onthe Moon. Rocks found in the lunarhighlands are generally rich in thecommon white mineral feldspar thatis composed of calcium andaluminum silicates. A highlyunusual class of rocks found in thehighlands was given the nicknameKREEP after its unique enrichmentin potassium (K), rare-earth ele-ments (REE), and phosphorus (P).

The age of the lunar samples wasdetermined by the abundance ratioof radioactive isotopes (Meyer,1987). It has been found that theage of most highland rocks hasbeen reset by shock and heatproduced by huge meteoriteimpacts. From radioactivity dating,researchers can determine whenthese impacts occurred. Most of thesamples have ages centering around4.0 billion years, although somerocks have escaped meteoriticbombardment and are as old as theMoon itself (4.6 billion years).

The lunar samples were stored insealed “rock boxes” during thereturn to Earth, although somesamples were exposed to theatmosphere of the Lunar LandingModule and the Command Module.After arriving at the Johnson SpaceCenter in Houston, all lunarsamples were quarantined for 6

weeks in the Lunar ReceivingLaboratory to insure the absence ofextraterrestrial life forms. Duringthe quarantine period, the sampleswere cataloged, photographed, andgiven a preliminary examination.After more careful examination, itbecame evident that life forms,organic molecules, and water wasabsent from the lunar samples andthe quarantine was discontinued forthe later Apollo missions.

Lunar samples are stored andprepared for allocation to researchscientists in the Lunar CuratorialFacility at the Johnson SpaceCenter. This facility provides ahigh-efficiency air filtration systemthat serves to remove dust and otherparticles from the laboratory air to adegree of less than 1000 particlesper cubic foot. A slightly positiveair pressure is maintained withinthe building and the floor plan

Figure 11. Transmitted polarized light photomicrograph of a regolith breccia consisting of a wide variety of glass,mineral, and lithic fragments in a brown glass matrix. It was recovered on the slope of the Apennine Front at theApollo 15 landing site.


restricts access to the storage vaultsso that areas with the most trafficare separated from areas where thesamples are kept.

The samples are stored andexamined exclusively in nitrogen-filled glove boxes. Only ultra-highpurity nitrogen gas, produced byboiloff of liquid nitrogen, is used tostore the rocks. This somewhatinert atmosphere prevents chemicalchanges in the samples that wouldbe unavoidable in the open atmo-sphere. The most severe reactionwould probably be oxidation ofmetallic iron grains in the samples.The lunar samples are only allowedto come into contact with threematerials: aluminum, teflon, andstainless steel. Investigatorshandling the rocks use teflonovergloves and stainless steeltongs. These tools are periodicallycleaned with acid and rinsed infreon to avoid any cross-contami-nation of the samples. Contamina-tion by organic substances incontinuously monitored withultraviolet lamps. During storage,the samples are kept in sealedteflon bags that are placed instainless steel containers with bolt-on tops. A positive seal is providedby an aluminum gasket betweenknife edges on the top and bottomof the container lid. The samplecontainers are permanently storedin nitrogen-filled cabinets in abank-like vault with thick steel-reinforced concrete walls.

Lunar samples probably serve astheir own best containers so theyare not cut up unnecessarily in aneffort to avoid contamination(Meyer, 1987). When a sample isneeded for an investigation, it istransferred into a glove box andremoved from its sealed container.A rigid protocol, designed tominimize contamination, is used forremoving the various layers ofteflon bags. The glove boxes arecleaned with liquid freon betweensamples to prevent cross-contami-nation from sample dust, and onlyone parent sample is allowed tooccupy a glove box at any particu-

lar time. Also, separate glove boxesare used for each Apollo mission.

The procedure for processinglunar samples involves carefullyweighing, photographing, anddescribing each individual sample.As the rocks are cut apart, eachsubsample is also weighed andphotographed. The photographs allcontain the image of an orientationcube that relates the orientation ofthe subsample to that of the parentand also to the original lunarsurface orientation. This is essen-tial so that orientation parametersare available for solar flare, cosmicray and micrometeorite exposurestudies. During the cutting process,the samples are dusted with nitro-gen gas. Maps are made of the sawcuts and the resulting surfaces ofthe rocks using binocular micro-scopes. The large rocks are initiallycut apart with a diamond-edgedbandsaw inside the nitrogen filledglove box. Since it is impractical tocool the surface of the rock duringthis cutting procedure, some rocksprobably grow quite hot during thisprocess. A certain degree ofcontamination is probably intro-duced at this point because a metalsmear can be observed on the sawnsurfaces of some of the hardestrocks. The resulting slabs fromlarge rocks and smaller samples arebroken up with a stainless steelchisel. Immediately after comple-tion of the subdividing operation,the subsamples are reassembledinto their original positions andphotographed as a group for thepurposes of documentation. Thephotographs are used to constructthree-dimensional maps that locateany particular subsample so thatinvestigators will be aware of theposition of their samples in relationto others that might appear in thescientific literature.

To prepare thin sections formicroscopy, a sample is firstimpregnated with epoxy undervacuum. Next, the sample is cutwith a diamond blade using alcoholas a coolant and then ground flatwith silicon carbide followed by

polishing with diamond. Afterpolishing, the sample is affixed to aglass microscope slide with epoxyresin. The final preparationinvolves grinding the sample to anapproximate thickness of 100microns followed by hand polishingto a 30 micron thickness usingdiamond paste on bond paper.Absolute ethyl alcohol is used in allphases of this preparation to avoidunwanted hydration of the samples.

Examination of thin sectionsprepared from lunar samples is bestdone with transmitted polarizedlight (see Figures 9-11). Using thistechnique, investigators can get ahandle on many of the propertiesdisplayed by these samples.Characteristics such as crystalshape and twinning, melt inclu-sions, cracks, fractures, and chemi-cal zoning can be readily docu-mented with polarized light micros-copy. In addition, the birefringenceof the samples can be recorded withquarter-wavelength and onewavelength retardation compensa-tors. Many samples are quitecolorful under polarized light andcolor photomicrography can yieldbeautifully colored micrographs(Figures 9-11). Reflected lightmicroscopy also aids in the identifi-cation of minerals present in thesamples and can be used to theanisotropy and reflectivity of thesamples.

In spite of all that has beenlearned from examination of lunarsamples, there is yet many unan-swered questions about the Moon.For instance, the most fundamentalquestion is still how the Moon wasformed. Theories range fromformation with the Earth as adouble-planet system, fission fromthe Earth followed by establishmentof a new orbit, to capture by theEarth as the Moon traveled throughspace. Another central question isthe asymmetry of the Moon. Theside facing the earth is filled withthe dark mare regions while theopposite side is composed mostlyof highlands. What influence didthe presence of the Earth have on


this phenomena? These and manyother questions remain unanswereduntil more research can be con-ducted on the lunar samples and onthe Moon itself. For a completeaccounting, it may be necessary toventure back to the surface of ournearest neighbor.

CONCLUSIONSPhotomicrography is a very

powerful tool for investigatingsamples in geology and geologicalpetrography. Although the newmicroscopes available today arevery “user friendly” with respect tophotographic exposure parameters,the best results are obtained whenthe photomicrographer pays carefulattention to the photographic aspectof microscopy.

ACKNOWLEDGEMENTSThe authors would like to thank

Drs. John W. Dietrich and CharlesMeyer from the NASA JohnsonSpace Center for loaning lunar andchondrule samples and very usefuladvice. Tom Fellers has beenparticularly helpful in discussionsconcerning microscopy and photog-raphy. Support has been generouslyprovided by Dr. Jack Crow, directorof the Center for Materials Researchand Technology (MARTECH) andthe National High Magnetic FieldLaboratory (NHMFL) at FSU.Wallace W. Thorner has providedexcellent graphics advice andsupport. Microscopes used in thestudy were provided by Steven Pageand Lynda Ruf of the Nikon Instru-ment Group, Melville, New York.

ABOUT THE AUTHORS Michael W. Davidson is aresearch scientist holding jointappointments at the Institute ofMolecular Biophysics, the Centerfor Materials Research and Technol-ogy (MARTECH), the National HighMagnetic Field Laboratory(NHMFL), and the SupercomputerComputations Research Institute(SCRI) at the Florida State Univer-sity. His color photomicrographycareer started late in 1986 during

morphological studies on liquidcrystalline DNA. He has developedadvanced techniques at FSU forprocessing color film and paper andhas written numerous articles on thesubject. At last count, his photomi-crographs have appeared on thecovers of over 100 scientific andtrade periodicals. Gary E. Lofgren is a scientist withthe National Aeronautics and SpaceAdministration and is located at theJohnson Space Center in Houston.He has been with NASA 23 years.He first came to NASA to help planthe exploration of the moon and tostudy the Lunar samples. Hecurrently directs a laboratory at theJohnson Space Center in whichhigh temperature and pressurehistory of lunar and meteoritesamples are duplicated experimen-tally. These experiments help tounderstand the early history of oursolar system.

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Brice, W. R., and Lind, B. H., 1987, Amanual of photographs as a teachingtool in petrography: Journal ofGeological Education, v. 35, p. 206.

Davidson, M.W. and Rill, R.L., 1989,Photomicrography: Commonground for science and art: Micros-copy and Analysis, May (no volumenumber), p. 7.

Davidson, M.W., 1990a, Fabrication ofunusual art forms with multipleexposure color photomicrography:the Microscope, v. 38, p. 357.

Davidson, M. W., 1990b, Publishingyour photomicrographs: MicroscopeTechnology and News, v. 2, p. 4.

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Michael W. Davidson - Microscope · PDF fileJOURNAL OF GEOLOGICAL EDUCATION, 1991, V. 39, p. 403 2 illumination, differential interfer-ence contrast, and Rheinberg