The Polarized Ligh Microscopy - Should We Teach the Use of 19th

ABSTRACT

The polarizing light microscope (PLM) has no doubtcontributed more to our knowledge of minerals androcks than any other single instrument. Then why is theuse of the PLM, and the teaching of optical mineralogy ingeneral, decreasing? Probably one of the main reasons iseducators seek to present the newest, mosttechnologically advanced techniques and methods totheir students. Also, the geoscience curriculum haschanged to include environmental geology, essentiallyhydrogeology. However, in presenting these newermaterials, or new courses, we must exclude something,and it appears that one of the things excluded isinstruction in use of the PLM. Another possibility is theprofessors teaching these courses do not have anadequate understanding of optical mineralogy.

Excluding the subject of optical microscopy will bethe biggest mistake we ever make in the geosciencescurriculum. This statement is justified because of thefundamentally important concepts presented in opticalmineralogy: 1) three-dimensional visualization, 2)inquiry-based learning, and 3) hands-on use of ananalytical instrument. No other single course in ourcurriculum provides so many of these fundamental skillsto our students. An example of inquiry-based learning isdetermining the best technique to identify a mineral. Forexample, powder X-ray diffraction provides a diffractionpattern searchable in a database, but the result might notbe correct. In the past 20-30 years, the spindle stage hasallowed for more detailed single crystal studies than everbefore. Optical techniques are also used to study suchdiverse mineralogical problems as cation-diffusion inzeolites and asbestos identification, or they may beincorporated into other areas of research, such assynchrotron experiments on oriented single crystals.Also, any serious petrologic study must begin with athorough examination of the samples by PLM, beforeother types of characterization can proceed. A diversityof employment opportunities exist for students who aretrained in use of the PLM, such as in the fields offorensics, material science, manufacturing, the foodindustry, medical technology, and the emerging field ofenvironmental mineralogy.

INTRODUCTION

Our understanding of minerals and rocks is attributedmore to use of the polarized light microscope (PLM) thanany other single instrument. With the PLM, we canquickly identify and characterize minerals anddetermine textural relationships in rocks, includingmicrostructures used in petrofabric analysis. Why, then,are we abandoning teaching these skills to our studentsin favor of other subject matter? One of the mainproblems using a PLM, though, is the answers do notcome from a computer but must be determined by theuser. Thus, more education and experience are requiredto use the PLM correctly than what is required for

computer-based instruments (e.g., X-ray diffractometersand electron microscopes). The overarching theme is thatstudents, and researchers, must understand thelimitations of the various instruments and be able toselect the most appropriate one for the problem at hand.Also, equally important, we should teach how tointegrate several different analytical methods to answerthe question at hand.

The PLM is one of the few instruments students willever get to use independently. This itself warrants itsteaching as we strive for more interactive methods ofinstruction. No other single instrument provideschemical, structural, and morphological information ona single sample and with so little sample preparation,albeit the chemical and structural data are foundindirectly. It is this “indirect method” of inferringchemical and structural data that is the double-edgedsword for the use of the PLM. Microscopists must inferthis information based on their experiences. When dataare misinterpreted, it is often (incorrectly) themicroscopic techniques and not the microscopist who isblamed.

Clearly, the PLM cannot directly determine amaterial’s crystal structure or chemical composition;diffraction methods are commonly used for the formerand spectroscopic methods for the latter (e.g., energydispersive spectroscopy in an electron beam instrument).However, once the structure and composition areknown, generally speaking, they can be related to somefeature that is observable or measurable with a PLM. Forinstance, refractive index is related to compositionalvariation and optical class is related to crystal system.Students can quickly confirm the identity of a mineral ifthey are taught how to use oil immersion and grainmounts. For example, the refractive index can providethe species of a plagioclase feldspar much easier thanusing power X-ray diffraction, which is difficultinterpret. Also, chemical information from an electronmicroscope cannot differentiate among kyanite,sillimanite, and andalusite, or between quartz and opal.Working with minerals in thin sections, there are evensimpler optical properties such as color, relief, twinning,mineral associations, and alteration that give qualitativechemical information and guide the pertologist infurther characterization of the rocks. The examples arenumerous, and several more will be presented below.Most importantly, the PLM excels in determining samplemorphology, for instance to aid in differentiatingamphibole from amphibole-asbestos in grain mounts orthe textural relationships of minerals in rocks. Thesignificance of these relationships could be expandedalmost endlessly. And the cost of maintaining a PLMversus that of an X-ray diffractometer or electron-beaminstrument is several orders of magnitude less. Thislower cost allows institutions with modest budgets tostill teach the use of the PLM, while the maintenance onother instruments may be cost prohibitive.

Mineralogists often only think of using the PLM forgeological materials, but the uses outside of ourdiscipline often have important societal implications.Again, the examples are numerous, and several will be

34 Journal of Geoscience Education, v. 52, n. 1, January, 2004, p. 34-44

The Polarized Light Microscope: Should We Teach the use of a19th Century Instrument in the 21st Century?

Mickey E. Gunter Department of Geological Sciences, University of Idaho, Moscow, Idaho

83844-3022, [email protected]

presented. Despite this, other disciplines have alsodiscontinued instruction of the PLM. We not only shouldcontinue teaching polarized light microscopy but betterintegrate its use into our research, both in the study ofgeology and other areas.

Probably the main reason for the decline in theteaching of optical mineralogy, and in fact mineralogy ingeneral, is that many geologists view this discipline as asurrogate to igneous and metamorphic petrology, orgeochemistry. That is to say, that mineralogy is not astand-alone discipline but merely leads to more“important areas” in the field of geology. Unfortunatelyby thinking like this they ignore the geologicalinformation gained by consideration of the crystalchemical aspects of a mineral. A close corollary is thatbecause so many petrologists have been trained in thefield of optical petrography (i.e., using thin sections tostudy rock texture), they teach optical mineralogycourses without a full understanding of the power ofoptical mineralogy outside of the field of petrology (i.e.,they would always favor “cutting a thin section” toexamine a rock over using the immersion method to aidin identification of a mineral). This is also unfortunatebecause geological information could be gained by singlecrystal optical studies of, for instance, the structure stateof feldspars (Su et al.,.1984).

This paper will provide a brief history of the PLMand its historical uses in mineralogy and geology,followed by a review of some basic optical methods (e.g.,what can be measured with a PLM with the aid of aspindle stage). Next, a series of mineralogical exampleswill be given, some of which could only be solved with aPLM. Finally, several examples of on-going integrationof optical techniques to study the orientationaldependence of physical properties of minerals will behighlighted. It is not the intention of this paper to discussall the uses of the PLM but rather to concentrate on theauthor’s research in an attempt to show the evolution ofhis work and how he has incorporated classical opticalmineralogy into newer analytical methods inmineralogy. This paper is based on a seminar the authorgave as a Mineralogical Society of America distinguishedlecturer.

BRIEF EVOLUTION OF THE USE OF THEPLM

The main impetus for development of the PLM was tostudy crystalline materials. In the 1800’s, scientistsstudying minerals noted light interacted with minerals inunique ways. They determined that certain mineralswould cause light to become polarized, and they alsoobserved that light traveled at different speeds indifferent directions for some minerals, while for othersthe speed of light was the same regardless of propagationdirection. They attributed these differences to varyingcrystal structures of minerals and sought ways to studythis in more detail. The PLM would become their maintool for studying minerals for the next hundred years.For a complete discussion of the development of the PLMand many of the accessories associated with it (e.g., theU-stage for study of petrofabrics) see Kile (2003).

Two separate methods were developed to studyminerals. One was the immersion method, wherecrystals of minerals were crushed and placed in a liquidthen observed with a microscope. This was first done in1815 using water as the liquid. The other method wasdevelopment of thin sections, first crudely done in 1831.

It was not until 1856 that the first true PLM was built, anda theoretical understanding of how light interacts in amineral was quickly developed. Advances were madeboth in the immersion and thin section methods until theearly 1900’s, when these techniques were basicallyperfected. Little has changed in their use since. Theimmersion methods allow for a fairly rapid identificationof single crystals, while thin section methods permit notonly mineral identification but also the study of texturalrelationships of minerals in rocks. Currently, thin sectiontechniques are more commonly taught than theimmersion methods. This is probably because thinsection techniques can be applied to the study of rocks,whereas the immersion method is basically used foridentification of minerals. Also, many scientistsincorrectly believe the commercially available refractiveindex liquids in use today contain PCB’s and arecarcinogenic; this was true of formulations used decadesago but not the newer formulations.

In the early 1900’s, the PLM was the main instrumentused by mineralogists and petrologists to study mineralsand rocks. The discovery of X-rays in 1912 led to a majorbreakthrough by which the crystalline properties ofminerals could be directly studied by X-ray diffraction,instead of indirectly studied by the interaction of lightwith a mineral. The development of X-ray diffraction ledmany mineralogists to abandon use of the PLM, becausenow they could use powder diffraction to aid in theidentification of minerals and single crystal X-raydiffraction to determine the crystal structures. Electronbeam instruments were also being developed in themiddle 1900’s, but it was not until the invention of themicroprobe in 1959 that these instruments became adominant force in the field of mineralogy and petrology.In the 1960’s, the chemical composition of micron-sizedminerals could be determined with an electronmicroprobe. This replaced the laborious technique ofusing larger size samples and performing wet chemicalanalysis. Mineralogists then had at their disposalinstrumentation capable of directly studying the atomicarrangement of atoms in minerals and preciselydetermining the chemical composition of minerals.

MAJOR USES OF THE PLM

Identification and characterization of minerals -Mineral samples are identified based on their structuraland chemical characteristics. X-ray and electrondiffraction techniques directly provide data about theatomic structure of a mineral. Diffraction occurs whenX-ray radiation or electrons pass through and interactswith the atoms of a crystalline material. Analysis ofdiffraction patterns are used to determine structuralproperties such as space group, unit cell parameters, andpositions of atoms. Energy dispersive spectroscopy(EDS) and wavelength dispersive spectroscopy (WDS)directly provide information about the chemistry of asample. A sample is placed in the path of a high-energyelectron beam. These electrons collide with the innershell electrons of the atoms comprising the sample, ejectan inner shell electron, and create a vacancy. Outer shellelectrons move into the vacant inner shell and, in theprocess, release energy in the form of X-ray radiation.The specific energy, or wavelength, of X-ray radiationproduced by an atom is characteristic of each element.EDS methods determine the energy of these X-rays, andWDS determines the wavelengths of the X-rays. By using

Gunter - The Polarized Light Microscope 35

EDS or WDS, it is possible to determine the quantity ofeach element in a sample. EDS is more rapid and requiresless sample preparation than WDS; however, WDSproduces more precise and accurate quantitativechemical analysis. Often, the correct identification of amineral requires that both structural and chemicalinformation be obtained.

Structural and chemical characteristics can also beindirectly observed using polarized light microscopy.Structural properties are obtained by the correlation of amineral’s optic class to its crystal system. Opticalclassification of minerals is based on whether they areisotropic or anisotropic. If a sample is isotropic, it hasonly a single refractive index value and belongs to theisometric crystal system, or it is amorphous. If a sample isanisotropic, it is further classified based on its optic class.Anisotropic minerals can be either uniaxial, whichbelong to either the hexagonal or tetragonal crystalsystem, or biaxial, which belong to the orthorhombic,monoclinic, or triclinic crystal system. Both of theseclasses are further divided into positive and negativegroups based on the relationship of refractive indicesand their principal vibration directions.

Chemical characteristics of minerals can be related tothe optical properties of the minerals (e.g., refractiveindices, 2V, sign of elongation, extinction, pleochroism,etc.). A century’s worth of careful optical and chemicalmeasurements has provided many empiricalrelationships, whereby a quick check of refractive indexor a determination of 2V can provide an approximationof chemical composition often required for mineralidentification. Thus, chemical data of minerals are foundindirectly by measurement of some basic optic property,usually the refractive index.

SOME IMPORTANT SPECIFIC USES OFTHE PLM

Asbestos - Without a doubt the most important use ofthe PLM for the past 20 to 30 years is in the identificationof asbestos. This is true for two reasons. First, the sizerange of concern for identification of asbestos fits the

resolution of a light microscope. Second, thedistinguishing characteristic of asbestos is itsmorphology, which is fibrous. It could be argued that theemerging filed of environmental mineralogy has its rootsin the use of the PLM for asbestos identification andcharacterization. (For an overview of the asbestos issuein the U.S.A. refer to Gunter, 1994.) A material couldhave the exact same chemical composition and X-raydiffraction pattern and in one case its morphology mightbe fibrous and the other case it might be non-fibrous. Sothe only method to distinguish between fibers andnon-fibers is the morphology, which can be observed inthe magnification range of that for a PLM. For example,Figure 1 shows amphibole samples from Libby,Montana, in an oil immersion mount. Clearly, theparticle on the left, labeled fragment, is non-fibrous andthe particle on the right is fibrous. This distinction canonly be made based on morphology. Chemical analysisof these two materials would be identical, and if we wereto perform a powder X-ray diffraction experiment, thediffraction patterns would also be identical.

Asbestos regulations are currently being revised inthe U.S.A. This is mainly the result of the amphibole andamphibole asbestos minerals occurring in the formervermiculite mine near Libby, Montana (Bandli et al.,2003; Gunter et al., 2003). The first change in theregulations will no doubt be to regulate amphibolesbased on group name and not species name. This isnecessary because the amphiboles at Libby are winchitesand richterites which, as of this time, are non-regulatedbut are just as harmful as the regulated amphiboleminerals. At first this seems like a fairly simplemodification to the regulations. However, the issue willthen become how to distinguish asbestos fromnon-asbestos amphiboles. Many of the regulatoryagencies have used the aspect ratio to make thisdistinction. For instance, if an amphibole particle has alength-to-width ratio greater than 3, it would beconsidered a fibrous particle. Thus, what would clearlybe a non-fibrous sample (Figure 1) would, based on thisaspect ratio rule, become regulated as an asbestosmaterial. Clearly, to regulate these materials correctlythere must be close interaction between thoseindividuals trained in polarized light microscopy andthe regulatory agencies.

Today there are many more jobs in asbestosidentification laboratories and in regulatory agenciesthan there are qualified microscopists to fill them. Thisscarcity of trained microscopists has occurred becausewe have stopped teaching classical optical mineralogy inour geology curriculum. The void has been filled in theprivate sector with one-week short courses in polarizedlight microscopy, optical mineralogy, and asbestosidentification. For instance, the McCrone ResearchInstitute in Chicago has trained hundreds ofmicroscopists in the past 20 to 30 years. If the privatesector is teaching these courses because of a nationwidedemand, it would be logical for us to include them as partof our curriculum. Any geology student successfullycompleting a semester long course in optical mineralogycould be practically guaranteed a job in one of these labs.

The spindle stage - One of the major advances in opticalcharacterization of minerals has been the refinement ofspindle stage techniques by Bloss and coworkers (forexample, Bloss, 1982; Bloss 1999; Gunter et al., 1988;Gunter and Twamley, 2001; Su et al., 1987). The spindlestage, which is basically a one-axis rotation device, is a


Figure 1. A photomicrograph of amphibole andamphibole-asbestos from Libby, Montana showingdiffering morphologies. (The photo was taken inplane-polarized light with 1.58 refractive indexliquid; the scale bar is 1 mm.)

microscope attachment that aids in obtaining largeamounts of optical data quickly and efficiently. A briefintroduction to the spindle stage and its uses follows; for

a thorough discussion of spindle stage techniques, seeBloss (1981; 1999).

The basic design for the spindle stage is a singlecrystal mounted on a glass fiber, or sewing needle, that isplaced on the rotation device (i.e., the spindle stage). Thecrystal can be placed in an immersion liquid, and routineoptical measurements and observations can be made.Figure 2 is a photograph of three different spindle stages.The Supper spindle stage is often used for researchpurposes, while the small detent spindle stage (Bloss,1999) and poster board spindle stage (Gunter, 1997) canbe used for teaching purposes. In all cases, however, thesetup is the same: small (50 to 200 �m) single crystals aremounted on the end of the spindle stage and placed on aPLM. Figure 3 shows the materials required to mountsingle crystals. Commonly, a single crystal is mounted,with the aid of a binocular microscope, on the end of aglass fiber with fingernail polish. The glass fiber is, inturn, placed in a 3 mm diameter brass pin, which isinserted into an X-ray goniometer head. Next, the X-raygoniometer head is screwed onto the Supper spindlestage shown in Figure 3. Now the crystal can beimmersed in refractive index liquid contained in the oilcell mounted on the spindle stage. Figure 3B and C showan amphibole crystal affixed to the end of a glass fiberand immersed in refractive index oil. The crystal hasbeen rotated 90° between B and C.

Figure 4A shows the spindle stage X-ray goniometerhead combination mounted on the stage of a PLM. Alsoshown in this image is a temperature-controlled oil cellused to record and change the oil’s temperature. At thesame time, the wavelength of light can be changed with amonochrometer. By changing both temperature andwavelength, the refractive indices of a material aremeasured to within plus or minus two to five in thefourth decimal place by the double-variation method(Bloss, 1981; Su et al., 1987).

One of the major advances of Bloss and coworkers isthe ability to orient biaxial minerals easily on the spindlestage with the aid of the computer program EXCALIBR(Bloss, 1981; Gunter et al., 1988). All that is required is


Figure 2. A) The necessary “equipment” to mountsingle crystals for optical examination with a spindlestage. Under a binocular microscope, a small singlecrystal (ca. 50-200 �m) is glued with fingernail polishto a (ca.) 100 �m glass fiber. Then the glass fiber isplaced in a brass pin that fits into the X-raygoniometer head, also pictured. Acetone can be usedto remove the crystal for further study if desired. B &C) Photomicrographs of an amphibole single crystalmounted on a glass fiber. The sample has beenrotated 90° on the spindle axis between B & C, thusallowing three-dimensional viewing of the singlecrystal. (The photos were taken in plane-polarizedlight with a 1.58 refractive index liquid; the scale baris 100 �m.)

Figure 3. Photographs of three different types ofspindle stages: Supper, detent, and poster board. Thefigure shows the X-ray goniometer head (with anaffixed crystal) mounted onto the Supper spindlestage and the oil cell supplied with this spindle stage.The oil cell for the poster board and detent spindle isalso shown.


Figure 4. Photographs of three different experimentalsetups for single crystal study; in all cases, an X-raygoniometer head is used and thus the same crystalcan be easily moved between experimental setups. A)The Supper spindle stage, with goniometer head, ismounted on the rotating stage of a PLM to observecrystal morphology in three dimensions and tomeasure optical properties (e.g., refractive index,pleochroism, 2V, orientation of the indicatrix, etc.).B) A goniometer head mounted on a single-crystaldiffractometer used to obtain structural data (e.g.,unit cell parameters, crystal structure, orientation ofthe crystallographic axes, etc.). C) A modified spindlestage placed in a synchrotron beam source for studiesof the orientational dependence of absorptionspectra.

Figure 5. Sketches showing the optical orientationfor andalusite and “Mn-andalusite.” Note that forandalusite the small refractive index (X) is parallel tothe long dimension of the crystal, while forMn-andalusite the optical orientation has changedand Z (the largest refractive index) is parallel to c. Thelower part of the figure is a polyhedral representationof the structure of andalusite. The main structuralunit is an edge-sharing chain of octahedral parallel tothe c crystallographic axis.

extinction measurements are determined for differentspindle stage settings. A mathematical relationshipexists between these extinction data and the opticalindicatrix. In other words, given extinctionmeasurements, a computer program will provide themicroscopist the spindle stage and microscope stagesettings to place the crystal so any of its three principalrefractive indices can be directly determined withoutappreciable error due to misorientation. Another majoradvantage of this method is that all refractive indices aredetermined on the same crystal. This, in turn, lends itselfto studies whereby the optically characterized singlecrystal can also be used for X-ray diffraction or chemicalanalysis. All the physical properties of a mineral can bedetermined on the same single crystal.

Isotropic andalusite - An example of an integratedoptical, X-ray, and chemical study was performed onandalusites with varying compositions (Gunter andBloss, 1982). Andalusite, Al2SiO5, can have appreciablesubstitution of Mn and Fe for the six-coordinated Al in itsstructure. There is also a change in the optical orientationin andalusite as this substitution occurs (Figure 5). Figure5 also shows a polyhedral representation of a portion ofthe structure of andalusite. Andalusite is an orthosilicatemineral and the isolated Si tetrahedron is shown inFigure 5. There are two Al sites: a five-coordinated sitelabeled M2 and a six-coordinated site labeled M1. Mn

and Fe substitute for Al in the M1 site that formedge-sharing chains of octahedrons parallel to the ccrystallographic axis. Notice how the structure ofandalusite controls its morphology; these edge-sharingchains are the main structural unit, thus elongatingandalusite parallel to its c crystallographic axis.

Gunter and Bloss (1982) obtained a suite ofapproximately 50 andalusite samples and selected 20 ofvarying compositions. For each of these 20 samples, weprecisely measured the refractive indices with the PLMand spindle stage, cell parameters by X-ray diffraction,and chemical composition by electron microprobe. Thesedifferent measurements were made on the same singlecrystals; thus, the change in the optical orientation couldbe tracked as a function of chemical composition. Wefound that the refractive index direction parallel to the ccrystallographic axis was the smallest of the refractiveindices for pure andalusite; however, it increases at agreater rate than the other two refractive indices withsubstitution of Mn and Fe for Al. Upon close examinationof Figure 6, notice a void in our data in the area wherethese three curves of refractive index values cross. Tworeasons were postulated for this: that samples of thiscomposition do not exist in nature (although this did notseem reasonable to us), and that andalusites with thiscomposition had been overlooked in thin section becausethey would appear isotropic, or near isotropic. Shortlyafter our work was published, Grambling and Williams


Figure 6. Relationship between refractive indices and Mn + Fe content for 20 andalusite samples (modifiedfrom Gunter and Bloss, 1982). Because there is a change in the optical orientation as a function of Mn + Fecontent (x+y on graph), the refractive index curves are labeled n||a, n||b, and n||c to correspond to therefractive index directions in the a, b, and c crystallographic directions, respectively. Note that when thecurves cross, a sample of that composition would appear isotropic.

(1985) found andalusites in this compositional range thatindeed appeared isotropic.

While this discovery may seem like a mere opticalcuriosity (i.e., that orthorhombic minerals may appearisotropic), it has significance to the metamorphicpetrology community. Grambling and Williams (1985)showed how changes in the triple point for thealuminosilicates could be related to the Mn and Fecontents of those andalusites, thus helping to account forsome of the variability in location of the triple-point inthe aluminosilicate phase diagram.

Retardation and optical orientation of natrolite groupzeolites - Based upon my previous work on andalusite, Isearched the literature for other possible effects ofchanges in optical orientation that could provideworthwhile information. Also, in general, a portion ofmy research is directed toward the understanding ofhow the optical properties of materials are related totheir crystal structure and chemical composition. I firstinvestigated zeolites because of their many industrialapplications and wide-spread geologic occurrences.Also, zeolites have low birefringence because they areframework silicates; thus, a small change in somestructural or chemical property may cause a large changein the easily observable retardation.

Gunter and Ribbe (1993) collected optical data fromthe existing literature for the natrolite group zeolites(natrolite, mesolite, and scolecite). After some obviouscorrections to the optical orientations incorrectlyprovided in some of the standard reference books, a plotof the refractive indices for these three minerals could bemade as a function of their Ca and Na content. In asimilar manner to andalusite, the rate of change of therefractive indices were different for the differentcrystallographic directions. In this mineral group, therefractive index direction parallel to the ccrystallographic axis, which corresponds to chains oftetrahedrons parallel to the c crystallographic axis,

increases at a lower rate than the refractive indicesparallel to the a and b crystallographic axes (Figure 7).Thus, natrolite has its largest refractive index directionparallel to its long dimension, mesolite has itsintermediate refractive index direction parallel to its longdirection, and scolecite has its smallest refractive indexparallel to its elongation. It is simple to distinguish thesethree minerals in thin section or grain mount based upontheir sign of elongation.

Figure 7 also shows that the retardation for natroliteand scolecite, while being low, would still appearanisotropic in thin section or grain mount. Mesolite, onthe other hand, exhibits very low retardation and wouldappear isotropic, or near so. During the preparation ofGunter and Ribbe (1993), we collected samples of fibrouszeolites from a nearby basalt flow (Gunter et al., 1993).Figure 8 shows one of these fibrous zeolites affixed to aglass fiber. In the plane-polarized light photograph, therefractive index fluid matches natrolite, so it exhibitsvery low relief. Mesolite has a slightly higher refractiveindex and thus stands out in higher relief at the tip of thecrystal. In cross-polarized light the photomicrographreveals how the retardation clearly shows an anisotropiccrystal, but because the retardation for mesolite is so low,it appears isotropic. The study again shows how subtlechanges in chemistry can cause large effects on theretardation. Because retardation is so easily observable, itcan be used for other purposes than simply to identifyminerals in thin section or grain mount.

Cation diffusion in heulandite group zeolites - Inanother study by Gunter et al. (1994) on Pb-exchangeheulandite group zeolites, we used optical data to aid indetermining the extent of Pb exchange. Pb substitutionincreases the birefringence, and thus the retardation.Figure 9A shows a non-exchanged sample exhibiting


Figure 7. Relationships of the refractive of the fibrouszeolites - natrolite, mesolite, and scolecite (modifiedfrom Gunter and Ribbe, 1993). This graph wasproduced from data synthesized from the literature –no data were collected. Based on this graph, it is easyto distinguish between these three samples by theirsign of elongation.

Figure 8. Photomicrographs of an approximately 500�m long natrolite sample capped with mesolite takenin both plane-polarized and cross-polarized light. Thecrystal is mounted on a glass fiber and immersed inindex-matching fluid for natrolite. In the plane-polarized photo, the mesolite tip has higher reliefthan the natrolite portion of the crystal. In thecross-polarized photo, the mesolite tip appears nearisotropic because of the lower birefringence ofmesolite (Figure 7).

first-order grey retardation, and Figure 9B shows a fullyPb-exchanged sample exhibiting second-order redretardation. (A color rendition of this figure can be foundin Gunter et al., 1994.)

Another method that can be used to determinewhether exchange has occurred, or to measure diffusionof Pb into the structure, is backscattered electron imaging(BSE) with a scanning electron microscope. Figure 9Cshows a BSE image where the Pb has diffused a few tensof microns on the rim of the grain. Figure 9D shows asimilar grain except this time as observed with a PLM.Note the high order retardation on the lower rightportion of the grain; this indicates Pb has diffused intothis portion of the mineral. Pb not only diffuses into therims of these minerals, but also through cracks. Figure 9Eis a BSE image showing areas of Pb diffusion, and Figure9F is a PLM image showing areas of Pb diffusion. Thus,again, in this study optics is very sensitive indetermining chemical changes and much more efficientthan performing a chemical analysis with an electronbeam instrument to see spatial variation in chemistry.

A natural extension of this project was to measurediffusion rates of different cations in heulandite groupzeolites by observing changes in optical properties. In aseries of exchange experiments by Yang et al. (1997), we

were able to determine the diffusion coefficients for Na,K, Rb, and Cs by making simple observations in changesof optical properties with a PLM. No other single methodavailable could be used as easily and efficiently toqualitatively observe exchange in zeolites or toquantitatively determine their diffusion coefficients.

EXAMPLES OF CURRENT RESEARCH

A major theme in this paper is the orientationaldependence of refractive indices and morphology ofminerals. And, as discussed in detail above, the spindlestage is the perfect tool to study this dependence. Whatfollows are three examples of how we have recentlyintegrated the spindle stage into other analyticalmethods with the underlying theme to characterize thedirectional dependence of various physical properties.

Single-crystal X-ray diffraction and optical orienta-tion - The optical orientation of minerals relates the crys-tallographic axes to the optical indicatrix. It is also usefulto identify the mineral either in immersion mounts or inthin section and is an important physical property of themineral. While X-ray diffraction is used to locate thecrystallographic axes, it cannot be used to locate the opti-cal directions. Thus if one wants to collect spectroscopicdata along the optical directions of a mineral, the spindlestage is used to determine these orientations. The opticalorientation is basically nonexistent for isotropic mineralsand trivial for uniaxial minerals. However, for biaxialminerals the relationship is harder to determine, espe-cially for monoclinic and triclinic minerals. Determiningthe optical orientation of these minerals requires an inte-grated study using both X-ray diffraction to locate thecrystallographic axes and the PLM to determine the ori-entation of the biaxial indicatrix.

This entire process can be simplified by using aspindle stage equipped with an X-ray goniometer head(Figure 4A). The X-ray goniometer head, with crystalaffixed, can be transferred between the spindle stage andan X-ray diffractometer (Figure 4B). Also, as describedabove, the orientation of the biaxial indicatrix can bedetermined with the measurement of extinctionpositions and use of the computer program EXCALIBR.Thus, the orientation of the biaxial indicatrix is foundand cast into the coordinate system of the microscope.Next, the goniometer head is transferred to the X-raydiffractometer and the orientation matrix is obtained.The orientation matrix contains information on theorientation of the crystallographic axes a, b, c cast into thecoordinate system of the X-ray diffractometer. Next, tofully determine the optical orientation, these twocoordinate systems are transformed into each other andthe relationship of the biaxial indicatrix and thecrystallographic axes mathematically determined(Gunter and Twamley, 2001). This entire process takesabout one hour, as compared to older methods thatmight require several days.

Synchrotron radiation and absorption spectroscopy- The advent of synchrotrons with their high-energywhite X-ray beams have created a major new analyticaltechnique with which to examine minerals. X-rayabsorption spectroscopy is one of many techniques thatrequires synchrotron radiation. In this method, theenergy of the X-ray beam is changed in such a way as toencompass the absorption-edge of an element of interest.


Figure 9. A series of images of natural andPb-exchanged heulandite zeolites showing changes inretardation (in the PLM images) and back-scatteredelectron (BSE) intensity (in the BSE images) as afunction of Pb exchange (modified from Gunter et al.,1994, which are in color). (All scale bars are 100 �m.)A) PLM image of a natural sample exhibiting lowretardation. B) PLM image of a fully Pb-exchangedsample showing a drastic increase in retardation. C)BSE image of a partially Pb-exchanged sample; the Pbhas only diffused into the edges of the sample. D) PLMimage showing how increased retardation can be usedto monitor Pb exchange around the edges of thecrystal; the greatest exchange is in the lower righthand corner of the grain. E) BSE image of a partiallyPb-exchanged sample, with Pb diffusing in along theedge and cracks in the grain. F) PLM image showingPb diffusion along the edges and cracks in a sample asshown by increases in retardation.

For example, for Fe absorption the energy of the X-raybeam is scanned over the main absorption edge at 7111eV. Information about the coordination, valence, andother bonding characteristics can be obtained frominterpretation of these spectra. The majority of the workin this area has been on millimeter-scale powders ofminerals, as well as on single crystals of similar size. Ourcurrent research (Dyar et al., 2002a, b) is exploring newways to make these measurements on micron-sizedsingle crystals. However, one of the concerns in this areaof research is the orientational dependence of theabsorption spectra. Some previous researchers have tried

to ascertain the orientational dependence of theseabsorption spectra by using oriented thin sectionminerals. There are technical difficulties with thismethod, along with a considerable amount of time beingrequired for sample preparation. For instance, study ofan orthorhombic mineral would require at least twooriented samples.

To study the orientational dependence of the spectraand fully characterize them on a 100 micron-size crystal,we modified a spindle stage so it could be placed in a


Figure 10. X-ray absorption near-edge spectroscopy(XANES) spectra of a garnet and olivine with thepolarized synchrotron beam parallel to each of thethree crystallographic axes. For garnet, all spectraexhibit the same features, while for olivine they varyas a function of orientation. Thus, the X-rayabsorption indicatrix is similar to the opticalindicatrix.

Figure 11. Photographs showing integration ofspindle stage methods with a scanning electronmicroscope (SEM). A) Stage of SEM with a 3 mm brasspin used for crystal mounting in a goniometer head(Figure 3A) inserted into a normal SEM sample holder.B) Photomicrograph of an acicular amphibole particlemounted on the end of a glass fiber. C) SEM image ofthe same sample in B in approximately the sameorientation. D) SEM image of a bundle of amphibolefibers mounted on a glass fiber. E) Magnified andreoriented image of the white box area in D to betterobserve the morphology of the amphibole. (Imagesmodified from Bandli and Gunter, 2001.)

polarized synchrotron beam (Figure 4C). We could thenplace an X-ray goniometer head on the spindlestage/synchrotron combination and make the requiredmeasurements. To test the system we developed, wemade measurements on samples that should show nopreferred orientation (e.g., an isotropic mineral, such asgarnet) and samples that should show orientationaldependence (e.g., an orthorhombic mineral, such asolivine). The garnet was oriented using X-ray diffractionas described above. The olivine was oriented using thespindle stage as described above. Based on the spectra inFigure 10, it is clear there is no orientational dependenceof the absorption spectra for garnet, as would bepredicted, and there is an orientational dependence ofthe absorption spectra for the olivine, as also would bepredicted based on the analogy with the behavior of theminerals and polarized light. Prior to the collection ofthese spindle stage assisted spectra, it was unclear whatthe orientational dependence of the spectra would be.Thus, integration of the methods of classical opticalmineralogy aids in understanding these new analyticalmethods and provides for better data collection methods.

Scanning electron microscope and three-dimen-sional imaging - When higher magnifications are re-quired, the scanning electron microscope (SEM) can beused in lieu of, or in conjunction with, a PLM. It is alsopossible to obtain chemical information directly if theSEM is equipped with an energy dispersive detector. TheSEM has a greater depth of field at similar magnificationswhen compared to a PLM. Standard sample preparationfor the SEM is placement of mineral grains on an alumi-num stub or glass slide. With this type of mount,two-dimensional imaging of the sample is possible.However, if a single crystal is affixed to the brasspin/glass fiber combination that fits into the X-raygoniometer head, as used on the spindle stage, this brasspin can be placed in the sample holder stage of the SEM(Figure 11A) to achieve three-dimensional imaging of theparticle (Bandli and Gunter, 2001). Combined optical andscanning electron microscope studies can be performedon the same single crystal; this is a great aid in identifica-tion of the sample because chemical information can alsobe obtained. It is also a very useful method to handle100-micron size particles without losing them. Once acrystal has been affixed to a glass fiber and observed inthe PLM (Figure 11B), it can then be easily transferred tothe scanning electron microscope (Figure 11C).

The main advantage of this method is to be able tomanipulate the crystal in the SEM to gain morphologicalinformation in different directions. Figure 11D shows abundle of amphibole asbestos fibers affixed to the end ofa glass fiber. In Figure 11E the sample has beenmanipulated to view approximately down the long axisof the fibers. In this orientation the crystal faces of theamphibole particle can be seen. If the sample weremounted on a flat surface, one would not be able tomanipulate it to observe the morphology in differentdirections. In the case of amphibole minerals, differentsurfaces may have an impact on the minerals’ healtheffect. For instance, amphibole asbestos particles tend toexpose the 100 surface, amphibole cleavage fragmentsexpose the 110 surface, and small single crystals ofamphibole expose the 100 surface. Clearly, from amineralogical perspective these different surfaces wouldhave different reactivities. Using the SEM with a spindlestage style sample mount helps to characterize themorphology of these particles.

CONCLUSIONS

I hope I have convinced the reader that the answer to thequestion posed in the title is “yes.” From the teachingperspective, the main reason we should continueteaching polarized light microscopy is that it is one of thefew hands-on, interactive methods of learning aboutanalytical instrumentation students get to participate in.From an employment perspective, there is no othersingle course students can take that will secure them a jobas rapidly as one that teaches the use of a PLM inenvironmental applications, especially in the field ofasbestos identification. In mineralogical research, thereare still many new discoveries and innovations waitingto be made using well-established, existing methods inoptical mineralogy, and efforts should be made in themineralogical community not to lose sight of thesemethods in favor of the latest instrumentation, whichmay not provide the best and most accurate data.

ACKNOWLEDGMENTS

I would like to thank Emeritus Professor F.D. Bloss, VPI& SU, for his major contributions to the field of opticalmineralogy as a textbook author, teacher, and researcher,and for mentoring me in the field of optical mineralogy. Iwould also like to thank the MSA (Mineralogical Societyof America) for inviting me to be a distinguished lecturerin 2002-03; this paper was based on a seminar presentedin that series. I would like to thank Professor OsamuTamada of Kyoto University for providing a visitingprofessorship, which enabled me to contemplate thesignificance of polarized light microscopy in thegeoscience curriculum and work on my MSA lecture.Lastly, I would like to thank Professor Charles Guidottiof the University of Maine for a thorough review of thismanuscript, providing suggestions for how to improveit, and confirming my views of the importance ofteaching the use of the PLM. This work was partiallysupported by funds from the National ScienceFoundation (NSF-CCLI 0127191).

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