Optical Mineralogy in a Modern Earth Sciences Curriculum · Optical mineralogy is a subject firmly integrated into ... and practical skills in polarized-light microscopy, such ...

ABSTRACT

Optical mineralogy is a subject firmly integrated intogeoscience programs that offer mineralogy and petrol-ogy modules. Polarized-light microscopy remains apowerful and cost-effective analytical method, both atthe educational and the professional level. It is the idealanalytical tool for the teaching laboratory. Virtually anypetrographic work that does not specifically require elec-tron-microscope-scale analysis involves an optical mi-croscope, whether in conjunction with other analyticalequipment, or not. However, changes in the perspectivesof geoscience education and the necessity to accommo-date students with interdisciplinary interests alongsidethose who opt for a classic geology degree create a needfor an optical mineralogy course that is concise, but stillmeets the demands of subsequent course modules thatbuild on it. There is a range of resources that we can makeuse of to maintain reasonably high levels of theoreticaland practical skills in polarized-light microscopy, suchas application-focused lab materials and prac-tice-oriented teaching with a strong interactive compo-nent, as well as computer-based teaching aids.

INTRODUCTION

Polarized-light microscopy is of potential interest to anyscience that is concerned with crystalline materials(geology, mineralogy, materials science, biology,forensic science, to name the most obvious ones). It istraditionally taught as a mineralogy module, eventhough optical crystallography makes no distinctionbetween natural minerals and synthetic crystallinematerials. This article emphasizes geoscience aspects,simply because that is still the main field of application ofpolarized-light microscopy. Bloss (1999) aptly outlinesthe significance of mineral optics for geoscientists withhis statement “The polarizing light microscope remainsthe premier tool for rapidly identifying the minerals andmineral reactions that occur in petrographic thin sectionsof rocks”. However, it must be kept in mind that the useof this analytical tool is by no means restricted topetrography, or even geoscience.

What organisms are for biology, what chemicalelements and their compounds are for the chemist, earthmaterials are for geoscientists (if we, for the purpose ofclarity, restrict the term “geoscience” or “earth science”to subjects concerned with the solid earth, includingunconsolidated sediments). There is a clear andindisputable demand on geoscientists to have afundamental understanding of earth materials,irrespective of one’s preference for basic research or forapplied aspects of geoscience. The fact that earthmaterials, with few exceptions (such as melts, fluids,glasses, and organic substance), are composed ofminerals underlines the significance of mineral scienceeducation for any aspiring geoscientist. Thecharacterization of rocks and minerals remains a basicobjective of geoscience education.

We have means to identify minerals on the basis ofchemical composition (e.g., electron microprobe), orstructure (e.g., X-ray diffraction), or both. Optical

mineralogy employs specific physical properties thatreflect both composition and structure. These are opticalproperties in the strict sense (refractive indices, color,birefringence, optic class, optic sign, optic axial angle),but also morphological-structural characteristics (form,habit, cleavage, twinning) and the relation between thetwo (sign of elongation, extinction behavior). I willrestrict myself here to the discussion of transmitted-lightmicroscopy, even though much of what is stated wouldapply to reflected-light microscopy as well. However,reflected-light microscopy is a more specialized subjectcommonly taught in conjunction with ore deposits, andis not necessarily part of a standard geoscience educationprogram.

OPTICAL MINERALOGY: STAPLE DIET ORLUXURY SIDE DISH?

For many decades, optical mineralogy has been a coresubject in most earth sciences departments. The routineexamination of rocks or grain mounts fromunconsolidated materials was typically performed witha microscope. With the advancement of other analyticaltools, electron microprobe and electron microscopy inparticular, the role of the polarizing microscope had beenredefined in some ways (e.g., universal stage methodshave been largely abandoned), but its significance hasnot been reduced by that. Rather, for many applicationsthe combination of different analytical tools proves to bemore powerful than each one by itself. It is also a matterof working efficiently to study samples with themicroscope before using more specialized equipmentsuch as a microprobe. The importance of thepolarized-light microscope for microstructural studiesremains unchallenged. Methods of collecting data mayhave changed (e.g., from measuring quartz c-axispatterns on the universal stage to image analysismethods), but the instrument of choice is still the same.

Yet, where questions are raised about theappropriateness of mineralogy courses in a geosciencecurriculum, optical mineralogy appears to be one of theprime targets. The reasons for that are not entirelyrational, it seems. To suggest that the availability ofapparently more sophisticated methods has made“classic” petrographic microscopy redundant isnonsensical. Such a perception merely indicates a(possibly widespread) misconception about thecapabilities and the range of applications of thepolarized-light microscope.

The questioning of the appropriateness of teachingoptical mineralogy in general is a relatively recentphenomenon which mostly relates to the restructuring ofgeoscience departments and programs. It is evident thatmany departments have focused on environmentaland/or technical aspects, some perhaps to attract morestudents, some perhaps reacting to the periodicalresurfacing of the demands for teaching more appliedtopics and less basic science. Geoscience is alsobranching out into new areas of research (such asgeobiology and biomineralogy). Has all this madeoptical mineralogy less important in any way? Theanswer is clearly “no”, as will be discussed below.

60 Journal of Geoscience Education, v. 52, n. 1, January, 2004, p. 60-67

Optical Mineralogy in a Modern Earth Sciences Curriculum

Jürgen Reinhardt School of Geological & Computer Sciences, University of Natal, Durban, 4041,South Africa, [email protected]

There are other aspects that may have an impact ondiscussions about the viability of such a course module.Firstly, optical mineralogy has a reputation of beingdifficult to master, mainly on the theoretical side, wherethe indicatrix concept has almost legendary statusamongst geologists. However, this is a teaching-learningproblem that should not affect a rational decision aboutthe significance of a course. As long as the course istaught with expertise, within an appropriate time frame,there is no reason why it should have a low rating withstudents. Secondly, furnishing a complete laboratorywith good quality polarizing microscopes is anenormous expense which is partly balanced, though, bythe low running costs once the laboratory is set up.Nevertheless, it must be conceded that the equipmentcosts may be prohibitive for smaller geosciencedepartments.

Hence, apart from the cost factor, three mainquestions need to be addressed in the discussion aboutoptical mineralogy course modules:

� How relevant is the an optical mineralogy coursewithin a modern geoscience education program?

� What could be the appropriate format of an opticalmineralogy course?

� How can we teach the essence of this subject within alimited timeframe, without cutting too manycorners?

RELEVANCE OF TEACHING OPTICALMINERALOGY

Optical mineralogy as a course module cannot beassessed out of its context within the geosciencecurriculum. It is (and must be) firmly interlocked with avariety of courses. Geology modules at lower leveltypically include macroscopic mineral and rockidentification. Mineralogy in a stricter sense is taught onthe next level, before or simultaneously with opticalmineralogy. Petrology courses, including petrography ofigneous, metamorphic and sedimentary rocks, follow.All three of the “petrologies” would commonly build onmicroscope skills. For petrography teaching, thepolarized-light microscope is the instrument of choice inthe classroom. Additional teaching of other analyticalmethods and equipment is desirable by any means, but itcannot replace microscopy at the interface betweenmineralogy and petrography.

What has been outlined above is a classic set-up ofgeology modules. However, we would grosslyunderestimate the versatility of the polarized-lightmicroscope if we would see its use restricted topetrology. The important role of this instrument in basicand applied mineralogical research is underlined by themany examples discussed by Gunter (2004; this issue).The asbestos example in particular stresses the demandfor skills in optical mineralogy in the expanding field ofenvironmental mineralogy.

The optical microscope bridges the viewing rangebetween macroscopic examination (eye, hand-lens) andthe electron microscope. As grain sizes of typical meta-morphic and igneous rocks fall into the tens-of-micronsto centimeter range, the optical microscope is an obviouschoice for studying rock textures. Examples are studiesof the order of crystallization, mineral intergrowths andcrystal alignment in igneous rocks, mineral sequences,reaction textures, and deformation microstructures in

metamorphic rocks. In structural geology, the use of po-larized-light microscopy in the study of microstructuresremains an obvious necessity. The bandwidth of applica-tions has even expanded here, on the experimental side(e.g., in situ deformation studies of natural or syntheticmaterials; Means, 1989) as well as on the analytical side(e.g., analysis of crystallographic-preferred orientationby image processing; Panozzo Heilbronner & Pauli,1993). In economic and applied geology, transmit-ted-light microscopy is essential for the characterizationof alteration styles, assessment of rock-mechanical prop-erties, weathering index and weathering behavior. Theseare just a few selected examples.

Evidently, the strength of the polarized-lightmicroscope lies in its versatility and cost-effectiveness.For the teaching lab, the following aspects are ofparticular relevance:

� Virtually all major minerals can be identified anddescribed by optical means as long as the crystals arelarge enough, even to the extent that mineralcompositions can be estimated (if not determinedprecisely);

� Most rocks have a grain size range that allows anoverview of the rock on the scale of a thin section, aswell as allowing detailed examination of singlegrains;

� Bulk rock composition can be estimated from themodal proportions of the constituents;

� Reaction textures and deformation-relatedmicrostructures (shape-preferred orientation,crystallographic-preferred orientation, grain-scaledeformation, recrystallization) can be studied;

� Compositional and textural material properties canbe examined simultaneously;

� Microscopes are easy to handle, with no need forcontinuous supervision, once the initial skills havebeen taught;

� Teaching of a large group is possible while eachstudent works on his/her own microscope;

� The operating costs (replacement costs excluded) arevery low.

When teaching systematic mineralogy, thepolarizing microscope helps to form visual images ofminerals in addition to what we observemacroscopically, in hand specimen or in outcrop. Formany typically fine-grained minerals, it is the commonimage to recall. Also, properties such as cleavage or colormay be easier to observe in the microscope than in amacroscopic specimen.

From both the professional and educationalperspectives, we can safely state that polarized-lightmicroscopy remains a powerful and cost-effectiveanalytical method in the geosciences. The extent ofinformation that a well-trained person can extract from asimple rock thin section in a short amount of time isexceptional, even taking into account the obviouslimitations with determining mineral compositions.Importantly, polarized-light microscopy is a method thatis almost exclusively taught in geology and mineralogy,as opposed to any other analytical method of similarsignificance in earth sciences. A graduate applying for aposition that involves laboratory work is likely to facecompetition from non-geoscientists in almost all areas of

Reinhardt - Optical Mineralogy in a Modern Earth Sciences Curriculum 61

analysis - unless skills in polarized-light microscopy areasked for. It seems irresponsible to give up thatcompetitive edge for no good reason at all.

The answer to whether optical mineralogy should betaught to geoscience undergraduates is quite simple. Aslong as petrography and mineral analysis are part of thecurriculum, there is no real choice. Without an opticalmineralogy course in the curriculum, the teaching ofpetrology courses is without substance. Furthermore, itmakes the teaching of mineralogy for geologists muchless practical.

The basic question is therefore whether mineralogyand petrology remain core subjects in geoscience. Aslong as geologists and mineralogists are expected toprovide expertise on earth materials, they will. However,geoscience branches out in very different directions,towards chemistry, physics, biology, materials science,and engineering. Many teaching institutions wouldstruggle to cover all this variety. Nevertheless, thoseinstitutions specifically offering general geology degreesmust consider very carefully whether they want to giveup teaching what many consider as fundamental skills ofa geologist.

CONSIDERING THE OPTIONS: HOW BRIEFOR HOW EXTENSIVE?

Assuming that the optical mineralogy course is nottaught as a subject completely detached from systematicmineralogy, we may consider three possibilities:

1. A basic course teaching a very restricted selection of10 to 15 of the most abundant minerals.

2. A single course integrating mineral optics theoryand systematic mineralogy, covering a standardrange of minerals sufficient to form the basis ofsubsequent petrography courses. These wouldcommonly include igneous, metamorphic andsedimentary petrography, but perhaps alsomicrostructural studies as part of a structuralgeology course.

3. A multi-semester sequence of interrelated modules,starting with a course on optical principles andcrystal optics, followed by a course on systematicoptical mineral determination, then proceeding toadvanced analytical methods and/or petrographycourses with emphasis on microscopic work (as in 2).

Option 3 is or was, in one variety or another, theclassic arrangement of teaching mineral optics andfollow-up courses in many geology and mineralogydepartments. Provided the courses are delivered in acompetent fashion, this “luxury edition” of opticalmineralogy is still the best and most logical way ofdeveloping professional skills with the polarizingmicroscope. However, students who follow a career thatdoes not suggest microscope work will be of muchimportance, could indeed question the amount of timespent on this subject during education. Hence, it is thiscourse structure which commonly sparks mostcontroversy.

The geosciences education programs of manydepartments cannot accommodate option 3 anymore, orperhaps never could. A basic course (option 1) satisfiesthe needs of most students who do not study for a

geology degree, but take geology courses as part of theirprogram. Option 1 would even allow students tocontinue with introductory sedimentary and igneouspetrography. It is clearly insufficient, however, to studymetamorphic rocks, or to do advanced petrographicwork in general. This dilemma could be solved, at least inlarger departments, by teaching different types ofmicroscopy courses for geology majors and non-geologymajors (or wherever the dividing line is drawn). Appliedgeoscience programs in which petrology and structuralgeology do not feature prominently, but where teachingobjectives include mineral-analytical skills (e.g., forenvironmental mineralogy) may be served best by anoption 1 course extended on the methodical side.However, special quantitative methods that demand athorough understanding of optical principles should stillbe taught in advanced mineralogy courses rather than inthe introductory module.

Generally, the actual contents of an optical mineral-ogy course - whatever its extent - would depend verymuch on the associated modules and the nature of theprogram. A strong petrological component in the pro-gram calls for a sound knowledge of rock-forming min-erals. Emphasis on the general mineralogical-analyticalside, on the other hand, would perhaps render the sys-tematic approach to minerals teaching less appropriate,with the methodical component then carrying more im-portance.

In the following, the specific aspects of teaching aconcise one-semester course that accommodatesstudents from a variety of programs are discussed indetail, based on personal experience over many years ofteaching. In comparison with a full-scale education inoptical mineralogy (as in option 3), some of the morespecialized topics have been reduced, but the simplesolution of leaving out enough material to fit the rest intoa tighter time-frame is clearly impractical. There is acertain minimum of theory that is needed and there is aminimum set of minerals to be discussed, in order tomeet the entry requirements of higher-level courses.Hence, we have to look at methods and resources toincrease the effectiveness of our teaching.

COURSE OBJECTIVES

The specific objectives of an introductory opticalmineralogy course are to learn to identify andcharacterize minerals in thin sections and grain mounts.This encompasses routine recognition of the mostcommon minerals in igneous, metamorphic andsedimentary rocks and unconsolidated materials(minerals of submicroscopic size such as clay mineralsexcepted), as well as the ability to use optical propertiesto determine mineral species or varieties with which astudent or a professional is less familiar. Thedevelopment of these skills cannot rely on courseworkonly. Further practical training and experience are vitallyimportant.

As with any other analytical equipment, the user hasto understand the basic functions of the polarizingmicroscope in order to be able to use it sensibly, and hasto have some knowledge about the methods of samplepreparation. The polarizing microscope should be taughtas being one of several analytical instruments availableto the earth scientist for analyzing minerals and rocks.Each of these instruments has its specific uses whichneed to be understood.


Even though an introductory course in polarizingmicroscopy concentrates on single minerals and theirproperties, these minerals are commonly observed in anatural, polymineralic environment (unless we usemonomineralic concentrates). Therefore, by looking atthin sections in particular, the experience at themicroscope extends beyond identifying single minerals.As the course advances, students will start to get a feelfor mineral associations and mineral abundance incommon rock types, random and preferred orientations,grain-size variations and more, without being explicitlytaught these subjects at that stage.

THEORETICAL ASPECTS OF MINERALOPTICS

Petrographic microscopy, like so many other analyticalmethods, is rarely and end in itself, but rather a means toother ends. We should not forget this when we teach thetheoretical side of mineral optics. A one-semester courseserving a mixed clientele from different scienceprograms cannot dive into every detail of opticalcrystallography, as interesting as that may be for thespecialist. On the other hand, without any theory ofcrystal optics, even the most fundamental opticalproperties of minerals cannot be understood. At the verycore of this theoretical framework are basiccrystallography, principles of optics, and indicatrixtheory.

The latter is commonly perceived by students as themost difficult part of crystal optics. Yet, there is no betteror easier way to sensibly describe, and communicateabout, light propagation in anisotropic materials andresulting optical phenomena. We may distinguishbetween “inherent” optical properties that are entirelycontrolled by mineral composition and structure - andhence cannot vary once composition and structure aredefined - and “acquired” material properties (mainlygrain morphology, including grain surface character,habit and twinning) that are influenced to at least somedegree by factors other than structure or composition.All the inherent properties are expressed in the geometryof the indicatrix, apart from cleavage and color. Withoutthe indicatrix concept, the optical properties ofanisotropic minerals can be observed, but notunderstood. This is unsatisfactory for both teachers andstudents. Science teaching should generally aim to closethe gaps between observation and comprehension.

In our present curriculum, optical mineralogy isintegrated into a one-semester mineralogy module.Lectures are primarily on theoretical aspects, andpractical sessions are taught in the microscope lab. Thelectures cover the following topics:

� Basic aspects of optics(nature of light, frequency, velocity, wavelength,electromagnetic spectrum, refraction, refractive in-dex, color of minerals, polarized and non-polarizedlight, optical isotropy and anisotropy)

� Behavior of light in optically isotropic materials(light propagation, microscopic identification ofoptically isotropic materials)

� Behavior of light in optically anisotropic materials:uniaxial and biaxial minerals

(light propagation, ray velocity surfaces, indicatrixconfiguration, indicatrix orientation in a crystalplate, relationship between crystallographic axesand optical orientation)

� Examination of anisotropic minerals with thepolarized-light microscope(birefringence, retardation, interference colors, effectof polarization on fast and slow rays for variablecrystal orientation, determination of vibrationdirections of fast and slow rays, sign of elongation,extinction characteristics, optic axis interferencefigures)

A total of about 15 full hours of lecturing is requiredfor theory. There is, however, little time to spend onoptical experiments. In the practical sessions that runparallel to the lectures, students see the “experimental”side while they work systematically through the variousminerals. The one important experiment that isconducted during lectures is the calcite experiment asdescribed in Nesse (1991). The optical principles that canbe taught with relatively simple means - a calciterhombohedron of optical quality and two polarizingfilters - are the polarization effect, the splitting of lightrays, their vibrations directions, and the geometricrelations of these optical phenomena to crystallographicorientation (see also Stoddard, 1997, and Zimmermann,1997, for details). An additional advantage of the calciterhombohedron with its sets of lower and upper parallelfaces is that the conditions of vertical light incidence andlight propagation through a crystal plate can be directlyrelated to the orthoscopic operation mode of thepolarizing microscope. Thus, a whole range ofobservations can be used to start developing thetheoretical framework of crystal optics and itsapplications to the microscope.

COURSE EQUIPMENT, SAMPLEMATERIAL, REFERENCE MANUALS

Ideally, a polarized-light microscopy course is equippedwith a sufficient number of quality binocularmicroscopes with a minimum of three objectives, ademonstration facility such as a petrographicmicroscope with a video camera, standard teaching aidssuch as indicatrix models and perhaps ray velocitysurface models, interference color charts (Michel-Lévycharts), and all the necessary samples. The basic designof polarizing microscopes has remained very much thesame over many decades except for improved opticalsystems and light sources.

The best technical set-up is wasted if the materialused is inferior, difficult or unattractive. Teachingphilosophies may differ in what type of material is mostsuitable for beginners. It is generally a sensible concept toprovide different examples for the same mineral species,time permitting. Students should be aware of variations,such as morphological characteristics or color, which canbe demonstrated using different samples. The classresponse is probably more enthusiastic if samples areattractive, easy to work with (not badly weathered), andtechnically well prepared. Low-quality samples aredefinitely not a good starting point for novices and arelikely to dampen the students’ interest in the subject.

Concerning textbooks, relatively compact mineral-ogy courses require an economic solution that combines


“classic” mineralogy and crystallography, analyticalmethods in theory and practice, as well as systematicmineralogy including mineral properties (as in Nesse,2000). There is a range of more detailed textbooks thatspecialize on theoretical and practical sides of opticalmineralogy (such as Bloss, 1961, 1999, Wahlstrom, 1979,Nesse, 1991), which can be recommended to students forconsultation. Undoubtedly, there will also be increasingcompetition from learning software in the future.

Whatever the preferred solution may be for learningaids on the theory side, a manual with mineral data isindispensable when actually using the microscope.There are two principal ways to operate: (1) to teachtextbook-based practicals, or (2) to provide students witha suitably designed, self-prepared course manual. If weoperate in the microscope lab with any of the books thatcontain extensive compilations of mineral data (such asTröger et al., 1979, Phillips & Griffen, 1981, Ehlers, 1987,Deer et al., 1992, Gribble & Hall, 1992), then all studentsshould have the same book on the workbench. To ensurethat this is indeed so, we would have to make a purchasecompulsory, or we provide the class with a set of copiesfor the duration of the course.

The use of any type of book with a systematic sectionlisting optical properties implies a specific approach toteaching new minerals. Unless the teacher chooses not touse the book temporarily as a new mineral is introduced,all mineral properties will be accessible at any time.Hence, students work on the microscope comparingtheir observations with the data in the textbook, whichwould normally correlate perfectly. The learning processis hence restricted to confirmation of what the book says,and errors are virtually excluded. This method can easilymislead a student in believing that he or she is masteringthe subject.

After teaching optical mineralogy courses in variousdepartments over many years, I stopped using opticalmineralogy books in the microscope lab altogether,except for higher-level petrology practicals. Apart fromthe problems discussed above, mineral datacompilations have one serious disadvantage for theinexperienced: they aim to be comprehensive. Thenumber of minerals described is far beyond what isnecessary for a student to know. Furthermore, due totheir comprehensiveness and their systematic approach,the descriptions of mineral properties inevitablyincludes information that is not critical for routinemicroscope work. It is difficult, though, if not impossiblefor a novice to judge what is important and what is not.We also need to remind ourselves that beginners havedifficulty in judging how common or uncommonminerals are, and hence how likely or unlikely certainminerals are they can think of.

For all purposes of a condensed-format microscopecourse, only a strictly reduced set of minerals makessense. Without extensive microscope experience, it ismuch easier to handle a smaller data compilation ofessentials rather than a complete minerals optics book.There is also much less danger in getting lost andspending more time than necessary on unidentifiedminerals. Perkins & Henke (2000) have explicitlyaddressed this need with a manual that contains detaileddescriptions of some 60 minerals and mineral groups(plus a few opaque minerals). In comparison, MacKenzie& Adams (1994) restrict the description to a mere 15minerals and mineral groups, which is only justsufficient for a very basic course as outlined above inoption 1. The emphasis of this book is onphotomicrographs of minerals and rocks, and opticaldata are not provided. Hence, it can hardly serve as asearch manual for routine petrography in geology, eventhough it is a visually attractive introduction topetrographic microscopy.

Is there yet another alternative? After having taughttextbook-based microscope courses for a while, I startedto work along two basic lines of approach: (1) If the rangeof minerals discussed in the course encompasses most ofthose which students (and professionals, for that matter)have a good chance of observing in common rock typesand surface depsoits, we might just as well restrict the labmaterials to these minerals; (2) a better learning effect canbe achieved if the students determine the opticalproperties of newly introduced minerals themselves,rather than extracting the properties from books.

Even though the teaching conditions varied betweendifferent departments, I found that I can get through amaximum of about 30 minerals and mineral groups in asingle-semester course. This forms a fairly solid basis forthe subsequent petrology courses. Evidently, the rangeof minerals that are most relevant for sedimentology andigneous petrology is much smaller than what is requiredto teach even an introductory course on metamorphicpetrology. Still, there is no need to be comprehensive. Itmust be clear from the beginning that the minerals of theoptics course are only a selection and that an importantobjective of the course is to be able to describe the opticalproperties of any mineral, whether present on that list ornot. Eventually, comprehensive data compilations asthose cited above would have to be consulted for lesscommon minerals not discussed in the course.

How can we operate if we need a systematic mineralmanual at the microscope, but choose not to provide allthe optic properties of newly introduced minerals from


Figure 1. Center: mineral sheet from laboratorymanual. Explanations for the various blocks have beenadded here. Data are entered into the sheet by eachstudent once the mineral has been examined and theproperties have been discussed with the class.

the start? A workable solution that I have tested oversome years now is a laboratory manual into which dataare entered as the course advances. The one I designedfor optical mineralogy classes consists of a methodicalpart and a systematic part. The systematic part comprisesa set of mineral sheets (one page for each mineral ormineral group), as in Figure 1. Only the mineral name isprinted on top, and a figure is included. The rest of thesheet has to be filled in by the student as the courseproceeds. To encourage a methodical approach, thesheets are subdivided into blocks: general information atthe top (mineral name, chemical formula, crystalsystem), properties that are primarily (though notexclusively) examined in orthoscopic mode andplane-polarized light, properties that are observed inorthoscopic mode with polarizers crossed, andproperties that are observed in conoscopic mode.Additional information of importance is added in the lastblock. If appropriate, graphs (for example for variation ofproperties in solid solution series) are added on thebackside.

The methodical part on microscopic mineral analysislists and explains the different features that areimportant for optical mineral characterization andidentification. The theoretical background of mineraloptics as covered by lectures is not included. Thehandbook is intended for use at the microscope; hence,the emphasis is on practical aspects. The sections of the

methodical part essentially follow the subdivision on themineral sheets:

� General aspects: Mineral composition, crystalsymmetry, and optics

� Orthoscopic mode / plane-polarized light:relief and refractive indices, form, cleavage, color

� Orthoscopic mode / crossed polarizers:birefringence, sign of elongation, extinction,twinning

� Conoscopic mode:optic character and optic sign, optic axial angle

� Additional information:retrogression - alteration - decomposition, occur-rence

In special “What-to-do” sections, the operationscarried out at the microscope are explained step by step.This is to help students refresh their memory ifnecessary, and let them develop confidence with thetechnical operation of the microscope (which in turnhelps to avoid unnecessary damage to microscopes).

PRACTICAL SESSIONS

Before the start of the main systematic section of themineralogy practicals, there is a general introductory


Figure 2. Routine procedure for introducing a new mineral in the microscope laboratory.

session on relief and refractive index, isotropy andanisotropy. This exercise is done with single- andmulti-mineral grain mounts (e.g., quartz, garnet, fluorite,rutile) and a range of liquids with different refractiveindices. All other practicals are dedicated to specificminerals, starting with quartz. In principle, the coursefollows the standard mineral classification of H. Strunz“backwards”, from framework silicates to orthosilicatesand then to non-silicates. By doing this, the mostabundant minerals are taught early in the course (quartz,feldspars, micas, pyroxenes). This has the distinctadvantage that, within a few weeks, students examiningthin sections can recognize most of the mineralsassociated with any new mineral introduced andtherefore see the mineral in its natural context. The onlyminor disadvantage with such an approach is that thefirst hours will be spent on framework silicates that arevery similar in many respects (colorless, low relief, lowbirefringence).

The various optical and morphological propertiesare introduced successively, whenever a suitable mineralis next on the list. Starting with quartz, relief, form,birefringence, optic character and sign are given specialconsideration. With the introduction of feldspars,extinction, twinning, cleavage, alteration anddecomposition are added. Once the course has advancedto the mica group, all the principal properties have beendiscussed using real mineral examples, and a moregeneral routine can be followed from there.

New minerals are introduced as follows (Figure 2):First, mineral composition and crystal system arediscussed (which would commonly be a refresher ofprevious mineralogy lectures). Then the class is givenexplicit instructions which mineral properties toexamine with the microscope. Emphasis is generallyplaced on critical mineral data. It is too time-consumingand simply unnecessary to go through the entire set ofproperties for each mineral.

Examples of the mineral are shown on thedemo-microscope-video system and if appropriate, hintsare provided how to find the mineral in the thin sections.Then students are left to work on their own for some timeto make notes on their observations (no entries into themanual at that stage). Discussions between students areencouraged as part of the co-operative learning effort.Support at the microscopes is generally providedthroughout the practicals. Commonly, thin sections froma range of different samples are supplied.

After a specified time, the results obtained by thestudents are discussed, and the correct properties arethen entered into the manual’s mineral sheets, takinginto consideration the full range of variations to ensure ageneral applicability of the data. The easiest way for theinstructor is to either use reproductions of the mineralsheets for an overhead projector, or to use a dataprojector. Less critical data as well as information onoccurrence and breakdown products are provided by theinstructor to complete the data set, but there is nonecessity to include properties that are normally notchecked during routine petrography. Once the sheet iscompleted, every student has an identical data set in hisor her manual.

At the end of the course, the manual is complete. It isfurther used for practical assignments and thinsection-based exams. Students keep the manual after thecourse and use it in other modules if they continue inearth sciences. As it is ring-bound, more minerals can be

added at any time. A mineral sheet template forphotocopying is included in the original manual.

The above outlined system of teaching minerals atthe polarized-light microscope has a substantialinteractive component between teacher and class; itprovides a very good control for the teacher on theprogress and level of understanding of the class and ofindividual students. The condensed course format doesrequire a relatively concentrated effort on all sides, and,without additional exercise, the necessary level ofexperience and confidence at the microscope may not bereached. Hence, students are encouraged to use themicroscope lab outside formal course hours as much aspossible. As the final assessment in practical skills beforethe exams, each student is given a different thin sectionon which a complete optical-mineralogical analysis hasto be performed.

COMPUTER-AIDED LEARNING PACKAGES

What we will see in the future is increasing support fromcomputer resources. Presently available computer-aidedlearning (CAL) packages include the optical mineralogymodule of the UKESCC software package (Emley et al.,1998) and the digital microscope of Palmer et al. (1999).We will also see an increasing amount of photomicro-graphs being made available through the internet or ascompilations on CD. These resources are welcome fordemonstration purposes in the labs and provide impor-tant learning resources for students outside actual coursehours.

There is indeed a large potential in CAL to supportoptical mineralogy teaching, both on the theoretical sideand on the practical side. One standard line to follow isrepetition of lecture material in an interactive way. Theparticular advantage of computers in mineralogy andpetrography, however, lies in 2-D and 3-D graphicalvisualization of structures, optic phenomena andconcepts, as well as in animated graphics. Even thoughall of this can be used for classroom demonstrations onceteaching practice has advanced to using a data projector,it is important for students to be able to revisit suchsections on their own. 3-D concepts in particular aretypical problems where students can have a verydifferent levels of comprehension.

One of the technical challenges of optical mineralogysoftware is the simulation of the polarized-lightmicroscope. This also incorporates the question whetherwe could teach optical mineralogy without the realmicroscope. While the “microscope simulator” is avaluable tool for clarifying practice-related procedures, itis unlikely to ever replace the reality of examining rockthin sections and minerals with the optical microscope.Polarized-light microscopy is a typical hands-on subject,and there is no need to change that. Just as a geologistcannot acquire and test the essential skills in geologicalmapping without physically going into the field,microscopy can neither be taught nor learned withoutusing a microscope. Real progress in teaching this topiclies in the sensible combination of the strengths of CALtools with the advantages of personal teaching (such asflexibility and direct interaction with students), printedcourse materials, and hands-on exercises using realequipment and real samples.


FINAL COMMENTS

The students’ response to the course and the teachingconcept has been positive, as is evident from course eval-uation questionnaires. Perhaps surprisingly positive,considering that mineral optics and systematic mineral-ogy are commonly not at the top of the list of students’ fa-vorite courses. The prime target of the courserestructuring has always been to meet the objectives inan effective way. Not surprisingly, the workload in thecourse is perceived as relatively high. Class assessmentand examinations yield positive results, with pass ratesbetween 80 and 100%. The “home-made” lab manual isgenerally very well received by the students. As it is pro-duced once a year, it can be continuously updated andmodified by the instructor.

A positive aspect of offering a relatively compactoptical mineralogy course is that students who are stilluncertain about the final route of their studies may rathertry a short option than opt for a multi-semester sequenceof modules. Some may then find the subject attractiveenough to include mineralogy in one form or another intheir further studies. The integration of mineralogycourses into programs other than mainstream earthscience would certainly help to attract students andpromote mineral science in general.

ACKNOWLEDGMENTS

Thanks to Mickey Gunter and George Rowbotham forcritical and helpful comments on the originalmanuscript. Thanks also to Tanja Reinhardt for technicalsupport.

REFERENCES

Bloss, F.D., 1961, An Introduction to the Methods ofOptical Crystallography, Holt, Rinehart andWinston, New York, 294 p.

Bloss, F.D., 1999, Optical Crystallography, MineralogicalSociety of America, Washington, D.C., 239 p.

Deer, W.A., Howie, R.A. and Zussman, J., 1992, AnIntroduction to the Rock-Forming Minerals (2ndedition), Longman, London, 696 p.

Ehlers, E.G., 1987, Optical Mineralogy, Vol. 2. MineralDescriptions, Blackwell Scientific Publ., Palo Alto,286 p.

Emley, D.W., Rowbotham, G. and Lees, G.J., 1998,Optical Mineralogy (Version 2), UK Earth ScienceCourseware Consortium, University of Manchester.

Gribble, C.D. and Hall, A.J., 1992, Optical Mineralogy:Principles and Practice, UCL Press, London, 320 p.

Gunter, M.E., 2004, The polarized light microscope:Should we teach the use of a 19th century instrumentin the 21st century?, Jounal of Geoscience Educationv. 52, p. 34-44.

MacKenzie, W.S. and Adams, A.E., 1994, A Colour Atlasof Rocks and Minerals in Thin Section, MansonPubl., 192 p.

Means, W.D., 1989, Synkinematic microscopy oftransparent polycrystals, Journal of StructuralGeology, v. 11, p. 163-174.

Nesse, W.D., 1991, Introduction to Optical Mineralogy(2nd ed.), Oxford University Press, New York, 335 p.

Nesse, W.D., 2000, Introduction to Mineralogy, OxfordUniversity Press, New York, 442 p.

Palmer, D.C., Tindle, A.G. and Crompton, S., 1999,Digital Microscope, CD S260 Geology Block 2, TheOpen University, Milton Keynes.

Panozzo Heilbronner, R. and Pauli, C., 1993, Integratedspatial and orientation analysis of quartz c-axes bycomputer-aided microscopy, Journal of StructuralGeology, v. 15, p. 369-382.

Perkins, D., and Henke, K.R., 2000, Minerals in ThinSection. Prentice Hall, Upper Saddle River, 125 p.

Phillips, W.R., and Griffen, D.T., 1981, OpticalMineralogy. The Nonopaque Minerals. W.H.Freeman, San Francisco. 677 p.

Stoddard, E.W., 1997, Minerals and Light, in: Brady, J.B.,Mogk, D.W. & Perkins, D. (eds.), TeachingMineralogy, Mineralogical Society of America,Washington, D.C., p. 291-296.

Tröger, W.E., Bambauer, H.U., Taborszky, F. andTrochim, H.D., 1979, Optical Determination ofRock-Forming Minerals, Part 1: DeterminativeTables, Schweizerbart, Stuttgart, 188 p.

Wahlstrom, E.E., 1979, Optical Crystallography (5th ed.),John Wiley & Sons, New York, 488 p.

Zimmermann, H.D., 1997, Experiments in CrystalOptics, in: Brady, J.B., Mogk, D.W. & Perkins, D.(eds.), Teaching Mineralogy, Mineralogical Societyof America, Washington, D.C., p. 297-308.


Optical Mineralogy in a Modern Earth Sciences Curriculum · Optical mineralogy is a subject firmly integrated into ... and practical skills in polarized-light microscopy, such ...

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