ABSTRACT

It is possible in a single term undergraduate mineralogycourse to complete a project involving thecharacterization and identification of mineralunknowns. These identifications are based on physicalproperties, unit cell parameters or d-spacingsdetermined from powder X-ray diffraction scans,chemical composition determined on an electronmicrobeam instrument, and optical propertiesdetermined with a petrographic microscope. Each phaseof the project is timed to compliment the sequence ofconcepts covered in mineralogy lecture and lab. Such aproject serves to (1) illustrate practical applications ofmethods discussed in lecture; (2) illustrate theconnection between mineralogy and topics covered inancillary courses such as chemistry and physics; (3)pique the interest of some students for pursuingmineralogic and petrologic research projects later in theircareers.

INTRODUCTION

Trends in earth science instruction have resulted inreduction in the amount and form of coverage inmineralogy courses and, to a lesser but equallydisturbing extent, all of the petrology sub-disciplines(sedimentary, igneous and metamorphic). Single-termcourses in “Earth Materials” have widely supplanted themineralogy-petrology sequence (Brady, 1995), in aneffort to make room in the earth science curriculum forcourses viewed as more timely or relevant: geophysics,low temperature/environmental geochemistry, hydro-geology, etc. As with many educational trends, this onemay return to haunt our discipline in a generation asprofessional geoscientists are found to lack thebackground necessary for thorough site and materialcharacterization. Ultimately, regardless of the directiongeoscience instructional emphasis swings, minerals,rocks, and surficial materials will be the fundamentalmaterials on and in which we will be performinggeophysical, chemical and isotopic analysis, with whichthe hydrosphere is interacting, and from which we willbe fabricating technologically advanced materials. Asolid background in mineral science is no less relevantnow than it was for previous generations of geoscientists.

A second disappointing trend is the decrease innumber of students going on to post-baccalaureateresearch in mineralogy and petrology. Anecdotalremarks from colleagues in the U.S.A. have made it clearthat recruiting students for mineralogy and petrologyresearch has become very competitive. At our institutionat least, the vast majority of applications for the graduateprogram are in hydrogeology. Part of this dearth ofstudents interested in mineralogy and petrology may berelated to the experience of students in mineralogycourses. Chances are they weren’t shown the practicalapplications and implications of what they were taught,how what they learned may be relevant toenvironmental science, or they weren’t given hands-on

experience with analytical methods in mineralogy andpetrology.

A single course in Mineralogy can succeed inproviding an adequate background in mineralogy thatprepares students for a diverse range of applications inupper division courses (e.g., hand sample and thinsection identification for the Ig/Met/Sed sequence;chemical composition and structure for low Tgeochemistry and hydrogeology). It can serve as aconnection with the ancillary sciences that are requiredby most geology curricula (chemistry, physics,mathematics), often bemoaned by students as notrelevant to a geoscience major. The manner in which thisis accomplished in our undergraduate Mineralogycourse is through the analysis of unknown materialscomprised primarily of minerals. As we progressthrough the semester from physical properties tosymmetry to crystal structure to crystal chemistry tooptics, students apply the principles of each section totheir unknown. They record their observations for eachstep of the process. By the end of the term, they havecompletely characterized their sample and prepare afinal report compiling all their information. In theprocess they obtain hands-on experience withinstrumentation such as X-ray diffraction and electronbeam microanalyzers (SEM, probe). Written evaluationsindicate that students find this exercise useful andvaluable for tying together all the material covered in theterm.

Mineralogy exercises that include some of theindividual aspects of the exercise outlined here arepresented in the volume “Teaching Mineralogy” (Bradyet al., 1997), published by the Mineralogical Society ofAmerica. For example, Dutrow (1997) describes a lab foridentifying common abrasives in household cleansers byXRD, and Cheney and Crowley (1997) propose acombined secondary electron microscopy/energydispersive lab for imaging and chemical analysis offine-grained minerals.

THE PROCESS

The idealized process of sample collection, characteriza-tion, and identification is outlined below. Fortunately,we are located in an area that, although not geologicallydiverse, contains a sufficient diversity of minerals occur-ring in carbonate veins in Paleozoic platform carbonates(calcite, dolomite, quartz, sphalerite, galena, fluorite,barite). What few minerals we find are usually aug-mented by samples that have been donated by citizenswho regularly drop-off materials, both natural and fabri-cated, they find on their property. Slag from coal-firedpower plants is a common material in this area that peo-ple find along train tracks and bring to us for identifica-tion. Slags are often the most interesting samples as theycontain a diverse mineralogy with quench textures andglass. They often generate interest because the donor ofthe sample often believes that they’ve found a meteorite!Local industries also produce materials (bricks, concreteproducts) that may serve the purposes of the exercise.The brick may be compared with the pre-fired raw mate-rial to examine the mineralogical and textural changesthat accompany baking of the sample at high tempera-ture. Archeological artifacts (pottery sherds, worked

Moecher - Characterization and Identification of Mineral Unknowns 5

Characterization and Identification of Mineral Unknowns: AMineralogy Term Project

David P. Moecher University of Kentucky, Department of Geological Sciences, Lexington KY40506-0053, moker@uky.edu

stone, flint) provide a cultural context that some studentsfind more relevant than geological materials. Identifica-tion of the components of a pottery sherd may be used todiscriminate among potential sources of raw materials,such as a particular clay/soil horizon or chert bed. Soilsand the bedrock on which they formed make particularlyinteresting projects. Students can compare the startingmaterial and end product of soil-formation, inferringwhat components are depleted or enriched in the pro-cess. If a sufficient number of students examine a soil,then clay mineralogy may be emphasized in lecture dur-ing the term.

David Smith, a student in my Spring 2002Mineralogy class, selected a sample of vesicular slagbelieved to have formed in a coal power plant boiler.Selected results from David’s analysis of the unknownare included here as an example of the process.

Sample Collection/Preliminary Identification - Ideally,students collect their unknown materials on a half-dayfield trip to an easily accessible locality. This could be alocal quarry or outcrop in which mineral veins occur, oran excavation or construction site in which variousfabricated materials are used, or surficial materials orbedrock are exposed. Alternatively, the materials may becollected on a field trip in another course early in theterm. Students usually have samples in their owncollections that they have collected over the years onschool or family trips, and have often been puzzled bytheir identification. The important point is for there to besome identifiable geologic context for the source of thematerial that may aid in the preliminary identification ofthe sample, or to which the students have some personalconnection. This preliminary identification is a workinghypothesis that will be tested, evaluated, and re-testedthroughout the course of the project. It is important tohave as much material as is needed to prepare apowdered fraction for X-ray diffraction and preparationof a polished thin section for optical and electronmicroscopy.

David’s unknown was a hand sample of a slag-likematerial found along the railroad tracks near a coal-firedpower plant. One of the by-products of coal combustionis silicate material that does not burn, but rather melts orforms a vesicular basalt-like or brick-like material. Thismaterial can coat the pipes through which water andsteam pass in the boiler, reducing the efficiency of theheat transfer process. The combustion process isessentially an experiment in pyrometamorphism. Hightemperature, silica undersaturated phases typically formfrom decomposition and melting of the silicatecomponent of coal, referred to as “ash” (clays andfine-grained detrital minerals).

Physical Properties - Many mineralogy courses involvesome component early in the term on hand sampleidentification of minerals based on their physicalproperties. This would be an opportunity to incorporatethe unknowns into any such labs. For students withsufficiently coarse-grained samples, and those luckyenough to have crystal faces preserved, some symmetrymay be deduced. However, well-formed crystals arerarely found. For those with finer grained samples, abinocular stereomicroscope may be required. Habit,color, streak, hardness, smell, taste (with due caution),cleavage (for those with sufficiently abundant material),and density (for those with access to a balance) may all beevaluated. However, it is rare for a student to have asufficient amount of material on which all characteristicproperties may be completely evaluated. In the case ofvery fine-grained aggregates or fine-grained crystals,

few physical properties may be identified. The studentwill thus have to resort to other means of identification offine-grained materials, as outline below.

The sample consisted of a light gray, glassy,vesicular mass (slag), of average density, containingabundant phenocrysts of a clear, bladed mineral withgood cleavage. The phenocrysts were the volumetricallymost abundant component of the sample (~90 %phenocrysts, 10 % glass). We did not attempt to separatethe phenocrysts from the matrix. The phenocrysts werepreliminarily identified as alkali feldspar.

Sample Preparation - Upon completion of preliminaryidentification and evaluation of physical properties, theinstructor and student must decide how to best preparethe sample for further analysis. In order to completelycharacterize the sample, a thin section chip should be cut.Enough material should remain from the scraps to crushand powder for X-ray diffraction. Students may berequired to prepare their own section, if the requisiteequipment is available, or chips can be sent out forpreparation of a polished thin section. The thin sectioncan be used for optical microscopy and electron probemicroanalysis. Fragments of very fine-grained materialscan be mounted on a glass slide or SEM mount foreventual electron microscopy.

David cut a slab and thin section chip from thesample in our rock preparation lab, which we sent out forpreparation of a standard uncovered thin section. Theremainder of the chip was ground into a powder to beused for X-ray diffraction scans. Upon crushing thesample emitted a strong sulfurous smell, a hint to one ofthe major chemical components of the material.

X-ray Diffraction - This step of the process permits theinstructor to introduce the principles of X-ray diffractionin determining crystal structures (measuring interplanarspacings and unit cell parameters), as well as the morepractical problem of materials characterization. Thebackground concepts required by students tounderstand X-ray diffraction include ionic radius, Millerindices, the electromagnetic spectrum (that may also beapplied later in optical mineralogy), the wave propertiesof X-ray photons, diffraction of radiation, constructiveand destructive interference of waves, and ultimately,the Bragg relationship for diffraction (n� = 2dsin�). Thederivation of the Bragg equation is an instructivein-class, small group exercise for the students. In ourcourse, students are introduced to the principles andmethods of X-ray diffraction, the process of indexing anXRD spectrum, and calculation of lattice parameters forisometric compounds (NaCl and KCl) in two labs duringthe course of the term.

Many institutions have a powder X-raydiffractometer at their disposal. Students may beinstructed in the operation of the diffractometer and runtheir own sample with or without instructor supervision,or the instructor can take the responsibility for collectingthe spectra. Students should at least go through theprocess of preparing their own powder mount. Theprocess of identification of unknowns has becomerelatively routine, aided by PC-based automation thatmatches an unknown spectrum to the catalog of knownspectra. However, this black-box approach robs thestudents of the educational experience of working-uptheir own diffraction pattern. Upon collection of thepattern, students measure the 2� for each peak(correcting each angle using an internal quartzstandard), calculate the d value, and rank the peaks inorder of decreasing relative intensity. At this point theycan compare the calculated d spacings with those of their

6 Journal of Geoscience Education, v. 52, n. 1, January, 2004, p. 5-9

preliminary identification. The instructor may requirethat some students determine the unit cell parametersusing many of the refinement methods available fordownload on the World Wide Web (e.g., several suchprograms are listed at http://www.ccp14.ac.uk/solution/unitcellrefine/; also see Holland and Redfern,1997). If a positive identification is obtained, studentsmay then move on to chemical analysis of the unknown.Alternatively, if no preliminary identification werepossible due to extremely fine grain size and/or texturalcomplexity of the sample, students can use the PowderDiffraction File or Hanawalt Search Manual to obtain amatch or eliminate alternative possibilities. It mayremain that the best that can be done as this point is toreduce the possible alternatives until confirmation bychemical analysis.

The X-ray diffraction pattern of the bulk unknown isshown in Figure 1. This is a relatively simple pattern, aswould be the case if a large component of the sample iscrystals of a single phase with minor glass. All the majorpeaks of the phenocrysts correspond to akermanite(Ca2MgSi2O7) or gehlinite (Ca2Al2SiO7), the end membersof the melilite solid solution series.

Electron Beam Imaging and Electron ProbeMicroanalysis - Qualitative (via energy dispersiveanalysis, EDS) and quantitative analysis (via wavelengthdispersive analysis WDS) by electron probe takesadvantage of the emission of characteristic X-radiationby electron interactions in the valence shell of atoms. Aswith XRD, electron beam micro-imaging andmicroanalysis permit the application of numerousprinciples of chemistry and physics. These include

atomic structure, quantum number, energy level, therelationship for photons between energy andwavelength (E = h�/�), electron transitions betweenenergy levels, characteristic and continuum radiation,and for the case of WDS, the Bragg relation fordiffraction. These concepts are introduced in twopreliminary labs, one on qualitative analysis by EDS, anda second on quantitative analysis by WDS. The labs runconcurrently with lectures on mineral chemistry,calculating mineral formulas, solid solution, etc. Forstudents with complex or very fine-grained samples,characterization of the sample by secondary electron (SE)and back-scattered electron (BSE) imaging may berequired. We don’t necessarily carry out a completequantitative analysis of all unknowns by WDS, becausein most instances the unknown consists of a single phaseand we obtain a definitive identification by XRD. Rather,we simply collect an EDS spectrum to confirm theelemental composition of the unknown.

Figure 2 is a BSE image of a representative area of thesample, with ED spectra of each component. The imagerevealed four major phases, two of which were notevident in hand sample: phenocrysts and glass (themacroscopic phases), a high average atomic numberopaque phase, and opaque dendritic crystallites in glass.The phenocryst is a Ca- and Mg-rich silicate with minorAl, the opaque phase is a Mn-Ca-Fe sulfide (absence of Oindicates it is not a sulfate), and the dendritic phase is aCa-rich sulfide with minor Mn. The very high Caabundance compared to Si, was consistent with thephenocrysts being a Si undersaturated Ca-Mg silicatesuch as a melilite solid solution, rather than a moreSi-rich mineral such as diopside. This conclusion was

Moecher - Characterization and Identification of Mineral Unknowns 7

Figure 1. X-ray diffraction pattern of bulk sample (phenocrysts + glass + quench crystals). The major peaks ofgehlenite are labeled.

8 Journal of Geoscience Education, v. 52, n. 1, January, 2004, p. 5-9

Figure 2. Backscattered electron image and energy dispersive spectra of components of unknown material.The goal of the project was to identify the phenocryst. Phenocrysts have very little gray scale contrastcompared to glass. Higher average atomic phases (shown as relative degrees of brightness in gray scaleimages) include a phase that is opaque in thin section (a Mn-Fe-Ca sulfide), and dendritic to feathery quenchcrystals (a Ca-Mn sulfide solid solution). The phenocryst is a Ca-Mg-Al silicate, whereas the glass containsthose elements and Ti, K, and S.

supported by the preliminary identification based on theXRD pattern.

Optical Microscopy - The final quarter of ourmineralogy course is dedicated to covering thetransmitted light optics of isotropic and anisotropicminerals. Optical mineralogy is a third opportunity toincorporate topics covered in a required second semesterphysics course such as the electromagnetic spectrum andrefraction (Snell’s Law). In labs associated with thiscomponent of the course, students work with grainmounts, oriented grain mounts, and eventually thinsections of rocks. Students are thus ready to characterizethe optical properties of their samples (isotropy, relief,retardation and birefringence, optic sign, etc.). Thiscomponent of the exercise serves primarily as a finalconfirmation of their unknown, as a definitiveidentification is usually completed by the time they haveconstrained the chemistry of their sample. However, it isa useful exercise for students to complete and bringsclosure to the project.

The phenocryst in the unknown occurs as euhedralrectangular blades exhibiting moderate relief, lowbirefringence (0.007), and it is uniaxial positive. Theoptical properties were consistent with the unknownbeing a melilite solid solution.

THE OUTCOME

Upon completion of the optical examination of samples,students prepare a succinct final report (2 to 3 pages) thatsummarizes their observations and presents the finalidentification of their unknown. The report includeshard copies of diffractograms, energy dispersive spectra,and/or SE/BSE images. The report should be edited bythe instructor, and returned to the student for revisionand preparation of the final draft. The instructor maywish to require each student to prepare an in-classpresentation to communicate the results to the entireclass, or to have each student read all the reports if classsize permits.

All the observations relating to David Smith’sunknown support the identification of the mineral as amelilite solid solution. The Mg-rich nature of the mineraland its optical sign are more consistent with akermanite,but the diffraction pattern corresponds more closely togehlenite. The opaque phases in the slag, whosecompositions were estimated from ED spectra (Figure 2),appear to be alabandite (MnS) – oldhamite (CaS) solidsolutions. These would be the minerals responsible forthe sulfurous odor emitted upon crushing and grindingof the sample.

PRACTICAL LIMITATIONS

There are three practical limitations to the incorporationof such a term project into the current single-termundergraduate mineralogy course. The most limiting isavailability of instrumentation. Most institutions willhave petrographic microscopes. However, fewer haveX-ray diffractometers, and fewer still have either ascanning electron microscope with an EDS detector orelectron probe microanalyzer. This type of project maytherefore be limited to larger institutions. A secondpractical limitation is instrument and instructor time.This may be made more efficient by scheduling sessionsin which groups of students run their samples on thediffractometer and probe one after the other. A teachingassistant may also supervise analytical sessions.Generally, instructor and instrument time have not beenissues as we typically have 10 to 15 students enrolled perterm, which is a manageable number. Such a project also

requires a very judicious selection of topics to be coveredin the mineralogy course. I have taken the approachrecommended by Brady (1995) that mineralogy cannotcover all the topics traditionally viewed as falling underthe mineralogy umbrella. Thus, symmetry andcrystallography are minimized. I cover a sufficientamount of symmetry that permits students to identifycrystal systems of minerals and that is needed for opticalmineralogy theory. Finally, cost may be an issue,primarily for the preparation of thin sections ifinstitutions do not have appropriate facilities. Polishedthin sections range from $10 to $20. The cost may bereduced by having students polish their own uncovered,unpolished sections.

CLOSING REMARKS

Although not a research project per se, this exercise givesstudents a taste of the approach and methods ofmineralogic research with applications in petrology. Theproject gives them a sense of the process, including ex-citement of discovering, revelation of the submicroscopicworld, of the power of the analytical methods at their fin-gertips, as well as the frustrations and stumbling blocksthat accompany the excitement of discovery. Undergrad-uate instruction using instrumentation is consistent withthe mission of research institutions, now strongly sup-ported by the U.S. National Science Foundation (Shapingthe Future, New Expectations for Undergraduate Education inScience, Mathematics, Engineering, and Technology:www.ehr.nsf.gov/ehr/due/docuents/review/96139/start.htm). Undergraduate use of instrumentation ob-tained with major equipment grants, commonly seen asthe purview of graduate instruction, is thus warrantedand justified. One further justification for introducingundergraduates to the use of instrumentation is that it of-ten stimulates an interest in mineralogic and petrologicresearch. Those whose interest is piqued by such meth-ods may wish to take the next step and complete a seniorthesis or research project in mineralogy or petrology, orbecome involved in the research program of faculty andsenior graduate students. Finally, it is hoped that a pro-ject of this nature will stimulate sufficient interest thatstudents may wish to continue on to post-baccalaureateresearch in mineralogy and petrology. At the time of thiswriting David Smith is undertaking a senior researchproject involving determination of shear sense alongductile faults using microstructural kinematic indicatorsviewed in thin section.

REFERENCES

Brady, J.B. 1995, Confessions of a mineralogy professor,Geotimes, September 1995, p. 4.

Brady, J.B., Mogk, D.W., Perkins, D. III, 1997, TeachingMineralogy, Mineralogical Society of America,Washington, D.C., 406 p.

Cheney, J.T., Crowley, P.D., 1997, Introduction to theSEM/EDS or “Every composition tells a story”, InBrady, J.B., Mogk, D.W., Perkins, D. III (eds.),Teaching Mineralogy, Mineralogical Society ofAmerica, Washington, D.C., p. 319-321.

Dutrow, B., 1997, Better living through minerals: X-raydiffraction of household products, In Brady, J.B.,Mogk, D.W., Perkins, D. III (eds.), TeachingMineralogy, Mineralogical Society of America,Washington, D.C., p. 349-359.

Holland, T.J.B., Redfern, S.A.T., 1997, Unit cellrefinement from powder diffraction data: the use ofregression diagnostics, Mineralogical Magazine v.61, p. 65-77.

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