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Measurement of Refractive Index in Thin Section Using Dispersion

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Measurement of Refractive Index in Thin Section Using Dispersion American Mineralogist, Volume 64, pages 440445, 1979 Measurementof refractiveindex in thin sectionusing dispersion staining and oil immersion techniques Tsoues E. LesrowsKI,De.vID M. Scornonp Departmentof Geology,Miami Uniuersily Oxford, Ohio45056 nNoDoNnLo E. Lnsrowsrt Departmentof Pathology,Auhman Hospital Canton,Ohio447l0 Abstract With the aid of a 10X dispersionstaining objective adapted for apertural and central screening,and a liquid-graininterface produced with a cuttingtool, the indexof refractionof a crystalin thin sectioncan be measureddirectly. Accuracy of i0.001 is attained. Introduction objectiveto makethe grain to appearto havecolored Petrographersare commonlyfaced with the neces- borders.The color observedis indicativeof the wave- grain sity or desirabilityof obtainingthe indexof refraction lengthfor which a match betweenn of the and (n) of a specificcrystal in a thin section,yet with no liquid has been obtained (Grabar and Principe, wayof doingso withoutremoving the grain from the 1963).Thus, dispersion staining aids markedly in the grain (Las- thin section,which can be a laborioustask, or by rapididentification of mineralsin mounts making a mineral separationfrom the rock sample. kowski, 1965). Apertural screeningand central In the latter casethere is no assurancethat the sepa- screeningwith a lOX dispersionstaining objective rated grains are identical with the specificone of enablesone to determinenD of crystals,not only in interestin the thin section.The dispersionstaining grainmounts, but alsoin petrographicthin sections. method,described and evaluatedhere, offers a rela- Our purposehere is to reportan evaluationof the in- tively rapid techniquefor determiningat least one method,based on the determinationof refractive indexofrefraction for a selectedgrain in thin section. dicesof albite,labradorite, quartz, and microclinein The procedureis particularlyuseful, for example,for thin section,and to describethe techniquein suf- obtainingthe n of a memberof an isomorphous ficient detail to encourageits use. series,which commonly allows chemicalcharacter- Theory izationof the mineral. Petrographersare familiar with the theory and The theory of the dispersionstaining objective has methodologyof index of refraction determination beendescribed (Cherkasov, 1960; Brown et al., 1963; using dispersioncolors (Bloss, 1967).If the n of a Grabarand Principe,1963).A standardlOx objec- grain is closeto a match with that of the oil in which tive is fitted with two types of removablescreens it is immersed,a dispersionof white light is observed placedin the rear focal plane of the objective. which producescolor fringesaround the mineral. screening Observationof thesecolors can lead to a reasonably Apertural precisedetermination of n. The aperturescreen is a diaphragmwith an open A lOX dispersionstaining objective (W. C. Mc- spaceabout 2 mm in diameter.This diaphragmis CroneInc., Chicago, Illinois) can be used as an aid in placed in the rear focal plane of the lOX objective making this determination.In caseswhere the n of a (Fig. I ). To adjustthe systemfor properillumination, grain and liquid match at one wavelength,but not at the following conditionsare met: others,it is possiblewith the l0X dispersionstaining l. Nicolsuncrossed offi3-ffi4x / 79 / 0304-0440$02.00 LASKOWSKI ET AL.: REFRACTIVE INDEX MEASUREMENT 441 Color Fring€ Central screening Centralscreening can be usedto definefurther the resultsobtained in the aperturalscreening measure- ment.The central screen is an opaquedot (l-3 mm in diameter)which is mountedon a glassplate. This Aporlure Focol Plone Areo screenis alsoplaced in therear focal plane ofthe lOX objective(Fig. 2). To adjust the systemfor proper illumination,the followingconditions are met: L Removethe aperturescreen 0bjective Lens 2. Nicolsuncrossed 3. High-powercondenser out 4. Light intensityhigh 5. Bertrandlens in 6. Insertcentral screen so that the opaquedot is in the centerof the crosshairs 7. Adjust the substagediaphragm of the micro- scopeso that the radiusof light visiblethrough the Bertrandlens is equalto theradius ofthe opaquedot on the screen ! Light 8. Adjust the opaquedot so that it completely coversthe radiusof light createdin step7 Fig.. l. Schematic diagram showing the effects of apertural 9. Openthe substage diaphragm ever so screening on dispersed rays of white light as viewed in focus. slightlyto allow a faint rim of light to be barelyvisible around the peripheryof the opaquedot 2. High-powercondenser out 10. Now takethe Bertrandlens out 3. Light sourceat high intensity ll. The systemis now arrangedfor 4. Bertrandlens in measurement with the centralscreen. 5. Aperturescreen in Figure 2 showsthe systemschematically. parallel 6._Substage diaphragm of the microscopeclosed bundlesof whitelight strikethe liquid-grainbound- so that the radiusof light visiblethrough the Ber_ ary and are dispersedinto variousrays (A, B, C, D, trand lensis slightlysmaller than the .udius of. and E). WavelengthC is not deviatedat the liquid-grain locatedwithin, the aperturescreen boundary,thus wavelengthC is the wavelengthfor 7. Now takethe Bertrandlens out which the index of refractionof the liquid and the 8. The systemis now arrangedfor measurement index of refractionof the grain with the aperturescreen. are equal.Wave- Figure I showsthe systemschematically. parallel Table bundlesof white light strikethe liquid-grainbound_ l Colors observ.O;;,:ffi;:."1 screeningand relared ary and are dispersedinto variousrays (A, B, C, D, E). Raysof wavelengths A, B, D, and E aredeviated Wevelength of liquitl-grain n natch at the liquid-grainboundary, but wavelengthC is not deviated.As light of wavelengthC is not deviated,it Deep Violet to Black Less thm looo Ao is the wavelengrhor color of light for which the liquid Duk Blue violet !ooo-l+5ooAo and grain have the same refractiveindex. All the RoyeJ. Blue j+6oo-lr8ooAo BIue green l+800-5200Ao Green 5zoo-5\oolo Yelfov green 5l+oo-5?odAo Yellor t?oo-5ooono YeLlov olange to Red. ormge 5ooo-5200Ao Red orege to Red 5zoo-5500ao Red to Bron red creater tbm 6500 Ao 42 LASKOI4/SKIET AL.: REFRACTIVE INDEX MEASUREMENT Table 2. Colors observed in central screening and related wavelensths fn focuB Becke lines Wavelength of llquid- greln n Datch Ligbt Blue to fihite Light Blue and Hhite Greater thm ?000 Ao Focol Plono Areo Cenlrol Greenish azue G!een 55OO-?OOOlo BIue md Green 63oo-5500 lo Royal Blue Royel Blue 51oo-5300 Ao Bl-uish violet Blue stl Red 5?oo-6roo lo 0blsctlYs Lsng Crinson Blue violet ed SZOO-5?OOAo Pai ^Frnda Light or@ge vloLet sd Orege Sooo-5200no Golden yellov Falnt Violet 4d )+l+oo-500oAo GoIal PaLe Yel1ov Faint Yeffow Less thu l+\oo Ao carefullypry the coverslipoffwith a sharprazor. Use of safetyglasses is advised. WhiteLighl 2. Selectthe largest grain of a specificmineral specieswhich is orientedfor the particularindex de- Fig. 2. Schematic diagram showing the bffects of central screening on dispersed rays of white light as viewed in focus. siredfor measurement. 3. Placethe thin sectionon the cutting apparatus (Fig. 3), and placethe cuttingapparatus on the mi- lengthsA, B, D, and E are deviatedat the liquid- croscopestage. Relocate the grain desiredfor mea- grain boundary.As all of the wavelengthsof dis- surementusing a low-powerobjective. persedlight are collectedand passthrough the rear 4. Placethe brassguide acrossthe grain to be focal plane of the objective,they encounterthe cen- measured,and hold the guidefirm with the magnets. tral screen.The centralscreen blocks out wavelength Make sure the indentationon the brassguide is C and allowswavelengths A, B, D, and E to passto acrossthe grainto be measured. theimage plane. The grain borders of thecrystal thus 5. Take the cuttingapparatus and attachedslide becomeilluminated and colored.When viewedin focus,the color at the grain border is relatedto the d RGctongleWllh Gloss actualwavelength (as the complementarycolor to the Plole Com€nf6d To Undsrside wavelength)for which the index of refractionof the liquidand that of thegrain are equal (Table 2). When Sh6efMelol Perlmslor de-focusing,two Beckelines are produced.The col- ors of theseBecke lines are similarly relatedto the oss Guido is wavelengthat whichthe liquid-grainindex match gn€t obtained(Table 2). Procedurefor nD measurementin petrographicthin lo ng le Cut In Cordboord To Hold section Sllde Measurementof an indexof refractionof a specific mineral (in this casenD-beta of an albite will be Fig. 3. Cutting apparatus.A guide which allows one to orient the measured)can be performeddirectly on the thin sec- cutting tool to a selectedgrain. A slide is placed in the central tion. The followingdescription of the determination rectangle,and the assemblageis placed directly on the microscope grain of the nD for the beta index of plagioclaseserves as stage. The slide is viewed with a low-power objective, and a for measurement is located. The brass guide is placed across the an example: desired grain and held in place by tlvo magnets. The entire l. It is necessaryto work with a thin sectionwith- assemblageis then removed from the stage of the microscope for out a coverslip.If the desiredslide has a coverslip, cuttlng. LASKOWSKI ET AL.: REFRACTIVE INDEX MEASUREMENT 443 off the microscopestage and placeon a flat table. of acetoneeach time liquidsare changed. As the rD 6. Apply three dropsof acetoneto the slidein the of a given liquid approachesthe nD of the grain, vicinity of the desiredgrain and rinsewith water.Be dispersioncolors will becomevisible at the cut-grain carefulnot
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