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 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 ,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.: 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 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 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 to disturbthe brassguide. boundaryin normalillumination with low intensity. 7. Using a high-speeddrilling tool fitted with a l- Make surethe grainis in the desiredoptical orienta- inch-diametercarborundum disc, make a verticalcut tlon. alongthe brass guide across the grain to bemeasured. I L lnsert the aperturescreen and setthe illumina- This cut must be executedin one downwardmotion tion as previouslydescribed. and must penetratethrough the rock sectioninto the 12. Place the crosshairsat a point on the grain glass.The idea hereis to createa trough or slit in the where color fringes are observed,and recheckthe grain to be measured. centeringof the screen. 8. Removethe slide from the cutting apparatus 13. Recordthe color observedthrough the aper- and add one drop of acetoneto the freshcut with a ture screen,and refer to Table I for the wavelength small pipette.Allow this drop to evaporatepartially, for which the indicesof refractionof the grain and then wash the entireslide with water. Dry the slide. liquid areequal. [The albite crystal described in Table 9. Beforeproceeding, check to be surethat the cut 3 yieldedgreen borders as viewedthrough the aper- producedis acrossthe grainin question. ture screenwhen a liquid with nD : 1.532at 25"C 10. Placea drop of indexliquid (Cargilleliquids was used.This indicated(Table l) a crystal-liquid are preferred)on the cut, making surethat the tem- index match at some wavelengthbetween 5200 and peratureof the liquid is about25'C. A liquid-grain 5400A.1 boundaryhas now beenestablished. Using successive 14. Removethe aperturescreen and insertthe cen- mounts,find a liquid which producesdiscernable dis- tral screen.Set the illumination for centralscreening persioncolors. Clean the cut on the slidewith a drop as previouslydescribed. Perform step 14 without al-

Table3. Resultsof dispersionstaining measurements on known crystals

MateriaL Type of 4-D liquid Colors observed duing screening nD Published uout (25oc) grain n natch measueCl nD Apertual Central

r.508 Green Violet+0rqge 52ooLo 01ess I so. Grain I.5I2 Yellov oruge Royal Blue 5too A' t.5rl t.)r. rruarglrre./ f.ouo teffov green Blue+Red. :too ll Glass I so. Grain I.5I2 Ormge red Blue+Green 6\00 A" r..5095 LOI (UAg1Ile,/ Arooniu t. oje ureen Violet+0range :zoo ll Chloride I so. craln 1.535 Yetlov green 81.vio].+Red orng. ,55o A' r.6396 1.5392 (casilre) Sodiw 1,608 31ue green VioIet+0rmge :roo l] Bronate I so. Grah f.bfz Green-Ye1. grn. BI.viol,+Red. orng. 5\oo A" r.6\67 f,olo) (usgalfel Low I.544 Royal Blue Gold+Violet )+Too.no Quutz Grain I.552 creen B1.viol.+Red orng. 5300A' r.5525 1.553 (B1oss,1957) Lov l. )4u uleen 81.viol+Red orng. 53oo.ro Grain 1,544 Yellov Blue+Red 5850A" r'l\tr6 1.5)+l+(Bross, r95?) Lov Tnln t. >4u Bfue green 81.Yiol.+Red orng. :zoo l! Section 1,544 Yel1ov green Blue+Red 57ooA" 1.5\\9 1.51+l+ Lov rnan I. >4o Stue gleen B1.vio1.+Red orng. :eoo al beclaon a. >>o leilov orange Royal Blue 6150A- r.ii39 1.532 creen Violet+Orege 5zooll A-lbtte R Graln I.536 Red orange Blue 6200 A" r.53r2 I q?t-t q?7 lrarr toco\ Thin L.532 creen Violet+0range :eoonl A.1bite Section ]-136 orege yeLlov Blue+Red 5o5oA- r,i35i r.531-1.t37 r. ))o ureen Violet+Ormge 5eooll Labradorite R Grain f . 5b0 Orilge real Blue 5200A" 7.5592 f . >)o-f .)oy (trerr, t9>9, thin f.556 Green-Blue grn. Violet+0range :eoo ll Labrailorite R Dectlon f. >ou Ietfov ormge Blue 5r5o A" t.sig\ r.558-! .569 High Ca f. )o4 laue green Violet+Orange lroo ll Labradorite B crein I.558 Yeltov orange Blue+Red 5o5oA" r.5617 r. jj3-r, j69 High Ca Thin I.56\ creen VioIet+0renge :eoo ll Labradorite Section I.558 Yellov-Lt.Ormge Blue+Red. 6000A" L.5679 r.558-r .569 Thin L,520 Yellov green 81.viol.+Red orng. >:>o,o.l Microcline R Section I.52\ Ormge Blue+Red. 6100A" r.5226 r.>zz-!,>zo (bfoss, lyol, LASKOWSKI ET AL: REFRACTII/E INDEX MEASUREMENT tering the position of the slide on the microscope Discussion stage,or changing the liquid. Table3 showsresults of digpersionmeasurements 15. Observethe colors produced by central screen- made on varioussamples. All measurementswere ing, and refer to Table2 for the wavelengthat which conductedat about 25oC. Indicesof refractionof the indices of refraction of the grain and liquid are ammoniumchloride, sodium bromate, and two opti- equal. This wavelength or range of possible wave- cal glasses(all Cargillestandards) were measured lengths should coincide with the results obtained using grain mounts and the dispersionstaining from apertural screening. [The albite crystal de- methodpreviously described. Accuracies of equalto scribed in Table 3 yielded violet and orange color or better than t0.001 were obtainedin all cases. fringes viewed through the central screen when a Theseresults are comparablewith accuraciesob- liquid with nD : 1.532at 25oC was used.This in- tainedby Cherkasov(1960) and Grabar (1963). dicated (Table 2) a crystal-liquid index match at The indexof refractionfor sodiumD light of the some wavelengthbetween 5000 and 5200A.1 epsilonray and rD-omega for low quartz,zD-beta of 16. For the albiteexample, steps l3 and l5 yielded afbite, nD-beta of microcline,nD-beta of labradorite, a crystal-liquid index match at some wavelength and rD-beta of high-calciumlabradorite were mea- within the range of 5200-54004 (step l3) and within sured directly from petrographicthin sections.In the range of 5000-52004 (step l5). Thesetwo wave- eachcase, a specificgrain in a specificorientation was length ranges overlap at 5200A, thus 52004 was chosenfor measurement.The indexof refractionfor chosen as the wavelength for which the index of sodium D light of the omegaray and nD-epsilon refractionofthe crystaland the liquid (in this casenD valuesof 1.5449and L5539measured for low quartz :1.532 at 25'C) are equal.In somecases the results comparefavorably with the valuesgiven by Bloss attained from Tables I and 2 will overlap over a (1967).The indexof refractionfor sodiumD lightof range of wavelengths.If this occurs, choose a mid- the betaray for albitewas measured in thin sectionto point within the overlapping wavelengths as the be 1.5355.A grainmount of the samesample gave an wavelength for which the n of the crystal and the nD-betaof 1.5352.These values fall within a rangeof liquid are equal. values(Kerr, 1959)for the albitefield. The indicesof 17. Plot the liquid (at 25'C) on a graph of n us. refractionfor sodiumD lightof thebeta rays for low- wavelength(Fig. a). Plot the match wavelength,at- calcium labradoriteand high-calciumlabradorite tained in step 16, on the liquid curve. For the albite weremeasured in thin sectionto be 1.5594and 1.5679 example, the match wavelength attained in step l6 respectively.Grain mountsof the samesamples gave (5200A) was plotted at point ,4 on Figure 4. nD-betasof 1.5592and 1.5677respectively. Kerr 18. Clean the slidewith a drop of acetone,rinse in (1959) placed the labradoritefield at nD-beta : water, and dry. 1.558-1.569.The indexof refractionfor sodiumD 19. If a wavelength match of 54004 or less was light of the betaray of microclinewas determined to attained above, choose a higher index liquid for the be 1.5226from a thin sectionmount. Bloss(1967) next measurement.If a wavelengthmatch of greater placedthe microclinefield at nD-beta: 1.522to than 54004 was attainedabove, choose a lower index t.526. liquid for the next measurement. Compositionis relatedto nD-betafor the plagio- 20. Repeat steps I l-18 with the new index liquid; clasesolid solutionseries (Chayes, 1952). Using his be sure the crystal is in the same orientation as be- graph for nD-betaus. plagioclasecomposition, the fore. (With liquid nD : 1.536at 25oC,6050A was albite,labradorite, and high-calciumlabradorite in determinedto be the wavelengthfor which the index Table 3 are estimatedto have compositionsof 8 of refraction of the albite in Table 3, and liquid rD : percentanorthite, 53 percentanorthite, and 68 per- 1.536at 25"C, are equal. This match wavelengthis centanorthite respectively. plotted at point B on Figure 4.) In casesdiscussed above, liquid-grain boundaries 21. Draw a line through the two match points asin were createddirectly on thin sectionslides. Color Figure 4. This line is an accuraterepresentation of the fringes were produced.To interpret these color change in index of refraction of the crystal in the fringes(some of themfaint) produced in thin section, orientation presentwith respectto a changein wave- the dispersionstaining method is preferredover other length. The point on the Y axis where the resulting standardmethods because of the accuracyattained line intersectsthe sodium D line is the nD of the (+0.001),quality of colorsproduced (a color fringe grain, in the orientationtested (point C on Figure 4). whichappears very faint in regularlight is generally LASKOIYSKI ET AL.: REFRACT,IVE INDEX MEASUREMENT 445

Liquld l* 2 0.1.336ot 25oC

Liquid l* | ll D:t 532 ot

AinA" Fig' 4' Determinationof nD of an albite.Liquids I and 2 areplot ted,n us.wavelength, according to valuesfor zF, rD, andn C givenfor eachliquid' Line ACB represents the changein indexof refractionof the albite with respectto a changein wavelength.The crystal point is orientedto measurethe betaindex. c is the nD-beta(1.5355) of the albitecrystal.

quite clear when the dispersionstaining lens is em_ r05. ployed),and easeof interpretationof the colorspro_ Cherkasov,Yu. A. (1960)Application of focal screeningto mea- duced. surementof indicesof refractionby the immersionmethod. 1zr. Geol.Reu.,2.218-235. References Grabar, D. G., and A. H. Principe(1963) Identification of glass fragmentsby measurementof refractiveindex and dispersion. Bloss, F. D. (1967) An Introduction to the Methods of Opticat "/. ForensicSci., 8, 54-67. Crystallography.Holt, Rinehart, and Winston, New york. Kerr, P. F. (1959)Optical Mineralogy.McGraw-Hill, New york. Brown,K. M., W. C. McCrone,R. Kuhn and L. Forlini(1963) Laskowski,D. E. (1965)Chemical microscopy of urinary calculi. Dispersionstaining part II-The systematicapplication to the Anal Chem.,37, 1399-1404. identification of transparent substances.The Microscope and Crystal Front, 14, 39-54. Chayes,F. (1952) Relationsbetween compositions and indicesof Manuscript receiued,March 27, 1978; refraction in natural plagioclase. g5_ Am. J. Sci.,Bowen Volume. acceptedfor publication,May 22, 1978.