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American Mineralogist, Volume 64, pages 440445, 1979
Measurementf refractivendex n thin section singdispersiontaining
ndoil immersiontechniquesTsoues E. LesrowsKI,De.vIDM. Scornonp
Department f Geology,Miami UniuersilyOxford,Ohio45056nNoDoNnLoE.
Lnsrowsrt
Department f Pathology, uhmanHospitalCanton,Ohio447l0
AbstractWith the aid of a 10X dispersion tainingobjectiveadapted
or aperturaland centralscreening,nd a iquid-grain nterface
roducedwith a cutting ool, the ndexof refractionofa crystal n thin
section an be measured irectly.Accuracyof i0.001 is attained.
IntroductionPetrographers re commonly acedwith the neces-sity or
desirability f obtaining he ndexof refraction(n) of a specific
rystal n a thin section, et with nowayof doingsowithout
emovinghegrain rom he
thin section,whichcan be a laborious ask, or bymaking a
mineralseparationrom the rock sample.In the lattercasehere s no
assurancehat the sepa-rated grains are identical with the
specificone ofinterest n the thin section.The dispersion
tainingmethod,described nd evaluated ere,offersa rela-tively rapid
technique or determiningat least oneindexofrefractionfor a
selectedrain n thin section.The procedures particularlyuseful, or
example,orobtaining he n of a memberof an isomorphousseries,which
commonly allows chemicalcharacter-izationof the
mineral.Petrographers re familiar with the theory andmethodologyof
index of refraction determinationusingdispersion olors
(Bloss,1967). f the n of agrain s close o a matchwith that of the
oil in whichit is immersed, dispersion f white ight is
observedwhich produces olor fringesaround the mineral.Observation f
these olorscan ead o a reasonablyprecise etermination f n.A lOX
dispersion tainingobjectiveW. C. Mc-Crone nc.,Chicago,ll
inois)canbeused san aid nmaking this determination.n caseswhere he
n of agrain and iquid match at onewavelength,ut not atothers, t is
possiblewith the l0X dispersion taining
objectiveo make hegrain o appear o havecoloredborders.The color
observeds ndicativeof thewave-length or which a match between of
the grainandliquid has been obtained (Grabar and Principe,1963).
hus,dispersiontaining idsmarkedly n therapid dentificationf
mineralsn grainmounts Las-kowski, 1965). Apertural screeningand
centralscreeningwith a lOX dispersion taining objectiveenables ne o
determine D of crystals, ot only ngrainmounts, ut also n
petrographichin sections.Our purpose ere s to reportan evaluation f
themethod,based n the determination f refractiven-dicesof albite,
abradorite,quartz,and microclinenthin section,and to describe he
technique n suf-ficientdetail to encouragets use.
TheoryThe theoryof the dispersion
tainingobjectivehasbeendescribedCherkasov, 960;Brown et al.,
1963;Grabarand Principe,1963). standardOx objec-tive is fitted with
two types of removablescreensplaced n the rear focalplaneof the
objective.
Apertural screeningThe aperturescreens a diaphragmwith an
openspaceabout 2 mm in diameter. his diaphragmsplaced n the rear
focal planeof the lOX objective(Fig. I ). To adjust he systemor
proper llumination,
the following conditionsaremet:l. Nicolsuncrossedoffi3-ffi4x /
79 0304-0440$02.0
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LASKOWSKI ET AL.: REFRACTIVE NDEX MEASUREMENT 441C o l o r F r i
n g
Ap o r l u re
CentralscreeningCentralscreeningan be used o define urther
heresults btainedn the apertural creening easure-ment.Thecentral
creens an opaque ot (l-3 mm n
diameter)which is mountedon a glassplate.Thisscreens alsoplacedn
the ear ocalplane fthe lOXobjective Fig. 2). To adjust the system
or properil lumination,he ollowingconditions remet:L Remove he
aperture creen2. Nicolsuncrossed3. High-power ondenserut4. Light
intensityhigh5. Bertrandens n6. Insert entral creeno hat heopaque
ot s nthe centerof the crosshairs7. Adjust the substage iaphragmof
the micro-scope o that the radiusof l ight visible hrough
heBertrandens sequal o the adiusofthe opaque oton the screen8.
Adjust the opaquedo t so that it completelycovers he radiusof light
createdn step79. Open hesubstageiaphragm ver oslightlyoallowa faint
rim of l ight to be barelyvisible roundthe periphery f theopaque
ot10 . Now take he Bertrandensoutll . The systems now arrangedor
measurementwith the centralscreen.Figure2 shows he system
chematically.arallelbundles f white ightstrike he
iquid-grainbound-ary and are dispersednt o various ays A, B, C,
D,E). Wavelength is not deviated t the iquid-grainboundary,hus
wavelength is the wavelengthorwhich the indexof refractionof the
liquid and theindex of refractionof the grain are equal.Wave-
Table l Colors observ.O;;,:ffi;:."1 screening nd relared
Wevelength of liquitl-grain n natch
F o c o l P l o n e A r e o
0 b j e c t i v e L e n s
! L i g h tFig.. l. Schematic diagram showing the effects of
aperturalscreeningon dispersedrays of white light as viewed in
focus.2. High-power ondenserut3. Light source t high ntensity4.
Bertrandens n5. Aperture creenn6._ ubstage iaphragm f the
microscope losedso that the radiusof light visible hrough he
Ber_trand ens s slightlysmaller han the udiusof. andlocatedwithin,
the aperturescreen7. Now take he Bertrandensou t8. The systems now
arrangedor measurementwith the aperturescreen.Figure shows he
system chematically.arallelbundles f white ightstrike he
iquid-grainbound_aryand aredispersednto various ays A, B, C, D,E).
Raysof wavelengths, B, D, andE aredeviated
at the iquid-grain oundary, ut wavelength is no tdeviated.As
light of wavelengthC is not deviated,tis thewavelengrhr colorof
light for which he iquidand grain have the samerefractive ndex. All
theDeep Violet to BlackDuk Blue violetRoyeJ. BlueBIue
greenGreenYelfov greenYellorYeLlov olange to Red. ormgeRe d orege
to RedRed to Bron red
Less hm looo Ao!ooo-l+5oo oj+6oo-lr8ooAol+800-5200o5zoo-5\oo
o5l+oo-5?odot?oo-5ooo o5ooo-5200Ao5zoo-5500 ocreater tbm 6500
Ao
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42 LASKOI4/SKIET AL.: REFRACTIVE NDEX MEASUREMENTTable 2. Colors
observed in central
wavelensthsscreening and related
fn focuB Becke lines Wavelength of llquid-greln n Datch
C e n l r o l F o c o l P l o n o A r e o
0 b l s c t l Y s L s n g
Wh i teL ig h lFig. 2. Schematic diagram s howing the bffects of
central
screening on dispersed rays of white light as viewed in
focus.
lengthsA, B, D, and E are deviated t the liquid-grain
boundary.As all of the wavelengths f dis-persed ight are
collectedand pass hroughthe rearfocalplaneof the objective, hey
encounterhe cen-tral screen. he centralscreen locksout wavelengthC
and allowswavelengths, B, D, and E to passothe mage lane.
hegrainborders f thecrystal husbecome lluminatedand colored.When
viewed nfocus, he color at the grain border s related o
theactualwavelengthas he complementaryolor o thewavelength) or
which the indexof refractionof theliquidand hat of thegrainareequal
Table ).Whende-focusing,wo Becke inesare produced.The col-ors of
theseBecke inesare similarly related o thewavelengtht which he
iquid-grain ndexmatch sobtained Table2) .Procedure or nD
measurementn petrographic hinsection
Measurement f an ndexof refractionof a specificmineral(in this
casenD-betaof an albitewill bemeasured) an beperformeddirectlyon
the thin sec-tion.The followingdescription f the determinationof
the nD for the beta ndexof plagioclaseerves san example:l. It is
necessaryo work with a thin sectionwith-out a coverslip. f the
desiredslide has a coverslip,
carefullypry the coverslip ffwith a sharp azor.Useof safety
lassess advised.2. Select he largestgrain of a
specificmineralspecies hich s oriented or the particular ndex
de-sired or measurement.3. Place he thin sectionon the cutting
apparatus(Fig. 3), and place he cuttingapparatus n the mi-croscope
tage.Relocate he grain desired or mea-surement singa
low-powerobjective.4. Place he brassguideacross he grain to
bemeasured,nd hold heguide irm with the magnets.Make sure the
indentationon the brassguide sacrosshe grain o bemeasured.5. Take
the cuttingapparatus nd attached lide
d R G c t o n g l e
Ligbt Blue to fihiteGreenish azue
Royal BlueBl-uish violetCrinson
Light or@geGolden yellov
PaLe Yel1ov
Light Blue and HhiteG!eenBIue md GreenRoyel BlueBlue stl RedBlue
violet ed
P a i ^ F r n d a
vloLet sd OregeFalnt Vio let 4dGoIalFaint Yeffow
Greater thm ?000 Ao55OO-?OOOo63oo-5500 o51oo-5300Ao5?oo-6roo
loSZOO-5?OOo
Sooo-5200no)+l+oo-500o o
Less thu l+\oo Ao
W l l h G l o s sU n d s r s i d el o eS h 6 e f
C o m n f 6 d T oM e l o l P e r l m s l o r
o s s G u i d og n t
l o n g e C ut I n C o r d b o o r d T oH o l d S l l d e
Fig. 3. Cutting apparatus.A guide which allows one to orient
thecutting tool to a selectedgrain. A slide is placed in the
centralrectangle,and the assemblages placed directly on the
microscopestage.The slide is viewed with a low-power objective, and
a grainfor measurement s located. The brass guide is placed across
thedesired grain and held in place by tlvo magnets. The
entireassemblage s then removed from the stage of the microscope
forcuttlng.
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LASKOWSKI ET AL.: REFRACTIVE NDEX MEASUREMENT 443off the
microscope tageand placeon a flat table.6. Apply threedropsof
acetoneo the slide n thevicinity of the desired rainand rinsewith
water.Becarefulno t to disturb he brass uide.
7. Usinga high-speed rilling tool fittedwith a
l-inch-diameterarborundum isc,makea vertical utalong hebrass
uideacrosshegrain o bemeasured.This cut mustbe executedn one
downwardmotionand must penetratehrough he rock section nto
theglass.The deahere s to createa troughor slit n thegrain o be
measured.8. Remove he slide from the cutting apparatusand add one
drop of acetone o the freshcut with asmallpipette.Allow this drop
to evaporate artially,then wash he entireslidewith water.Dry the
slide.9. Beforeproceeding,heck o be sure hat thecutproduceds
acrosshe grain n question.10. Placea drop of index iquid
(Cargilleiquidsarepreferred)on the cut, making sure hat the
tem-perature f the liquid s about25'C. A liquid-grainboundaryhasnow
beenestablished. singsuccessivemounts, ind a liquidwhichproduces
iscernable is-persion olors.Clean hecut on theslidewith a drop
of acetone ach ime iquidsarechanged. s the Dof a given iquid
approacheshe nD of the grain,dispersion olorswill become isibleat
the cut-grainboundaryn normal l luminationwith low intensity.Make
sure hegrain s n the desired pticalorienta-t lon.I L lnsert he
aperture creen ndset he llumina-tion aspreviously escribed.12.
Place he crosshairs t a point on the grainwherecolor fringes are
observed,and recheck hecentering f the screen.13. Record he color
observedhrough he aper-ture screen, nd refer o Table I for the
wavelengthfor which the indicesof refractionof the grain andliquid
areequal. Thealbitecrystaldescribedn Table3 yieldedgreenbordersas
viewed hrough the aper-ture screen hena liquid with nD : 1.532 t
25"Cwas used.This indicated Table ) a crystal-l iquidindex match at
somewavelength etween5200and5400A.114. Remove he aperture creen nd
nsert he cen-tral screen. et he illumination or centralscreeningas
previouslydescribed. erformstep 14without al-
Table3. Results f dispersion tainingmeasurementsn known
crystalsMateriaL Type of 4- D liquid Colors observed duing
screeninguou t (25oc)Apertual Centra l grain n natch
PublishednD
nDmeasueCl
01ess
G la ssArooniuChlorideSodiwBronateLowQuutzLov
Lov
Lov
A-lbtte
A.1bite
Labradorite
Labrailor iteHigh CaLabradoriteHigh CaLabradorite
Microcline
Violet+0rqgeRoyal BlueBlue+Red.Blue+GreenViolet+0range8 1 . v io
] . + R e d o r n g .VioIet+0rmgeBI . v io l , + R e d . o r n g
.Gold+VioletB1 . v io l . + R e d o r n g .81.viol+Red
orng.Blue+Red8 1 . Y io l . + R e d o r n g .Blue+RedB1 . v io 1 .
+ R e d o r n g .Royal
BlueViolet+OregeBlueViolet+0rangeBlue+RedViolet+OrmgeBlueViolet+0rangeBlueViolet+OrangeBlue+RedVioIet+0rengeBlue+Red.8
1 . v io l . + R e d o r n g .Blue+Red.
52ooLo5too A' t .5r l:too ll6 \00 A" r. .5095:zoo ll,55o A' r
.6396:roo l]5 \oo A" r.6 \67)+Toono5300A' r.552553oo ro5850A" r ' l
\ t r6:zoo l!57ooA" 1.5\ \9:eoo al6150A- r. i i395zoo l6200 A"
r.53r2:eoo l5o5oA- r, i35 i5eoo l5200A" 7.5592:eoo ll5r5o A" t .s
ig \lroo ll5o5oA" r.5617:eoo ll6000A" L.5679>:>o,o. l6100A"
r.5226
I so .
I so .
I s o .
I s o .
r . 5 0 8 G r e e nGrain I .5I2 Yellov oruge
f .ouo te f fov g r e e nGrain I .5I2 Ormge red
t . o je u reencraln 1.535 Yet lov green
1,608 31ue greenGrah f .bfz Green-Ye1. grn.
I . 5 4 4 R o ya l B lu eGrain I.552 creen
l . ) 4u u le e nGrain 1,544 YellovTnln t. >4 u Bfue greenSe
c t i o n 1 , 5 4 4 Ye l1 o v g r e e nrnan I. >4 o Stue
gleenbeclaon a. >> o le ilov orange
1.532 creenGraln I.536 Red orangeThin L.532 creenSect ion ]-136
orege yeLlov
r . ) ) o u reenGrain f . 5b 0 Orilge realt h in f . 5 5 6 G r e
e n - B lu e grn .Dect lon f. >o u Ietfov ormge
f. )o 4 laue greencrein I .558 Yeltov orangeThin I.56\ creenSect
ion I .558 Yellov-Lt .OrmgeThin L,520 Yellov greenSect ion I .52\
Ormge
t . ) r . r r u a r g l r r e . /
LO I ( U Ag 1 I l e , /
1.5392 casi l re)f ,o lo ) (usga l fe l1 . 5 5 3 ( B 1 o s s , 1
9 5 7 )1 .5)+ l+ Bross, r95?)
1 .51+ l+
I q ? t - t q ? 7 l r a r r t o c o \
r . 5 3 1 -1 . t3 7f . > ) o - f . ) o y ( t r e r r , t 9
> 9 ,
r .558-! .569r. jj3-r, j69r.558-r 569r . > z z - ! , > z o
( b f o s s , l y o l ,
R
R
R
B
R
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LASKOWSKI ET AL: REFRACTII/E NDEX MEASUREMENTtering the position
of the slide on the microscopestage,or changing he l iquid.15.
Observe he colorsproducedby central screen-ing, and refer to Table2
for the wavelengthat whichthe indicesof refraction of the grain and
l iquid areequal. This wavelengthor range of possiblewave-lengths
should coincide with the results obtainedfrom apertural screening.
[The albite crystal de-scribed in Table 3 yielded violet and orange
colorfringes viewed through the central screen when al iquid wi th
nD : 1.532at 25oC was used.This in-dicated (Table 2) a crystal-l
iquid index match atsomewavelengthbetween5000 and 5200A.116. For
the alb i teexample, teps 3 and l5 y ie ldeda crystal-l iquid index
match at some wavelengthwithin the rangeof 5200-54004 (step l3) an
d withinthe range of 5000-52004 (step l5). These wo wave-length
ranges overlap at 5200A, thus 52004 wa schosen as the wavelength
for which the index ofrefractionofthe crystaland the l iquid (in
this case D:1.532 at 25 'C) are equal . n somecases he resul
tsattained from Tables I and 2 will overlap over arange of
wavelengths. f this occurs,choosea mid-point within the overlapping
wavelengthsas thewavelength for which the n of the crystal and thel
iquid are equal .17. P lot the l iquid (at 25'C) on a graph of n
us.wavelength Fig. a) . Plot the match wavelength,at-tained in step
16,on the liquid curve. For the albiteexample, he match
wavelengthattained in step l6(5200A) wa s plotted at point ,4 on
Figure 4.18. Clean he s l idewi th a drop of acetone, inse nwater,
and dry.19. If a wavelengthmatch of 54004 or less wasattained
above, choosea higher index l iquid for thenext measurement. f a
wavelengthmatch of greaterthan 54004 wa sattainedabove,choosea
lower ndexliquid for the next measurement.20. Repeat steps l-18
with the new index l iquid;be sure the crystal is in the same
orientation as be-fore. (Wi th l iquid nD : 1.536at 25oC,6050A
wasdetermined o be the wavelength or which the indexof refractionof
the albite n Table 3, and liquid rD :1.536at 25"C, are equal. This
match wavelength splotted at point B on Figure 4.)21. Draw a line
through the two match points as nFigure4. This l ine s an accurate
epresentation f thechange in index of refraction of the crystal in
theorientation presentwith respect o a change n wave-length. The
point on the Y axis where the resultingline intersects he sodium D
line is the nD of thegrain, n the orientation ested point C on
Figure4).
DiscussionTable3 shows esults f digpersion easurementsmade on
varioussamples. ll measurementsereconducted t about 25oC. ndicesof
refractionofammonium hloride, odiumbromate, nd wo opti-ca l
glassesall Cargil lestandards)were measuredusing grain mounts and
the dispersionstainingmethodpreviously escribed.ccuracies f equal
oor better han t0.001 were obtained n all cases.These esultsare
comparablewith accuracies b-tained y Cherkasov1960) ndGrabar
1963).The indexof refraction or sodiumD lightof theepsilonayand
D-omega or low quartz, D-betaofafbite,nD-beta f microcline, D-beta
f labradorite,and rD-beta of
high-calciumabradoriteweremea-sureddirectly rom petrographichin
sections.neach ase, specific rain n a specific rientation
aschosenor measurement.he ndexof refractionorsodium D light of the
omega ay and nD-epsilonvalues f 1.5449 nd L5539measuredor low
quartzcompare avorablywith the valuesgivenby Bloss(1967). he ndexof
refractionor sodiumD lightofthe beta ay or albitewasmeasuredn thin
sectionobe 1.5355. grainmountof thesame ample aveannD-beta f
1.5352. hese aluesall within a range fvaluesKerr, 1959)or the
albite ield.The ndices frefractionor sodiumD lightof thebeta ays or
low-calcium labradoriteand high-calciumabradoriteweremeasuredn thin
sectiono be1.5594 nd 1.5679respectively.rain mountsof the same
amplesavenD-betasof 1.5592and 1.5677 espectively. err(1959)placed
he labradorite ield at nD-beta:1.558-1.569.he indexof refract ionor
sodiumDlightof the beta ay of microcline asdeterminedobe 1.5226 rom
a thin sectionmount. Bloss 1967)placed he microcline ield at
nD-beta 1.522 ot .526 .Compositions related o nD-beta or the
plagio-clase olid solutionseriesChayes, 952).Usinghi sgraph for
nD-betaus. plagioclaseomposition,healbite, abradorite, nd
high-calciumabradoritenTable 3 are estimatedo have compositions f
8percent northite, 3 percent northite, nd 68 per-centanorthite
espectively.In cases iscussedbove,
iquid-grainboundarieswerecreateddirectlyon thin section
lides.Colorfringes were produced.To interpret these
colorfringessome f them aint)producedn thin
section,thedispersiontainingmethod spreferred ve
rotherstandardmethods ecause f the accuracy
ttained(+0.001),qualityof colorsproduceda color fringewhichappears
ery aint n regular ight s generally
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LASKOIYSKI ET AL.: REFRACT,IVE NDEX MEASUREMENT 445
Liquld l* 20.1.336ot 25oC
Liquid l* |ll D:t 532 ot
A i n A "Fig' 4' Determination f nD of analbite.Liquids and
2areplot ed, us.wavelength,ccordingo valuesor zF, rD, andnC given
oreach iquid' Line ACB representshe changen indexof refraction f
thealbitewith respecto a changen wavelength. he crystal soriented o
measurehe beta ndex.point c is the nD-beta 1.5355) f the
albitecrystal.quite clear when the dispersion taining ens s
em_ployed),and ease f interpretation f thecolorspro_duced.
ReferencesBloss, F. D. (1967) An Introduction to the Methods of
OpticatCrystallography.Holt, Rinehart, and Winston, New
york.Brown,K. M., W. C. McCrone,R. Kuhn an d L. Forlini
1963)Dispersion tainingpart II-The systematic pplication o
theidentification of transparent substances.The
MicroscopeandCrystal Front, 14, 39-54.Chayes, . (1952)Relations
etween ompositions nd ndices frefraction n natural plagioclase. m.
J. Sci.,BowenVolume.g5_
r05.Cherkasov, u. A. (1960)Applicationof focalscreeningo
mea-surement f indices f refraction y the
mmersionmethod.1zr.Geol.Reu.,2.218-235.Grabar,D. G., and A. H.
Principe 1963) dentification f glassfragments y measurementf
refractivendexanddispersion."/ .ForensicSci.,8, 54-67.Kerr, P. F.
(1959)OpticalMineralogy.McGraw-Hill, New york.Laskowski,D. E.
(1965)Chemicalmicroscopy f urinarycalculi.Anal Chem., 7,
1399-1404.Manuscript receiued,March 27, 1978;acceptedor
publication,May 22, 1978.
