Microscope Setup for Uniaxial Interference Figures in Mineralogy
School
Stony Brook University**We aren't endorsed by this school
Course
GEO 306
Subject
Geology
Date
Dec 10, 2024
Pages
6
Uploaded by SuperResolve14279
Name: Mineralogy Lab Exercise X] — Uniaxial Interference Figures | wase start with part A so we know that everyone’s microscope is functioning and well-aligned. We have only limited number of slides for parts B-E; most should start on part F, after completing part A. Interference figure of phlogopite: Learn how to configure your microscope for conoscopic ob Phlogopite gives a biaxial interference figure, but it is casy to set-up and will quickly show w servation. is functioning properly. hether your microscope 1) Place a either a small flake of phlogopite on a glass slide or a larger flake directly on the microscope stage and focus, starting at low power, then working up to the highest-power objective. 2) Configure for conoscopic observation: analyzer in (x-polars), condensing lens in (located in substage assembly), bertrand lens in (located just below the ocular a.k.a. eyepiece). - You should observe a black cross that breaks apart and re-forms as the stage is rotated. If not, get help. ' - Small adjustments that can help: condensing lens should be as close to the sample as possible without touching the slide / flake; focus the bertrand lens (some models). - if your optical path is far off-center please inform your TA. Please do not try to center the optical path. 3) Recall that the center of the interference figure corresponds to the view in orthoscopic mode (with x-polars) a) rotate the stage until the interference figure closes up into a cross and note the color observed in the center of the field of view: Black ( (uith Y\K/MC.'.)"_'C ) b) keeping the stage in the same orientation (do not rotate), remove the condensing and bertrand lenses (orthoscopic mode) and report here the appearance of the flake: "PJ,( 1/ rf( ( ‘ Ctery . o | 0 afl a ‘ . how does it compare to part a)? T\/\p__x WA JADLDN 4w POIRL . . 5 i k Lo o . . ‘(:‘v." LN ehCG -2 ¢) re-configure for conoscopic observation, then rotate the stage 45°, umll_the isogyres are farthest apart anc’j’ report here the interference color observed in the center of the field of view: Vfljlfl"\ e - \/ L) b (G x’.‘\‘\ A d) keeping the stage in the same orientation (do not rotate), remove the condensin i 2 g and bertr; (orthoscopic mode) and report here the appearance of the flake (i.e. interferen and lenses ce color): \fi Ojf)u) / how does it compare to part c)? B()y—h - P UL o LOIMAR N~ Scanned with CamScanner
, k S . i ite; slide B-1) nimum retardation of olivine in a (hm.sc.cllon (dunite nd focus at low power. of the provided thin sections containing olivine on the stage, a ¢ agrain that shows the est retard -Ley artas a glli C5 i i 54 h ¢ d 5 . i i i llSI'] 4 'Our Mlcl]el e ' I s th l"gh t ret ation (hlgheS[ order color S), gy € C ) ge of lIltCl‘elellCe COIOIS, ‘O”()“"l alon on tlle ; ight in the chart): chart, so that you can name the highest interference color (furthest to the right in th ) highest interference color (e.g. 1° red): : ' % retardation (nm): IC ' 4) Assuming that the thin section has a thickness 1 = 45 um, determine the birefringence. This will be the maximum birefringence of the mineral birefringence: 2" 01O 5) The maximum birefringence corresponds to & for olivine, w hich varies linearly with composition. Use the result from part 4) and the chart in Nesse, p. 249 (242 in 31 edition) to estimate the olivine composition: 4’ a ', olivine composition (e.g., Fo30): Vfi D?/» C) Optical character of garnet: These thin sections contain garnets, 1) In the thin section, locate a garnet crystal (no cleavage) and the description in Nesse: crystal system for the garnet group: [ )02V (."‘7 (- 2) Examine the garnet crystal under crossed polars, as the stage is rotated Cubic Bl Ctupl als Gho yoAc) bt (,»/,.,—— ‘O.J'~"V— v VUZLLD LhCE and describe your observation here: Le s whi Gh CO e oo Ud 3) Examfil)e(gh’er garnet ¢ ; ) ; rystals, is there a range of birefringence or is it the same for all crystals? INQO ¢ ‘QJ I_Q_.:”) ”‘.r'f,j'U\'/‘()‘ )C;t/,/&”'/J f:“'.» y / /,v»;_{fiL (_‘] , ’JA" Qfi-(.” e /N L1 " ( i Tzl 4) Ex 1ai?md bOr\z,'w'%( ho fae ihde K stagc) E::r::ri:e fl:\‘?tlon nterms of the indicatrix for this crystal system (i.e., what type of section lies parallel to the o : I » as the stage is rotated and describe your observation here; A A J‘m';,f'((:‘ AL A0 v ¢ . ~ S CC ‘ , 7 N0 A O Yty /1 i - e Cof 7 rgnd ORY.0 2. G0 g Ehde s ; = A4 1A X e CP 210040 A s JI LKk ; ] 74 / Wi ~1 cQ ) 04 ((‘}’LQ_ OUIJ’).QC(J( (/O = o §— | 9 2 G—m - _ 5) Examine 1hecry5la, in ol LN DIt - O L O econ ; ane polarized light and describe the reliel (high vs, jovw : compared with g i 1ght.anc.ces A 2 1OW, positive vs. nepativ i ) cent mine ) ! af tHe:eraing drs TP S. negative). Relief is best With 1 ~ 1.54: J mineral grains or holes. Note that the grains are mmersed in (g holes filled with) epoxy Gwo) i lw‘g,‘- a e : ()T[/? | (1}[ (hey~ ) — N Hro~As Scanned with CamScanner o |
T — e ——ee— D) Centered interference figures from thin sections S . N Y H : A 1) Nearly monomineralic (uniaxial) rock section. Searcl for a grain that st crossed polars (watch out for holes!). a) Obtain an interference figure, a ays nearly extinct as the stage is rotated with nd sketch and label the result below (melatope, isogyres, isochromes (if present)) : : v X A - 7\ '&.‘,f"///, )22 b) Determine the optic sign: Insert the gypsum plate and determine whether addition or subtraction occurs in each quadrant (see Nesse p. 68). This mineral has low birefringence. If using a A-plate (A =550 nm) ~Addition: isochromes will appear to move in toward the melatope, 1° grey — 2° blue; Subtraction: isochromes move out: 1° grey — 1° vellow. Nw:_ CohbFhorbon NE: Jl\"\}ffl;{' ) ¢ o B sw.___ G O0IH it Optic sign of this mineral: PWL& T [ S e . » SE:_ UG O 2) Locate a grain that could give a nearly centered flash figure (Nesse Fig. 6.20). Such a grain will give the highest interference colors. The interference figure will consist of broad diffuse isogyres that nearly fill the field, then quickly disappear when the stage is rotated a by only few degrees. The isogyres depart in the direction of the optic axis. 1f ’ this is not what you observe (e.g., just a single isogyre that sweeps the field N-S then E-W; Nesse Fig. 6.19), examine other grains until you find one. a) In this orientation, which privileged directions of the crystal are in the light path for orthoscopic observation? c-Ww ditho oy $ay £ b) Why should this orientation give the highest interference coglors for its grain size? 7 ” n |3 1T = . . Bom hodp nk iAo/ fo utth fhe RoleGucro, | V)¢ ) q - q - 3 ¢ QL Ly 01 ~ KR 0 ohig oS 400 B pte LB L oY g e b PR : W AN (R e colondy, ¢) sketch the appearance of the interference figure at extinetion and afier rotating just a few de A LAY Jocation of the c-axis: o degrees CCW; inelude the ; | > (&n & & AN Scanned with CamScanner
e i o v vary i U R e, sxhibit E) Nearly monomineralic (uniaxial) rock section. This mlnclnfl has very h'gh birefringence. M”\I grains ‘~\h': 2 interference colors that are “off the chart”. Search for a grain that exhibits low-order interference colors (1° or 2°) s that gr i _ . e oy crossed polars, which should give a slightly off-centered figure. Try to orient the grain so that the field of view at higl power is uniform and free of a twin boundary. a) Obtain an interference figure, and sketch the result below, labeling the isochromes. b) Determine the optic sign: Insert the gypsum plate and determine whether addition or subtraction occurs in each quadrant (see Nesse p. 68). This mineral has high bireffingence. If using a A-plate (A =550 nm) Addition: isochromes will appear to move in toward the melatope; Subtraction: isochromes move out; The directions below denote quadrant with respect to the melatope: NW: ’,;*[.”f;_ ‘}1"""‘: > (Al S NE: 2 74 Q°-3 SW: Yo 16w CSulskhad SE: @, Is the optic sign of this mineral positive or negative? fonuzine F) Interference figures of an unknown: determination of optic sign and orientation 1) prepare a grain mount of your unknown in an oil having n.i 1 given by the TA, which lies between 7. and N Enter data given by your TA here: @A , / Unknown: _| —OAATE. Mol ) L 2) Locate a grain that will give you a centered optic axis interference figure — such a grain will have a circular section parallel to the stage. How do you recognize such a grain? } | n 200 \ : | z . ) \ 9.9) ( A 04721 A E A } (‘(7\;,‘,2.,-“* JONP | & a) Obtain an interference figure then sketch the figyre and labe| the melatope, isogyres, isoch i i e B o g . » 1I50gyres, romes and their colors (e.g., 1° yellow). [Ifthe figure is not well-centered (Nesse Fig. 6.15), examine other grains until you find one. Scanned with CamScanner SN
) determine the optic sign: Insert the gypsum plate and daseribe what h: he i Determine | ha ddition: whether addition or subtraction occurs in each quadrapt (see Nesse isochromes will appear to move in, toward the melatop . Qubtraction: isoch ol fringence is | (only grey isochromes), then addition: 1° grey — 2 5{1;;: subtraction: 1° gre 1° yellow. & . NW: SWAENO/E R \ ERROCUNAY SW: ] SE: (SO /)2 From this result, is the optic sign of your unknown positive or negative? ¢) return to orthoscopic observation, plane polarized light 4nd observe the Becke line and describe the results: Nol| >N d) Which of your unknown’s indices of refraction are you observing? e) Does the result of the Becke line test does agree with the optic sign determined in part b) (assuming that ni lies between 7, and 7;)? Possible situations are: optically *+: ng > noil > 1o ; OF optically = 1e <ol < Mo N 1 10 . 1 S J ) } center OA figure (see Nesse, Fig. 6.18). Such a 3) Obtain an interference figure for a grain that gives a slightly off- pe that lies in, or just beyond, the field of viewn grain will exhibit low-order interference colors, and give a melato a) Sketch the figure and label the melatope, isogyres, isochromes and their colors. , ) escribe how the int rence fig P o b) D ribe Qlf(.an Uure cha tate es s d lE 1 ] nges as y0u 1o ate the stage (words and/or Sketch) 8% 0 AV f . & ( : e Qur 1S 1| - — i e Scanned with CamScanner
¢) Rotate the stage so that a N-S isogyre is centered in the field of view, then lo k the stage (if possible) and ente the angular position: (read from scale on the side of the stage) ettt d) Change to orthoscopic configuration and uncross the polars. which crystal privileged direction (g’ or ) is oriented parallel to your pol (Nesse Figs. 6.15, 6.21 might help) arizer? ) _ r this result is consistent the result of part 2c) describe the results of a Becke line test and whetle Db L ¢ thelstage so that one of the E-W isogyres is centered in the field of rite down the angular position here: 9 ) X e) Return to conoscopic observation and rotat view. then lock the stage (if possible) and w b o f) Change to orthoscopic configuration and uncross the polars. 7 > 7 % - 5 i 5 x . { » Now, which crystal vibration direction (&, ', 0t ) is oriented parallel to your polarizer? i | ne test in this ofientation, and what this result indicates about the relative g) describe the results of a Becke li values 7oil , Mo » Ne (€.8. Noil » Mo < Moil < ng'): n BRSO ¢ mode and rotate the stage 45° so that the melatope lies in the NW quadrant. Then, mode, with x-polars, and describe what happens to the observed interference color when BT e TR 1° gray = ;0 l)![m. addition). Explain your result in terms of the orientation e s fast () and slow (N) ray du'cc.tlons relative to the slow ray direction (y) of the accessory P\z;le by drawing a schematic indicatrix section with major axes having correct relative lengths and labelled (g, £, ). h) Return to conoscopi return to orthoscopic Scanned with CamScanner