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# GEOL 585 GEOL 585

WVU

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This 75 page Class Notes was uploaded by Jessica Braun MD on Saturday September 12, 2015. The Class Notes belongs to GEOL 585 at West Virginia University taught by Helen Lang in Fall. Since its upload, it has received 16 views. For similar materials see /class/202708/geol-585-west-virginia-university in Geology at West Virginia University.

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Date Created: 09/12/15

Dr Helen Lang Del t of Geoloh amp Geovral hr West Virginia University SPRING 2009 GEOLOGY 585 OPTICAL MINERALOGY amp PETROLOGY A Mineral must be crystalline Crystalline means that it has an ordeer and re etitive atomic structure The external Shape of minerals re ects their internal structure Crystal Shapes are best descrlbed 1n terms of Symmetry Symmetry is the repetitive arrangement of features faces comers and edges of a crystal around imaginary lines points or planes Re ects internal ordering of atoms in the m1nera1 structure All shapes and properties must conform to the m1neral s symmetry Crystal Shape growth forms of 1nc11V1c1ua1 grams Special named shapes cube Pyrite dodecahedron octahedron Crystal Shape a 55 V p General Shapes v a prismatic tabular bladed platy blocky 6 v Cleavage forms Must contorm to m1nera1 symmetry 0 Shiny smooth planar 0 By quality breakages perfect Between weakly good bonded planes in fair mineral structure By Shape or number Sometimes hard to cubic tell from growth rhombohedral faces which commonly octahedral have imperfections not prismatic as smooth Cleavage Examples Fluorite perfect octahedral cleavage 4 directions 8 sides Note difference from Fluorite growth faces cubes Rotational Symmetry in Minerals Name Shorthand Angle Symbol lIola 2fold 3fold 4fold 6fold l 2 3 4 6 3600 1800 1200 600 A 900 l 0 Only these ve are possible Types of symmetry possible in Minerals 1 2 3 4 6 proper rotations mirror planes center of symmetry rotoinyersion rotoinyersion Demonstration of and 3 These can be combined in 32 ways to make crystal shapes Minerals are Grouped into SiX Crystal Systems based on Symmetry System Characteristic Symmetr Isometric Cubic System four 3 or S Hexagonal System one 6 6 3 or 39l etragonal System one 4 or 4 Orthorhombic System three 2 andor m Monoclinic System one 2 andor m Triclinic System i w r Stratevies for identify my 539 mmetr39 Grasp crystal or block with thumb and nger on opposite corners edges or face centers turn block 60 90 120 or 180 check to see if it looks the same Check for other s mmetr39 axes and mirror planes perpendicular to the ads you nd All symmetry elements intersect at center Use at hand or card to check for mirror planes Use knowledge of systems I ll explain A Form is A set of similarly shaped faces That are related tu eaehother by the symmetry of the crystal Forms can be open or Closed Isometric System Four 3 or 3 comercomer of reference cube All isometric shapes also have three perpendicular 4 71 or 2 axes These are the crystallographic axes a1 a2 a3 all equal length All isometric forms are equidimehsiorial Highest symmetry system The Cube and Octahedron are simple common Isometric Forms octahedron More Isometric Forms Isometric Minerals Fluorite CaFZ To What mineral group does uorite belong Name Cleavage Form Isometric Minerals Garnet CaFeMgMn3A128i3O12 1139 y All isometric minerals are isotropic which means Name of Garnet Growth F orm Isometric Minerals Formula Mineral Group Names of Common Non isometric Forms Open Forms Pedion single face Pinacoid 2 parallel faces Prisms 3 4 6 8 or 12 faces all parallel to a common line Pyramids 3 4 6 8 or 12 faces that intersect at a point Use pre x to indicate System or symmetry Some Open Forms tetragonal hexagonal ditrigonal prism 3 pyramid prism fold symmetry What mineral Names of Common Non 1sometr1c borms Closed Forms dipyramid disphenoid rhombohedron scalenohedron two 3 4 6 8 or 12 sided pyramids top and bottom related by a horizontal mirror plane 4 nonequilateral triangular faces 6 rhombshaped faces 8 or 12 scalene triangle shaped faces Nonisometric Closed Forms Hexagonal Hexagonal Tetragonal dipyramid disphenoid scalenohedron Rhombohedra Q 6 faces related by a 3 bar axis common Iorrn tor carbonates Point Groups There are 32 possible combinations of the allowed sd mmetrd elements in minerals They re oalled Point Groups or Crystal Classes more later Can be grouped into 6 Crystal Systems Grouped in SiX Crystal Systems System Characteristic Symmetr Isometric Cubic System four 3 or S Hexagonal System one 6 6 3 or Tetragonal System one 4 or 4 Orthorhombic System three 2 andor m Monoclinic System one 2 andor m Triclinic System 1 or l Crystallographic Axes Reference axes Conventional ways to hold and refer to faces on crystals Different convention for each system Crystallographic Axes Isometric System Three perpendicular axes Coincide with three 4fold 0r 2fold axes All equal length Called a1 a2 a3 Crystallographic Axes Tetragonal System c Three perpendicular axes Vertical axis c coincides with 4 or 4bar axis One axis c is longer or shorter than other two a1 and a2 which are equal Tetragonal Examples Crystallographic Axes Orthorhombic System c Three perpendicular axes coincide with 2fold axes or are perpendicular to mirror planes o All different lengths called a b e 51 Crystallographic Axes Monoclinic System c All axes different lengths Called a b c b aXis coincides with 2 fOld axis 01 mirror plane B gt 90 f c is parallel to long edges a slants down to the front a alb bl c anvle between a and cgt90O Crystallographic Axes Triclinic System e N0 perpendicular axes All different lengths Q 7 b Called a b e Crystallographic Axes Hexagonal System Four axes c Vert1cal ax1s c 1s longer or shorter and coincides with 6fold or 3fold axis Three horizontal axes coincide with 2fold axes arelto c and 1200 to eachother Three horizontal axes are equal lengths a1 a2 a3 Hexagonal Examples g m V SiX Crystal Systems System Axial Relationships Isometric Cubic System Hexagonal System Tetragonal System Orthorhombie System Monoelinie System Trielinie System Dr Helen Lang Del t of Geoloh amp Geovral hr West Virginia University SPRING 2009 GEOLOGY 585 OPTICAL MINERALOGY amp PETROLOGY Symmetry and Mineral Optics Optical properties obey and re ect the symmetry of the crystal structure Minerals are Grouped into SiX Crystal Systems based on Symmetry System Characteristic Symmetr Isometric Cubic System four 3 or S Hexagonal System one 6 6 3 or 39l etragonal System one 4 or 4 Orthorhombic System three 2 andor m Monoclinic System one 2 andor m Triclinic System i w r Crystallographic Axes Reference axes Conventional ways to hold and refer to faces on crystals Different convention for each system Crystallographic Axes Isometric System Three perpendicular axes Coincide with three 4fold or 2fold axes All equal length Called a1 a2 a3 Garnet halite pyrite and uorite are isometric Crystallographic Axes Tetragonal System Must have one 4 or 4bar axis Three perpendicular axes Vertical axis c coincides with 4 or 4bar axis One axis c is longer or shorter than other two a1 and a2 which are equal Crystallographic Axes Orthorhombic System Has three 2fold axes andor one rnirror plane Three perpendicular axes coincide with 2fold axes or are perpendicular to rnirror planes a All different lengths called a b c Crystallographic Axes Monoclinic System Has one 2fold axis or mirror All axes different lengths Called a b c b axis coincides with 2fold axis or is mirror plane c is parallel w long edges a slants down to the front a b b c angle between a and cgt90O gt90 c 0 Crystallographic Axes Triclinic System e N0 perpendicular axes All different lengths K b Called a b e W 32 oint 39rou s or cr stal classes HermannMauguin HM Symbols are a simple numeric way to represent the symmetry of each class Threeplace symbols Different content for each system isometric hexagonal tetragonal orthorhombic monoclinic triclinic SYSTEM SYMMETRY four 3 or one 4 or RELATIONS HIPS a1aza3 0c wF90 a1aza3 c ac900 a1az1200 azxa31200 a1az c FE y90 a b c cgtbgta FE y90 a b c a b c gt lt SYMBOL CLASSES CLASSIFICATION lisymmetry a axes 273 or 3 diagonal 13m 432 34ther edge to edge c 27symmetry of a axes 34ther between a axes 6mm 622 E 6 3 c 27s39 mmetr39 of a axes 34ther between a axes 4mm 422 4 lisymmetry a 27symmetry of b axes 34ther c axes y mmetrv 222 mm2 171 or The Petrographic Microscope Planepolarized light from below polarizer EW or N e is darkest parallel to polarizer Removable analyzer above sample Rotatable stage Various microscopes New Leica Microscopes The Petrographic polarizing Microscope Analyzer NS Objectives Rotating Stage Polarizer EW perpendicular to Analyzer s 5 z L p 5 V5 zaw g Q 2 235 25383 quot quotC X 7 3939T7Jjr 7 l A Lv Polarized light travels ght through isotropic minerals and travels at the same speed n all directions In anisotropic minerals polarized light is split into two rays vibrating at 90 to each other in two special allowed vibration directions the two rays travel at different speeds Light passing thru an anisotropic mineral If recombined wave is per pendicular to Analyzer no light passes mineral is dark If recombined wave ANALYZER is parallel to the Analyzer all light passes mineral appears brightest fast ray long 7 r slow ray L 7 k short 7 2 Lagging of the slow ray behind the fast ray is called Retardation When the two rays recombine at the Anal zer they interfere constructiveld or destructively with each other and there is Venerallv a com onent of livht A arallel to the Analyzer Different cow of light experience different amounts of Retardation Retardation and Interference uartz Wedve between Crossed Polaroid Films in Monochromatic NaD 9t590nm Light Note constructive and destructive interference llnterferenee Colors llquothilar m m m Constructive bright and Destructive Interference black for different colors sums to the interference colors at the bottom for white light Phillips 1971 IntCI fCI CI lCC pu1u15 ucpcuu U11 The amount of retardation caused by the mineral in a certain direction How anisotropic is the mineral 8 maXimum difference in refractive indeX And the thickness of the mineral typically a 30 nm thin section for I106 Colors bire ingence Olivine M uscovite 003 E E V1 V1 0 G 4 O E H retardation Properties viewed in Plane polarized light PPL Relief relative Becke lines bright line moves toward medium with hivher refractive index distance increased Color Pleochroism Grain shape Cleavages Alteration Others Properties in Crosspolarized light XPL Birefringence None zero gt isotropic Interference colors gt anisotropic How high low medium high extremely high Ithnctlon parallel extinction extinction angle Others C KR 1quot 39r39 quotPiix 39 lt quotvim xi Cg l i39 ix Ii i i 4 cK J j yii V A itquot 213 z A f 3 lt15 Isotropic Same properties in all directions Light travels at the same speed in all directions Isometric symmetry Uniaxial One unique axis one direction in which the mineral appears isotropic optic axis c axis Tetragonal or Hexagonal symmetry Biaxial Three principal refractive indices two directions in which the mineral appears isotropic optic axes angle between them is 2V Orthorhombic Monoclinic or Triclinic symmetry Uniaxial and Biaxial Optics David Hirsch Uniaxial Indicatrix Movie David Hirsch Biaxial Indicatrix Movie A really good Optical Mineralogy course Dr Greg Finn Brock University Canada The optic axis direction along which mineral appears isotropic is always parallel to c 0 the ordinary ray vibrates perpendicular to the c axis a the extraordinary ray vibrates parallel to the c axis optic axis c optic aXIs c V Ema bgtltenotime uniaxial Positive Negative 8 gt 0 a lt 0 Vs g V0 V8 2 V0 0 is fast ray 0 is slow ray Quartz Beryl Leucite Zircon Rutile Tourmaline Apatite Calcite Dolo Corundum Z optic axis c axis Circular Saciiun O Radius n Principie Secliuns Radii n and nquot Unlaxlal Paslllve lndlcalrlx Elongated along the optic axis X s pllc axl 0 c s c aXis Z Indicatrlx axis Greg FinnBrock U X optic axis c axis wk 3 714quot Uniaxial Negative Indicatrix Flattened along the optic axis caxis op in axis c axis X indioetrix axis Obtaining an interference figure Conoscopic Light microscope setup Uniaxial Minerals Centered Optic Axis figure Looking down the caxis Biaxial Minerals Several or tions Bxa 8x0 or O tic Axis CA suggest Optic Axis figure easiest to find likely grains Uniaxial Figure amp Optic Sign if a gt co 00 is the fast ray and the optic sivn is negative if 00 gt a a is the slow ray and the optic sign is positive Uniaxial Mineral Conoscopic Ligh Ccnvergsm tht passes through We m nera Mm the res ys a wbrsts a unu radxa nes 1mm me alape an mums langannany m m c rcu ar iscchramss Where the vlbrallun alvecuans DI the and x rays are pmum m m pmanzahon dwemmns s interference gure appears Mack and farms kha sugyr s Greg FinnBrock U 3 0 e alt mxii I m Su 8 we 23938 Qiw Positive Negative 8 gt m NW SE m gt 8 NW SE fast on slow slow on slow gt VwgtVE subtraction V5 V0 addition mfast yellow wSIOW blu X vibration direction of fastest ray or refractive index of light vibrating in X direction or lowest refractive index Y vibration direction Of light travcm lg dlUl lg UH 3 refractive index of light vibrating J to Optic Axis 3 intermediate refractive index Z vibration direction of slowest ray y refractive index of light vibrating in Z direction y highest refractive index on lt 3 lt y always birefringence 5 y or always 39 39 The Optic Axes GA The Optlc Axes are are perpendicular to z sym at the m rieal about 0A 0A th Z indicatrix axis IndlcatIIX and lie within the xz a ne the indicatrix t the Optic Angle or 2 angie The two iwith radius quot5 Intersect aiong the Y indicatrix axis Greg FinnBrock U Biaxial Relationships 13 u Biaxial Positive Biaxial Neiative 3 closer to a 3 closer to quot1 Z is Bxa X is Bxa Greg FinnBrock U T 1 i i i Conoscopic light Several possibilities Bxa on and 0A Optic Axis figure easiest to use OA easiest to ick likeI vrains looking down Optic Axis grain appears isotropic pick lowest birefringence grain you can find rotate stage you want one that stays blackgray Melatope stays in center isogyre spins around Biaxial Optic Axis Figure Biaxial Optic Axis Figure centered at 45 0from extinction I Posmve Negative Gypsum piam Gypsum piam sznnm pizm 530nm piste Fuiirwave piam Fuiirwave piaie abo Mago Qmmmu SYSTEM SYMMETRY four 3 or one 4 or RELATIONS HIPS a1aza3 0c wF90 a1aza3 c ac900 a1az1200 azxa31200 a1az c FE y90 a b c cgtbgta FE y90 a b c a b c gt lt SYMBOL CLASSES CLASSIFICATION lisymmetry a axes 273 or 3 diagonal 13m 432 34ther edge to edge c 27symmetry of a axes 34ther between a axes 6mm 622 E 6 3 c 27s39 mmetr39 of a axes 34ther between a axes 4mm 422 4 lisymmetry a 27symmetry of b axes 34ther c axes y mmetrv 222 mm2 171 or Uniaxial sign and interference figure Uniaxial Interference Figure centered m mxg 8 3 m 8 8 m a F w 8 x 8 8X x 03 Positive Negative 8gtm NWSE mgtg NW SE fast on slow slow on slow VwgtVE subtraction V5gtV addition 03 fast yellow 13 SIOW blue

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