Optical Properties in Plane Polarized Light PDF

OPTICAL PROPERTIES IN PLANE POLARIZED LIGHT (PPL)  Twinkling Twinkling is the changes of relief as the stage is turned (only in minerals with large differences in nz' and nx’). The birefringence (∆n) of minerals is commonly not large enough to create distinct changing effects, with the except...

OPTICAL PROPERTIES IN PLANE POLARIZED LIGHT (PPL)  Twinkling Twinkling is the changes of relief as the stage is turned (only in minerals with large differences in nz' and nx’). The birefringence (∆n) of minerals is commonly not large enough to create distinct changing effects, with the exception of carbonates. The Fig. shows the striking change of relief in calcite and dolomite due to their extreme birefringence (∆n = 0.172 Cal resp. 0.177Dol). Change of relief in calcite and dolomite during a 360° rotation of the stage. Crystal sections are oriented parallel to the c axis. Shown are the four positions where the vibration directions of the two waves in the crystal coincide exactly with the polarizer directions. In these positions, only the E-W vibrating wave is passing through the crystal. The large difference between the refractive indices of the O- and E-waves causes the change in relief ( Δn = 0.172 for calcite; 0.177 for dolomite).  Colour and Pleochroism The intensity of white light can be variably decreased due to absorption as it passes through the thin section. If absorption wavelengths are outside the visible spectrum, glass phase and minerals appear colourless (white to greyish). Colours are generated if selective absorption affects certain parts of the Garnet spectrum more than others. Colour and colour intensity are important mineral-specific properties. In optically isotropic substances (glass phase, cubic minerals) light absorption is identical in all directions. Glasses and cubic crystals therefore show a single colour, no matter how the crystals are sectioned, as long as they are chemically homogeneous. The colour Leucite does not change if the stage is turned.  Colour and Pleochroism In optically anisotropic substances (non-cubic minerals), absorption is a directional property. Thus, crystals of a single mineral species can display different colours dependent on their orientation and corresponding light vibration directions. Colours and pleochroism are mineral-specific properties. The majority of minerals do not show any, or only weak, pleochroism. Exceptions displaying strong pleochroism are tourmaline, members of the amphibole group, Fe-Ti-rich biotites. Pleochroic minerals of tetragonal, hexagonal and trigonal symmetry show two specific absorption colours (dichroism), parallel to the vibration directions of the E- and O-waves. Sections orthogonal to the crystallographic c-axis (= optic axis) generally show the absorption colour of the O-wave as the microscope stage is rotated. Sections parallel to the c-axis show an alternation between the absorption colour of the E-wave (E-W orientation of the c-axis) and the O-wave (N-S orientation of the c-axis) for every 90° rotation of the stage and example tourmaline). Change of absorption colour of tourmaline during a 360° rotation of the microscope stage. Shown are the four positions of a crystal section in which the vibration directions of the two waves coincide exactly with the directions of the polarizers. In these orientations, only the E-W vibrating wave passes the crystal; the N-S wave is not activated.  A B C D E F Examples of distinct to strong pleochroism. A: Chloritoid. B: Biotite. C: Ti-rich biotite. D: Lazulite. E: Aegirine augite. F: Fe - rich epidote.  Colour and Pleochroism Pleochroic minerals of orthorhombic, monoclinic and triclinic symmetry possess three characteristic absorption colours (trichroism) related to vibration directions parallel to the principal indicatrix axes X, Y and Z. The determination of these colours must be performed on selected crystal sections that are parallel to two of these axes. As the example of the amphiboles demonstrates beautifully, there is a correlation between absorption colour and chemical composition. Colours of amphiboles of different composition, in relation to the principal vibration direction parallel b. The amphibole grains are oriented such that the b-axis is in alignment with the lower polarizer. Optical Indicatrix The three-dimensional propagation of light in both isotropic and anisotropic materials can be presented graphically as 3-D models for optical indicatrix, which describes the refractive Isotropic index as a direction- dependent variable. For each ray direction, the refractive index is represented by a specific distance from a chosen origin. The geometric form representing the refractive index for all ray propagation directions is a Anisotropic sphere in isotropic material Anisotropic Uniaxial and an ellipsoid in Biaxial anisotropic material.  Isotropic Indicatrix All minerals belonging to the cubic crystal system are isotropic with respect to their optical properties. In thin section microscopy, glasses and cubic minerals generally show a single specific refractive index and colour independent of orientation. The x, y, and z, crytallographic axes are symmetry related to each other in a cubic material. Therefore the R.I. parallel to x, y, and z must be equal. The only ellipsoid that fits these constraints is a sphere. All sections through the indicatrix are circles and the light is not split into two rays. Birefringence may be considered to be zero. Another important characteristic of optically isotropic materials is that light waves do not experience any change in vibration direction. This means that E-W vibrating plane-polarized light waves maintain their E-W orientation after passing through the isotropic materials (glass, mineral). Therefore, they are blocked by the analyzer, which is a N-S oriented polarizer. Indicatrices of Anisotropic Minerals Variable values of n within mineral Has property of double refraction Light entering material usually split into two rays Two rays vibrate perpendicular to each other Caveat: Special section of minerals: light not split and behaves like isotropic mineral – called optic axis. All other orientations: light splits into two rays. Optic Axis Special direction where rays not split into two rays Wave normal and ray path coincide Hexagonal and tetragonal have one optic axis (Uniaxial Minerals) Orthorhombic, monoclinic, and triclinic have two optic axes (Biaxial Minerals)  View parallel to Uniaxial Indicatrix View perpendicular to optic axis optic axis Minerals belonging to the group of hexagonal, trigonal and tetragonal symmetry are optically uniaxial. The R.I. in the plane perpendicular to the main symmetry axis (z) must be constant. R.I. parallel to z can be different. Result is an ellipsoid of revolution, with one isotropic section parallel to (001) and one optic axis parallel to z. Circular Section: ⊥ optic axis: all 's Principal Sections: have  and true : Positive max & min n's All sections have !! Negative Propagate light perpendicular to c-axis n n c=Z  n n a=X n - n > 0 n - n = 0 b=Y Grain therefore,   0: changes therefore,  =0: color upon grain stays black rotation. (same as the n Grain will isotropic case)  go black n n whenever Propagate light along indicatrix the c-axis, note what n axis is E-W happens to it in plane or N-S of thin section  For light travelling ⊥ c get elliptical principal section of indicatrix: get 2 rays O-ray with n =  E-ray with n =  this  (parallel c) is the maximum possible deviation in n from  (true ) Random Sections: same situation as above, except that E-ray has some n between  and  All intermediate values are called ’ (a variable value between  and ); (' and )  Z= Optic axis = C- axis Uniaxial Indicatrix Z= Optic axis = C- axis n < n Elongated along the X optic axis Flattened n > n along the Uniaxial Positive Indicatrix optic axis Uniaxial Negative Indicatrix  Tangerine = Uniaxial (-); n e.g. Calcite n n n n n n Uniaxial n Spaghetti squash = Uniaxial (+); e.g., Quartz Indicatrix  Summary oCircular Section o(⊥ optic axis: all 's) o extinct oPrincipal Sections o(have  and true ε : max & min n's) largest birefringence! o Random Sections (ε' and ) oalways have !! oAny cut through center of a uniaxial indicatrix will have  as one semiaxis and ' (or true ) as the other  Biaxial Indicatrix  long The indicatrix of crystals with orthorhombic, monoclinic and triclinic symmetry is represented in a coordinate system with axes X, Y, Z that are orthogonal to each other. The geometry of the triaxial ellipsoid is defined by the proportional lengths of the refractive indices n, n on X, Y and Z. Thus, the symmetry of the α Short ellipsoid is generally orthorhombic. The refractive indices are defined as nz > ny > nx (or n  > nβ > nα). There are two circular sections with the radius n. y β Medium Orthogonal to these circular sections, light propagates with the same velocity as in an isotropic substance. These two directions correspond to the two optic axes. Biaxial Indicatrix orthogonal The optic axes lie in the ZX plane (= optic axial plane OAP). Y is to the OAP. The angle between the optic axes (2V) is mineral-specific and can attain values between 0° and 90°. biaxial-positive; If Z dissects the acute angle (2Vz < 90°), the crystal is if X dissects the acute angle (2Vx < 90°), the crystal is biaxial-negative. If the axial angle is 90°, the crystal is optically neutral. crystal's The spatial orientation of the indicatrix in a crystal is defined by the symmetry: correspond In crystals of orthorhombic symmetry, the ellipsoid axes (X, Y, Z) to the crystallographic axes (a,b,c). Which indicatrix axis is parallel to which crystallographic axis depends on the specific mineral. In crystals of monoclinic symmetry, it is only the crystallographic b axis and one of the indicatrix axes that are parallel (commonly Y). The other two axes lie in the symmetry plane (010) and form angles with the crystallographic axes a and c. In triclinic crystals, none of the axes of the orthorhombic indicatrix is parallel to any of the crystallographic axes. The indicatrix axes form mineral-specific angles with the crystallographic axes.  Biaxial Indicatrix Positive Negative Thank You