Optical Properties Using Crossed Polarizers Mode (CPL) PDF

Optical Properties Using Crossed- polarizers Mode (CPL)  Orders of Interference Colors “Intermediate” interference colors Interference colors one observes when polars...

Optical Properties Using Crossed- polarizers Mode (CPL)  Orders of Interference Colors “Intermediate” interference colors Interference colors one observes when polars are crossed (CPL). Color can be quantified numerically:  = nhigh – nlow. To describe interference colours we must specify both a hue and an order. To do this we use Interference Colour Chart. The orders of the chart are separated by a purplish red colour. The interference colour of the mineral can be estimated by counting the number of red bands at the wedge-shaped grain edges. The determination of interference colours may be difficult in “Low interference colors” highly birefringent minerals (such as carbonates: so-called high- order white). Wedge-shaped grain boundaries provide a means to observe the colour spectrum, in relation to decreasing crystal thickness, down to first-order black. The interference colours of coloured minerals may be masked significantly as they overlap with the minerals' own colour (examples: riebeckite, biotite). In strongly coloured minerals of high birefringence, the primary mineral colour will be dominant under crossed polarizers.  B C A B A D (A) Variation of interference colours in differently oriented forsterite grains; (B) to (D) Decreasing interference colours at wedge-shaped grain edges of forsterite grains in different crystallographic orientation. The crystal section D shows the highest interference colour (2nd order yellow-green) and is oriented orthogonal to c, i.e. parallel to (001) with principal vibration directions Z//a and X//b. C  Calcite grains in a thin section of marble appear in high-order Quartz grains sectioned parallel to c white. B. The wedge-shaped margin of a calcite grain shows a show the highest interference colour spectrum of decreasing interference colours from the interior (1st order creamy white); these are sections containing the principal plateau to the outer edge as the crystal thickness decreases vibration directions E = Z//c and O from 25 to 0 µm. Five colour orders can be recognized using the = X⊥c. red bands as a reference. What interference color is this? 3rd order interference colour Anomalous interference Colours The interference colours of certain minerals deviate from the normal colour scheme. Instead of the grey to white colours in the first order of the interference colour spectrum, leather brown, ink blue to grey-blue colours are observed. Such "anomalous" interference colours are generated by a strong dispersion of birefringence, which means that the latter attains distinctly different values for different wavelengths (colours). In the melilite example, the value of birefringence becomes zero for wavelengths in the range orange-yellow-green. Hence, these colours are not contributing to the interference colour. “Anomalous” interference “Anomalous” interference colors in Melilite colors in Chlorite Determination of Birefringence Determination of birefringence using the interference colour: Birefringence is an important property of anisotropic minerals and is crucial for mineral determination. However, the amount of birefringence of a specific mineral depends on the orientation of the section. In optically uniaxial minerals it varies from zero in the section perpendicular to the optic axis (direction of optical isotropy), to a maximum value (|ne-no|) in sections parallel to the optic axis. In optically biaxial minerals it varies from zero in sections perpendicular to one of the two optic axes, to a maximum value (nz-nx) in the section parallel to the optic plane. For this reason, the grains of each anisotropic mineral show different interference colours in thin section dependent on their crystallographic orientation. For routine mineral identification, only the maximum birefringence (∆n = nz-nx) values are critical, and these values are commonly listed in mineral-optical tables or compilations. Determination of birefringence (nz-nx) of  forsterite (section D) in a standard thin section of known thickness (25 µm) taking the maximum interference colour observed in thin section [(∆ - d colour chart after Michel-Lévy].  Procedure for Determining Birefringence Maximum  is a useful diagnostic value Gives grain the n maximum and n miminum of Easily determined in thin section with known thickness Find grain with highest interference colors – most likely to have fastest and Retardation  (nm) slowest n values Find retardation on the basis of the color (bottom of chart) Calculate the birefringence using equation:  =/d Orchartfind maximum birefringence from Thickness = 30 microns Use 30 micron line + color, follow radial line through intersection to margin & read Estimate the birefringence of birefringence this mica crystal Quartz and Microcline have low birefringence  Olivine has high relief (in PPL) Olivine has Moderate-High Birefringence (in CPL)  Pyroxene has Moderate Birefringence (in CPL) Pyroxene has Moderate Relief  Sphene has Extremely High Birefringence (in CPL) Calcite has Extremely High Birefringence (in CPL) Plagioclase has low (white to grey) birefringence Determine the birefringence of olivine mineral in this section by using the interference colour  Twinning Twins Microcline with characteristic are generated through crystal-structure- cross-hatched twinning controlled intergrowths of two or more individual crystal segments with a defined symmetrical relationship. Twinning can also result from deformation (as in calcite). The individual parts of a twinned mineral are commonly intergrown such that they either mirror each other's orientation (the symmetry plane being the twin plane), or they are rotated against each other by a specific angle (the rotation axis being the twin axis), or both. For many mineral species twinning is an important property for identification. There are different kinds Polysynthetic or lamellar twinning in plagioclase of twinning such as contact twins, penetration twins, simple twins, multiple twins, polysynthetic (or lamellar) twins. Inrecognized thin section, twinning is commonly easily under crossed polarizers if the mineral is anisotropic. The individual parts of twinned crystals show different brightness and interference colour, and on turning the microscope stage different extinction positions are revealed. Twinning in minerals A: Chloritoid, lamellar twins on (001). B: Mg-rich chlorite (clinochlore), lamellar twins on (001). C: Cummingtonite, thin twin lamellae on (100). D: Plagioclase laths with polysynthetic, lamellar twins. E-G: Polysynthetic twinning in plagioclase. H: Sanidine, Carlsbad twins. I: K-feldspar, Baveno growth twin. J,K: Cordierite, cyclic twinning on {110} (triplet). First- order red plate inserted in K for better birefringence contrast. L: Lamellar twins in cordierite. Twinning in minerals A: Staurolite, penetration twin on (320). B: Kyanite, simple twin on (100). C: Titanite, simple twin on (100). D: Simple and lamellar twins in chondrodite grains. E-G: Simple and lamellar twins on (100) in augite. H: Pigeonite twin on (100) inverted to orthopyroxene with augite exsolution lamellae on (001). I: Amphibole, simple twin on (100 Twinning in minerals A: Microcline with characteristic cross- hatched twinning (section approximately parallel to (001)). B: Microcline, twin set (section approximately parallel to (001)). C: Chessboard albite, formed by albitization of microcline. D-F: Leucite with complex lamellar twinning. J,K. Calcite with polysynthetic glide Twins on {0112}. L: Corundum with twin lamellae on{1011}.  Zoning In minerals with extensive solid solution, the optical properties vary with chemical composition, including birefringence. Chemical zoning in crystals can then be recognized from the conspicuous zoning of the interference colour. Particularly impressive examples are plagioclases and titanaugites with oscillatory zoning, as found in certain volcanic rocks. Polysynthetic or lamellar Zoned Plagioclase (XPL) twinning in plagioclase Zoned Amphibole (PPL)  Pyroxene Plagioclase Olivine Pyroxene