6. summary optics
TRANSCRIPT
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Summary of optical properties SARAH LAMBART
Appolo12: Credits: Open University Virtual Microscope Project
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ISOTROPIC, UNIAXIAL OR BIAXIAL?
u Mineral is:
u Isotropic if all grains are extinct under crossed polars during 360o rotation.
u Uniaxial if it gives a uniaxial interference figure (crossed or straight lines with Bertrand lens).
u Biaxial if it gives a biaxial interference figure (curves with Bertrand lens).
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BIREFRINGENCE u Estimation of Birefringence - in thin section with a given
thickness of minerals (0.03 mm), birefringence is estimated using interference color chart.
u Note that only absolute birefringence is diagnostic:
u |ω - ε| for uniaxial minerals
u (γ - α) for biaxial minerals
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INTERFERENCE FIGURES u Uniaxial mineral: cross or line
u Biaxial mineral: one or 2 curves
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INTERFERENCE FIGURES u Section parallel to c: Flash figure
Uniaxial Biaxial
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OPTIC SIGN
u from uniaxial centered or near centered interference figure for uniaxial minerals. u Positive if blue in NW-SE quadrants
u from biaxial BXA or OA centered or near centered interference figure.
u Melatopes in NW-SE quadrants: positive if blue inside the isogyres
u for biaxial minerals only, estimated with BXA or centered OA figure
2V
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RELIEF
u From comparison with surrounding minerals or cement in which the crystals are mounted (Becke line method), or with oil in immersion method.
u Constant for isotropic minerals or for circular section of the optical indicatrix:
u Uniaxial mineral: basal section – Φ = ω
u Biaxial mineral: section perpendicalar to OA - Φ = β
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PLEOCHROISM u Absorption color - if present, may be observable in isotropic, uniaxial,
and biaxial minerals with analyzer not inserted. (i.e., color seen in transmitted light)
u Uniaxial minerals may have pleochroic formula: ω = color1, ε = color2. If optic axis is perpendicular to the stage i.e. we are looking at the basal section), only one color will be observed.
u Biaxial minerals may have pleochroic formula α= color1, β = color2, γ = color3, but only 2 colors will be observed in any one grain, unless the optic axis is perpendicular to stage - then only one color.
In transmitted light!!!!!!!
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SIGN OF ELONGATION u Uniaxial minerals: when elongated (long direction parallel
to the C axis):
u 1) Turn the mineral on the stage such as the direction of elongation is NE-SW
u 2) insert the gypsum plate: + if the interference color increases
u Biaxial minerals: u 1) Position the long direction parallel to N-S
u 2) Turn the stage clockwise until the mineral go extinct
u 3) turn the stage clockwise by 45°
u 4) insert the gypsum plate: + if the interference color increases ⇔ the long direction is parallel to γ
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EXTINCTION ANGLE u Uniaxial minerals:
u Parallel extinction on prismatic section (parallel to c-axis)
u Symmetrical extinction for all other faces if the cleavage intersect)
u Biaxial minerals: u Orthorhombic crystal: same as uniaxial minerals: parallel and
symmetrical extinction
u Monoclinic and triclinic crystals: inclined and asymmetrical extinction
u Extinction angle: maximal acute angle
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OTHER OPTICS PROPERTIES u Zoning
u Twinning
u Exsolution lamellae
u Undulatory extinction
Thin section of gabbro with twinned plagioclases and exsolution lamellae of two pyroxenes.
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NEXT WEEK
u Reflected light optics – Chapter 8