Optical Mineralogy Flashcards
A geologist analyzes a thin section of basalt and observes that several phenocrysts appear dark brown, nearly opaque, even under high magnification. What microscope adjustment should they use to better see internal details of these crystals?
A. Increase the condenser aperture
B. Decrease the condenser aperture
C. Switch to a lower-power objective
D. Rotate the polarizer 90 degrees
ANSWER: B
EXPLANATION: Decreasing the condenser aperture narrows the cone of light illuminating the sample. This reduces the amount of light absorbed by the mineral, potentially allowing more light to transmit through it and reveal internal features.
You are examining a mineral grain in a thin section. When you rotate the microscope stage, the grain displays dramatic changes in color. This phenomenon is best explained by:
A. Isotropism
B. Anisotropism
C. Pleochroism
D. Birefringence
ANSWER: C
EXPLANATION: Pleochroism is the ability of a mineral to absorb different wavelengths of light in different crystallographic directions, leading to color changes when the mineral is rotated.
After focusing on a mineral in a thin section under plane-polarized light (PPL), you switch to crossed-polarized light (XPL). The mineral completely disappears from view. This most likely indicates that the mineral is:
A. Highly birefringent
B. Isotropic
C. Opaque
D. Possessing perfect cleavage
ANSWER: B
EXPLANATION: Isotropic minerals do not interact with polarized light and therefore remain dark under XPL.
You observe a mineral with two distinct directions of cleavage intersecting at approximately 90 degrees. Under XPL, you notice that this mineral displays first-order grey interference colors. You can conclude that the mineral has:
A. Low birefringence
B. Moderate birefringence
C. High birefringence
D. Variable birefringence
ANSWER: A
EXPLANATION: Minerals with low birefringence display first-order gray, white, or yellow interference colors.
A mineral in a thin section displays extremely bright, high-order interference colors. To reduce the color intensity and make identification easier, you should:
A. Insert the Bertrand lens
B. Insert the accessory plate
C. Remove the upper polarizer
D. Slightly defocus the image
ANSWER: B
EXPLANATION: An accessory plate (like a gypsum or quartz wedge) introduces a set amount of retardation, subtracting from or adding to the mineral’s interference colors and helping to determine its properties.
The substage diaphragm on a petrographic microscope is primarily used to:
A. Adjust light intensity
B. Control relief
C. Improve resolution
D. Enhance birefringence
ANSWER: B
EXPLANATION: The substage diaphragm controls the angle of the light cone reaching the sample, which impacts the appearance of relief (how much features stand out from the background).
When using the Becke Line test to determine relative refractive indices, the bright line of light will move when raising the microscope stage.
A. Outward, into the higher index medium
B. Inward, into the higher index medium
C. Outward, into the lower index medium
D. Inward, into the lower index medium
ANSWER: A
EXPLANATION: The Becke line will move into the medium with the higher refractive index as the stage is raised.
You notice a mineral under XPL that shows “flash figures” — alternating bright and dark sweeping bands as you rotate the stage. This indicates the mineral is:
A. Uniaxial and displays a centered optic axis figure
B. Uniaxial and displays an off-center optic axis figure
C. Biaxial and displays a centered optic axis figure
D. Biaxial and displays an off-center optic axis figure
ANSWER: A
EXPLANATION: Flash figures are characteristic of uniaxial minerals showing a centered optic axis interference figure.
A mineral displays parallel extinction under XPL. This could mean that the mineral belongs to which crystal systems?
A. Tetragonal, hexagonal, or orthorhombic
B. Orthorhombic, monoclinic, or triclinic
C. Monoclinic, triclinic, or cubic
D. Isometric, trigonal, or hexagonal
ANSWER: A
EXPLANATION: Minerals in the tetragonal, hexagonal, and orthorhombic systems display parallel extinction.
The speed of light is slower in a medium with:
A. Low refractive index
B. High refractive index
C. Zero refractive index
D. Constant refractive index
ANSWER: B
EXPLANATION: The refractive index is a measure of how much light slows down in a material. Higher refractive index means slower speed of light.
To obtain the sharpest image of a mineral grain under high magnification, you should adjust the:
A. Fine focus knob
B. Coarse focus knob
C. Bertrand lens
D. Condenser height
ANSWER: A
EXPLANATION: The fine focus knob allows for precise adjustments to achieve maximum clarity, especially at high magnifications where even slight movements can blur the image.
- While examining a thin section, you notice internal reflections obscuring details on the surface of a mineral. The best way to reduce this effect is to:
A. Increase the refractive index of the mounting medium.
B. Change to reflected light microscopy.
C. Close the substage diaphragm slightly.
D. Lower the upper polarizer.
ANSWER: C
EXPLANATION: Closing the substage diaphragm narrows the cone of light illuminating the sample. This reduces the amount of light reflecting off internal surfaces and improves the visibility of details on the mineral’s surface.
A mineral grain displays undulatory extinction under XPL. This characteristic indicates that the mineral likely has:
A. Very low birefringence
B. Multiple twin planes
C. Deformation in the crystal structure
D. Strong pleochroism
ANSWER: C
EXPLANATION: Undulatory extinction is often associated with deformation features within the mineral crystal structure, causing variations in light extinction patterns as the stage is rotated
You want to determine the optic sign of a biaxial mineral. Which accessory plate would be the best choice?
A. Gypsum plate (1st order red)
B. Mica plate (1/4 wavelength)
C. Quartz wedge (variable retardation)
D. Full-wave plate
ANSWER: C
EXPLANATION: A quartz wedge introduces variable retardation, allowing you to observe interference color changes and determine the optic sign of a biaxial mineral.
- To enhance the visibility of a mineral grain with very low relief, you should:
A. Slightly defocus the image.
B. Immerse the sample in oil with a refractive index close to the mineral.
C. Lower the stage slightly.
D. Narrow the condenser aperture
ANSWER: B
EXPLANATION: Immersing the thin section in a liquid with a refractive index close to the mineral can help reduce the difference in relief, making it appear more prominent.
A uniaxial mineral displays a centered flash figure. You rotate the stage until the optic axis is oriented north-south. Which color will you likely see when inserting a gypsum accessory plate?
A. Addition of blue/yellow colors (higher order interference)
B. Subtraction of blue/yellow colors (lower order interference)
C. No change in interference color
D. Transition into second-order colors
ANSWER: A
EXPLANATION: When a gypsum plate is inserted with a uniaxial mineral’s optic axis oriented east-west, it introduces additional retardation, often causing higher-order interference colors with blue and yellow components.
An accessory plate is designed with a specific retardation value. This value will be most accurate when using light of which wavelength?
A. Red light (650 nm)
B. Yellow light (589 nm)
C. Green light (550 nm)
D. Blue light (450 nm)
ANSWER: B
EXPLANATION: The specified retardation value of an accessory plate is typically calibrated for a specific wavelength, often around the yellow light spectrum (around 589 nm). Using light closer to this wavelength will yield the most accurate results.
You are observing a mineral with anomalous interference colors. This might be due to:
A. High birefringence of the mineral.
B. Extreme thinness of the section.
C. Strong absorption by the mineral.
D. Misalignment of the polarizers.
ANSWER: C
EXPLANATION: Anomalous interference colors can arise due to strong absorption of certain wavelengths of light by the mineral, leading to deviations from the expected interference color sequence.
. When performing conoscopic observation, why is it essential to use a high-power objective lens?
A. To collect a wider cone of light from the mineral.
B. To achieve better image resolution.
C. To increase specimen-to-objective distance.
D. To prevent damage to the front lens element.
ANSWER: A
EXPLANATION: High-power objective lenses have shorter focal lengths, allowing for the collection of a wider cone of light from the mineral. This wider cone of light is crucial for observing the interference figure during conoscopic observation
The speed of light in a mineral depends on:
A. Only the refractive index of the mineral.
B. Only the orientation with respect to crystal axes.
C. Both the refractive index and crystal orientation.
D. Neither the refractive index nor crystal orientation.
ANSWER: C
EXPLANATION: The speed of light varies in a mineral based on both its refractive index and the orientation of the light with respect to the mineral’s crystallographic axes. Anisotropic minerals have different refractive indices depending on the direction light travels through them.
You’ve switched to a high-power objective lens but cannot obtain a focused image of a mineral grain. This likely indicates that:
A. The working distance of the objective is too short.
B. The refractive index of the immersion oil is incorrect.
C. The condenser aperture requires adjustment.
D. The upper polarizer is not fully inserted.
ANSWER: A
EXPLANATION: High-power objectives have short working distances (the space between the lens and the coverslip). If this distance is too short, you may not be able to bring the sample into focus.
You notice a thin, bright halo around a mineral grain under partially closed condenser aperture. This halo is likely due to:
A. Diffraction at the interface between two different media.
B. Internal reflections within the thin section mount.
C. Oblique incident light passing through the mineral.
D. A mismatch in the thickness of the slide and coverslip.
ANSWER: A
EXPLANATION: Diffraction effects are more pronounced when light passes through narrow openings (like a partially closed aperture), often leading to fringes or halos around objects.
An opaque mineral obscures features in the thin section. To better observe the surrounding minerals you should:
A. Use a lower power objective.
B. Employ reflected light microscopy.
C. Immerse the slide in a higher refractive index oil.
D. Insert an accessory plate into the microscope
ANSWER: B
EXPLANATION: Opaque minerals are better examined using reflected light, which illuminates them from above rather than having to transmit light through them.
. While analyzing a mineral grain with two intersecting cleavages, you rotate the stage to align one cleavage parallel to the microscope crosshairs. You notice maximum brightness through the eyepieces. This suggests that the cleavage plane is:
A. Aligned with a polarizer direction
B. An optically isotropic direction
C. A principal vibration direction
D. Perpendicular to the optic axis
ANSWER: C
EXPLANATION: In anisotropic minerals, light will be brightest when a cleavage or other internal plane aligns with one of the mineral’s principal vibration directions.
You’re trying to distinguish between two minerals with very similar refractive indices. Which technique would likely be the most effective?
A. Comparing the birefringence of the minerals
B. Employing the Becke Line method
C. Analyzing extinction characteristics
D. Observing pleochroism under plane-polarized light
ANSWER: B
EXPLANATION: The Becke Line test allows for precise determination of relative refractive indices, which can help differentiate minerals with very subtle differences
. A mineral displays vivid interference colors that appear to shift as you rotate the polarizer slightly. This phenomenon is likely caused by:
A. Optical activity of the mineral
B. Very high birefringence of the mineral
C. Oblique illumination of the sample
D. Dispersion of light within the mineral
ANSWER: A
EXPLANATION: Optical activity is the rotation of polarized light as it passes through a substance. This rotation can cause interference colors to shift as the polarizer is rotated.
A mineral with inclined extinction always shows its maximum interference color when:
A. The stage is rotated 45 degrees from extinction
B. One of the cleavage directions is parallel to the crosshairs
C. The optic axis is aligned with a polarizer
D. The thickness of the slide is precisely 30 microns
ANSWER: A
EXPLANATION: Minerals with inclined extinction display their maximum interference colors when their fast and slow vibration directions are oriented at 45 degrees to the polarizer and analyzer.
You observe a mineral with an unusual “blinking” effect as you slowly rotate the polarizer under plane-polarized light. This could indicate:
A. Polysynthetic twinning within the mineral
B. Optically active mineral with strong rotation effects
C. High birefringence with first-order interference colors
D. Misaligned polarizer and analyzer within the microscope
ANSWER: A
EXPLANATION: Polysynthetic twins have different extinction positions, causing parts of the mineral to go dark under PPL while other areas of the same mineral remain bright
You notice two minerals with nearly identical interference colors under XPL. To help differentiate between them, you should:
A. Measure the 2V angle of both minerals
B. Determine the optic sign of both minerals
C. Observe their behavior under plane-polarized light
D. Immerse the thin section in different refractive index liquids
ANSWER: B
EXPLANATION: Minerals with the same interference color might have different optic signs (positive vs. negative), which can be a vital distinguishing factor.
The resolution of a petrographic microscope is primarily determined by:
A. The numerical aperture of the objective lens
B. The brightness of the light source
C. The wavelength of light used for illumination
D. The thickness of the thin section preparation
ANSWER: A
EXPLANATION: Resolution refers to the ability to see fine details. In a petrographic microscope, the numerical aperture (NA) of the objective lens is most crucial for resolving tiny features in a mineral grain. Higher NA lenses collect more light and allow you to distinguish between closely spaced details.
When examining a rock with a fine-grained, interlocking texture in XPL, you find it difficult to identify individual minerals. Which adjustment could improve the visibility of crystal boundaries?
A. Switch to a lower-power objective for a wider view.
B. Rotate the Bertrand lens into the light path.
C. Close the substage diaphragm slightly.
D. Replace the oculars with higher magnification eyepieces.
ANSWER: C
EXPLANATION: Closing the substage diaphragm slightly increases the contrast, which can help with defining crystal boundaries in fine-grained textures.
A mineral in PPL displays a uniform color with varying degrees of brightness as the stage is rotated. This suggests that the mineral likely:
A. Is isotropic
B. Has oblique extinction
C. Possesses strong dispersion
D. Exhibits low birefringence
ANSWER: A
EXPLANATION: Isotropic minerals remain uniformly dark or uniformly bright under PPL regardless of stage rotation, as there’s only one refractive index and no polarization effects.
To examine fine-scale textures with the highest possible detail, you should choose an objective lens that prioritizes:
A. High numerical aperture and low magnification
B. Low numerical aperture and high magnification
C. High numerical aperture and high magnification
D. Long working distance and low magnification
ANSWER: C
EXPLANATION: A high numerical aperture provides better resolution, essential for fine details. High magnification is also necessary to see these details clearly.
You’re performing the Michel-Levy technique to roughly estimate the birefringence of a mineral. If the accessory plate introduces additional retardation equivalent to 550 nm, the target mineral would display compensation when it shows:
A. First-order yellow interference color.
B. Third-order blue interference color.
C. Second-order orange interference color.
D. No change in its current interference color
ANSWER: A
EXPLANATION: For compensation to occur, the retardation added by the accessory plate and the retardation of the mineral must sum to zero. Yellow of the first order is approximately 550 nm retardation.
You’re examining a thin section in XPL and notice a mineral displaying anomalous blue interference colors despite seemingly thick grains. The best explanation for this phenomenon is:
A. Extremely low birefringence in the mineral.
B. The mineral likely being strongly pleochroic.
C. Misalignment of the analyzer and polarizer.
D. An error in the preparation of the thin section.
ANSWER: A
EXPLANATION: Anomalous interference colors with lower-order colors than expected often suggest exceptionally low birefringence of the mineral.
While attempting extinction angle measurements, you find the position of total darkness to be slightly offset when switching stage rotation directions. This could be caused by:
A. Stage backlash affecting rotation accuracy.
B. Strong dispersion of light within the mineral.
C. Bent crosshairs within the microscope eyepiece.
D. Poorly calibrated condenser height on the microscope.
ANSWER: A
EXPLANATION: Stage backlash refers to a slight misalignment that can occur when switching rotational directions, potentially leading to slight offsets in extinction position measurements.
A mineral displays a bright, first-order red interference color. You remove the Bertrand lens and insert the upper polarizer. The interference color changes to a deep blue. This indicates that the mineral is likely:
A. Biaxial with a positive optic sign.
B. Biaxial with a negative optic sign.
C. Uniaxial with a positive optic sign.
D. Uniaxial with a negative optic sign.
ANSWER: D
EXPLANATION: This color shift with crossed polars and removal of the Bertrand lens is typical for a uniaxial negative mineral, where fast and slow rays switch roles.
You notice that focusing on objects at the top and bottom of the field of view requires slightly different focus knob adjustments. This suggests an issue with:
A. Stage leveling on the microscope.
B. Uneven thickness of the thin section slide.
C. Unequal curvature of the objective lens surface.
D. Misalignment between the objective and ocular lenses.
ANSWER: A
EXPLANATION: Stage misalignment would cause parts of the sample to be further or closer in the field of view, necessitating different focus knob positions.
You’re observing two opaque mineral grains. Grain A appears brighter and more reflective than Grain B. This suggests that Grain A likely has a:
A. Higher refractive index than Grain B
B. Rougher surface compared to Grain B
C. Lower refractive index than Grain B
D. Larger grain size compared to Grain B
ANSWER: A
EXPLANATION: The reflectivity of opaque minerals is strongly correlated with their refractive indices. Higher refractive index minerals generally display higher reflectance.
- The maximum observable birefringence of a mineral depends on:
A. The mineral’s absolute refractive indices.
B. The orientation of the mineral with respect to the light path.
C. The thickness of the thin section preparation.
D. The wavelength of light used for illumination.
ANSWER: B
EXPLANATION: Birefringence is the difference between a mineral’s highest and lowest refractive indices. This difference is greatest when the light travels perpendicular to the mineral’s optic axis, so the observed birefringence depends on how the mineral grain is oriented within the thin section.
