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Correlation: DAY 2 > Optical Mineralogy > Flashcards

Optical Mineralogy Flashcards

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1
Q

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

A

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.

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2
Q

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

A

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.

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3
Q

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

A

ANSWER: B
EXPLANATION: Isotropic minerals do not interact with polarized light and therefore remain dark under XPL.

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4
Q

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

A

ANSWER: A
EXPLANATION: Minerals with low birefringence display first-order gray, white, or yellow interference colors.

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5
Q

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

A

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.

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6
Q

The substage diaphragm on a petrographic microscope is primarily used to:
A. Adjust light intensity
B. Control relief
C. Improve resolution
D. Enhance birefringence

A

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).

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7
Q

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

A

ANSWER: A
EXPLANATION: The Becke line will move into the medium with the higher refractive index as the stage is raised.

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8
Q

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

A

ANSWER: A
EXPLANATION: Flash figures are characteristic of uniaxial minerals showing a centered optic axis interference figure.

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9
Q

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

A

ANSWER: A
EXPLANATION: Minerals in the tetragonal, hexagonal, and orthorhombic systems display parallel extinction.

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10
Q

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

A

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.

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11
Q

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

A

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.

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12
Q
  1. 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.
A

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.

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13
Q

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

A

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

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14
Q

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

A

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.

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15
Q
  1. 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
A

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.

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16
Q

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

A

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.

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17
Q

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)

A

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.

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18
Q

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.

A

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.

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19
Q

. 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.

A

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

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20
Q

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.

A

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.

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21
Q

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.

A

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.

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22
Q

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.

A

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.

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23
Q

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

A

ANSWER: B
EXPLANATION: Opaque minerals are better examined using reflected light, which illuminates them from above rather than having to transmit light through them.

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24
Q

. 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

A

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.

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25
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
26
. 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.
27
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.
28
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
29
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.
30
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.
31
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.
32
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.
33
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.
34
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.
35
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.
36
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.
37
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.
38
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.
39
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.
40
40. 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.
41
. A mineral grain remains dark throughout a complete 360-degree stage rotation under crossed polars (XPL). This suggests that the mineral is likely: A. Uniaxial and oriented with the optic axis perpendicular to the stage. B. Biaxial with an optic axis within the plane of the thin section. C. Isotropic or very weakly birefringent. D. Exhibiting first-order grey interference colors.
ANSWER: C EXPLANATION: Isotropic minerals remain dark under XPL regardless of orientation. Minerals with extremely low birefringence may also appear nearly dark.
42
You switch from PPL to XPL. A mineral grain changes from colorless to a vibrant blue interference color. This suggests that the mineral is: A. Uniaxial and exhibits strong dispersion. B. Isotropic, showing anomalous interference colors. C. Anisotropic with moderate birefringence. D. Opaque and reflecting crossed polarized light.
ANSWER: C EXPLANATION: Anisotropic minerals exhibit interference colors under XPL. The color intensity depends on the mineral's birefringence.
43
A mineral under PPL shows distinct changes in absorption color as the stage is rotated. This phenomenon is called: A. Interference B. Dispersion C. Pleochroism D. Extinction
ANSWER: C EXPLANATION: Pleochroism is the ability of a mineral to absorb different wavelengths of light differently depending on the crystallographic direction.
44
44. Under XPL, you notice a mineral showing straight extinction. This observation suggests that: A. The mineral belongs to the triclinic crystal system. B. The mineral is uniaxial with the optic axis vertical. C. A crystallographic axis is parallel to a polarizer direction. D. The mineral has first-order grey interference colors
ANSWER: C EXPLANATION: Straight extinction occurs when the mineral's principal vibration directions align with the polarizer and analyzer directions.
45
You are examining a mineral in XPL and notice a slight color shift as you slightly de-focus the image. This color shift is likely due to: A. Oblique extinction of the mineral grain. B. Absorption of the mineral varying with thickness. C. Dispersion of light within the mineral. D. An error in the thin section preparation process.
ANSWER: C EXPLANATION: Dispersion refers to the variation of a mineral's refractive index with the wavelength of light. This can lead to subtle color shifts when the focus is changed slightly.
46
You are analyzing a mineral grain with two distinct cleavages in PPL. However, the interference colors under XPL appear abnormally low compared to its thickness. This suggests that: A. The mineral is close to extinction under XPL. B. One cleavage direction is nearly parallel to the optic axis. C. The mineral is strongly pleochroic and is absorbing light. D. The light source in the microscope is malfunctioning.
ANSWER: B EXPLANATION: If a cleavage is almost parallel to the optic axis, its effective birefringence is very low, leading to weaker interference colors.
47
You notice two adjacent mineral grains that extinguish at slightly different stage rotation positions. This likely indicates that the minerals: A. Have similar crystallographic orientations. B. Belong to the same mineral species with twinning. C. Share a cleavage plane between the two grains. D. Have different birefringence values.
ANSWER: B EXPLANATION: Twinning creates different extinction orientations within a single crystal, leading to adjacent areas extinguishing at slightly different angles.
48
. Under XPL, you observe a mineral with sweeping bands of alternating bright and dark colors as you rotate the stage. This is a characteristic feature of: A. A euhedral mineral with oscillatory zoning. B. Multiple twin lamellae in a single mineral grain. C. Biaxial minerals showing centered interference figures. D. Uniaxial minerals displaying flash figures.
ANSWER: D EXPLANATION: Flash figures are a hallmark of uniaxial minerals, appearing as the optic axis moves in and out of alignment with the polarizers.
49
A mineral grain shows a strong color change from yellow to colorless under PPL as you rotate the stage. This could be best explained by: A. An interference effect dependent on mineral thickness. B. Strong pleochroism within the crystal structure. C. Optical activity with rotation of PPL direction. D. A thin section preparation artifact.
ANSWER: B EXPLANATION: Pleochroism causes colors to change with orientation in PPL due to differential absorption of light.
50
Under XPL, a mineral grain flashes into brilliant colors when you slightly insert the Bertrand lens, then immediately goes dark. This observation suggests that: A. The mineral has very high birefringence. B. The mineral is uniaxial with a small 2V angle. C. The mineral is biaxial with inclined extinction. D. The mineral is opaque and misaligned in the thin section.
ANSWER: B EXPLANATION: When a uniaxial mineral with a small 2V angle (the angle between the two optic axes) is slightly tilted, the optic axes briefly pass through the field of view as you insert the Bertrand lens. This creates a flash of high-order colors before the grain goes to extinction.
51
. You've carefully measured a mineral's extinction angle relative to its cleavage. However, your results deviate significantly from values reported in the literature. The most likely explanation is that: A. The microscope stage rotation direction is misreported. B. The mineral is displaying anomalous interference colors. C. The thin section has been cut off-orientation. D. You've incorrectly identified the mineral species.
ANSWER: C EXPLANATION: Extinction angles are measured relative to crystallographic features like cleavage. If the thin section is cut off-orientation, measured extinction angles will be inaccurate.
52
. While examining a mineral under XPL, you notice its color changes slightly depending on whether you focus slightly above or below its optimal focus position. This suggests: A. Strong interference colors masking true absorption color. B. Polysynthetic twinning with minor orientation differences. C. Significant dispersion within the mineral crystal. D. Overlapping mineral grains with different properties.
ANSWER: C EXPLANATION: Dispersion, the dependence of refractive index on wavelength, can cause subtle color shifts at different focal positions.
53
With crossed polarizers inserted, you notice a colorless mineral grain displaying an unusual "speckled" or "mottled" appearance. This effect is most likely due to: A. Abundant fluid inclusions within the mineral. B. Zoned growth variations causing slight birefringence shifts. C. An undulose extinction pattern from crystal deformation. D. An error arising from the substage illumination source.
ANSWER: C EXPLANATION: Strain can disrupt the regular crystal structure of a mineral, leading to undulatory extinction with a speckled appearance under XPL.
54
You suspect a mineral is quartz but want confirmation. Under crossed polars, it shows a uniform interference color across the grain with no significant change during stage rotation. This supports the idea that the mineral is likely: A. Quartz, due to its uniaxial nature. B. Calcite, due to its strong rhombohedral cleavage. C. Microcline, due to its diagnostic twinning pattern. D. Feldspar, which has similar optical properties to quartz.
ANSWER: A EXPLANATION: A uniaxial mineral like quartz displays uniform interference color throughout when the optic axis is not parallel to the thin section. However, other properties are needed for a definitive identification of quartz.
55
55. You are working with an unfamiliar biaxial mineral in XPL. To estimate its birefringence, you would ideally want to find a grain oriented to show: A. Parallel extinction and high-order interference colors. B. Oblique extinction and first-order grey colors. C. Maximum interference color with inclined extinction. D. Parallel extinction under PPL with strong pleochroism.
ANSWER: C EXPLANATION: Maximum interference colors are displayed when the difference between the refractive indices along the observed vibration directions is greatest, which provides a good approximation of the mineral's birefringence.
56
While examining a thin section, you notice two mineral grains in direct contact. Grain A is bright under XPL, but Grain B appears dark. This suggests Grain B could be: A. Oriented with an optic axis parallel to the stage. B. A mineral with much lower birefringence than Grain A. C. Optically isotropic or extremely weakly birefringent. D. Strongly pleochroic and absorbing most of the light.
ANSWER: C EXPLANATION: Isotropic minerals or those with very low birefringence will appear dark or nearly dark under XPL, even if adjacent to a brightly colored anisotropic mineral.
57
A mineral under XPL displays a distinct change from blue to yellow interference colors as you rotate the stage approximately 90 degrees. This is most consistent with a: A. Uniaxial mineral exhibiting strong dispersion. B. Uniaxial mineral with positive optic sign. C. Biaxial mineral with high birefringence. D. Biaxial mineral showing centered optic axis figure.
ANSWER: C EXPLANATION: A large shift in interference color with a rotation of 90 degrees implies strong birefringence, often seen in biaxial minerals.
58
A mineral appears nearly opaque under PPL. However, when switched to XPL, it transmits some light and displays a faint interference color. This suggests that the mineral: A. Has a high refractive index and low birefringence. B. Is misaligned in the thin section. C. Possesses strong absorption with some light transmitted. D. Is biaxial and close to an extinction position.
ANSWER: C EXPLANATION: Strongly absorbing minerals often appear opaque under PPL but might transmit some light and weakly display interference colors under XPL.
59
You observe two minerals with identical birefringence. Mineral A has a thin, plate-like shape, while Mineral B is more cube-like. Under XPL, you'd expect: A. Mineral B to display a higher-order interference color. B. Mineral A to display a higher-order interference color. C. Both minerals to display the same interference color. D. Neither mineral to show a distinguishable interference color
ANSWER: A EXPLANATION: Interference color depends on both birefringence and thickness. The thicker mineral (B) will show a higher-order interference color, even if the birefringence is the same.
60
A mineral displays bright, high-order colors under XPL. To definitively determine if it's uniaxial or biaxial you should: A. Insert a quartz wedge accessory plate. B. Employ conoscopic observation techniques. C. Switch to PPL and observe any pleochroism. D. Apply the Becke Line test to estimate refractive index.
ANSWER: B EXPLANATION: Conoscopic observation allows you to view the interference figure of a mineral. Uniaxial minerals display a characteristic "cross and rings" pattern, while biaxial minerals show a more complex figure with two optic axes. This is the most definitive way to distinguish between uniaxial and biaxial minerals.
61
You observe a mineral in a thin section displaying straight extinction under XPL. This observation alone could be consistent with a mineral belonging to which crystal systems? A. Isometric, hexagonal, or triclinic B. Monoclinic, triclinic or cubic C. Triclinic, tetragonal or orthorhombic D. Tetragonal, hexagonal, or orthorhombic
ANSWER: D EXPLANATION: Minerals in the tetragonal, hexagonalonal, and orthorhombic crystal systems exhibit straight extinction along specific crystallographic directions.
62
62. Under XPL, you notice a mineral grain with two sets of cleavage intersecting at approximately 60/120 degrees. This cleavage pattern is strongly suggestive of a mineral belonging to which crystal system? A. Hexagonal B. Monoclinic C. Orthorhombic D. Triclinic
ANSWER: A EXPLANATION: The 60/120 degree cleavage intersection is characteristic of minerals crystallizing in the hexagonal system.
63
You are using the Becke Line method to compare a mineral to a mounting medium with n = 1.54. When the stage is raised, the bright line moves into the mounting medium. This suggests that the mineral likely has: A. A refractive index slightly higher than 1.54 B. A refractive index significantly higher than 1.54 C. A refractive index slightly lower than 1.54 D. A refractive index significantly lower than 1.54
ANSWER: C EXPLANATION: The Becke line moves into the medium with the higher refractive index. Since the line moved into the mounting medium, the mineral must have a slightly lower refractive index.
64
Under PPL, you notice a mineral displaying two distinct refractive indices within the same grain due to varying relief along different directions. This mineral is most likely: A. Isometric B. Uniaxial C. Biaxial D. Opaque
ANSWER: C EXPLANATION: Biaxial minerals possess three principal refractive indices, and depending on the grain orientation, may display two distinct values under PPL
65
While using the oblique illumination method, you slightly block part of the condenser light cone from one side. A mineral grain displays a distinct shadowing effect opposite the direction of the blocked light. This suggests: A. The mineral has a higher refractive index than the medium. B. The mineral has a lower refractive index than the medium. C. The mineral is displaying strong pleochroism. D. The mineral surface is uneven or fractured.
ANSWER: B EXPLANATION: With oblique illumination, a mineral with a lower refractive index than the mounting medium will have a shadowing effect on the opposite side from where the light was blocked.
66
You identify a mineral with a cubic form and a single refractive index. To confirm its crystal system, you should check for: A. Parallel extinction under crossed polars. B. Isotropism when the stage is rotated. C. Oblique extinction under crossed polars. D. Undulatory extinction under crossed polars.
ANSWER: B EXPLANATION: Isotropic minerals (like those in the cubic system) remain dark throughout rotation under XPL, confirming their single refractive index.
67
A mineral has nw > 1.7 and ne < 1.7. To definitively determine if it's uniaxial or biaxial by optical techniques, you would need to: A. Find a grain showing a centered interference figure. B. Measure the exact values of nw and ne. C. Test if the mineral is pleochroic under PPL. D. Analyze the extinction angle relative to cleavage.
ANSWER: A EXPLANATION: Observing an interference figure is the only way to definitively distinguish between uniaxial and biaxial minerals. Options B, C, and D could be helpful but don't always give a conclusive answer.
68
An orthorhombic mineral has its principal refractive indices aligned with its crystallographic axes. You observe a grain with a refractive index of 1.65. With this single measurement, you CANNOT determine if this value corresponds to: A. na (lowest refractive index) B. np (intermediate refractive index) C. ny (highest refractive index) D. Any of the above; more measurements are needed.
ANSWER: D EXPLANATION: Orthorhombic minerals have 3 refractive indices. Without knowing the grain's orientation or other relief observations, you cannot identify whether the measured value corresponds to the highest, lowest, or the intermediate refractive index
69
You suspect a mineral might be quartz but aren't certain. Which of the following observations would provide the strongest support for confirming its identity? A. Distinct rhombohedral cleavage under PPL. B. Parallel extinction and high-order interference colors under XPL. C. Uniaxial positive interference figure under conoscopy. D. Low relief and lack of pleochroism under PPL.
ANSWER: C EXPLANATION: The uniaxial positive interference figure is a unique characteristic of quartz and is the most definitive way to confirm its identity.
70
A mineral grain with high relief shows a strong shift in its Becke line position when changing from yellow light to blue light. This indicates that the mineral exhibits: A. Significant birefringence B. Anomalous interference colors C. Low refractive index D. Strong dispersion
ANSWER: D EXPLANATION: Dispersion refers to a mineral's ability to refract different wavelengths of light (colors) at slightly different angles. A strong shift in the Becke line position with a change in light source (yellow vs. blue) suggests significant dispersion in the mineral.
71
You notice a mineral with a distinct color under PPL but remains uniformly dark throughout rotation under XPL. This suggests the mineral is: A. Pleochroic and close to an extinction position. B. Isotropic or has extremely low birefringence. C. Uniaxial and oriented with the optic axis vertical. D. Composed of overlapping crystals with different orientations.
ANSWER: B EXPLANATION: Isotropic minerals don't interact with polarized light, and very weakly birefringent minerals may appear dark even with crossed polars.
72
An uniaxial positive mineral has its optic axis oriented horizontally within a thin section. Under XPL, you'd expect to observe its: A. Lowest refractive index (nw) B. Highest refractive index (ne). C. An intermediate refractive index between nw and ne. D. Birefringence (ne — nw)
ANSWER: A EXPLANATION: When a uniaxial mineral has its optic axis horizontal, you're observing the vibration direction corresponding to nw (the ordinary ray).
73
Using a mounting medium with n = 1.55, you determine a mineral has a refractive index slightly lower than the medium. To narrow the estimate, you should: A. Remount the sample in a significantly higher index liquid. B. Switch to a mounting medium with a lower index. C. Use the oblique illumination method instead. D. Measure the 2V angle with conoscopic observation.
ANSWER: B EXPLANATION: To get a more precise measurement, you need to use a mounting medium closer in refractive index to the mineral. This will improve the Becke line's visibility.
74
While examining a thin section, you notice a mineral grain with two intersecting cleavages seemingly at 90 degrees and low-order interference colors. The mineral could belong to which crystal systems? A. Isometric, hexagonal, or tetragonal B. Tetragonal, orthorhombic, or monoclinic C. Orthorhombic, monoclinic, or triclinic D. Monoclinic, triclinic, or cubic
ANSWER: B EXPLANATION: Tetragonal, orthorhombic, and monoclinic minerals can display an apparent 90-degree cleavage angle and low-order interference colors.
75
To unambiguously determine the optic sign of a uniaxial mineral you will need to use: A. An accessory plate like the gypsum plate. B. Conoscopic observation and identification of the quadrants. C. The oblique illumination technique D. The Michel-Levy interference color chart.
ANSWER: B EXPLANATION: The optic sign (positive or negative) of a uniaxial mineral can be determined by observing the interference figure under conoscopic observation and how the figure shifts in certain quadrants.
76
A mineral under PPL displays significant relief and strong color changes as the stage is rotated. The most likely explanation is: A. Anomalous interference colors due to thin section effects. B. Strong dispersion, causing color shifts with light refraction. C. Interference between overlapping mineral grains. D. Pleochroism, where absorption varies with direction
ANSWER: D EXPLANATION: Strong color changes with rotation under PPL highlight pleochroism, which is the differential absorption of light along different crystallographic directions.
77
You observe a grain showing maximum interference color under XPL when its long axis is 30 degrees from a polarizer direction. This suggests: A. Oblique extinction of approximately 30 degrees. B. Biaxial mineral with a moderate 2V angle. C. Misalignment of the Bertrand lens in the microscope. D. Zoned crystal with compositional variations.
ANSWER: A EXPLANATION: Maximum interference color occurs at 45 degrees from extinction. An angle of 30 degrees suggests an approximate extinction angle of 15 degrees (90-45-30).
78
If a mineral has ne = 1.70 and nw = 1.65, an estimate of its maximum possible birefringence would be: A. 3.35 B. 0.05 C. 1.675 D. Insufficient information to estimate
ANSWER: B EXPLANATION: Maximum possible birefringence is the difference between the highest and lowest refractive index, in this case, 1.70 - 1.65 = 0.05
79
Under XPL, you notice a dark mineral grain displaying a distinct, bright halo. The halo appears to shift outwards when the stage is raised. This suggests: A. The mineral has a higher refractive index than the medium. B. The mineral has a lower refractive index than the medium C. An error in Becke line interpretation due to oblique light. D. Strong internal reflections within the mineral crystal
ANSWER: B EXPLANATION: An outward moving halo with rising stage is a Becke line effect indicating a mineral with a lower refractive index than the mounting medium.
80
You find a grain displaying a centered flash figure under XPL that remains centered even when tilting the stage. This tells you that the mineral is: A. Biaxial with a small 2V angle. B. Biaxial with a high 2V angle. C. Uniaxial with low birefringence. D. Uniaxial with the optic axis nearly vertical.
ANSWER: D EXPLANATION: A centered flash figure under XPL suggests a uniaxial mineral. If the figure remains centered while tilting the stage, it indicates the optic axis is nearly perpendicular to the plane of the thin section (almost vertical).
81
After examining your thin section, you notice the mineral grains appear "fuzzy" and lack sharp boundaries. The most likely cause of this is: A. Misalignment of the upper polarizer B. The rock sample was cut too thin for ideal viewing. C. Poor polishing of the thin section surface. D. Using an immersion oil with incorrect refractive index.
ANSWER: C EXPLANATION: Rough or poorly polished surfaces scatter light, causing blurry or indistinct mineral boundaries.
82
A diamond saw is preferred for cutting rock samples during thin section preparation because it: A. Provides a smoother cut, minimizing polishing time. B. Can easily cut rocks containing very hard minerals. C. Introduces minimal damage to the rock structure. D. Allows for cutting precise, ultrathin samples.
ANSWER: B EXPLANATION: Diamond's exceptional hardness allows it to cleanly cut through even the toughest rock samples, which is essential for many geological materials.
83
You've finished grinding your thin section close to the desired thickness, and your mineral grains are showing faint interference colors. To reach the standard 30-micron thickness you should ideally: A. Continue grinding with the current coarse abrasive. B. Switch to a fine-grained lapping plate. C. Apply a polishing compound to the current surface. D. Mount the sample and grind the opposite side.
ANSWER: C EXPLANATION: With faint interference colors, you're close to ideal thickness. Polishing removes minor scratches leaving a smooth, optically flat surface.
84
Using the Michel-Levy chart, you determine a mineral has a birefringence of around 0.035. To confirm this, you would ideally use an accessory plate introducing a retardation of approximately: A. 100 nm B. 300 nm C. 550 nm D. 1000 nm
ANSWER: C EXPLANATION: To determine the sign or compensation with the Michel-Levy method, you want an accessory plate with a retardation close to the estimated birefringence of the mineral. 550 nm (1st order yellow) is the closest match.
85
You are trying to differentiate between two minerals with very similar optical properties. Using the Michel-Levy chart, the most reliable feature to distinguish between them would be: A. Their refractive index relative to the mounting medium. B. The specific order of their interference colors. C. The thickness of the thin section preparation. D. The presence or absence of twinning within the minerals.
ANSWER: B EXPLANATION: While both minerals might show similar colors, the precise sequence of colors as thickness increases (moving down the Michel-Levy chart) will differ.
86
During a lab session, you accidentally added excess epoxy during thin section mounting. This could cause: A. Cracking of the thin section during grinding. B. Introduction of air bubbles under the coverslip. C. Uneven thickness across the area of the slide. D. Difficulty in obtaining sharp focus under the microscope.
ANSWER: B EXPLANATION: Excess epoxy makes it difficult to firmly seat the coverslip without trapping air bubbles, which would interfere with microscopy.
87
While analyzing a fine-grained rock, you suspect you might be misinterpreting overlapping mineral boundaries. To improve visibility, you could: A. Use a higher magnification objective lens. B. Decrease the condenser aperture slightly. C. Switch from XPL to PPL. D. Remount the sample in a higher-index medium.
ANSWER: B EXPLANATION: Closing the condenser aperture slightly enhances contrast, which can help differentiate overlapping grains in fine-grained textures.
88
You notice that a mineral displaying high-order interference colors appears significantly duller than expected. The most likely explanation is: A. Excessive thin section thickness masking colors. B. Interference from abundant inclusions within the grain. C. Misaligned polarizer and analyzer leading to light loss. D. Strong absorption characteristics of the mineral.
ANSWER: D EXPLANATION: Strongly absorbing minerals often appear dull or muted even when displaying theoretically bright interference colors.
89
Using the Michel-Levy chart, you estimate your thin section to be slightly thinner than the standard 30 microns. This means most minerals will likely appear to have: A. Slightly higher refractive indices than expected. B. Stronger absorption characteristics. C. Slightly lower birefringence than expected. D. Anomalous interference colors due to thickness
ANSWER: C EXPLANATION: Apparent birefringence is directly related to thickness. A thinner section would slightly reduce the observed interference colors.
90
When making a frosted glass slide for thin section mounting, the purpose of the frosted side is to: A. Ensure better adhesion of the rock sample. B. Reduce reflections from the slide itself during microscopy. C. Indicate the correct orientation for mounting the coverslip. D. Minimize the amount of polishing necessary.
ANSWER: B EXPLANATION: Frosting the glass slide creates a slightly roughened surface that helps prevent internal reflections within the glass itself. This improves the clarity of the image observed through the microscope.
91
While preparing a thin section, you notice your rock sample is too friable to cut and grind effectively. This could be addressed by: A. Switching to a softer abrasive during grinding. B. Using a thicker glass slide for better support. C. Impregnating the sample with resin before cutting. D. Applying higher pressure during the grinding process.
ANSWER: C EXPLANATION: Resin impregnation helps solidify friable rocks, making them stable enough for the cutting and grinding processes of thin section preparation.
92
After polishing, you observe faint, concentric rings on your thin section under the microscope. This is most likely caused by: A. Improper cleaning, leaving debris on the slide. B. Zoned crystal growth affecting optical properties. C. Uneven polishing creating thickness variations. D. An error resulting from double-sided polishing.
ANSWER: C EXPLANATION: Concentric rings under the microscope typically indicate unevenness in the thickness of the thin section caused by variations during polishing.
93
Under crossed polars, you identify a mineral displaying an interference figure with very tightly spaced isochromes (rings). This suggests that the mineral has: A. Low birefringence B. High birefringence C. Significant dispersion D. Strong pleochroism
ANSWER: B EXPLANATION: A high birefringence leads to tightly spaced isochromes in the mineral's interference figure, making the rings appear closer together.
94
. For preliminary mineral identification using the Michel-Levy chart, the most helpful characteristics to determine are: A. Relief and presence of pleochroism. B. Maximum interference color and extinction angle. C. Interference figure and refractive index. D. Cleavage pattern and crystal shape.
ANSWER: B EXPLANATION: The maximum interference color (related to birefringence and thickness) and the extinction angle are helpful for quickly narrowing down the possible mineral options on the Michel-Levy chart.
95
. Under XPL, an isotropic mineral accidentally included in your thin section will likely appear: A. Uniformly dark at all stage rotation angles. B. Colorful due to strong dispersion effects. C. Bright white with high-order interference colors. D. Mottled with variations in grayness due to inclusions
ANSWER: A EXPLANATION: Isotropic minerals remain dark throughout an entire 360-degree stage rotation under crossed polarized light.
96
You're mounting a thin section with a coverslip. To ensure ideal optical properties during microscopy, you should select a coverslip with a thickness close to: A. 0.17 mm B. 0.5 mm C. 1.0 mm D. The thickness of the coverslip is unimportant.
ANSWER: A EXPLANATION: Most petrographic microscopes are calibrated for coverslips approximately 0.17 mm thick. Using a significantly different value can introduce aberrations to the image.
97
To improve the precision of your birefringence estimations using the Michel-Levy chart, you should ideally: A. Employ the Becke line method with various refractive index liquids. B. Determine the mineral's optic sign using an accessory plate. C. Measure the exact thickness of your thin section. D. Analyze the mineral with oblique illumination under PPL.
ANSWER: C EXPLANATION: Birefringence estimations from the Michel-Levy chart depend on an assumed thickness. Measuring the actual thickness will improve the accuracy of these estimations.
98
When attempting conoscopic analysis on an extremely small mineral grain, your primary challenge will likely be: A. It will be difficult to find the grain with a high-power objective. B. The grain will likely display first-order grey interference colors. C. Locating the optic axis within the field of view will be difficult. D. Determining the optic sign will require multiple accessory plates.
ANSWER: C EXPLANATION: Small grain sizes limit how much you can center the optic axis when switching to the conoscopic view, making the technique challenging.
99
A thin section preparation artifact is most likely to be mistaken for which genuine mineral characteristic? A. Cleavage B. Zoning C. Twinning D. Pleochroism
ANSWER: C EXPLANATION: Twinning can be complex, and some preparation artifacts - like strain shadows or uneven polishing - can sometimes mimic twinning patterns.
100
You're analyzing a thin section of sedimentary rock containing fine calcite grains. To enhance the visibility of these grains, you should: A. Stain the thin section to highlight carbonate minerals. B. Lower the condenser to increase diffraction effects. C. Employ reflected light microscopy instead of transmitted light. D. Immerse the slide in a high refractive index liquid.
ANSWER: D EXPLANATION: Calcite has a relatively low refractive index. Immersing the slide in a liquid with a higher refractive index will reduce the contrast between the calcite and the mounting medium, making the fine calcite grains stand out more clearly.

Correlation: DAY 2 (2 decks)

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