Exam Review Flashcards
How are optical properties related to crystal symmetry?
Higher symmetry (cubic) → Isotropic behavior (single refractive index, no birefringence).
• Medium symmetry (tetragonal, hexagonal) → Uniaxial behavior (two refractive indices, one optic axis).
• Lower symmetry (orthorhombic, monoclinic, triclinic) → Biaxial behavior (three refractive indices, two optic axes).
• Lower symmetry leads to more complex optical properties, including inclined extinction and variable birefringence.
How are the axes in the 6 crystal systems relates to the principle refractive
indices. e & w or a, b, & g?
• Higher symmetry (Cubic) → One refractive index (isotropic).
• Intermediate symmetry (Tetragonal, Hexagonal) → Two refractive indices (uniaxial).
• Lower symmetry (Orthorhombic, Monoclinic, Triclinic) → Three refractive indices (biaxial).
• In uniaxial minerals, n_e corresponds to the optic axis (c-axis), while in biaxial minerals, n_a, n_b, and n_c are oriented along different crystallographic directions.
What is the difference between Isotropic vs. Isometric vs. Anisotropic?
Isotropic- minerals remain extinct(dark) for the full 360 degrees of stage rotation
Isometric- are all isotropic and have equal dimensions
Anisotropic- minerals that appear extinct for 4 times during a 360 degree stage rotation. They show birefringence when they are not extinct.
How can you identify parallel, symmetrical, and inclined extinction?
When crystal faces or cleavages are parallel to the N-S and
E-W polarizers (or cross hairs) at extinction, it is called
“parallel extinction”.
When crystal faces or cleavages are at 45o to the N-S and
E-W polarizers (or cross hairs) at extinction, it is called
“symmetrical extinction”.
When crystal faces or cleavages are at an angle to the N-S and
E-W polarizers (or cross hairs) at extinction, it is called
“inclined extinction”.
What does the presence of parallel, symmetrical, and inclined extinction tell
us about the most likely crystal system of a crystal?
if a mineral shows parallel extinction, it is likely from a higher symmetry system (tetragonal, hexagonal, or orthorhombic). If it shows inclined extinction, it is likely monoclinic or triclinic, which have lower symmetry.
Know the location and function of: the upper polarizer (or Analyzer); lower
polarizer; the condensing lens; objective lenses; ocular lenses; the Bertrand
lens; the accessory plate; the accessory slot; focus. Which of these are
almost always IN, or sometimes IN and sometimes OUT?
Upper polarizer is sometimes in or out
Lower polarizer is usually always in
Bertrand lens is usually out, but is put in when we want to view optics
Accessory plate is sometimes in or out, and adds 1 wavelength
Condensing lens is usually on, and directs light into a mineral
Objective lens are usually in and provide different levels of zoom into a mineral
Ocular lenses are usually in and are the piece you look through to view the specimen
Focus is always on, and can be adjusted to fine tune a zoom
How would you set up Orthoscopic vs. Conoscopic viewing using a
petrographic microscope?
Orthoscopic viewing is achieved by leaving the cross polars off and bertrand lens out
Conoscopic viewing is achieved by inserting the bertrand lens and crossing the polars
What is the relationship between the Refractive index and the speed of light?
RI=(speed of light in vacuum/speed of light in a material)
How does a diamond work? What two properties make it sparkle?
A diamond works by cutting it into specific angles, which utalize refraction and dispersion to make it sparkle. The point at the bottom of the diamond should be 98.5 degrees.
How does a prism work? What two characteristics must it have?
By bending white light into different angles based on the wavelength. Separating white light into the rainbow. Requires Dispersion and nonparallel sides.
What is the “critical angle” for Diffraction vs. Reflection?
The critical angle for reflection occurs when the angle of incedence is equal to 90 degrees. The critical angle for diffraction occurs when light is bent in such a way that it is parallel to another surface.
What is the difference between Isotropic, Uniaxial, and Biaxial materials in
terms of the crystal systems involved?
Isotropic- All amorphous materials, and all minerals belonging to the isometric system
-materials with 1 refractive indices. Light travels the same speed in all directions.
Anisotropic
-materials with more than 1 refractive indices. Light travels at different speeds in different directions.
-Uniaxial: Hexagonal and tetragonal
-RI=E&w
-Biaxial: Orthorhombic, monoclinic and triclinic
-RI=a,b, and g(y)
What is the difference between Pleochroism vs. Birefringence?
Pleochroism is the false colors observed through plane polarized light as the stage is rotated. Birefringence is false colors observed through cross polarized light as the stage is rotated.
What is the difference between Positive and Negative Uniaxial crystals?
Uniaxial:
• Positive: € > w.
• Negative: € < w.
What is the difference between Positive and Negative Biaxial crystals?
Biaxial:
If a < b «_space;g the Optic Axes are closer to g and the crystal is Biaxial Positive
If a «_space;b < g the Optic Axes are closer to a and the crystal is Biaxial Negative
How do we determine the Uniaxial sign? The Biaxial sign?
Uniaxial- when the cross is formed we look at the NE corner, insert the accessory plate and determine the sign based on which color is closest to the cross (Blue +) (Yellow -)
Biaxial- Rotate the stage so the isogyres are concave to the north east corner. Insert the accessory plate and record the closest color to the isogyres in the NE corner (Blue +) (Yellow -)
What three techniques can be used to Measuring the Biaxial 2V?
- Kamb’s method = measure the amount of stage
rotation for the isogyres to leave the field of view. - Tobi’s Method = measure the separation of the
isogyres = measure the distance between the
melatopes (OAs) when the isogyres are at maximum
separation (45o position) for BXa figures. - Examine the angle of isogyre curvature =>
the straighter the isogyres, the larger the 2V.
How can we tell the difference between a figure with an Uniaxial single
isogyre vs. a figure with a Biaxial single isogyge?
- Rotate the microscope stage:
• If the isogyre disappears quickly and does not split → Uniaxial.
• If the isogyre remains visible longer and curves/splits → Biaxial.- Observe the shape:
• A straight, broad isogyre suggests a uniaxial flash figure.
• A curved, hook-like isogyre suggests a biaxial figure.
- Observe the shape:
How can we distinguish between an Uniaxial Cross, a BXA, a BXO, and a Flash
figure?
• Uniaxial Cross: Sharp, centered cross that remains stationary during rotation.
• BXA (Biaxial Acute): Isogyres form a cross that splits and moves apart upon rotation.
• BXO (Biaxial Obtuse): Isogyres move out of the field of view, and move perpendicular to the BXA.
• Flash Figure: Occurs when the optic axis is near parallel to the light path, and the stage is rotated, the field will brighten almost completely.
How do we do the Beckie Line test?
Observe a mineral grain in an oil under the microscope. When we put the grain in focus and begin to zoom out, we can see white lines (becke lines) move either into or out of the grain. This allows us to estimate the (RI) of the grain if we use an oil with a known (RI).
How do we determine if a crystal is Length Fast vs. Length Slow?
We can determine fast or slow length by aligning the crystal so the elongated side runs at a 45 degree angle, or from the SW-NE of the microscope. Then we add 1 rot, which add 550nm to the view. If Birefringence increases then the slow length is aligned NE if it decreases than the fast length is aligned NE.
How do we determine if we have High relief vs. Low relief?
Relief is used to describe how well we can see the shadows/outline of a mineral grain within oil. High relief means that the (RI) of the grain is higher than the (RI) of the oil, and means we will see a well defined outline. Low relief means the (RI) of the grain is lower than the (RI) of the oil, and shows that the shadows/outline of the grain will be less visible.
What is the standard thickness for a thin section?
0.03mm
How do we use the Michael-Levy chart to determine birefringence?
First begin by observing a color from a mineral and estimating or calculate the wavelength for that color. Next measure or estimate the thickness of the grain and use this info to plot a mark where thickness and wavelength intersect. Next follow the diaganol line towards the top of the chart to give you an number for Birefringence.
