Week 4 - Optical Mineralogy Flashcards
What happens when minerals are viewed under Crossed Polarized Light (XPL)?
They exhibit extinction, which can reveal crystal structure.
How can you determine if a mineral is cubic under XPL?
If it remains dark in all orientations, it is cubic or viewed along its c-axis. Wont be dark in PPL.
What does it mean if a mineral alternates between light and dark four times per 360° rotation under XPL?
It is not cubic and belongs to another crystal system.
What is parallel extinction?
When cleavage or elongation is parallel to the polarizer (N/S or E/W).
What is inclined extinction?
When cleavage or elongation is at an angle to the polarizers.
What is symmetrical extinction?
When extinction occurs at angles symmetrical to cleavages or crystal faces.
What does “no extinction angle” mean?
The mineral lacks cleavage or elongation.
How are extinction angles measured?
Based on the mineral’s orientation in thin-section.
What does “length fast” mean in extinction angles?
The fast ray vibrates parallel to elongation (negative elongation).
What does “length slow” mean in extinction angles?
The slow ray vibrates parallel to elongation (positive elongation).
What is undulose extinction?
When different parts of a mineral grain go extinct at different times due to deformation.
What is chemical zoning?
Variations in mineral composition create different extinction angles in different parts of the mineral.
What are isotropic minerals?
Minerals where light moves at the same speed in all directions (e.g., cubic minerals, glass).
What shape is the indicatrix for isotropic minerals?
Sphere
What are anisotropic minerals?
Minerals where light splits into two polarized components traveling at different speeds.
What shape is the indicatrix for anisotropic minerals?
Ellipsoid.
What crystal systems do uniaxial minerals belong to?
Tetragonal, hexagonal, and trigonal systems.
How many optic axes do uniaxial minerals have?
One
Give an example of a uniaxial mineral.
Quartz (Uniaxial Positive) or Nepheline (Uniaxial Negative).
What crystal systems do biaxial minerals belong to?
Orthorhombic, monoclinic, and triclinic systems.
How many optic axes do biaxial minerals have?
Two
Give an example of a biaxial mineral.
Feldspars (Biaxial).
What does a positive uniaxial mineral look like?
Prolate indicatrix (stretched along the c-axis).
What does a negative uniaxial mineral look like?
Oblate indicatrix (compressed along the c-axis).
What is the 2V angle in biaxial minerals?
The angle between the two optic axes.
What is the optic sign for biaxial minerals if “Blue Upper Right” appears?
Positive (BURP).
What is the optic sign for biaxial minerals if “Orange Upper Right” appears?
Negative
What is twinning in minerals?
The symmetrical intergrowth of two or more crystals of the same mineral.
What is simple twinning?
Two intergrown crystals (e.g., hornblende, K-feldspar).
What is polysynthetic twinning?
Multiple parallel twins (common in plagioclase feldspar).
What is zoning in minerals?
Changes in color or extinction angle from core to rim.
What causes zoning in minerals?
Chemical variations during mineral growth (common in tourmaline).
What is the indicatrix?
Geometric representation of how light propagates through a mineral, showing variations in refractive indices in different directions
What is the purpose of the Becke line test?
To determine the relative refractive index of a mineral compared to the surrounding medium
How does the Becke line behave when the microscope stage is lowered?
The Becke line moves into the material with the higher refractive index. Vise Versa
What is the key principle behind the Becke line test?
Light is refracted toward the material with a higher refractive index, causing the bright Becke line to move into it when the stage is lowered.
What is birefringence?
The difference in refractive indices (n₁ - n₂) of a doubly refracting mineral, observed under cross-polarized light (XPL).
How can birefringence be estimated in thin section?
By observing interference colours using crossed polars and comparing them to a Michel-Lévy interference colour chart.
What factors influence the observed birefringence of a mineral?
The mineral’s crystallographic orientation, thickness of the thin section, and the difference in refractive indices.
What does BURP stand for in axial figure analysis?
Blue Upper Right Positive
What does the presence of a black cross in an interference figure indicate?
uniaxial
What type of minerals exhibit uniaxial optic figures?
Tetragonal, hexagonal, and trigonal minerals.
What does it mean if you are looking straight down the C-axis of a mineral?
The mineral will appear opaque in conoscopic view.
What is the role of the conoscopic light lens in axial figure tests?
It is a concave lens that helps scatter light to produce an interference figure.
