Week 4 - Optical Mineralogy

Question 1

What happens when minerals are viewed under Crossed Polarized Light (XPL)?

Answer 1

They exhibit extinction, which can reveal crystal structure.

Question 2

How can you determine if a mineral is cubic under XPL?

Answer 2

If it remains dark in all orientations, it is cubic or viewed along its c-axis. Wont be dark in PPL.

Question 3

What does it mean if a mineral alternates between light and dark four times per 360° rotation under XPL?

Answer 3

It is not cubic and belongs to another crystal system.

Question 4

What is parallel extinction?

Answer 4

When cleavage or elongation is parallel to the polarizer (N/S or E/W).

Question 5

What is inclined extinction?

Answer 5

When cleavage or elongation is at an angle to the polarizers.

Question 6

What is symmetrical extinction?

Answer 6

When extinction occurs at angles symmetrical to cleavages or crystal faces.

Question 7

What does “no extinction angle” mean?

Answer 7

The mineral lacks cleavage or elongation.

Question 8

How are extinction angles measured?

Answer 8

Based on the mineral’s orientation in thin-section.

Question 9

What does “length fast” mean in extinction angles?

Answer 9

The fast ray vibrates parallel to elongation (negative elongation).

Question 10

What does “length slow” mean in extinction angles?

Answer 10

The slow ray vibrates parallel to elongation (positive elongation).

Question 11

What is undulose extinction?

Answer 11

When different parts of a mineral grain go extinct at different times due to deformation.

Question 12

What is chemical zoning?

Answer 12

Variations in mineral composition create different extinction angles in different parts of the mineral.

Question 13

What are isotropic minerals?

Answer 13

Minerals where light moves at the same speed in all directions (e.g., cubic minerals, glass).

Question 14

What shape is the indicatrix for isotropic minerals?

Answer 14

Sphere

Question 15

What are anisotropic minerals?

Answer 15

Minerals where light splits into two polarized components traveling at different speeds.

Question 16

What shape is the indicatrix for anisotropic minerals?

Answer 16

Ellipsoid.

Question 17

What crystal systems do uniaxial minerals belong to?

Answer 17

Tetragonal, hexagonal, and trigonal systems.

Question 18

How many optic axes do uniaxial minerals have?

Answer 18

One

Question 19

Give an example of a uniaxial mineral.

Answer 19

Quartz (Uniaxial Positive) or Nepheline (Uniaxial Negative).

Question 20

What crystal systems do biaxial minerals belong to?

Answer 20

Orthorhombic, monoclinic, and triclinic systems.

Question 21

How many optic axes do biaxial minerals have?

Answer 21

Two

Question 22

Give an example of a biaxial mineral.

Answer 22

Feldspars (Biaxial).

Question 23

What does a positive uniaxial mineral look like?

Answer 23

Prolate indicatrix (stretched along the c-axis).

Question 24

What does a negative uniaxial mineral look like?

Answer 24

Oblate indicatrix (compressed along the c-axis).

Question 25

What is the 2V angle in biaxial minerals?

Answer 25

The angle between the two optic axes.

Question 26

What is the optic sign for biaxial minerals if "Blue Upper Right" appears?

Answer 26

Positive (BURP).

Question 27

What is the optic sign for biaxial minerals if "Orange Upper Right" appears?

Answer 27

Negative

Question 28

What is twinning in minerals?

Answer 28

The symmetrical intergrowth of two or more crystals of the same mineral.

Question 29

What is simple twinning?

Answer 29

Two intergrown crystals (e.g., hornblende, K-feldspar).

Question 30

What is polysynthetic twinning?

Answer 30

Multiple parallel twins (common in plagioclase feldspar).

Question 31

What is zoning in minerals?

Answer 31

Changes in color or extinction angle from core to rim.

Question 32

What causes zoning in minerals?

Answer 32

Chemical variations during mineral growth (common in tourmaline).

Question 33

What is the indicatrix?

Answer 33

Geometric representation of how light propagates through a mineral, showing variations in refractive indices in different directions

Question 34

What is the purpose of the Becke line test?

Answer 34

To determine the relative refractive index of a mineral compared to the surrounding medium

Question 35

How does the Becke line behave when the microscope stage is lowered?

Answer 35

The Becke line moves into the material with the higher refractive index. Vise Versa

Question 36

What is the key principle behind the Becke line test?

Answer 36

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.

Question 37

What is birefringence?

Answer 37

The difference in refractive indices (n₁ - n₂) of a doubly refracting mineral, observed under cross-polarized light (XPL).

Question 38

How can birefringence be estimated in thin section?

Answer 38

By observing interference colours using crossed polars and comparing them to a Michel-Lévy interference colour chart.

Question 39

What factors influence the observed birefringence of a mineral?

Answer 39

The mineral’s crystallographic orientation, thickness of the thin section, and the difference in refractive indices.

Question 40

What does BURP stand for in axial figure analysis?

Answer 40

Blue Upper Right Positive

Question 41

What does the presence of a black cross in an interference figure indicate?

Answer 41

uniaxial

Question 42

What type of minerals exhibit uniaxial optic figures?

Answer 42

Tetragonal, hexagonal, and trigonal minerals.

Question 43

What does it mean if you are looking straight down the C-axis of a mineral?

Answer 43

The mineral will appear opaque in conoscopic view.

Question 44

What is the role of the conoscopic light lens in axial figure tests?

Answer 44

It is a concave lens that helps scatter light to produce an interference figure.