Optical Mineralogy

Question 1

Isometric crystal system

Answer 1

Axes: 3
Angles: 3 at 90 degrees
Sides: a1 = a2 = a3
Optical properties: isotopic
Optic axes: infinite

Question 2

Hexagonal crystal system

Answer 2

*EXTINCTION: Parallel
Axes: 4
Angles: 3 at 60 degrees
Sides: a1 = a2 = a3 != c
Optical properties: UNIaxial (type of anisotropy)
Optic axes: 1

Question 3

Tetragonal crystal system

Answer 3

*EXTINCTION: Parallel
Axes: 3
Angles: 3 at 90 degrees
Sides: a1 = a2 != c
Optical properties: UNIaxial (type of anisotropy)
Optic axes: 1

Question 4

Orthorhombic crystal system

Answer 4

**EXTINCTION: Parallel
Axes: 3
Angles: 3 at 90 degrees
Sides: a != b != c
**Optical properties: BIaxial (type of anisotropy)
**Optic axes: 2

Question 5

Monoclinic crystal system

Answer 5

*EXTINCTION: Angular
Axes: 3
Angles: 2 at 90 degrees
Sides: a != b != c
Optical properties: BIaxial (type of anisotropy)
Optic axes: 2

Question 6

Triclinic crystal system

Answer 6

*EXTINCTION: Angular
Axes: 3
Angles: 0 at 90 degrees
Sides: a != b != c
Optical properties: BIaxial (type of anisotropy)
Optic axes: 2

Question 7

Uniaxial optical properties & associated crystal systems

Answer 7

Uniaxial = 1 optic axis
(Type of anisotropy)

Angle size varies
# of angles = 3
Axis c length != axes a’s
# of axes varies

Uniaxial —> parallel extinction
BUT, not the other way around!
Parallel extinction != uniaxial (b/c orthorhombic is biaxial, but has parallel extinction)

Question 8

Biaxial optical properties & associated crystal systems

Answer 8

Biaxial = 2 optic axes
(Type of anisotropy)

# of axes: 3
Lengths of axes: ALL UNEQUAL
Angle size: varies (n/a or 90 degrees)
# of angles = varies (0-3)

Angular extinction —> biaxial
BUT, not the other way around!
Biaxial != angular extinction (b/c orthorhombic is biaxial, but has parallel extinction)

Question 9

Constructive interference

Answer 9

Light waves w/similar behaviors interact & combine resulting in AMPLIFIED light wave

Question 10

Destructive interference

Answer 10

Light waves that are opposite/off interact & cancel each other out —> no light waves

Question 11

Opaque minerals

Answer 11

No light passes through the mineral, regardless of sample thickness

Question 12

Isotopic

Answer 12

Isotopic = infinite optic axes

Speed of light is equal in all directions

Question 13

Anisotropic

Answer 13

Cause of extinction

In most directions, light splits into two rays w/two different speeds; vibrating at RIGHT angles

Uniaxial/biaxial: only specific directions: light isn’t split, i.e. behaves isotropically in specific directions

When optic axes are aligned with cross hairs of microscope: extinction occurs!

Question 14

Amphibole vs. pyroxene, in thin section

Answer 14

Amphibole:

• 60/120 degrees
• double chain
• WIDER I-beam structure b/c of A site and OH

Pyroxene:

• 90/90 degrees
• single chain

Question 15

Pleochroism

Answer 15

Change in colors as mineral is rotated on stage in PPL

Question 16

Birefringence

Answer 16

Change in BRIGHTNESS of color as mineral is rotated on stage in XPL