1. Introduction to Mineralogy Dr
Introduction to Mineralogy Dr. Tark Hamilton Chapter 13: Lecture 20 Optical Mineralogy & Petrography: Snell’s Law
Camosun College GEOS 250
Lectures: 9:30-10:20 M T Th F300
Lab: 9:30-12:20 W F300

2. Optical Parameters for Amphiboles
Klein (1964)
Wabush Iron Fm
Labrador
Cummingtonite
Monoclinic 2/m
γ =
β =
α =
δ = 0.02+/-.003
(Mg,Fe)2(Mg,Fe)5(OH)2O22
Anthophyllite
Orthorhombic mmm
γ =
β =
α =
δ = 0.02+/-.003
Grunerite
Beware Crocidolite
Fibrous amphiboles
fig_13_01

3. Light will travel according to this vector in “Field Free Space”
Electrical Vector of Plane Polarized Light: Time varying potential field at x
fig_13_02
Light will travel according to this vector in “Field Free Space”
Vector addition occurs in crystal fields according to “Optical Density”

4. Snell’s Law & Refractive Index
Light is refracted towards the vertical for
any angle < 90° entering denser media.
fig_13_03

5. Refractive Index, Optical Density & the Speed of Light in Media
C = 3 x 108 m/s or 3 x 1017 nm/s in vacuo
Cm < in denser media (more electrons w/mass)
n = C / Cm or n = Cair / Cmineral (Refractive Index)
i = angle of incident PPL
r = angle of refracted PPL
Sin i / Sin r = ni / nr = ci / cr
Sin r = (ni /nr ) Sin i
C = λ x ν , velocity = wavelength x frequency

6. Dispersion: different wavelengths refract at different angles
For light of a fixed frequency, since C = λ x ν , red light
has a greater velocity than violet due to its longer wavelength
fig_13_04

7. Total Internal Reflection & the Critical Angle
Air, n = 1.0
DOD’ = Critical Angle, ni / nr sin i = 1.0
No emergent light
(non destructive testing of gems)
fig_13_05

8. Pulfrich Refractometer
Unknown mineral
telescope
Glass hemisphere
Nglass = 1.90 , C.A. = 50°  nmineral = Opal !
fig_13_06

9. Circularly or randomly polarized light
Plane Polarized Light
Circularly or randomly polarized light
fig_13_07

10. Polarizing Substances:  transmission directions
Superposed Tourmaline Crystals
Superposed “Ray-ban” sheets
fig_13_08

11. Polarized Light by Reflection & Refraction
Planar Surfaces (including minerals) absorb or scatter
vibration directions which are not parallel to the surface.
Reflected or transmitted light becomes plane polarized.
fig_13_09

12. Isotropic Minerals & Relief in n=1.54
Fluorite
Sodalite
Pyrope
Diamond
Index of Refraction
1.433
1.483
1.714 –
1.750
2.417
Relief
- Very low
- Low
+ High
+ Extreme

13. Isotropic Crystals & the Becke Line
Mineral in Focus
Lowered stage
nmineral < noil
Becke Line moves out
Mineral in Focus
Showing Relief
nmineral > noil
Mineral in Focus
Lowered stage
nmineral > noil
Becke Line moves in
fig_13_11