Introduction to Mineralogy Dr. Tark Hamilton Chapter 13: Lecture 20 Optical Mineralogy & Petrography: Snell’s Law

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

Optical Parameters for Amphiboles

• fig_13_01

• Anthophyllite
• Orthorhombic mmm
• γ = 1.664-1.708
• β = 1.647-1.689
• α = 1.639-1.671
• δ = 0.02+/-.003

• Cummingtonite
• Monoclinic 2/m
• γ = 1.615-1.697
• β = 1.605-1.685
• α = 1.598-1.674
• δ = 0.02+/-.003

• Klein (1964)
• Wabush Iron Fm
• Labrador

• (Mg,Fe)2(Mg,Fe)5(OH)2O22

• Grunerite

• Beware Crocidolite
• Fibrous amphiboles

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”

Snell’s Law & Refractive Index

• fig_13_03

• Light is refracted towards the vertical for
• any angle < 90° entering denser media.

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

Dispersion: different wavelengths refract at different angles

• fig_13_04

• For light of a fixed frequency, since C = λ x ν , red light
• has a greater velocity than violet due to its longer wavelength

Total Internal Reflection & the Critical Angle

• fig_13_05

• Air, n = 1.0

• DOD’ = Critical Angle, ni / nr sin i = 1.0
• No emergent light
• (non destructive testing of gems)

Pulfrich Refractometer

• fig_13_06

• Nglass = 1.90 , C.A. = 50°  nmineral = 1.455 Opal !

• Unknown mineral

• Glass hemisphere

• telescope

Plane Polarized Light

• fig_13_07

• Circularly or randomly polarized light

Polarizing Substances:  transmission directions

• fig_13_08

• Superposed Tourmaline Crystals

• Superposed “Ray-ban” sheets

Polarized Light by Reflection & Refraction

• fig_13_09

• Planar Surfaces (including minerals) absorb or scatter
• vibration directions which are not parallel to the surface.
• Reflected or transmitted light becomes plane polarized.

Isotropic Minerals & Relief in n=1.54

Mineral	Fluorite	Sodalite	Pyrope	Diamond
Index of Refraction	1.433	1.483	1.714 – 1.750	2.417
Relief	- Very low	- Low	+ High	+ Extreme

Isotropic Crystals & the Becke Line

• fig_13_11

• Mineral in Focus
• Showing Relief
• nmineral > noil

• Mineral in Focus
• Lowered stage
• nmineral > noil
• Becke Line moves in

• Mineral in Focus
• Lowered stage
• nmineral < noil
• Becke Line moves out