Introduction to MineralogyDr. Tark Hamilton

Chapter 13: Lecture 20Optical 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

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Optical Parameters for Amphiboles

AnthophylliteOrthorhombic mmm

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

CummingtoniteMonoclinic 2/mγ = 1.615-1.697β = 1.605-1.685α = 1.598-1.674δ = 0.02+/-.003

Klein (1964)Wabush Iron Fm

Labrador

(Mg,Fe)2(Mg,Fe)5(OH)2O22

Grunerite

Beware CrocidoliteFibrous amphiboles

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Electrical Vector of Plane Polarized Light: Time varying potential field at x

Light will travel according to this vector in “Field Free Space”Vector addition occurs in crystal fields according to “Optical Density”

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Snell’s Law & Refractive Index

Light is refracted towards the vertical forany 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

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Dispersion: different wavelengths refract at different angles

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

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Total Internal Reflection& the Critical Angle

Air, n = 1.0

DOD’ = Critical Angle, ni / nr sin i = 1.0No emergent light

(non destructive testing of gems)

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Pulfrich Refractometer

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

Unknown mineral

Glass hemisphere

telescope

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Plane Polarized Light

Circularly or randomly polarized light

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Polarizing Substances: transmission directions

Superposed Tourmaline Crystals Superposed “Ray-ban” sheets

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Polarized Light by Reflection & Refraction

Planar Surfaces (including minerals) absorb or scattervibration 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

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Isotropic Crystals & the Becke Line

Mineral in FocusShowing 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