Lecture 16 (11/20/2006)Lecture 16 (11/20/2006)

Analytical MineralogyAnalytical Mineralogy

Part 3:Part 3:

Optical Properties of Biaxial Optical Properties of Biaxial MineralsMinerals

Optical IndicatricesOptical Indicatrices• A 3-d map of the indices of refraction for various vibration directions of light rays• Orientation of the indicatrix within a mineral is symmetrical with the

crystallographic axis

IsotropicIsotropic

IsometricIsometric

Anisotropic – UniaxialAnisotropic – Uniaxial

TetragonalTetragonalHexagonalHexagonal

Anisotropic-Anisotropic-BiaxialBiaxial

OrthorhombicOrthorhombicMonoclinicMonoclinic

TriclinicTriclinic

Biaxial IndicatrixBiaxial IndicatrixPrincipal vibration axes

greatest nn

lowest nn

intermediate nn

< < ’<’<<<’<’<

Circular Sections and Optic AxesCircular Sections and Optic AxesCircular Circular SectionSection

Circular Circular SectionSection

Optic Optic AxesAxes

Optic Optic PlanePlane

2V and the Optic Sign2V and the Optic Sign

+ -

Trace of CircularSections

Random Section through the Random Section through the Biaxial IndicatrixBiaxial Indicatrix

Vibration plane parallel to stage

Double refractionrays

Variable Variable Birefringence Birefringence

within a within a Biaxial Biaxial MineralMineral

=0

=max

Biaxial Optic Axis FiguresBiaxial Optic Axis Figures

Look for a mineral with the Look for a mineral with the lowest interference colors, i.e. lowest interference colors, i.e. ~0~0

Acute Bisectrix Figures (Bxa)Acute Bisectrix Figures (Bxa)

Melatope (emergence of optic axes)Melatope (emergence of optic axes)

Off-centered Bxa figureOff-centered Bxa figure

Determining the Optic Sign of Biaxial MineralsDetermining the Optic Sign of Biaxial Minerals

DD

++

DD

--UU

UU

DD

DD

DD

DD

’ ’ is fast rayis fast ray

is intermediateis intermediate

’ ’ is slow rayis slow ray

--

++

X

X

UU

UU

UU

UU

Estimating 2V by Curvature of Estimating 2V by Curvature of IsogyreIsogyre

Estimating 2V by Separation of Estimating 2V by Separation of IsogyresIsogyres

Extinction AngleExtinction Angle

SymmetricalSymmetrical ParallelParallel InclinedInclined

Sign of ElongationSign of Elongation

slowslowrayray

Example – Length slowExample – Length slowInterference Interference colors increasecolors increase

Slowing down Slowing down the slow ray the slow ray

Next Lecture 11/22/06Next Lecture 11/22/06

Principles of X-ray MineralogyPrinciples of X-ray Mineralogy

Read: Read: Klein p. 309-321Klein p. 309-321