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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
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