Transcript Lecture 16 (11/20/2006) Analytical Mineralogy Part 3: Optical Properties of Biaxial

Lecture 16 (11/20/2006)
Analytical Mineralogy
Part 3:
Optical Properties of Biaxial
Minerals
Optical 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
Isotropic
Isometric
Anisotropic – Uniaxial AnisotropicBiaxial
Orthorhombic
Tetragonal
Monoclinic
Hexagonal
Triclinic
Biaxial Indicatrix
Principal
vibration
axes
greatest n
lowest n
intermediate n
< ’<<’<
Circular Sections and Optic Axes
Circular
Section
Optic
Plane
Optic
Axes
Circular
Section
2V and the Optic Sign
Trace of
Circular
Sections
+
-
Random Section through the
Biaxial Indicatrix
Vibration
plane
parallel to
stage
Double
refraction
rays
=0
=max
Variable
Birefringence
within a
Biaxial
Mineral
Biaxial Optic Axis Figures
Look for a mineral with the
lowest interference colors,
i.e. ~0
Acute Bisectrix Figures (Bxa)
Melatope (emergence of optic axes)
Off-centered Bxa figure
Determining the Optic Sign of Biaxial Minerals
U
+
U
D
U
X
D
D
+
D
U
D
-
U
X
’ is fast ray
 is intermediate
’ is slow ray
U
D
Estimating 2V by Curvature of
Isogyre
Estimating 2V by Separation of
Isogyres
Extinction Angle
Symmetrical
Parallel
Inclined
Sign of Elongation
Example – Length slow
slow
ray
Slowing down
the slow ray
Interference
colors increase
Next Lecture 11/22/06
Principles of X-ray Mineralogy
Read:
Klein p. 309-321