Optical Techniques

1
Optical Techniques

• Biaxial Minerals

Sphene
2
Biaxial Minerals

• Anisotropic
• Orthorhombic, Monoclinic, Triclinic

Goethite
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Procedure for Interference Figures

• Focus on the grain of interest using a low power
  objective
• Switch to a higher powered objective
• Flip the Bertrand Lens
• Insert the accessory plate

4
Biaxial Sign

• Resemble /- sign in Uniaxial Minerals
• quadrant colors are the same as uniaxial
  minerals
• Determined in the same manner
• Biaxial Positive
• Biaxial Negative

5
Centered Optic Axis Figure

• One melatope is centered under the crosshairs

Melatope
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2V angle

• Angle between the acute optic axes
• Estimated from the curve of the isogyre

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2V Angle

• Generally, the straighter the isogyre, the larger
  the 2V angle
• 0 lt 2V lt 90

90
melatope
2V 30
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Optic Angle

• Acute Bisectrix
• Bxa
• Seen when the acute angle is oriented - to the
  stage
• Obtuse Bisectrix
• Bxo
• Seen when the obtuse angle is oriented - to the
  stage

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Acute Bisectrix

• BXa
• Bisects the acute (2V) angle
• Isochromes appear as an oval or figure 8 around
  the melatopes

10
Obtuse Bisectrix

• BXo
• Bisects the obtuse angle
• Isochromes appear to curve outward