OPTICAL MINERALOGY AND PETROGRAPHY (CONOSCOPIC BIAXIAL) Flashcards
Conoscopic Setup
Light Source In
Polarize In
Condenser In
Aperture Diaphragm Open
HIGHEST POWERED OBJECTVE
Analyzer In
Bertrand Lens IN
Without Bertrand Lends, How can IF be observed?
Remove Ocular or
Insert Pinhole Ocular
Biaxial Indicatrix
Ellipsoid
Relationship of the three indices of Redraction?
Na
Maximum Birefringence
Nc-Na
Parts of a Biaxial Indicatrix
2 Optic Axis (OA)
1 Optic Plane
2 Circular Sections
Optic Angle (2V)
Acute Bisectrix (Bxa)
Obtuse Bisectrix (Bxo)
Optic Plane (Plane parallel to the Z-axis)
Optic Normal (Plane perpendicular to the Z-axis)
Biaxial Interference Figures (5)
Centered Acute Bisectrix
Centred Obtuse Bisectrix
Centered Optic Axis
Centered Optic Normal
Random Orientations
What does the isochrome distance to the melatope imply in terms of birefrigence
Nearer the meltope, light experiece lower birefirefringence and has a lower retardation
What can you expect to see in a consocopic observation when the mineral has thick crystals and high birefringence?
Numerous Isochromes
Distinctive Characteristics of a Centered Acute Bisectrix
Most often have 2 melatopes
w/ tear drop or 8-shaped ischrome
Cross formed by isogyres where arms are thinner along the melatope and thicker along the optic normal
When stage is rotated 45 deg, the isogyre cross will separate to form hyperbolas towards the optic plane
Angle at which the two melatopes will still be si the Field of view in centered acute bisectrix
2V<40 deg
When the one of the optic axis is vertical or perpendicular to the stage the interference figure is said to be ________________
Centered Biaxial Optic Axis
If 2V>50 what ca you expect to see in a centered biaxial optic axis?
Only one melatope with a straight horizontal isogyre
When you rotate the stage towards the 45deg in a centered biaxial optic axis, Curvature is at its?
Maximum
In the 45 deg position where the isogyre curvature is at its maximum, where lies the Bxa?
at the convex side (Nasa tuktok)
Using the Isogyre Curvature 2V ange can be approximated, if the isogyre is straight 2V is (Observed at 45 deg position)
90 deg (The more the isogyre arms is straighter, the higher the angle in the 45 degree position)
This is the interference figure when the obstuse bisectrix is perpendicular to the stage
Centered Obtuse Bisectrix
Since the angle between the two optic axis is greater than 45 degrees, the image of a centered obtuse bisectrix will be
No melatopes since both will be outside the field of view with thicker arms of the cross both NS EW isochrome is still teardrop but more zoomed
An optically neutral mineral has a 2V of?
90 deg
When will a Biaxial flash figure or Bixial Optic Normal show?
When the Optic Normal is vertical to stage or the field of view and Bxo, Bxa, and optic axes lie horizontal or along the stage
at this angle no isogyre will show in the field of view
45 deg
Interference figure where there is any combination not mentioned above
Off centred figures or Random Orientations
A biaxial mineral is optically positive if
Bxo = X (shortest axis, fast ray Na)
A biaxial mineral is optically negative id
Bxo = Z (longest axis, slow ray Nc)