OPTICAL MINERALOGY AND PETROGRAPHY (CONOSCOPIC UNIAXIAL)

Question 1

Conoscopic Setup

Answer 1

Light Source In
Polarize In
Condenser In
HIGHEST POWERED OBJECTVE
Analyzer In
Bertrand Lens IN

Question 2

Without Bertrand Lends, How can IF be observed?

Answer 2

Remove Ocular or
Insert Pinhole Ocular

Question 3

A combination of EXTINCTION and INTERFERENCE COLORS Produced by rays going through the sample at different angles which consists of STRONGLY CONVERGING LIGHTS under the conoscopic illumination

Answer 3

Interference Figure

Question 4

Uses of Interference figure

Answer 4

Distinguish between Uniaxial and Biaxial
Determine Optic Sign

Question 5

Two arms of the cross in N-S, E-W direction where vibration are perpendicular

Answer 5

Isogyres

Question 6

The point where isogyres corss and where the optic axis emerges in the interference figure

Answer 6

Melatope

Question 7

Bands of Equal retardation which increases from the center or
Interferece colors indentical to those on the colour char increases order from the melatope outwards

Answer 7

Isochromes

Question 8

?Circular/Basal Section
?Vertical Optic Axis
?centred black cross superimposed on circular bands of Int. Colors
?Does not move when stage is rotated (Stationary)
?Since optical Axis is vertical, optical properties vary symmetrically
?Number of isochromes depends on retardation and thickness
?Birefringence 0

Answer 8

Centered Optic Axis Figure

Question 9

In a centered optic axis the Ordinary ray (w) vibrates in what direction
Like Latitude on the surface of the indicatrix

Answer 9

PERPENDICULAR to the C-axis conincides with the isochrome

Question 10

In a centered optic axis the Extraordinary (e) ray vibrates in what direction
Like Longitude on the surface of the indicatrix

Answer 10

PARALLEL or RADIAL to the C-axis

Question 11

?Random Section
?Melatope lies outside the FOV the isogyre fattens as the distance from the melatope increases
?Optic axis is not vertical
?On rotation, melatope swing in a circle around the center of the field of view
?Birefringence is intermediate

Answer 11

Off Centered Optic Axis Figure

Question 12

?Mineral grain is oriented with its optic axis horizontal
?Birefringence is Maxmimum
?result because vibration of the directions are nearly parallel to the polarisation of the microscope
?Horizontal or Principal Section

Answer 12

Uniaxial Flash Figure

Question 13

When will the isogyre cross be split in two and rapidly leave the FOV in the quadrants into which the Optic Axis is aligned

Answer 13

<5 deg rotation rotation

Question 14

at this position no isogyres will be present and the field of view may exhibit some interference colors or isochromes

Answer 14

<10 deg rotation

Question 15

Optically Positive

Answer 15

w=fast e=slow

Question 16

Optically negative

Answer 16

w=slow e=fast

Question 17

Set up to determine optic sign

Answer 17

Conoscopic with Centered Optic Axis
Insert Gypsum Plate (Compensator)
Look at the NE quadrant of the interference figure

Question 18

Why NE Quadrant

Answer 18

It corresponds to the Extraordinary Ray Vibration

Question 19

If the interference color of the isochrome in the NE quadrant increases in order (Yellow to blue), the Extraordinary wave is the slow ray

Answer 19

Optically positive

Question 20

If the interference color in the NE quadrant decreases Blue to yellow, the extraordinary ray is the fast ray

Answer 20

Optically Negative

Question 21

Using quartz wedge if the ischromes move outward away from the melatope (NE-SW)

Answer 21

Optically Negative

Question 22

Using quartz wedge if the isochromes move inwards towards the melatope (NE-SW Direction)

Answer 22

Optically Positive