OPTICAL MINERALOGY AND PETROGRAPHY (CONOSCOPIC BIAXIAL)

Question 1

Conoscopic Setup

Answer 1

Light Source In
Polarize In
Condenser In
Aperture Diaphragm Open
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

Biaxial Indicatrix

Answer 3

Ellipsoid

Question 4

Relationship of the three indices of Redraction?

Answer 4

Na

Question 5

Maximum Birefringence

Answer 5

Nc-Na

Question 6

Parts of a Biaxial Indicatrix

Answer 6

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)

Question 7

Biaxial Interference Figures (5)

Answer 7

Centered Acute Bisectrix
Centred Obtuse Bisectrix
Centered Optic Axis
Centered Optic Normal
Random Orientations

Question 8

What does the isochrome distance to the melatope imply in terms of birefrigence

Answer 8

Nearer the meltope, light experiece lower birefirefringence and has a lower retardation

Question 9

What can you expect to see in a consocopic observation when the mineral has thick crystals and high birefringence?

Answer 9

Numerous Isochromes

Question 10

Distinctive Characteristics of a Centered Acute Bisectrix

Answer 10

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

Question 11

Angle at which the two melatopes will still be si the Field of view in centered acute bisectrix

Answer 11

2V<40 deg

Question 12

When the one of the optic axis is vertical or perpendicular to the stage the interference figure is said to be ________________

Answer 12

Centered Biaxial Optic Axis

Question 13

If 2V>50 what ca you expect to see in a centered biaxial optic axis?

Answer 13

Only one melatope with a straight horizontal isogyre

Question 14

When you rotate the stage towards the 45deg in a centered biaxial optic axis, Curvature is at its?

Answer 14

Maximum

Question 15

In the 45 deg position where the isogyre curvature is at its maximum, where lies the Bxa?

Answer 15

at the convex side (Nasa tuktok)

Question 16

Using the Isogyre Curvature 2V ange can be approximated, if the isogyre is straight 2V is (Observed at 45 deg position)

Answer 16

90 deg (The more the isogyre arms is straighter, the higher the angle in the 45 degree position)

Question 17

This is the interference figure when the obstuse bisectrix is perpendicular to the stage

Answer 17

Centered Obtuse Bisectrix

Question 18

Since the angle between the two optic axis is greater than 45 degrees, the image of a centered obtuse bisectrix will be

Answer 18

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

Question 19

An optically neutral mineral has a 2V of?

Answer 19

90 deg

Question 20

When will a Biaxial flash figure or Bixial Optic Normal show?

Answer 20

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

Question 21

at this angle no isogyre will show in the field of view

Answer 21

45 deg

Question 22

Interference figure where there is any combination not mentioned above

Answer 22

Off centred figures or Random Orientations

Question 23

A biaxial mineral is optically positive if

Answer 23

Bxo = X (shortest axis, fast ray Na)

Question 24

A biaxial mineral is optically negative id

Answer 24

Bxo = Z (longest axis, slow ray Nc)

Question 25

Therefore to find if a mineral is positive or negative, one must

Answer 25

Find if the Bxa or Bxo correspods to X or Z axis

Question 26

Steps to identify Optic sign using Centered Acute Bisectrix

Answer 26

1) Obtain a centred acute bisctrix where the cross if formed (at extinction or where optic plane is at 90 degrees
2) Rotate the stage so that optic plane will be inclined to the 45deg where the cross of isogyres should split
3) Insert Gypsum plate and observe the change in interference color in between the two melatopes (sa labas ng curves) and in the concave part (sa loob ng curve)

Question 27

If the interference color in between the melatope becomes blue and the interference colors in the concave side becomes yellow

Answer 27

Optically Negative

Question 28

If the interference color in between the melatope becomes yellow and the interference colors in the concave side (sa loob ng curves) becomes blue then it is

Answer 28

Optically Positive

Question 29

When using qtz wedge and the isochromes move inwards to the center of the curve the mineral is said to be

Answer 29

Optically positive

Question 30

When isochromes move outwards away from the center of the curve (FOCUS LANG sa loob ng curve na nasa NE,SW diretcion) the mineral is said to be

Answer 30

Optically Negative

Question 31

If 2V=90 can optic signe be distinguushed?

Answer 31

No, because mieral is optically Neutral

Question 32

If 2V=0 can optic signe be distinguished?

Answer 32

No, because mineral is uniaxial (only 1 Optic Axis)

Question 33

If a mineral is biaxial how may pleochroic color will it have?

Answer 33

3

Question 34

Pleochroism of Hornblende

Answer 34

X=yellow (fast)
Y=Pale green (Intermediate)
Z=Green (Slow)

Question 35

Extinction of Bixial minerals is a function of

Answer 35

Cleavage and crystal habit (NOT TWININNING!)

Question 36

if X-axis is parallel to the length the sign of elongation is?

Answer 36

Length Fast

Question 37

If Zaxis is parallel to the length the sign of elongation is?

Answer 37

Length Slow

Question 38

The indices of refraction is measured by?

Answer 38

Grain Mount Techniques

Question 39

Is it possible to determine the exact index of refraction in this section?

Answer 39

No