Quiz:

1.  In a crystal cut normal to the optic axis, what will the indices of refraction be?

2.  In a crystal cut parallel to the optic axis, what will the indices of refraction be?

3.  In random section through the mineral, what do we call the indices of refraction? (one is n_w)

4.  If the mineral is optically positive, which is the fast ray in a random section through the mineral?

5.  In what section through the mineral will we observe the highest interference color?

6.  What section will never show pleochroism, even in a pleochroic mineral?

7.  What crystal systems are uniaxial?

8.  A mineral is gray in XP and the omega ray is oriented NE-SW. If it turns blue when the gypsum plate is inserted, is the mineral optically positive or negative?

9.  In a uniaxial negative mineral, is the indicatrix a plain or peanut m&m shape?

Geo 406 Lecture 07 - Conoscopy and uniaxial interference figures

Reading: Ch. 65-73

next: pp. 77-78, 80-84, 86-87, 104-107

Announcements

Midterm on Thursday Feb 7

Goals

Know how to set up the microscope for conoscopy

Understand how an interference figure is formed

Understand the features of a uniaxial interference figure

Be able to identify types of uniaxial figures

Be able to identify optic sign.

One more indicatrix detail

Think of a random cut through a uniaxial mineral

Draw indicatrix cut

Show epsilon and omega.

When mineral is extinct, what is orientation of indicatrix section?

Where is optic axis?

Conoscopy setup

High-power objective

Crossed polars

Condenser in

Bertrand lens in

or remove ocular and look down tube

(pinhole aperture)

Interference figure

[quartz (0001) section] shows Optic Axis figure

what happens when you rotate the stage

Terminology:

Isogyres - black bands

Melatope - where optic axis emerges

Isochromes - color rings

How figures form (Fig. 6.14, p. 67)

Condensor is like the inverse of a fish-eye lens

takes parallel light, and makes it converge

fish-eye takes converging light and makes it parallel

Each point corresponds to a direction through mineral

center is vertical

edges are inclined

Vibration directions:

recall earlier discussion: on random path through mineral, omega vibrates tangentially to optic axis, and epsilon prime vibrates radially (p. 55)

So draw figure, and mark different vibration directions in each region.

Isogyres

Show which directions are extinct

Isochromes

Show different paths that create isochromes

More inclined paths show higher interferene colors

Longer path length (greater d)

Greater n_s-n_f

Will it change when you rotate the stage?

Show vibration directions on figure.

Types of figures & optic sign

Centered optic axis figure

What grains in XP will produce this figure?

Show slow & fast directions for uniaxial positive mineral.

What happens when you insert the gypsum plate?

What would you see for a negative mineral?

Flash figure

What grains in XP will produce this figure?

Off-center optic axis figure

What grains in XP will produce this figure?