lab 4- plain polarized light

Isotropic mineral properties under microscope

-the velocity of light that passes through mineral is equal in all directions

-no double refraction

-PPL:see true color of mineral

XPL: dark (extinct)

Anisotropic mineral properties under microscope

-velocity of light is not equal in all direction

-double refraction

How to use the microscope under
Plane Polarized Light?

1. set the microscope; hold base and arm

2. turn on light and adjust

3. place slides on the microscope stage, right side up

4. Always start on lowest mag.

5. focus with course focusing know until you can clearly see mineral in slide

6. rotate to next highest power, sont skip middle one

7. focuse with fine focusing knob

Observable Properties under
PPL

Visual properties for ID:

-color (including pleochroism

-cleavage

Refraction related properties:

-relief

-Becke line

Color and pleochroism

-Isotropic minerals: generally no Pleochroism

-isometric minerals

Anisotropic minerals: may or may not have pleochroism (weak or strong), color changes as stage is rotated

Cleavage

Perfect: the cleavage is cutting through the whole grain, dense cleavage lines

Good: the cleavage is clear but not that dense and consistent

Fair: the cleavage is clear but relatively sparese and may not be that consistent

poor: the cleavage is very sparse and sometimes accompanied with fracture or parting.

none: no cleavage, nut sometimes shows fracture or parting

Refraction related properties

refractometry refers to the measurement of index of refraction

Relief and Beacke lines are used to compare the index of refraction of mineral and the epoxy or surrounding minerals

Relief

Becke line

A rim of light alonf the edge of grain

best seen under highest mag power

lower the stage to see movement of beckeline

tips: 1. focus the mineral before doing test

2. adjust the light intensity to best reveal becke line

negative vs positive relief

inward: positive relief

outward: negative relief