Earth Materials Lecture 5: Introduction to Petrography

Question 1

Define Petrography

Answer 1

Laboratory based technique for studying
rocks and minerals in thin section.

Question 2

How is Petrography conducted?

Answer 2

By use of tools:

− Petrological (polarizing) microscope – for making
routine observations using transmitted light that is
polarized = plane polarized light (PPL).
− Rocks cut into thin section = 30 μm thick.
− Upper analyser in place = cross-polarized light (XPL)

Question 3

What is the purpose of petrography/main uses

Answer 3

Allows us to observe rocks and minerals in a level of detail not available in hand specimens. Features and colours of minerals are made more obvious under microscope.

Used in rock classification.

Question 4

Describe lights role and nature in petrography

Answer 4

Light is a transverse waveform
Light leaving a source vibrates in all directions.

Minerals can polarize light.

Question 5

How do electrons affect the behavior of light in minerals

Answer 5

Electric component of light is affected by electron
clouds surrounding the atoms that comprise the crystal
lattice.

Density and distribution of clouds affects the behaviour
of light in that mineral.

In these crystal lattices, light can vibrate better in some directions than others. (Permitted ray vibration directions).

Question 6

Define Refraction

Answer 6

Incident light passing from one
substance to another is bent at the
interface if the two substances are
made from materials with different
properties.