Optical Properties

Question 1

Colour

Answer 1

PPL

Question 2

Pleochroism

Answer 2

Change in intensity of in colour as stage is rotated at 90°

Only applies to colored minerals in the thin section

Question 3

Relief

Answer 3

Degree to which mineral grain(s) appear to stand out compared to mineral next to it
(BECKE LINES ASSIST)
High: stands out strongly,
Moderate: does not strongly stand out but is still visible
Low: does not stand out from mounting medium
(+/- relief)

Question 4

Cleavage

Answer 4

Plane on which a mineral prefers to be split/parted. Appear as straight lines.

Question 5

Fracture

Answer 5

Exposure to mechanical stress (preferred orientation)
Uneven/irregular
Concohidal
Hackly

Question 6

Crystal habit

Answer 6

External shape.

Euhedral
Subhedral
Anhedral

Question 7

Birefringence and interference colours

Answer 7

Bire, maximum possible difference I reflective indices of a mineral, seen as interference colours under(XPL)

Question 8

Extinction Angle

Answer 8

Vibration direction parallel with lower polarizer

Question 9

Types of extinction

Answer 9

Parallel: cleavage aligned with one crosshair[ EA=0°]
Inclined: Cleavage angle to crosshairs[EA>0°]
Symmetrical: two cleavages, 2 extinction angles can be measured, if two angles are= then it’s Symmetrical extinction

Question 10

Twinning

Answer 10

Intergrowths of 2 or more minerals

Question 11

Zoning

Answer 11

Changes in crystal between its core and outer rim. Indicates change in composition of crystal