OPTICAL MINERALOGY

Question 1

What is the melatope in an interference figure?

Answer 1

The point of maximum birefringence

Question 2

What does an isogyre represent in an interference figure?

Answer 2

A colored pattern indicating birefringence

Question 3

What type of interference figure exhibits a stationary cross with concentric isochromes?

Answer 3

Biaxial figure

Question 4

In biaxial minerals, how many melatopes can be observed in an interference figure?

Answer 4

1

Question 5

Which accessory plate is commonly used to determine the optic sign of a mineral?

Answer 5

Quartz wedge

Question 6

When using a gypsum plate, how can you identify a uniaxial positive mineral?

Answer 6

Blue addition and yellow subtraction in NE-SW direction

Question 7

Which mineral is most likely to display a uniaxial interference figure?

Answer 7

Feldspar

Question 8

What is the significance of isochromes in an interference figure?

Answer 8

They indicate extinction positions

Question 9

What happens to isogyres in a biaxial interference figure as the stage rotates?

Answer 9

They remain stationary

Question 10

In a uniaxial interference figure, where is the melatope located?

Answer 10

At the edge of the isogyre

Question 11

What is the optic axis of a mineral?

Answer 11

The line along which the mineral displays double refraction

Question 12

What happens in a flash figure?

Answer 12

The isogyres appear as a diffuse cross

Question 13

What does the solvus line represent on a phase diagram?

Answer 13

The boundary between solid and liquid phases

Question 14

What does the solidus line represent?

Answer 14

The limit of solubility between two solid phases

Question 15

In a phase diagram, where does 100% liquid exist?

Answer 15

Below the solidus

Question 16

What is the function of a tie line on a phase diagram?

Answer 16

It separates solid from liquid phases

Question 17

What reaction occurs at the eutectic point?

Answer 17

Solid1 + Solid2 → Liquid

Question 18

What is the peritectic reaction?

Answer 18

Liquid → Solid1 + Solid2

Question 19

Which phase diagram shows a eutectic system?

Answer 19

Pb-Sn alloy

Question 20

What is the angle between the melatopes in a biaxial interference figure known as?

Answer 20

2V angle

Question 21

In a binary eutectic system, what occurs at the eutectic point?

Answer 21

Complete solidification into one solid phase

Question 22

What is pleochroism?

Answer 22

Change in mineral color depending on thickness

Question 23

Which type of pleochroism is found in uniaxial minerals?

Answer 23

Dichroism

Question 24

Which type of pleochroism occurs in biaxial minerals?

Answer 24

Dichroism

Question 25

Which mineral can show pleochroism from brownish-green to dark red?

Answer 25

Andalusite

Question 26

What is the cause of pleochroism?

Answer 26

Different absorption of light along different crystal axes

Question 27

Which mineral can display colors ranging from light purple to deep purple?

Answer 27

Spodumene

Question 28

What is the optical property of an isotropic mineral?

Answer 28

No birefringence and constant color in XPL

Question 29

Which crystal system typically displays trichroism?

Answer 29

Uniaxial system

Question 30

Which of the following is an idiochromatic mineral?

Answer 30

Malachite