OPTICAL MINERALOGY Flashcards
What is the melatope in an interference figure?
The point of maximum birefringence
What does an isogyre represent in an interference figure?
A colored pattern indicating birefringence
What type of interference figure exhibits a stationary cross with concentric isochromes?
Biaxial figure
In biaxial minerals, how many melatopes can be observed in an interference figure?
1
Which accessory plate is commonly used to determine the optic sign of a mineral?
Quartz wedge
When using a gypsum plate, how can you identify a uniaxial positive mineral?
Blue addition and yellow subtraction in NE-SW direction
Which mineral is most likely to display a uniaxial interference figure?
Feldspar
What is the significance of isochromes in an interference figure?
They indicate extinction positions
What happens to isogyres in a biaxial interference figure as the stage rotates?
They remain stationary
In a uniaxial interference figure, where is the melatope located?
At the edge of the isogyre
What is the optic axis of a mineral?
The line along which the mineral displays double refraction
What happens in a flash figure?
The isogyres appear as a diffuse cross
What does the solvus line represent on a phase diagram?
The boundary between solid and liquid phases
What does the solidus line represent?
The limit of solubility between two solid phases
In a phase diagram, where does 100% liquid exist?
Below the solidus
What is the function of a tie line on a phase diagram?
It separates solid from liquid phases
What reaction occurs at the eutectic point?
Solid1 + Solid2 → Liquid
What is the peritectic reaction?
Liquid → Solid1 + Solid2
Which phase diagram shows a eutectic system?
Pb-Sn alloy
What is the angle between the melatopes in a biaxial interference figure known as?
2V angle
In a binary eutectic system, what occurs at the eutectic point?
Complete solidification into one solid phase
What is pleochroism?
Change in mineral color depending on thickness
Which type of pleochroism is found in uniaxial minerals?
Dichroism
Which type of pleochroism occurs in biaxial minerals?
Dichroism
Which mineral can show pleochroism from brownish-green to dark red?
Andalusite
What is the cause of pleochroism?
Different absorption of light along different crystal axes
Which mineral can display colors ranging from light purple to deep purple?
Spodumene
What is the optical property of an isotropic mineral?
No birefringence and constant color in XPL
Which crystal system typically displays trichroism?
Uniaxial system
Which of the following is an idiochromatic mineral?
Malachite
