1. Identification of minerals with the petrographic microscope
Chapter 10 - A
Identification of minerals with the petrographic microscope

2. Content Sample preparation Microscope alignment
Determination of the refractive index
Use of interference colors
Conoscopic observation of interference figures

3. Microscopy Transmitted light microscopy Reflected light microscopy
Transparent crystals
Light transmits through mineral grains
Common rock-forming minerals
Reflected light microscopy
Opaque crystals
Light reflects from highly polished surface
Usually ore minerals
 This course: transmitted light microscopy

4. Sample preparation: Transmitted light microscopy
Grain mount:
Finely ground fragments; immersed in oil and scattered on glass plate; covered by thin sheet of glass
Thin section:
Cut slab from rock sample – area of interest
Bottom - polished and cemented onto glass slide
Top - ground to desired thickness; covered with balsam and thin cover glass
Rock-forming minerals now transparent

5. Why use a cover glass?

6. Microscope alignment Important in order to:
have light going through the center of all lenses, of the stage, the condenser
get two polarizers filtering light at vibration directions perpendicular to each other
Oculars – one or both adjusted for each eye; cross-hair in focus
Stage – center exactly in the optic axis; object not to move during stage rotation
Condenser – when switched on light beam should be centered around cross-hair
Polarizer – one set at 0º and one at 90º

7. Other settings
Brightness of light – comfortable for your eyes – very bright will give headaches and burns out filaments
Iris – determines the diameter of the light beam coming from the source – different setting for different magnifications
Condenser lens – use to get high resolution at high magnification
Focusing – to avoid collision: first bring sample close to objective lens (not against) and increase distance until sample in focus

8. Determination of the refractive index
Grain mount
Edges of crystal – act as small prisms which concentrate light as a ring of light – the Becke line
When increasing the distance from sample to objective (defocusing), the Becke line is always refracted in the direction of a medium of higher RI
In practice:
Change liquids until two adjacent liquids defines the range for the index of the mineral

9. Determination of refractive Index

10. Birefringence (δ)
When a ray of light is split into two separate polarized rays – each with a single vibration direction perpendicular to that of the other ray
True maximum birefringence value (δ) of mineral
Isotropic: δ = n – n = 0
Uniaxial: δ = nε – nω
Biaxial: δ = nγ – nα
Under the microscope:
Observed under crossed polarized light as:
Interference colors
Only in anisotropic minerals

11. Birefringence/double refraction
Doubly refracted waves are polarized but separate, vibrating in different planes – no interaction
Need interference – study interference colours and properties
To get interference – a second polarizer inserted – the analyzer:
Crossed polarizer/upper polarizer/crossed nichols
Used to analyze the interference effects of light in minerals

12. Interference colours
First order colors Second order colors Third order colors

13. Birefringence
A characteristic that all anisotropic minerals have, intensity differs
High birefringent minerals – third/fourth order interference colours
Med birefringent minerals – second order interference colours
Low birefringent minerals - first order interference colours
For specific mineral birefringence depends on orientation:
Maximum birefringence - orientation of grain shows highest possible interference colour for the specific mineral
Minimum or no birefringence – orientation of grain shows lowest or no interference colour for specific mineral
Intermediate birefringence – orientation of grains shows interference colours intermediate between minimum and maximum

14. Interference colours
Determine order of colour and so value for birefringence – interference color chart

15. Use of interference colors True birefringence
In sample: crystals in random orientations
 each grain different interference colors, each with corresponding birefringence
Minimum birefringence
Circular section (perpendicular to optical axis) give lowest order or no interference colours – refractive indices on both axes equal or almost equal
Also referred to as the isotropic section
True birefringence
Longest elliptical section (parallel to optical axis) give highest order colors
Refractive index on major axis = largest; on minor axis = smallest
THUS: to determine the true birefringence of mineral – choose grain with highest interference colors and read of the value of birefringence from the color chart

16. Use of interference colors: Accessory plates (compensators)
Accessory plate is a crystal with known birefringence and orientation
Determine unknown mineral optical orientation by comparing with known crystal plate orientation
Crystal orientation in plate parallel with mineral orientation
Plate colors interfere constructively with colors of mineral
Addition – Positive (Red plate + color of mineral = blue)
Crystal orientation in plate perpendicular with mineral orientation
Plate colors interfere destructively with colors of mineral
Subtraction – Negative (Red plate - color of mineral = yellow)

17. Use of interference colors: Accessory plates (compensators)
POSITIVE NEGATIVE

18. Use of interference colors: Extinction
As an anisotropic crystal is rotated a full turn under crossed polarized light, it goes into extinction 4 times
I.e. – at every 90° rotation the mineral goes dark
This happens every time the two perpendicular vibrating directions falls parallel with the two polarizer directions

19. Use of interference colors: Extinction angle
When optical axis vertical (circular section) – mineral dark during rotation
When inclined – mineral go dark once every 90º
Angle of extinction can be measured for elongated minerals or minerals with strong cleavage
Parallel extinction
Inclined extinction
Symmetrical extinction
No extinction angle

20. Observation of interference figures using convergent light – conoscopic view
Insert condenser
lens
Gives convergent
light
Enters sample at
50º - 90º angles
See image of light
source
Interference effects at
different angles

21. Conoscopic observation of interference figures
Isotropic
No image

22. Conoscopic observation of interference figures
Uniaxial
Perpendicular to
optical axis

23. Conoscopic observation of interference figures
Uniaxial
At an angle to the
optical axis

24. Conoscopic observation of interference figures
Uniaxial
Parallel to the
optical axis