Optical Mineralogy

1
Optical Mineralogy

• Lab 13 Fall, 2012
• Uniaxial Interference Figures

2
Conoscopic Observation

• In order to observe an interference figure the
  microscope must be used in the conoscopic mode
• Conoscopic refers to the cone-shaped illumination
  obtained when the condenser lens is near the thin
  section
• This requires that the following conditions be
  met

3
Conoscopic Technique

• A. Analyzer inserted and crossed with respect to
  polarizer (CN)
• B. Objective lens with a numerical aperture
  (N.A.) ? 0.65 must be used
• C. The condensing lens must be moved (or
  swing-out lens inserted) to focus the light on a
  small area
• D. The Bertrand lens must be inserted

4
Choosing a Grain

• Choose a grain that stays in extinction or has
  very low colors
• You are trying to locate a grain with its optic
  axis perpendicular to the slide
• You want to be looking along the optic axis, or
  as close as you can possibly get this produces
  a centered optic axis figure
• How close that is depends on the birefringence of
  the mineral

5
Choosing a Grain, II

• For quartz, the grain must be almost black at all
  times, for olivine, first-order gray will do
• For calcite, any recognizable interference color
  will probably work
• Try to be at least in the lower 10 of the
  mineral's color range
• Sometimes you just can't do it with a given thin
  section, especially if the mineral you're dealing
  with has only tiny grains or very few of them

6
Conoscopic vs. Orthoscopic Observation

• Diagram compares the two types of viewing

7
Conoscopic Procedure

• Select a grain whose interference you wish to
  check
• Make sure the cover slip is facing up
• Move the grain to the center of the stage
• Be sure you are in CN (are the polars crossed?)
• Focus at low power
• Make sure you are not focused on a crack or
  impurity in the grain

8
Conoscopic Procedure, II

• Increase to medium power, double check focus
• Move up to high power and double check focus
• Be sure to raise or flip in the auxiliary
  condenser lens

9
Bertrand Lens

• Insert the Bertrand lens
• If your scope does not have a Bertrand lens,
  remove the eyepiece and look down the microscope
  tube
• An interference figure should appear rotate the
  stage to see if there is any change

10
No Interference Figure?

• Check that the microscope is in the correct
  configuration
• Check that the grain on high power is not focused
  on a crack or impurity
• Also check that the high power objective is
  properly centered

11
Uniaxial Minerals

• The optical class uniaxial has minerals from two
  mineral systems
• Tetragonal A4
• Hexagonal
• Rhombohedral division A3
• Hexagonal division A6
• Each system has a unique high order axis, as
  shown this is the optic axis

12
Quadrant Labels

• The quadrants are labeled starting in the upper
  right and going counterclockwise
• Roman numerals are used to designate quadrants

13
Optic Axis

• The optic axis is designated as the
  crystallographic Z axis
• When a thin section of a mineral is cut
  perpendicular to the optic axis, and then viewed
  perpendicular to the thin section, light is
  traveling along the optic axis
• Light traveling in this direction experiences a
  single index of refraction, ? (omega)

14
Optic Axis Figures
The isogyre has 1º color the area between the
isogyre arms is 1º white, unless isochromes are
present
15
Low vs. High Birefringence
Calcite, high birefringence

• Quartz, low birefringence

16
Origin of Isogyres
Figure 21, page 28, W. W. Moorhouse, The Study of
Rocks in Thin Section S marks the slow ray (for
the case)

• In conoscopic view, ? always vibrates to the z
  axis and tangential to the isochromes, whereas e
  always vibrates ? to the isochromes
• Whenever one of these vibration directions is
  parallel to the polarizer (i.e., E-W), extinction
  occurs

17
Origin of Isogyres, II

• The two bands of extinction form a centered cross
  for an optic axis section
• The point where the isogyres meet is called the
  melatope and represents the optic axis itself
• Melatope comes from Greek words meaning dark
  and place

18
Origin of Isochromes

• Light which travels along the optic axis is not
  split into two rays, nepsilon' nomega, and
  exits the mineral to form the melatope
• No retardation "between" rays

19
Origin of Isochromes, II

• Light following paths 2 4 experience moderate
  retardationnepsilon' lt nomega 550 nm
• Light following paths 3 5 experience moderate
  retardationnepsilon' ltlt nomega 1100 nm because
  light makes a larger angle with optic axis and
  must take a longer path through the sample

20
Photomicrograph of High-Birefringence Mineral

• The colored rings are isochromes
• Calcite highly birefringent

21
Accessory Plates

• Accessory plates are plates of anisotropic
  minerals ground to a thickness that gives a
  particular retardation of light
• When inserted into the light path, they change
  the retardation of light coming through the thin
  section by a specific amount and the resultant
  interference color helps to identify the mineral

22
1o Red Accessory Plate

• This is the compensator you will encounter most
  frequently
• The lab microscopes are equipped with one, and we
  will use it extensively
• The full wave plate is also called a gypsum
  plate, 1l plate, 550 nm plate, or 1o red plate
  (1o rot, in German) because it is usually made of
  gypsum and produces a 550 nm or 1o red
  retardation

23
Quarter Wave Plate

• This plate is found on your microscopes in lab,
  but we do not use it extensively
• As the name implies it produces a retardation of
  ¼l
• It is also called a mica plate, 150 nm plate, and
  1o gray plate,  because it is usually made of
  muscovite (glimmer in German) and produces a
  retardation of 150 nm, or 1o gray

24
Quartz Wedge

• This is a crystal of quartz cut into a wedge
  shaped
• Since its thickness varies along the wedge, it
  can produce a range of retardations that
  correspond to interference colors from 0 (1o
  black) up to about 3800nm (5o green)  - this
  varies from wedge to wedge 
• The wedge, like all compensators usually has its
  slow direction clearly marked, and is inserted
  into the microscope tube such that slow direction
  in the compensator is at a 45o angle to the
  polarizing direction

25
Uniaxial Positive Sign

• In a uniaxial mineral, the two principle indices
  of refraction are denoted e (epsilon) and ?
  (omega)
• If e gt ?, the mineral is uniaxial positive

26
Uniaxial Negative Sign

• If e lt ?, the mineral is uniaxial negative

27
Determination of the Optical Sign

• Accessory plates may be used to determine the
  optical sign
• Minerals with isochromes are usually treated
  differently than minerals without isochromes

28
Uniaxial Mineral, No Isochromes

• The 1º red (Rot 1) plate is inserted
• On most microscopes, this will be from the SE
• The slow direction of the accessory plate (N)
  should be aligned NE-SW
• A blue color appears in quadrants I III, which
  indicates addition
• A yellow color in quadrants II IV indicates
  subtraction
• This is a uniaxial positive mineral with low
  birefringence

29
Uniaxial Positive with 1º Red Plate

• Uniaxial positive mineral, with 1º red plate
• Note blue in quadrants I III, yellow in
  quadrants II IV
• The isogyres show the 1º red color of the
  accessory plate

30
Uniaxial Mineral, No Isochromes
Figure 24b, page 30, W. W. Moorhouse, The Study
of Rocks in Thin Section

• A mica or quarter ? plate may be used for
  minerals with low to moderate birefringence
• It produces a pair of black dots in quadrants
  where subtraction occurs

31
Uniaxial Mineral, with Isochromes

• The isochromes in quadrants I III move inward,
  and those in quadrants II IV move outward
• This is a uniaxial positive mineral with moderate
  to high birefringence

32
Multiple Isochromes

• If the interference figure displays numerous
  isochromes, color changes produced with the 1º
  red plate become difficult to detect
• In this case the quartz wedge is used
• Inserting the Quartz wedge results in the
  movement of the isochromes about the isogyres

33
Use of the Quartz Wedge

• In quadrants where the colors subtract, the
  isochromes move outward as lower order colors
  form near the melatope and displace higher order
  colors
• In quadrants where the colors add, the isochromes
  move inwards, towards the melatope
• The isogyre, on insertion of the accessory adopts
  the interference color corresponding to the
  retardation of the accessory

34
Uniaxial Mineral, with Isochromes, using Quartz
Wedge

• Left, positive right, negative

35
Uniaxial Mineral, No Isochromes

• A blue color appears in quadrants II IV, which
  indicates subtraction
• A yellow color in quadrants I III indicates
  addition
• This is a uniaxial negative mineral with low
  birefringence

36
Uniaxial Mineral, with Isochromes

• The isochromes in quadrants I III move outward,
  and those in quadrants II IV move inward
• This is a uniaxial negative mineral with moderate
  to high birefringence

37
Uniaxial Negative with 1º Red Plate

• Uniaxial negative mineral, with 1º red plate
• Note blue in quadrants II IV, yellow in
  quadrants I III
• The isogyres show the 1º red color of the
  accessory plate

38
Summary of Uniaxial Sign Determination

• The diagram summarizes the determination of
  uniaxial signs using a 1o red plate

39
Off-Center Figures

• Finding a grain with the optic axis oriented
  exactly perpendicular to the stage will sometimes
  be very difficult
• It would be much more common to find one wherein
  the optic axis is at a slight angle to being
  perpendicular to the microscope stage

40
Off-Center Figure Properties

• Such a grain will exhibit the following
  properties
• It is a grain that shows w refractive index and
  an e' refractive index that is close the w
  refractive index
• It would also show very low order (1o gray
  interference colors between extinction positions
  if the analyzer is inserted in orthoscopic mode 

41
Off-Center Figure Diagram

• On rotation of the stage, the melatope would
  rotate in a circle around the perimeter of the
  field of view, and the bars of the isogyres would
  remain oriented E-W and N-S

42
Rotation of an Off-center Figure

• Figure 22, page 29, W. W. Moorhouse, The Study of
  Rocks in Thin Section

43
Off-Center Orientation Diagram

• The melatope lies outside the field of view
• The vibration direction of the ordinary ray is
  tangential to the isochromes
• The vibration direction of the extraordinary ray
  is radial from the melatope

44
Photomicrographs of Off-Center Figures

• Thick Quartz Left 15º off center Right 30º off
  center

45
Positive Off-Center Figure

• For an optically positive crystal, all NE and SW
  quadrants will turn blue and the NW and SE
  quadrants will turn yellow, both colors replacing
  the 1ogray color present before insertion of the
  compensator

46
Negative Off-Center Figure

• For an optically negative crystal, all NE and SW
  quadrants will turn yellow and all NW and SE
  quadrants will turn blue, both colors replacing
  the 1ogray color present before insertion of the
  compensator

47
Flash Figure

• A mineral grain is oriented with it's optic axis
  horizontal
• This orientation exhibits the maximum
  birefringence, for this mineral in the thin
  section, and produces a flash figure

48
Flash Figure II

• The flash figure results because the vibration
  directions, of the indicatrix, within the field
  of view are nearly parallel to polarization
  directions of the microscope
• extraordinary rays vibrate parallel to optic axis
  
• ordinary rays vibrate perpendicular to optic axis
  

49
Flash Figure III

• With the grain at extinction, the optic axis is
  oriented either EW or NS in the resulting
  interference figure
• The interference figure produced occupies most if
  not all of the field of view and consists of a
  very broad, fuzzy isogyres cross
• Upon rotating the stage, lt 5 rotation, the
  isogyres will split and move out of the field of
  view in opposite quadrants

50
Flash Figure Diagram

• Diagram showing flash figure orientation, and a
  flash figure image

51
Flash Figure after Small Rotation

• The isogyre splits and quickly leaves the field
  of view
• The optic axis lies along the line connecting the
  isogyres