Refractometry

1
Refractometry

• Ness, 1991, Chapter 3

2
Refractometry
METHODS TO MEASURE R.I. (n) OF MINERALS

• 1/ Immersion method
• When the refractive index of the oil and the
  refractive index of the crystal grain are the
  same, light rays will not be refracted and the
  grain remains invisible
• When the refractive indices are different, the
  outline of grain will be visible. This is known
  as relief.

3
REFRACTOMETRY

• The invisible man had to change his refractive
  index to become not only transparent, but
  invisible.

4
Refractometry

• The outline becomes more pronounced with a
  greater difference in refractive index.

High relief
Moderate relief
NOTE If R.I. of mineral is greater than the
oil, it has positive relief If R of mineral is
lower than the oil, it has negative relief
Low relief
5
Refractometry

• 2/ Becke-line Method
• Slightly close iris diaphragm (less light)
• Change the focus of the microscope by lowering
  stage
• Light-line (Becke Line) moves to material of
  highest R.I. (n) when the distance between stage
  and objective is increased

Becke Line moves into oil of higher R. I.
Becke Line moves into mineral of higher R.I.
6
Isotropic materials

• Nesse, 1991 Chapter 4, p. 34-36

7
Isotropic materials
Fig 4.1

• Isotropic indicatrix
• An indicatrix is a geometric figure that shows
  the relationship between changes in Refractive
  Index and the vibration direction for light
  passing in any direction through the material

8
Isotropic materials
Think in 3-D !
(Fig 4.1)

• Isotropic indicatrix
• Consider ray a traveling along the X-axis
  vibrating parallel to the Z-axis, the Refractive
  Index is na, plotted on both sides of Z-axis from
  the origin
• Ray b traveling along Y-axis vibrating parallel
  to X-axis, nb plotted on X-axis.
• This is an Isotropic material , na nb X-Z
  plane shows a circle

9
Isotropic materials

• How to identify isotropic minerals in thin
  section
• Generally difficult for determination of
  refractive index
• In Cross-Polarised Light Black - on complete
  rotation of stage
• In Plane-Polarised Light
• Relief
• Determine if it is low-moderate-high
• Determine if it is positive or negative
• Color (v. useful BUT use with caution)
• Cleavage (determine how many what direction)
• eg 3 cleavages _at_ 90?