Composition of Sand at Sandy Hook

What is Refractive Index?

• Refractive Index (RI) is the ratio of the speed of light in a vacuum compared to the speed of light in another medium such as a mineral: – RI = (Speed of Light)VACUUM/(Speed of Light)MEDIUM

• The RI of a vacuum is 1 and the RI of all other media is >1, with translucent minerals falling with the range of approximately 1.4 (opal) to 2.5 (diamond).

Refraction Index: immersion oils of known refractive index and comparing the unknown mineral to the oil

How Do You Determine Refractive Index?

• When light travels between two materials of different refractive index (such as air and water) the path of the light abruptly bends (refracts) or alternatively it may be reflected off of the interface between the media.

• It is this refraction and reflection of light that make the boundary visible to us.

Refractive Index

• If a mineral grain is immersed in a medium that has a different RI, then the refraction and reflection will allow us to see the grain clearly.

• If a mineral is immersed in a medium that has an identical RI, then there would be no reflection or refraction from the edge of the mineral. Light would travel through as if the mineral grain was not there. 

Refractometry

• Refractometry involves the determination of the refractive index of minerals, using the immersion method.

• This method relies on having immersion oils of known refractive index and comparing the unknown mineral to the oil.

• If the indices of refraction on the oil and mineral are the same light passes through the oil-mineral boundary un-refracted and the mineral grains do not appear to stand out.

If noil <> nmineral then the light travelling though the oil-mineral boundary is refracted and the mineral grain appears to stand out.

RELIEF -

the degree to which a mineral grain or grains appear to stand out from the mounting material, whether it is an immersion oil, Canada balsam or

another mineral.

Strong relief

• mineral stands out strongly from the mounting medium,

• whether the medium is oil, in grain mounts, or other minerals in thin section,

• for strong relief the indices of the mineral and surrounding medium differ by greater than 0.12 RI units

Moderate relief

• mineral does not strongly stand out, but is still visible,

• indices differ by 0.04 to 0.12 RI units

Low relief

• mineral does not stand out from the mounting medium,

• indices differ by or are within 0.04 RI units of each other.

Anthophyllite fibers stand out clearly in 1.55 RI Oil

RI of anthophyllite does not equal 1.55

Anthophyllite fibers are subdued  in 1.60  RI Oil

RI of anthophyllite is close to 1.60

Anthophyllite fibers stand out clearly in 1.68 RI Oil

RI of anthophyllite does not equal 1.68 

EXAMPLE

BECKE LINE

• In order to determine whether the index of refraction of a mineral is greater than or less than the mounting material the Becke Line Method is used

• BECKE LINE - a band or rim of light visible along the grain boundary in plane light when the grain mount is slightly out of focus.

BECKE LINE

• Becke line will always move towards the material of higher RI upon lowering the stage.

• A series of three photographs showing a grain of orthoclase

• http://www.brocku.ca/earthsciences/people/gfinn/optical/becke4.htm

The grain is in focus, with the Becke line

lying at the grain boundary.

The stage is raised up, such that the grain boundary is out of focus, but the Becke line is

visible inside the grain.

The stage is lowered, the grain boundary is out of

focus, and the Becke line is visible outside the grain.

BECKE LINE

• When the RI of the mineral and the RI of the mounting material are equal, the Becke line splits into two lines, a blue line and an orange line.

• In order to see the Becke line the microscope is slightly out of focus, the grain appears fuzzy, and the two Becke lines are visible.

• The blue line lies outside the grain and the orange line lies inside the grain.

• As the stage is raised or lowered the two lines will shift through the grain boundary to lie inside and outside the grain, respectively.

Becke line of quartz, liquid n=1.544. http://users.skynet.be/jm-derochette/index_measurement/Becke.htm