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 oilmineral 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
Anthophyllite fibers
are subdued in
1.60 RI Oil
Anthophyllite fibers
stand out clearly in
1.68 RI Oil
RI of anthophyllite
does not equal 1.55
RI of anthophyllite
is close to 1.60
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