| This page and the 3 following ones
illustrate the use of the Raman microscope to
identify some of the minerals of a rock section. The pictures presented were
obtained unless otherwise specified with lateral illumination on a
polished slab. As usual, the apex of the red arrow indicates the
position of the laser spot for the Raman spectrum recording. The
comparison between those reflection images and the
thin section can be made
to help the minerals identification.
The dark big crystal at the center of this photograph was
identified as hornblende on the thin section.
|The blue curve is the Raman
spectrum from spot A, identified as Hornblende, in agreement with the
results of the observations on the thin section. The spectrum of spot I will be discussed
on page 2.
A lot of green transparent crystals are visible throughout this
rock section. The Raman spectrum of spot B was recorded and identified
as Muscovite. Mica is also clearly visible on the thin section with its
high birefringence color.
| The Raman spectrum of
spot B is similar to the reference Muscovite spectrum for wave numbers
less than 900 cm-1, that is not including the broad peak at 1040 cm-1.
If a database search is done with this peak included, no good match is
found so this broad peak at 1040 cm-1 is believed to be a fluorescence
peak. Note also its presence on the spot H spectrum to be discussed on
page 2. Muscovite frequently exhibits some high fluorescence sometimes
so intense that the Raman spectrum cannot be seen at all.
Calcite + Quartz
|The small brownish crystal
at spot C was selected for Raman recording.
| The spot C spectrum
has been interpreted as the sum of 2 minerals contributions : Quartz and
Calcite as can be seen by comparison with some other Raman spectra
recorded on pure single crystals. The brown color cannot be attributed
to anything at this step and will be explained later.
Opaque mineral : Ilmenite
Normal incidence reflection image. Opaque minerals appear white while
dark spots are defects in the polishing (holes).
Thin section in non polarized transmission light showing opaque
minerals as black spots.
|Small opaque minerals
particles can also be analyzed by the Raman microscope. On the image to
the left (normal incidence reflection), the light is directly reflected
from an opaque mineral which acts nearly as a mirror so it appears
Normal incidence reflection view with polarizer and analyzer.
On this image taken in normal reflection between crossed polarizers
(in fact slightly uncrossed to increase anisotropic effect), the
anisotropy from grey to dark brown of this opaque mineral can be seen
because the two crystals have different orientations.
The Raman spectrum help to identify this mineral as Ilmenite in
agreement with the optical anisotropic behavior and the reflection
coefficient estimated qualitatively.