The 632.8 nm laser has been used to
identify the black mineral associated with olivine in this section.
The red curve is the Raman
spectrum of area 1. It is a mixed spectrum of forsterite plus another
mineral. The blue curve is the difference between the red spectrum and a
pure forsterite Raman spectrum. Two major peaks are present. This
spectrum is not identified, it could be the spectrum of oxidation
products as opaque materials are very sensitive to the laser power. To
solve this problem, the thin section has been polished to allow the
examination of opaque minerals (see below).
The polished section
reveals the presence of a yellow mineral in reflection view. This
mineral is anisotropic. On the left, the laser damage is visible in the
area 3 because the full laser power has been used there. If the power is
reduced with a neutral filter of absorbance = 0.6, no damage of the
crystal can be seen in area 4.
The spectrum with the
reduced power is very weak, it could not be assigned to any mineral. If
the full power of the laser is used, the spectrum exhibits sharp peaks
similar to an hematite Raman spectrum. Thus this material is an iron
mineral. The reflection coefficient has been measured: 39.5% at
575 nm, lower than pyrite. This material could be Pyrrhotite.
Polished section of the black material within olivine crystals.
Below is the reflection view between crossed
The picture above is
the reflected light view of the polished section. It shows clearly that
the opaque minerals between the olivine crystals appear pure and
polished on some locations like the area 5. Raman has been used to
identify magnetite as the opaque component. To get the magnetite
spectrum, a reduced intensity laser beam must be used, otherwise the
magnetite is readily oxidized to hematite.