Gabbro opaques minerals Raman spectra.

            Wide field thin section of this rock.                 Raman of transparent minerals.  


            Area 1: opaque regions around olivine crystals.

     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 polarizers.

      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.