Peridotite. Nomarski Interference Contrast Images.

 

 

This page illustrates three imaging modes for the reflected light microscope:

1. The plane polarized light (LPNA) gives an idea of the reflection coefficient of the minerals. It is used mainly to recognize the opaque minerals. For transparent materials, the reflected light intensity varies according to the refraction index of the mineral. This kind of micrograph is very useful for the Raman microscopy to distinguish different minerals present at the surface.

2. The crossed polars view (LPA) gives an insight into the material (internal reflections). For opaque minerals, it indicates the anisotropy of the mineral, for the transparent crystals like those figured below, it is an internal view of the section.

3. Differential interference contrast in reflection (DIC) is an enhanced topography of the surface. It looks like an amplification of the distances perpendicular to the image plane. This view is obtained by inserting a Wollaston prism in the accessory slot of the microscope between crossed polars.

 
    LPNA view of a diopside crystal of a peridotite thick section. The gray color of the image is uniform due to the fact that the refractive index of the full area is nearly constant except some darker material . Parallel cleavage planes of the diopside are visible at the center of the field.
   LPA view of the same region. The internal colors of the material appear.

   Below, is a DIC image of the same area. It shows that, due to the polishing method, the surface is not completely flat and exhibits rounded shapes of each particles.  It can give an idea of the hardness of the different crystals present in the section. In particular on this section area, the cleavage traces are exemplified.

 
Another LPA view of a different location on this section. The field of view is mainly occupied by light gray olivine crystals. At some places, darker alteration products are present. Some very small white opaque particles can also be seen.
LPA view shows the different real colors of the materials. Brownish oxidation products are clearly visible.

The DIC images below illustrates the surface roughness at the microscopic scale. By tilting the Wollaston prism, different interference colors can appear as can be seen on the two last micrographs on this page.