The peridotite from Lanzo is heavily weathered in some
part of the rock. Near the rock surface, no original mineral remains,
all is transformed. This Raman study has been made with thick polished
sections so the images of the location where Raman has been measured are
reflected light pictures with or without crossed polarization.
The two images below illustrate the alteration of
olivine crystals due to weathering. The original minerals are hydrated
and transformed to other mineralogical materials, especially fine
grained minerals. The end product of this transformation is serpentine,
the low birefringence mineral seen just below on the crossed polars
image. On the plane polarized micrograph, the serpentine is completely
transparent without any structure due to its low refraction index.
Between the remnant olivine crystals is a microcrystalline material,
greenish in some places of the LPNA view, composed of a minerals
The areas 1, 2 and 3 exhibit increasingly refractive
index as can be seen on the left picture above: the reflection
coefficient is higher when the index of refraction increases. Thus we
expect three different minerals. On the double polarized image on the
right, the difference between regions 1 and 2 is not quite different.
This picture is an internal reflections image in a thick section so
material blow the surface is also imaged. The same is true for the Raman
microscopy. The microscope is not a confocal instrument so the thickness
being sampled is not absolutely restricted to the surface.
spectrum of the area 1 is mainly clinochlore but a small signal of
olivine is also visible. It is a characteristic of this section: most
spectra are not coming from a pure mineral but from a mineral assemblage
due to the weathering.
Area 2 is antigorite the serpentine material end member of the
Location 3, the blue curve, is the forsterite (olivine) as can be
seen in comparison with the RRUFF reference material. The spectrum
indicates the presence of the antigorite close to the high refraction
The image above illustrates the transformation of a big
crystal to a microcrystalline material. On the plane polarized image,
the homogenous gray crystal (area 4) is transformed to a granular
material (area 5). On the crossed polars micrograph, The green
transparent crystal becomes a grainy white crystals mixture.
crystal labeled 4 is diopside as seen on the Raman spectrum on the left.
The area 5 is a mineral mixture of diopside, antigorite and
clinochlore. The pure minerals spectra taken at different locations have
been added to the figure below for an easy comparison.
Different color as seen in the image above is not always
synonymous of different minerals. The big crystal with cleavage planes
(6) has the same index of refraction as the pale green crystal (7) as
seen on the left image and the composition of both areas are the same:
Some other areas (8, 9, 10 and 11) have also been
investigated with the Raman. The same minerals pure of in mixtures are
found. Low reflectance in LPNA images is mainly clinochore, intermediate
index is mainly antigorite and high index has a composition closer to
In this section, we can also find some opaque minerals
like magnetite and chromite as seen on the pictures and spectra included