Raman Spectra of Opaque Minerals

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Spectrum of an Anatase single crystal. (Blue and red curve with the Raman microscope described here). Notice the high influence of crystal orientation on the spectrum. Peaks at 400 and 520 cm-1 almost disappear when crystal is rotated. The peak at 150 cm-1 is still visible with our microscope even if it appears in the filter transition region due to its high intensity.

Polished section of a chromite. Our spectrum is closer to the magnesio-chromite from the RRUFF database  taken with a laser at 785 nm.
A polished slab has been cut in the rock shown here. Our Raman spectrum seems to be the sum of two minerals contributions : Hematite and Goethite. The double peak at 400 cm-1 and the peak at 550 cm-1 are found in the Goethite spectrum. Maybe reaction with moisture at the surface of the section. A new spectrum after repolishing can be seen below.
The rock shown on the left side was sold as ilmenite. The Raman spectrum is not similar to Ilmenite. With the help of  the RRUFF database  a first interpretation was the following:  the section on the left seems to be close to a mineral called Armalcolite, a magnesian Ilmenite. A careful polishing of the sample allowed a reflection coefficient to be measured: it is close to the Hematite reflecting power. A second spectrum with the new spectrograph is reported in the figure below. It seems now that the new spectrum could be interpreted as hematite in agreement with the reflection coefficient. In the same figure, a new spectrum of the  hematite above has also been recorded with the new spectrograph just after repolishing the sample.

 An Ilmenite spectrum can be found on the Hornblende schist.



New spectrograph recording.


One of the octahedron magnetite crystal found on the rock reproduced on the left side has been used to record the Raman spectrum. Magnetite has a 3 peaks spectrum quite similar to the reference.
A polished slab has been cut from the sample shown. The spectrum I have found on this sample is characteristic of pyrite rather than marcasite as can also be seen on the pyrite sample below. I should investigate further with other samples to find a real Maracasite Raman spectrum.

Spectrum of a polished Pyrite sample is shown above, quite similar to the reference spectrum from the RRUFF database.
Elemental Silicon is often used in the literature to calibrate the wavelengths of Raman spectrographs. I have polished a few silicon flakes as shown on the left to record this Raman spectrum nearly composed of one single peak.

The bright mineral Skutterudite on the left gave the Raman spectrum above. My spectrum is similar to the references but not identical. The high noise level of these spectra is due to the low Raman scattering  Skutterudite.

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