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Fe-Ti-Cr Oxides

correlations between Raman features and structure-chemistry

These minerals are important because:
    1. They contain information about rock petrogenesis and alteration, memories of oxygen fugacity.
    2. Their exsolution into companion mineral pairs give constraints on formation temperature and cooling rate.

There are very few systematic studies on Raman feature of Fe-Ti-Cr oxides .

Three groups of Fe-Ti-Cr oxides are important for common intrusive and extrusive rocks:
Chromite, Ulvöspinel - Magnetite, Ilmenite-Hematite, and related solid solutions.

Chromite (Fe2+, Mg2+)IV(Cr3+, Fe3+, Al3+)VI2O4

Fd3m – Oh7 symmetry, normal spinel structure,  A2+ take 1/8 of tetrahedral sites,  B3+ take ˝ of  octahedral sites ; Five predicted Raman modes,  A1g mode (680-770 cm-1) contributed mainly by B3+O6 group correlation between A1g Raman peak position and (Cr+Fe3+)/(Cr+Fe3++Al)

Magnetite (Fe2+2Fe3+O4)-- Ulvöspinel (Fe2TiO4):

Fd3m – Oh7 symmetry, inverse spinel structure Five predicted Raman modes, A1gmode (667 & 678 cm-1) contributed by TiO6, Fe3+O6, Fe2+O4 groups

Ilmenite (FeTiO3) -- Hematite (Fe2O3):

R3c-D3d6 symmetry, Corundum structure, Fe fill 2/3 of octahedral sites Seven  predicted Raman modes, Raman peak position is very sensitive to Mg/(Mg+Fe) ratio, but  less sensitive to Fe/(Fe+Ti) ratio R3 – C3i2 symmetry, Pseudo-Corundum structure, Fe & Ti orderly fill 2/3 of octa -sites, 10 predicted Raman modes

To identify Fe-Ti-Cr oxides in the spectra obtained by point-counting:

Classification can be achieved using only one Raman peak

Fe-Ti-Cr oxides obtained from Raman point-counting of EETA79001 Martian meteorite:

a. Chromites

Spectra from core and rim of chromite spectral variation of core chromite, Backscattering electron image (BEI) shows a zoned chromite
Conclusion based on Raman data:
Mineral proportion & grain size -- ~ 1 % in lithology B, appear as single-phase spectra, most are consecutive, i.e. xenocryst >300 μm
Compositional feature -- Raman peak position distribution suggests a  (Cr+Fe3+)/(Cr+Fe3++Al) = 0.75 -1.0

b. magnetite & ulvöspinel

magnetite and Mt-Ulvss peak position and pattern variations BEI shows very small grain size
Conclusion based on Raman data:

Proportion & grain size: Magnetite ~2%, Ulvöspinel <1%, Appearing in multi-phase spectra mostly => small grain sizes, with magnetite slightly bigger ;

Chemistry feature -- Nearly end-member magnetite , and ulvöspinel of variable Fe/Ti ratio.

c. Ilmenite & Hematite

Conclusion based on Raman data:

End-member ilmenite, Fe/Ti ~1.0, Mg/(Fe+Mg) ~ 0 - 0.1;

mineral proportion << 1%;

 

Hematite has not been found in this Martian meteorite;

Raman spectrum suggest -- it has a well crystalline structure;

It was only found once; 

The existing of hemetite indicate oxidized alteration at local scale.

 

Reference: Wang A., Kuebler K. E., Jolliff B. L., Haskin L. A., Fe-Ti-Cr-Oxides in Martian Meteorite EETA79001 Studied by Point-Counting Procedure Using Raman Spectroscopy. Abstract #1742, 34th LPSc, 2003.

 

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