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
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| 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):
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| 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):
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| 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:
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| Classification can be achieved using only one Raman peak |
Fe-Ti-Cr oxides obtained from Raman point-counting of EETA79001 Martian meteorite:
a. Chromites
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| Spectra from core and rim of chromite | spectral variation of core chromite, | Backscattering electron image (BEI) shows a zoned chromite |
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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 |
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b. magnetite & ulvöspinel
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| 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. |
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c. Ilmenite & Hematite
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Conclusion based on Raman data: End-member ilmenite, Fe/Ti ~1.0, Mg/(Fe+Mg) ~ 0 - 0.1; mineral proportion << 1%; |
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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.
