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| Titel |
The Raman spectrum of Ca-Mg-Fe carbonates; Applications in geobiology |
| VerfasserIn |
M. A. van Zuilen, N. Rividi, B. Ménez, P. Philippot |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250065919
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| Zusammenfassung |
| Carbonates form a very important mineral group in geobiological studies. They are a
common mineral matrix for putative carbonaceous microfossils in Archean greenstone belts,
form an important chemical deposit in seafloor hydrothermal systems, and are a common
product in biomineralization processes. In many geobiological studies there is a specific need
for simple characterization of carbonate composition while avoiding complex sample
preparation or sample destruction. Raman spectroscopy is a highly versatile non-destructive
technique enabling in-situ characterization of minerals and carbonaceous materials. It can be
combined with confocal microscopy enabling high-resolution Raman mapping of entire rock
thin sections, or can be integrated in submersibles and potentially Mars-rovers for
direct field-based mineral identification. It is thus important that well-established
spectral databases exist which enable unambiguous identification of a wide variety of
carbonate minerals. The most common carbonates in the Ca-Mg-Fe system include the
CaCO3 polymorphs calcite, aragonite, and vaterite, as well as the solid solutions
CaMg(CO3)2-CaFe(CO3)2 (dolomite-ankerite) and MgCO3-FeCO3 (magnesite-siderite).
Although various carbonate end-members have been studied exhaustively by Raman
spectroscopy, a simple protocol for rapid distinction of various carbonate solid
solutions is still lacking. Here we present a detailed study of Raman shifts in various
carbonate standards of known composition in the Ca-Mg-Fe system. Carbonates
with rhombohedral symmetry display a Raman spectrum with six characteristic
vibrational modes – four of these represent vibrations within the (CO3)2- unit
and two represent external vibrations of the crystal lattice. We show that Raman
band shifts of internal mode 2ν2 (range 1725-1765 cm-1), and external modes T
(range 170-215 cm-1) and L (range 285-330 cm-1) for siderite-magnesite and
ankerite-dolomite solid solutions display distinct and well defined positive correlations with
Mg number (Mg/Mg+Fe+Mn+Ca). Raman shifts calibrated as a function of Mg
number were used in turn to evaluate the chemical composition of natural carbonate
samples. In particular it is shown that detailed micron-resolution Raman maps can
be generated of carbonate crystal-zonation in hydrothermally altered sedimentary
deposits from Archean greenstone belts. Large spectral-range analysis (140-2000
cm-1) in static-mode (centered at 1150 cm-1) allows for combined Raman mapping
of both carbonate-composition (2ν2, T, L modes) as well as kerogen structural
ordering (D1-D4 and G modes in the range 1100-1700 cm-1), and therefore allows
for simultaneous characterization of putative organic microfossils and associated
carbonate matrix in metamorphosed Archean rock samples. Finally, it will be shown
that these carbonate solid solutions can be distinguished from other end-member
carbonates such as calcite, vaterite and the orthorhombic polymorph aragonite. |
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