A. Cavallo

Application of micro-FTIR imaging in the Earth sciences

AbstractIn this paper we describe recent applications of micro-infrared imaging in the Earth sciences. We address, in particular, the use of Fourier-transform infrared (FTIR) spectroscopy in characterizing the zoning and speciation of H and C in a variety of geological materials, including microporous minerals, nominally anhydrous volcanic minerals (NAMs), and crystal inclusions. These investigations show that use of the modern techniques of FTIR imaging enables detection of the zoning of volatile species across the studied samples, and possible configuration changes of structurally-bound carbon molecular species (e.g., CO2 vs CO3) during crystal growth. Such features, which are not accessible with other micro-analytical techniques, may provide information about the physicochemical properties which act as constraints in the genesis of the samples, and important information about the evolution of the geological system. Tests performed with focal-plane-array detectors (FPA) show that resolution close to the diffraction limit can be achieved if the amounts of the target molecules in the sample are substantially different. We also point out the possibility of using FTIR imaging for investigations under non-ambient conditions. FigureFTIR-FPA image of fluid inclusions within a haüyine matrix

Crystal structure of whiteite-(CaFeMg) from Crosscut Creek, Canada

Abstract The structural characterization of whiteite-(CaFeMg), a natural orthophosphate from Crosscut Creek, Yukon, Canada, with chemical composition (Ca0.86Na0.05)Σ = 0.91(Fe0.88Mn0.02)Σ = 0.90Mg2.17Al1.93(PO4)4(OH)2 · 8 H2O obtained by WDS-EMPA, was carried out by means of single crystal X-ray diffraction. The structure was solved in the P2/a monoclinic space group, with the following unit cell constants: a = 14.8700(15), b = 6.9785(5), c = 9.9268(10) Å, β = 110.110(1)°, V = 967.31(15) Å3. Structure refinement lead to the crystal chemical formula (Ca0.95Na0.11)Σ = 1.06Fe0.90Mg2.01Al2.05 (PO4)4(OH)1.99 · 8.17 H2O. Phosphorus atoms display tetrahedral (PO4) coordination, while magnesium, iron(II) and aluminum display regular octahedral coordination; calcium displays a complex CaO8 coordination. MgO2(H2O)4, FeO6, AlO4(OH)2, CaO8 polyhedra and PO4 groups are arranged such as to form a three-dimensional framework via edge- and vertex-sharing arrangements. Strong O—H … O hydrogen bonds contribute to stabilize the array: such interactions were also investigated by means of Libowitzky formula (Libowitzky, 1999) application, owing to the results of IR spectroscopy.