Garnet Raman

Calligaro, T., Colinart, S., Poirot, J. P., and Sudres, C. (2002). Combined external-beam PIXE and mu-Raman characterisation of garnets used in Merovingian jewelry. Nuclear Instruments and Methods in Physics Research B 189 320-327. 

Arredondo E.FI. and Rossman G.R. (2002) Feasibility of determining the quantitative OH content of garnet with Raman spectroscopy. Am. Mineral. 87, 307-311. 

Keywords ion beam analysis, Raman spectrometry, archaeometry, PIXE, RBS, NRA, garnets... 

The second provenance criterion is based on the identification of inclusions in gemstones. Micro-Raman spectrometry was used for this task in almandine garnets. Various inclusions were observed like apatite, zircon, monazite, calcite, and quartz and two of them, curved needles of sillimanite (Al2Si05) and 10-pm metamict radioactive crystals, were specifically found in archaeological garnets. Fig. 6 shows the Raman spectra of a sillimanite needle, which is a mineral formed under a high temperature and high pressure metamorphism. 

Before the invention of lasers in 1960 (Maiman), radiation emitted by the mercury arc, especially at 435.8 and 404.7 nm, has been u.sed for exciting Raman spectra (Brandmiiller and Moser, 1962). Today, most types of lasers ( continuous wave (cw) and pulsed, gas, solid state, semiconductor, etc.), with emission lines from the UV to the NIR region, are used as radiation sources for the excitation of Raman spectra. Especially argon ion lasers with lines at 488 and 515 nm are presently employed. NIR Raman spectra are excited mainly with a neodymium doped yttrium-aluminum garnet laser (Nd YAG), emitting at 1064 nm. 

Parkinson C. D. and Katayama 1. (1999) Present-day ultrahigh-pressure conditions of coesite inclusions in zircon and garnet evidence from laser Raman microspectroscopy. Geology 27, 979-982. 

Thus, a high frequency excitation source enhances the number of scattered Raman photons dramatically. As an example, let us consider the relative intensity of a 1000 cm Raman line excited at 1064 nm and also excited at 514 nm. The first wavelength is available from an Nd YAG (yttrium aluminum garnet doped with neodymium) diode laser, and the second from an argon ion laser. Using the excitation at 1064 nm the 1000 cm Raman band occurs at 1191 nm, and for the excitation at 514 nm the Raman line would occur at 542 nm. Therefore, the enhancement of the 514 nm excitation relative to the 1064 nm excitation would be by a factor (1191/542)" = 23.3. This means that there are 23.3 times as many photons to detect with the shorter wavelength excitation simply as a consequence of the nature of the scattering process. This can be a very important effect. 

P. McMillan, M. Akaogi, I. Ohtani, Q. Williams, R. Nieman, and R. Sato, Cation disorder in garnets along the Mg3Al2Sij0 2-Mg4Si40i2 join an infrared, Raman and NMR study, Phys. Chem. Min. 1989,16, 428-435. 

Figure 4.80 3D confocal Raman image of a fluid inclusion in garnet (60 x 60 x 30 iim red, garnet blue, water ... 

Several of the early problems associated with Raman have been overcome. Fluorescence emission from the sample was a problem when visible light sources were used. Now, NIR sources such as the NdrYAG (neodymium yttrium aluminum garnet) laser diminish fluorescence and provide variable power levels for measurement optimization. The major difficulty with Raman is the lack of experienced interpretive spectroscopists and detailed texts. Even NIR seems commonplace when compared with the number of Raman applications. Only time will solve these latter problems as more applications occur and more are published. 

Figure 36 Raman spectra of four different minerals found in a Maya ceremonial polished axe from Mexico (a) clinopyroxene (b) clinoamphibole (c) garnet (d) titanite. This association led to the nondestructive identification of the rock as an eclogite. (After Ref. 63.)...

BA Kolesov, CA Geiger. Raman spectra of silicate garnets. Phys Chem Minerals 25 142-151, 1998. 

The theory of the effect of crystal structure on k is still in a very rudimentary state. It is generally observed that the larger the number of optical branches in the phonon spectrum, the lower the thermal conductivity. In garnets, there are 80 atoms in the primitive unit cells, and there are 97 optical modes at the zone center (Hurrell et al., 1%8). The combination of infrared and Raman studies has... 

Laser is an acronym for light amplification by simulated emission of radiation. In SERS, as well as in other types of Raman scattering experiments, a continuous-wave (CW) gaseous ion laser is normally used, e.g., an argon-or krypton-ion laser. It is also possible to use a pulsed laser, such as a neodymium, Nd ", in yttrium-aluminum garnet (YAG) laser however, a much... 

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