Raman spectroscopy applied to Earth sciences and cultural heritage
In Earth Sciences and Cultural Heritage Science we can only understand the formation of the ‘objects’ if they are well characterized. Optical observation, including optical microscopy, is still the primary tool and is essential in obtaining a preliminary, qualitative determination of an object, to determine the relations between it and other objects, and to place it in a general context. Most of the time, however, optical observations are insufficient. Spectroscopic methods are the second “set of eyes” used to gain greater insight into these objects and to use physical chemistry, if applicable, to derive the mechanisms of formation. Spectroscopic methods are numerous and have been described in a previous volume (6) in the EMU Notes in Mineralogy series, edited by A. Beran & E. Libowitzky (2004). In chapter 7 of that volume, Raman spectroscopy was addressed by Nasdala et al. (2004). Though that volume provides a very useful means to gain a general understanding of the contribution of each spectroscopy to mineralogy, no details of the theory, instrumentation and applications to the different types of objects could be provided. In the past eight years, there have been many improvements in the instrumentation which makes Raman spectroscopy a versatile technique used in many Earth Science and Cultural Heritage laboratories and so it appeared appropriate to have a school and a book dedicated to Raman Spectroscopy alone. Four main topics are addressed here: (1) Theory: in Chapter 1, dedicated to the Raman effect, in Chapter 4, dealing with the modelling of Raman spectra, in Chapter 2 to the links between fluorescence and Raman spectroscopy, in Chapter 10 for the exploitation of Raman spectra of minerals at high pressure and temperature, and in Chapter 12 for the rationale behind Raman spectra of graphitic carbon compounds; the basic theory of the instrumentation is developed in Chapter 3. (2) Methodology including the instrumentation: in Chapter 3 and the Raman data analysis in Chapter 5. (3) Experimental aspects: for the investigation of Raman spectra at high pressure and temperature using diamond anvil cells for minerals (Chapter 10) and geological fluids (Chapter 7) and with fused silica capillary for fluids (Chapter 6). (4) Application: to different types of objects: geological fluids (Chapter 8), silicate glasses and melts (Chapter 9), biogeology and astrobiology (Chapter 11), graphitic carbons (Chapter 12), gemmology with a link with fluorescence spectroscopy (Chapter 13), and Cultural Heritage (Chapter 14). A given chapter may address several topics, as it is impossible to obtain relevant information from the Raman study of a given object without considering the theory for the interpretation of the spectra, or instrumental set-up including special cells, or handling raw spectra. It is also clear that the whole theory could not be developed thoroughly with all the details as it deals with quantum mechanics and group theory. This would require at least two further volumes! Thus, chapters focused on the theoretical aspects are written with the aim of giving the main steps and the results obtained from the theory and how theory can be used for the interpretation of Raman spectra. Readers who want to know the details of the theory and their associated calculations should consult specialized textbooks or publications which are referred to in the chapters hereafter.
Optical cells with fused silica windows for the study of geological fluids
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Published:January 01, 2012
Abstract
Two types of optical cells with fused silica windows are described for study of geological fluids at temperatures (T) up to 600°C and pressures (P) up to 1 kbar. One is the high-pressure optical cell (HPOC), in which fluids of known composition can be loaded directly into the cell and its pressure can be adjusted and measured. The other is the fused silica capillary capsule (FSCC), which contains sample fluid with both ends of the tube flame sealed. Both types of cells can be inserted into a heating-cooling stage (USGS-type, the newly developed stage from INSTEC or that from Linkam) for in situ observations and Raman spectroscopic analyses at various P-T conditions. The HPOC has been applied to measure the solubility and diffusion of methane in water, the solubility of methane hydrate in water, and methane pressures in fluid samples. It is also very useful for providing fluid standards with known composition and pressure for the calibration of Raman spectroscopic systems before quantitative analyses. The FSCC is particularly useful for samples which need to be reacted at elevated P-T conditions for long periods of time (days or weeks). These two types of optical cells with fused silica windows are particularly suitable for the study of organic systems and also for systems containing sulphur.