Applications of FTIR microspectroscopy to mineral sciences

FTIR microscpectroscopy can be used in mineral sciences for identification purposes, however modern applications involve mostly the analysis of volatile components in geological materials. Water and carbon dioxide exert strong influence on the chemical and physical properties of minerals; hence they have a controlling effect on many high-temperature geological processes (e.g. magma genesis, kinetic of phase transformations, etc.), as well as low-temperature alteration processes. Unfortunately, geological materials are characterized by extremely variable compositions; with this respect microanalysis of light and volatile elements is one of the main challenges of modern mineralogy. Conventional micro analytical techniques such as EMPA are unsuitable to quantify elements with Z < 8. SIMS, a technique that is not easily accessible and which requires careful sample preparation and complex, time-consuming, analytical procedures can address this problem. LA-ICPMS provides a micro analytical tool for some elements with Z < 8, as for example Li, B and Be; it does not require sample preparation, but it is unsuitable for H and C. X-ray single crystal diffraction (SREF) can be useful in locating and possibly quantifying light-elements, but the procedure is not conventional and requires combination with other techniques. A possible alternative is offered by spectroscopy, such as Raman, FTIR and NMR. FTIR is particularly suitable for volatile elements, since in their molecular configurations (e.g. C-H-O species) they usually absorb the infrared radiation very efficiently. However, the tremendous advantage of FTIR spectroscopy is the possibility of characterizing not only the amount but also the speciation and the crystallographic location of the probed element in the sample under interest. Coupled with its relatively low cost, ease of use and ease of sample preparation, this makes the FTIR technique an extremely interesting tool for the study of light-element-bearing minerals at micro-scales. The main defect of conventional FTIR is its relatively poor spatial resolution (50 m beam diameter). However, this problem can be currently overcome with the use of synchrotron-light FTIR spectroscopy (SR-FTIR), which now allows a spatial resolution of the order of Raman microprobes (~ 3 m beam diameter). We present here the recent developments of the FTIR activities at Roma Tre with the aim of showing, to the scientific community interested in the analysis of light-elements, the new facilities, which are currently available in our laboratories. The topics that will be treated, using as examples the most recent results obtained by our group, include: • Quantitative analysis of H in hydrous minerals • Quantitative analysis of H in nominally anhydrous minerals • Speciation of H • Quantitative analysis and speciation of B in minerals and glasses • Quantitative analysis and speciation of C in minerals • Polarized measurements and the crystallographic orientation of O-H, C-O and B-O species in minerals • In situ thermal behavior of H2O and CO2 • Study of phase-transitions: FTIR spectroscopy at non-ambient conditions (high-T and -P) • Synchrotron-light microspectrometry: mapping of H and C in feldspathoids.