A crystal-chemical study of cordierite, synthesis and stability at variable H2O and CO2 concentration: geological and technological applications

Abstract

Cordierite is a relatively widespread mineral, having a peculiar ability to trap H2O and CO2 up to very high pressure and temperature. This is a unique property for a rock forming silicate, and has a significant role in the mineralogical equilibria in HT and UHT metamorphism (Harley and Thompson, 2004). In this work, great attention was paid to the study of the diffusion mechanisms of carbon dioxide inside these channel-like structures. Considering that carbon dioxide is probably one of the major responsible for long-term climate change on Earth (IPCC, 2005), the ability of these minerals to entrap CO2 could provide insights for future research for the permanent CO2 storage in minerals. The aim of this Thesis was to investigate the diffusion of CO2 across cordierite, and address possible implications from both a geological and a technological point of view. The work was completed by the parallel study of beryl, which is structurally correlated to cordierite. The text is divided into four major sections: 1) In the first section (Chapter 1 and 2) I introduce the issues related to the qualitative and quantitative analysis of H2O and CO2 in cordierite by means of Fourier-Transform Infrared Spectroscopy (hereafter FTIR). In detail I show the crystal-chemical and spectroscopic study of chemically different samples, from an almost Mg-cordierite end-member to its Fe-analogue sekaninaite. Additionally I will discuss the calibration of the molar absorption coefficient ε, an indispensable coefficient for quantitative measurement in FTIR micro-spectroscopy. Both these chapters have been published in first-rank mineralogical and petrological journals and are thus reported here as they are published. 2) In the second section (Chapter 3 and 4) I’ll focus in mechanism of outward CO2 diffusion: to this purpose, oriented single-crystal cordierite slabs were heated by using a heating-stage under the FTIR microscope and investigated using in situ FTIR μ-spectroscopy. Part of this study was done using a synchrotron-light source (SR-FTIR) to improve the signal-to-noise ratio and attain higher spatial resolution in the data. In this section I studied in particular the absorbance variation at constant temperature as a function of time, and evaluated the kinetic and diffusion parameters for CO2 expulsion from the matrix. 3) In the third section (Chapter 5) I discuss the mechanism of inward diffusion of CO2 within the structural channel of cordierite and beryl under different pressure, temperature and time conditions. The experimental work was done using a piston cylinder apparatus. In this section I’ll make extensive use of high resolution single-crystal FTIR Focal Planar Array (FTIR-FPA) imaging to characterize possible inhomogeneity in the CO2/H2O across the samples, and identify the possible pathways for CO2 diffusion. As it will be shown, the spectroscopic imaging was also necessary to locate the analytical spots for CO2 measurements in the sample. 4) The last part (Chapter 6, 7 and 8) covers additional features observed during the work. In particular, chapter 6 deals with a multidisciplinary study of a peculiar diffusion pattern of CO2 across an hourglass zoned beryl. This chapter relates on advances in techniques such as Time Of Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) chemical imaging, polarized light FPA imaging and High Resolution SR-FTIR mapping. Chapter 7 deals with changes in coordination environment and physical state of H2O in low-water beryl and cordierite. Chapter 8 eventually presents a summary of the work and illustrates the technological applications of CO2 diffusion in beryl.