Dottorato di ricerca in Scienze della Terra Ciclo XXVI Linking crystal chemistry and physical properties of natural and synthetic spinels: an UV-VIS-NIR and Raman study Settore Scientifico Disciplinare GEO/06 Candidato Docente guida: Veronica D’Ippolito Prof. Giovanni B. Andreozzi Anni 2010-2013 “La mineralogia è una scienza di elite, ha pochi sbocchi professionali. Ma, come in tutte le cose della vita, se ci metti passione e determinazione, una strada vedrai che te la crei” Prof. Sergio Lucchesi “Success is getting what you want. Happiness is liking what you get.” H. Jackson Brown Index Introduction………………………………………………………………....... 1 Chapter 1- The Spinel Group……………………….…………………... 5 1.1- Crystal chemistry……………………………………………………………. 5 1.2- Structure……………………………………………………………………... 8 1.3- Relevance of spinels in the geological field……………………………… 16 1.4- Relevance of spinels in the gemological field …………………………… 19 1.5- Relevance of spinels in the technological field …...…………………….. 20 Chapter 2- Spectroscopic methods…………….……………………. 22 2.1- Introduction to spectroscopy………………………………………………. 22 2.2- Raman spectroscopy...…………………………………………………….. 25 2.2.1- Symmetry, Group theory, Normal Modes and Selection rules … 30 2.2.2- Applications………………………………………………….……….. 33 2.2.3- Raman spectra of Spinel Group compounds…………………….. 36 2.2.4- Luminescence spectroscopy using Raman spectrometer…….… 40 2.3- Optical absorption spectroscopy………………………………………….. 42 2.3.1- Crystal Field Theory (CFT)…………………………….…………… 45 One-electron systems…………………………….……………... 45 Crystal field splitting…………………………….……………….. 47 Many-electron dN systems…………………………………….… 50 Tanabe-Sugano diagrams………………………….…………… 50 2.3.2- Qualitative measurements in optical absorption spectra……….. 52 Causes of color………………………….……………………….. 53 2.3.3- Quantitative analysis in optical absorption spectra……………… 56 Structural relaxation……………………………………………... 57 i Chapter 3- Materials and methods………………………………....... 60 3.1- Materials………………………………………………………………….….. 60 3.2- Synthesis by Flux growth method……………………………………........68 3.2.1- Experimental conditions……………………………………….……. 70 3.3- Single-Crystal X-Ray Diffraction…………………………………………... 75 3.4- Electron microprobe analysis………………………………………........... 78 3.5- X-ray mapping technique in SEM/EDS analysis………………………… 79 3.6- Cation distribution procedure……………………………………………… 80 3.7- LA-ICP-MS instrument……………………………………......................... 81 3.8- Raman spectroscopy instruments……………………………………....... 82 3.8.1- Polarized Raman spectra………………………………….............. 84 3.9- Optical absorption spectroscopy instrument…………………………….. 86 3.9.1- Experimental conditions of synthetic spinels……………….......... 86 3.9.2- Experimental conditions of natural spinels………………………... 87 Chapter 4- Results and discussion…………………….................... 89 4.1- Synthetic blue Co-spinels……………..…………………………………… 89 4.1.1- Synthesis and chemical characterization………………………..... 92 4.1.2- Structural and spectroscopic characterization………................... 100 4.2- Natural multicolor spinels………………………………………................. 113 4.2.1- Chemical characterization…………………………………………... 114 4.2.2- Spectroscopic characterization…………………………………….. 126 Fourier transform infrared spectroscopy………………………. 126 UV-VIS-NIR-MIR spectroscopy………………………………… 132 4.3- Synthetic (Mg,Co,Fe)(Al,Cr)2O4 spinels: Raman study........................ 153 4.3.1- The spinel s.s. (MgAl2O4) end-member…………………………… 153 4.3.2- The MgAl2O4‒ CoAl2O4 spinel series……………………………… 157 4.3.3- The MgAl2O4‒ FeAl2O4 spinel series……………………………… 165 4.3.4- The MgAl2O4‒ MgCr2O4 spinel series…………………………….. 172 4.4- Raman spectra of natural spinels…………………………………………. 183 4.4.1- Comparison between natural and synthetic spinels…………...... 188 ii 4.5- A multi-analytical study of a natural blue gahnite (ZnAl2O4)…………... 194 4.5.1- Chemical and structural characterization………………………... 195 4.5.2 Spectroscopic characterization……………………………………. 201 Chapter 5- Conclusion and Implications…………………………........... 206 5.1- Optical properties…………………………………………………………… 206 5.2- Vibrational properties………………………………………………………. 208 5.3- Implications………………………………………………………………….. 209 Acknowledgments……………………………………………………………… 212 References…………………………………………………………………….… 214 iii Introduction The goal of the present work is the development of an empirical model of optical and vibrational properties of natural and synthetic spinels. Single crystals of natural and synthetic spinels have been studied in detail by a combination of complementary methods with the aim to explore their physical properties, which in turn have been interpreted on the basis of spinel crystal chemistry. Multiple oxides with spinel structure represent minerals largely occurring in rocks as well as materials of high relevance from a technological point of view. To understand their physical properties the complete chemical, structural and crystal-chemical study is necessary. This study can be inserted in a multidisciplinary context, shown schematically in the tetrahedron in Figure 1, where every vertex is closely correlated to another. Fig. 1. Streamlined scheme of the multidisciplinary context in which the study of spinels is inserted. Understanding the relationships among environmental conditions (P, T, etc.), crystal chemistry (e.g., cation distribution), cation distribution and physical properties can help to exploit the spinel potential, both in Earth Sciences and in Materials Sciences. 1 One of the characteristics of the spinel structure is the flexibility in the range of cations and cation charge combinations that can be accommodated, making it a structure adopted by over a hundred compounds, many of them either important minerals or important commercial materials. On the Earth, many varieties of spinels occur as main or accessory minerals in different rocks, from the upper mantle to the crust, and their properties need to be understood in order to model the Earth interior. Being sensitive to temperature, pressure, oxygen fugacity, and bulk rock and fluid composition, intra- and inter- crystalline equilibria in spinel are extremely useful for constructing mineral geothermometers, geobarometers and oxygen fugacity sensors (Della Giusta et al., 1986; Ballhaus et al., 1991; Sack and Ghiorso, 1991; Princivalle et al., 1999; Lenaz and Princivalle, 2005). Spinel oxides have been also discovered in extraterrestrial materials, as for example in Martian meteorites. The identification and compositions of the spinels on Mars provide information and constraints on conditions of rock genesis and alteration. In addition, they can provide evidence for magnetism on the “red planet” or can give information about oxygen fugacity and the possibility of life on Mars (Wang et al., 2004; Righter et al., 2009; Mayhew et al., 2013). Gem-quality natural spinels are appreciated as attractive and brilliant gemstone all over the world for their color range and vivid tone. Indeed thanks to the ability of accepting different cations with different valence in the structure, spinels exhibit a wide range of compositions and then a wide range of color. From a technological point of view, spinel oxides are suitable for a large variety of applications, from semi-conductors to catalysts, from refractory material to electrode for batteries, from magnetic devices to pigments for their electrical, mechanical, magnetic, thermochemical and optical properties (Shukla et al., 1999; Fernandez and de Pablo, 2002; Fierro et al., 2005; Gedam et al., 2009). Among the large variety of useful physical properties shown by spinels, this study deals with the optical and vibrational properties, through a multi-methodological approach using Electron MicroProbe, Single-Crystal X-ray Diffraction, Optical 2 Absorption and Raman spectroscopies. In particular, this study investigates how the minerals of the spinel group absorb or scatter radiation in the visible and infrared part of the electromagnetic spectrum, and the relationships with crystal chemistry. A large amount of natural spinels showing various colors and tone has been analyzed in the present work. Causes of color in minerals may be different, and Nassau (1987) provided a good overview of the various causes, most of them being related to transition metal ions, their valence and their coordination. However, more complex coloring mechanisms are difficult to characterize and often remain unexplained. To fully comprehend the origin of color in spinels, an accurate chemical and spectroscopic study has been performed using the optical absorption spectroscopy, which investigates the absorption of the ultraviolet, visible and infrared region of the electromagnetic spectrum (UV-VIS-NIR-MIR). It is concerned with transitions of electrons between outermost energy levels and can be used to identify the electronic transitions responsible for color and pleochroism in minerals. In addition this spectroscopy can provide information about the local structure around the chromophore ion such as the type of coordination, its symmetry and distortions, degree of covalency of bond distance and interactions with neighboring anions and cations. A larger set of spinel single crystals, including synthetic samples, has been investigated through the Raman spectroscopy, which is a non-destructive technique used to interpret atomic vibrations, with the aim to establish the correlations between the spectra and compositions. Raman spectra consist in unique vibrational fingerprints useful in routine identification of a multitude of materials, without necessarily knowing the