Aaron Palke

Aaron Palke
Gemological Institute of America

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Project
The solid Earth consists for the most part of minerals and rocks, but fluids, glasses, melts and other noncrystalline substances are also found and they play an important role in a number of geochemical and geophysical processes. The mineral sciences and the field of geochemistry are greatly concerned with investigating the nature of all geomaterials. Indeed, one wants to describe and understand their fundamental chemical and physical properties and also their behaviour under different physical conditions. In many cases a level of scientific understanding is best achieved when the atomistic-scale properties and interactions can be described or characterised. This is, for example, the case for investigating the adsorption behaviour of molecules or atoms on the surfaces of minerals or in studying the physical nature of viscosity of a silicate melt. Ultimately, it is the atomistic-scale properties that control the bulk macroscopic properties of a material and, thus, they have to be characterised and understood. One is interested in both the static and dynamic behaviour of atoms and molecules and their energetic properties and interactions with one another. This is where spectroscopy enters the picture, because spectroscopic measurements can provide local or atomistic-level information on a variety of different materials, whether they are gas, liquid or solid phase. This information can be compositional, structural, crystal chemical as well as dynamical. The traditionally trained mineralogist, petrologist or geochemist was instructed to use an optical microscope, a powder X-ray diffractometer and a couple of different devices for analytical chemistry determinations, such as the electron microprobe or an X-ray fluorescence unit, to characterise a rock or mineral, for example. Times have changed and this no longer suffices. Today, the research problems and directions are often different from those of the past and new approaches and ‘tools’ for study are required. Here, the development and application of various spectroscopic methods over the last approximately 30 years is most notable. The number of spectroscopic methods that are presently available to mineral scientists and geochemists is staggering. There has been both a birth of new spectroscopic techniques and rapid advances and refinements of the older more traditional methods (e.g., IR, Raman, XAS). Both developments are having a major influence on the type of investigations undertaken on a wide variety of geomaterials and, in addition, under different physical conditions. The range of methodologies and the types of investigations are so great that it is sometimes difficult to decide on what kind of spectroscopic measurement should be made to address a scientific question. In general, many spectroscopic methods deliver chemical or structural information that is narrow or specific in scope in comparison to the important X-ray diffraction experiment used in structural investigations of crystalline materials, for example. However, the information that they can deliver can be very quantitative and unique. For example, 29Si MAS NMR spectroscopy can be used to determine quantitatively the short-range distribution of Si atoms in silicates or the type of SiO4 polymerisation in silicate melts. Such information cannot be easily obtained by nearly any other experimental method. On the other hand, NMR spectroscopy can only be applied to nearly paramagnetic ion free substances and in the case of crystals one needs to know the crystal structure before a spectrum can be fully interpreted. A second example involves IR spectroscopy. If one wants to determine small concentrations of structural H2O or OH- in a nominally anhydrous mineral or in a silicate glass and also their energetic interactions with the environment, then IR spectroscopy is essentially unique in its ability to provide such information. Indeed, in the case of noncrystalline materials like gases or fluids or even nanoparticles, spectroscopy offers the best way to characterise their physical and chemical properties. A difficulty for the beginner is to decide what method should be used or how one should start a spectroscopic investigation. Experience shows that, in general and in many cases, the different spectroscopic methods should be used in a complementary fashion (Calas & Hawthorne, 1988). These authors state “one can view the different spectroscopies … , as a series of tools that one uses to solve or examine a problem of interest; a single tool is generally not sufficient for ones needs – you cannot drive a nail and drill a hole with just a hammer”. It can be stated further that one cannot build a house with just a saw. Hence, if the goal is to understand the physical and chemical nature of some material or a system of phases (e.g., a rock or a mineral-fluid interface) in a complete sense, one must apply a number of different techniques. Ultimately, one wants to understand the properties of a material or a physicochemical system from the atomistic level through the nano and microscopic scale up to the macroscopic state. In addition, the atomistic-level dynamic properties sometimes require a description over different time scales. Needless to say, there remains much work to be done in the Earth Sciences. Observed in a historical context, the general scientific problem has not changed greatly with time. The ancient Greeks struggled to understand the nature of matter, as did Kepler almost 2000 years later (Schneer, 1995). Today, mineral scientists and geochemists are still struggling to understand why various geomaterials behave the way they do under certain geologic conditions or why they display the properties that characterise them.
Project
A main research thrust of the mineral sciences lies in investigating the thermodynamic, crystal-chemical, and physical properties of minerals. In terms of the thermodynamic properties of end-member phases, a number of different compilations listing the standard functions Cp, V°, S°, and deltaHf° can be found (e.g., Holland and Powell, 2011). Crystal-chemical and other properties (e.g., compressibility, thermal expansion, magnetic and electronic, etc.) are given in a number of books and monographs (e.g., AGU ¬- Mineral Physics and Crystallography: A Handbook of Physical Constants and various MSA Reviews of Mineralogy and Geochemistry). It is a fact, though, that most rock-forming silicates are not compositionally simple end members, but are substitutional solid solutions, and it is imperative to investigate their thermodynamic, crystal-chemical and physical properties. In terms of the composition of many minerals, the element Fe is important. Because of its different electronic states (+2 and +3, as well as mixed valence charge-transfer states), Fe gives rise to complex and interesting behavior in minerals. The importance of the exchange of Fe2+ and Mg cations, as in many rock-forming silicate groups, has been recognized for many years (e.g., Ramberg, 1952) and much work has been done. However, a complete and precise understanding of the thermodynamic and physical properties (e.g., elastic constants and compressibility) of Fe2+-Mg silicate solid solutions is still not at hand. Microscopic- or atomistic-scale aspects determine the macroscopic behavior of minerals (Navrotsky and Kieffer, 1985; Geiger, 2001). It follows that electronic and magnetic states arising from Fe can significantly affect thermodynamic (e.g., heat capacity) and other properties (e.g. compressibility). However, in the mineral sciences, not much experimental research has been done and several areas are not well understood. Indeed, it can be argued, in an even broader scientific sense that an understanding of the relationships between microscopic crystal-structure properties and macroscopic thermodynamic behavior of minerals is still in its beginning phase. These issues are important and not just strictly for academic reasons, as applied aspects are also involved. Namely, if a more fundamental understanding of various Earth processes is to be achieved, whether they involve metamorphism, mantle melting, or deep earthquakes, to name but three, more quantitative research in terms of the thermodynamic, crystal-chemical and physical properties of rock-forming minerals is necessary.
Project
. To determine the primary origin and age of some alluvial gem quality Sapphire associated with alkali volcanism in Upper Benue Trough NE Nigeria To determine the condition of formation of the sapphire-zircon-spinel megacrysts associations. To develop a model for the origin of sapphire-zircon-spinel megacrysts associations in a rift setting which will then be evaluated within various genetic models for basalt-related alluvial sapphire-zircon-spinel megacrysts fields in a global context.