The study of α-quartz and α-cristobalite ballen in rocks from 16 impact structures (Bosumtwi, Chesapeake Bay, Chicxulub, Dellen, El'gygytgyn, Jänisjärvi, Lappajärvi, Logoisk, Mien, Popigai, Puchezh-Katunki, Ries, Rochechouart, Sääksjärvi, Ternovka, and Wanapitei) shows that ballen silica occurs mainly in impact melt rock and also in suevite, and more rarely in other types of impactites. Ballen α-cristobalite by itself was observed only in samples from the youngest craters studied here (at Bosumtwi and El'gygytgyn), but it occurs in association with α-quartz ballen in impactites from structures with intermediate ages (from ca. 35 to 120 Ma); thus, our observations suggest that α-cristobalite ballen are back-transformed to α-quartz with time. Transmission electron microscope observations show that α-cristobalite and α-quartz ballen have similar microtextures and are formed of several tiny angular crystals with sizes up to ~6 μm. The observation of toasted α-quartz ballen, notably at the Popigai impact structure, further supports the notion that toasting is due to vesicle formation after pressure release, at high post-shock temperatures, and, thus, represents the beginning of quartz breakdown due to heating. Our investigation increases the number of impact structures at which ballen silica has been found to 35.

Abstract The solid Earth consists for the most part of minerals and rocks, but fluids, glasses, melts and other non-crystalline 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 of a material 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 1 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.

Abstract The term luminescence (originally “luminescence glow”) is derived from lumen (Latin for light). It describes the ability of minerals to emit light after being excited with various kinds of energy (optical, electric, mechanical, chemical etc .). Luminescence is often described as the “cold glow” of minerals and other matter and, thus, it is not identical to the (temperature-induced) “black-body” light emission of red-hot minerals or melts. Another characteristic feature of luminescence is that the excitation process that finally causes luminescence is reversible and does not cause permanent changes or damage to a mineral sample. Luminescence emission is a remarkably widespread phenomenon; it is known from more than two-thirds of all insulator minerals ( McKeever, 1985 ). We will discuss below that luminescence is based on energetic transitions (on the order of several electron volts) in the electronic shells of atoms in materials. Therefore, this phenomenon is sensitively controlled by the short-range order of minerals. Luminescence has been a well-established technique in materials science research for decades. Up to the 1980's, there was already a wealth of luminescence studies on natural minerals (see Pagel et al. , 2000 ). Unfortunately, these problems with the interpretation seem to have made the whole luminescence field of geoscientific investigation appear an uncertain and speculative technique.

Abstract Chapters Chapter 3 and Chapter 4 deal with optical absorption spectroscopy and comprise two interrelated parts. Part I , the present chapter, is intended as an introduction for the beginners, e.g. undergraduate students of geosciences, in order to help them acquire an understanding of the basic theoretical principles, focussing on the crystal field (CF) concept. After a short introductory section including some “technical” information, the reader is guided step by step through the development of the qualitative principles of the CF theory (CFT), referring to several aspects important for geosciences. The necessary concepts and tools are briefly outlined, whereas references to a selection of relevant textbooks and publications are given for further reading. Concise tables and figures help to illustrate and summarise important topics. Examples of the actual spectra are provided, mainly concerning the “many-electron systems” (of the first-row transition ions, i.e. , 3 d 2,3,7,8 ), since they cover the full diversity of the crystal field based spectroscopic aspects. Part II (Chapter 4 in this volume – Andrut et al. , 2004 ) deals with the quantitative aspects of the crystal field theory and its applications. It highlights the power of semi-empirical methods for the calculation of energy levels of the transition metal complexes with arbitrary low symmetry.

Abstract In Part I (Chapter 3 in this volume - Wilder et al. , 2004) we described the basic principles of crystal field theory (CFT) based on group theory and symmetry. The usefulness of CFT resides in the fact that it can predict the type and number of electronic transitions and their relative energies for transition metal ions in crystals. Hence CFT enables interpretation of the optical absorption spectra. The crystal (or ligand) field induced on the central ion depends on the type and positions of the ligands (i.e., bond angles and distances R) and on the point symmetry of the resulting coordination polyhedron. The number of exited crystal field (CF) states and the type of the ground state arising from a given free-ion d N configuration depends solely upon molecular symmetry, i.e. the site symmetry in case of crystals, and is independent of any model used to describe the metal-ligand bonds. Although the exact energies cannot be calculated ab initio , it is possible to extract empirical parameters from experimental electronic absorption spectra which describe the interaction between metal and ligand. For a given d N X 6 complex with an ideal octahedral coordination (symmetry O h ), the cubic CF splitting parameter 10Dq, together with Racah parameters B and C, provide basis for a reasonably complete description of the electronic spectra ( Lever, 1984 ). In most crystals the site symmetry is, however, lower than O h . This requires introduction of additional, so-called distortion parameters to describe the lower symmetry CF components