Abstract Rutile is an important accessory mineral in metamorphic rocks and is used as a geothermobarometer or geochronometer. This study aims to bridge the gap between diffusion studies in simplified and complex natural systems by investigating the incorporation and mobility of Al in natural rutile and its high-pressure polymorph TiO 2 (II). Experiments were performed at 0.1 MPa to 7 GPa, 1223–1373 K, at buffered μ(Al 2 O 3) and with f O 2 constrained to ≤CCO, which is the equilibrium between graphite and a CO-CO 2 gas phase. Based on electron probe microanalysis, secondary ion mass spectrometry and Fourier transform infrared analyses, we suggest a complex combination of mechanisms to explain the incorporation of Al and H in natural rutile and TiO 2 (II). This includes: (1) the incorporation of Al 3+ on octahedral Ti-sites charge balanced by the formation of oxygen vacancies; and (2) the incorporation of oxygen in interstitial positions charge balanced by hydrogen interstitials. Determined Al-diffusivities in natural TiO 2 are approximately eight to nine orders of magnitude faster compared to previously published data. A possible explanation includes a significantly enhanced rate of ionic diffusion through the combined effect of hydrolytic weakening, enhanced Al-diffusion through extended defects and to a minor extent oxygen fugacity variations. Consequently, results of this study question that the inferred high closure temperatures for the Al-in-rutile geothermobarometer can be applied to all natural systems.

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 Spectroscopic methods provide information about the local structure of minerals. The methods do not depend on long-range periodicity or crystallinity. The geometric arrangement of atoms in a mineral phase is only one aspect of its constitution. Its vibrational characteristic, electronic structure and magnetic properties are of greatest importance when we consider the behaviour of minerals in dynamic processes. The characterisation of the structural and physico-chemical properties of a mineral requires the application of several complementary spectroscopic techniques. However, it is one of the main aims of this School to demonstrate that different spectroscopic methods work on the same basic principles. Spectroscopic techniques represent an extremely rapidly evolving area of mineralogy and many recent research efforts are similar to those in materials science, solid state physics and chemistry. Applications to different materials of geoscientific relevance have expanded by the development of microspectroscopic techniques and by in situ measurements at low- to high-temperature and high-pressure conditions.

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.