Natural crystals (minerals) have stories to tell about the conditions that prevailed during their growth, and so play a key role in the study of past climates, ancient and modern volcanism, the plate tectonic motions responsible for ancient mountain belts, among other problems. However, mineral chemistry must be interpreted through some understanding of how environmental properties get imprinted on compositions of growing crystals. It is generally assumed that equilibrium thermodynamics controls the mineral-chemical archive. But emerging evidence indicates that non-equilibrium processes, such as diffusion or irreversible reactions, dominate the chemistry of many key minerals. And, it is possible that the poorly understood thermodynamic properties of surfaces rather than the well-studied properties of bulk solids and fluids control compositions of some minerals. The goal of the Caltech-RPI proposal is to understand these phenomena at an atomic (molecular) scale. Its results will advance interpretations of mineral chemistry as constraints on the environments of mineral growth, and thereby advance the study of Earth?s chemical systems and past environments
This study will focus its attention on the zonation of isotopes (particularly naturally occurring isotopes of O and Si) and trace elements in the mineral quartz, grown from natural or synthetic water-rich solutions. Oxygen and silicon atoms make up most of the three dimensional structure of quartz (and most other minerals), and proportions of their ?light? and ?heavy? isotopes provide some of the most widely used records of the geologic past. The distribution of these isotopes in individual minerals are not always uniform. This heterogeneity, particularly differences in composition between adjacent simultaneously growing crystal faces, tells us that some influence other than environmental factors may have affected the growth of the crystal?some manifestation of non-equilibrium growth. The Caltech-RPI study will use the most recent advances in analytical geochemistry to document the co-variations of O and Si isotopes associated with intracrystalline heterogeneities, and their relationship to heterogeneities in trace element abundances. The resulting chemical data will yield a sort of fingerprint for mechanisms responsible for non-equilibrium growth. The study will then construct first-principles chemical-physics models of mineral growth, constructed to replicate and explain these chemical fingerprints.
A principal goal of the earth sciences is to use the chemical compositions of minerals and other geologial materials as records of conditions in past environments. This task is central to studies of past climate change, measurements of the temperatures, water pressures and other variables in the interiors of volcanoes, and many other subjects. All such studies must deal with the fact that mineral chemistry is a complex, indirect archive of past environmental conditions, an archive that can be read only if you know the 'rules' that govern the ways in which minerals tap components of the fluids and magmas from which they grow. These rules include several poorly understood processes that take place at scales not much larger than individual molecules, at and near the surfaces of growing minerals: diffusion through liquid or melt; chemical reactions within those fluids and melts to make constituents available for addition to a mineral surface, the actual attachment of chemcals to a growing mineral surface, and the reorganization of that surface to a stable crystal lattice. This grant supported fundamental studies of these processes, through laboratory experiment, theoretical models and measurements of natural crystals. We focused on the peculiar case of crystals that grow from a well-mixed fluid environment, but someone the processess acting near the crustal surface consipre to generate contrasts in composition from one crystal face to another. In particular, we explored the hypothesis that the chemistry of growing minerals is controlled by the structures of layers of newly grown minerals, which differ from the stable crystal lattice and can vary between crystal faces. Our work demonstrated that these crystal-face-specific factors significantly influence the abundances of trace elements encorporated into growing crystals, but have no measurable effect on isotopic compositions of major constituents of those crystals. Our findings suggest that this mechanism could be an important factor controlling trace element records of past environments, but likely plays little role in stable isotope records of those environments.
