Melanie Kaliwoda

New Raman Spectroscopic Data of the Almahata Sitta Meteorite

ABSTRACT Raman spectroscopy is a convenient method to classify polytypes plus polymorphs and to investigate different compositions, structures, and modifications of minerals. This information is required to classify meteorites of different mineral compositions. Our intention was to examine different graphite modifications and to determine the opaque phases within the Almahatta Sitta (AS) meteorite. Our investigation focused on specimens AS39 and AS4. Both are multicomponent breccias, composed of different meteoritic lithologies, that is, anomalous polymict ureilite material, different types of chondrites, and an iron meteorite.[ 1-5 ] The mix of material implicates a crash of variable asteroids within time and space. This means that a primary ureilitic asteroid body has been the target of multiple meteorite or asteroid impacts, which did not completely destroy the asteroid but added new material (from chondritic to iron) to the main mass. For instance, Gabriel and Pack[ 6 ] suggested that the vein metal of monomict ureilites is introduced by the impact of a Ni-poor iron meteorite in a similar scenario as described above. Raman spectroscopy helped to investigate different carbon materials, that is, graphite and diamond. We used single-spectrum acquisition and spatial mappings to identify the different modifications. Furthermore, we classified minor opaque phases such as schreibersite, suessite, daubreelite, cohenite, and kamacite. The investigation of these opaque phases is dealt with in further papers.[1,2]

On the effect of carbonate on barite growth at elevated temperatures

Abstract The effect of carbonate on the growth of barite {001} surfaces from aqueous solutions supersaturated with respect to barite (Ωbarite ~ 12) was studied by hydrothermal atomic force microscopy (HAFM) and Raman spectroscopy at temperatures ranging from 25 to 70 °C. The experiments showed that the effects of carbonate depend on the specific location of growth. For mono-layers growing on pristine barite, the carbonate-additive promotes growth and the spreading rate of two-dimensional islands increases with temperature. However, growth is inhibited in layers growing on surfaces, which grew in carbonate-containing solution. The threshold carbonate concentration necessary to completely inhibit growth is inversely correlated with temperature. Raman spectroscopy revealed the presence of carbonate within crystals, which grew in carbonate-containing solution. Judging by these findings, incorporation of carbonate into the structure of growing barite as a thermally activated process likely is a controlling factor, which inhibits barite growth. Thus the study shows that additives can exert opposing effects on growth not only depending on additive concentration but also depending on the specific growth location. The implication of this work, therefore, is that bimodal effects of additives on crystal growth occur more frequently than generally recognized. The insights into the mechanisms of such bimodal effects of additives can significantly contribute to the understanding and predictability of the kinetics of macro-scale processes such as barite scale formation or the behavior of barium sulfate in CO2-sequestration fluids