ABSTRACT
This work addresses the long-standing debate surrounding the origin of color variation in fluorite (CaF2) through a novel quantitative approach. By examining eight carefully selected fluorite samples having different hue of colors from the Amba Dongar mine in Gujarat, India, a rigorous quantitative analysis was conducted. This approach combined chemical compositional data and optical spectroscopic features to elucidate the relationship between elemental composition, concentration, and color variation in fluorite. Precise elemental concentration data for trace transition metals, alkali metals, and rare earth elements (REEs) were obtained through inductively coupled plasma mass spectroscopy (ICP-MS) analysis of powdered fluorite samples. Optical spectroscopic techniques, including UV-visible absorption, emission (photoluminescence and fluorescence), and Raman spectroscopy, were employed to capture characteristic spectral signatures for specific color of the study sample. The work unveils a strong correlation between specific elemental concentrations and observed spectral features, particularly influenced by alkaline metals, transition group elements, and REEs. Fluorite's optical absorption behavior lacks a clear pattern in UV and infrared wavelength ranges but correlates well with transition metal, alkaline element, and REE concentrations in visible wavelength regions, influencing coloration. Luminescent centers in the study fluorite samples correspond to specific REE concentrations, indicating a strong linkage between emission wavelengths with the presence of specific REE. UV-visible and fluorescence in fluorite result from trivalent REE or Eu2+ ions, with emission intensity affected by REE concentration and specific REE or combinations thereof. Raman spectroscopy identifies characteristic modes related to F-substitution and REE impurities, providing insights into fluorite's structural composition. This quantitative correlation between elemental composition and spectroscopic characteristics represents a novel contribution for understanding color variation mechanisms in fluorite. The comprehensive analysis of this present work underscores the intricate interplay of mineral composition, and element concentration particularly alkaline metals, transition group elements, and REEs, for variation in spectral signatures with variation in fluorite's visual attributes.
ABSTRACT
Bridgmanite, the most abundant mineral of the Earth's lower mantle, has been reported in only a few shocked chondritic meteorites; however, the compositions of these instances differ from that expected in the terrestrial bridgmanite. Here, we report the first natural occurrence of Fe-bearing aluminous bridgmanite in shock-induced melt veins within the Katol L6 chondrite with a composition that closely matches those synthesized in high-pressure and temperature experiments over the last three decades. The Katol bridgmanite coexists with majorite and metal-sulfide intergrowths. We found that the natural Fe-bearing aluminous bridgmanite in the Katol L6 chondrite has a significantly higher Fe3+/ΣFe ratio (0.69 ± 0.08) than coexisting majorite (0.37 ± 0.10), which agrees with experimental studies. The Katol bridgmanite is arguably the closest natural analog for the bridgmanite composition expected to be present in the Earth's lower mantle. Textural observations and comparison with laboratory experiments suggest that the Katol bridgmanite formed at pressures of â¼23 to 25 gigapascals directly from the chondritic melt generated by the shock event. Thus, the Katol L6 sample may also serve as a unique analog for crystallization of bridgmanite during the final stages of magma ocean crystallization during Earth's formation.
ABSTRACT
Red clays of Central Indian Basin (CIB) under influence of trace of Rodriguez Triple Junction exhibited chemoautotrophy, low temperature hydrothermal alterations and photoautotrophic potential. Seamount flank TVBC-08, hosting such signatures revealed dominance of aerobic anoxygenic phototroph Erythrobacter, with 93% of total 454 pyrosequencing tags. Subsequently, enrichments for both aerobic (Erythrobacter) and anaerobic anoxygenic phototrophs (green and purple sulphur bacteria) under red and white LED light illumination, with average irradiance 30.66Wm-2, were attempted for three red-clay sediment cores. Successful enrichments were obtained after incubation for c.a. 120 days at 4°± 2°C and 25°± 2°C, representing ambient psychrophilic and low temperature hydrothermal alteration conditions respectively. During hydrothermal cooling, a microbial succession from anaerobic chemolithotrophy to oxygenic photoautotrophy through anaerobic/aerobic anoxygenic phototrophic microbes is indicated. Spectral absorbance patterns of the methanol extracted cell pellets showed peaks corresponding to metal sulphide precipitations, the Soret band of chlorosome absorbance by photosystem II and absence of peaks at Qy transition band. Dendritic nano-structures of metal sulphides are common in these sediments and are comparable with other sulphidic paleo-marine Martian analogues. Significant blue and redshifts have been observed for the experimental samples relative to the un-inoculated medium. These observations indicate the propensity of metal-sulphide deposits contributing to chemiluminiscence supporting the growth of phototrophs at least partially, in the otherwise dark abyss. The effects of other geothermal heat and light sources are also under further consideration. The potential of phototrophic microbial cells to exhibit Doppler shift in absorbance patterns is significant towards understanding planetary microbial habitability. Planetary desiccation could considerably influence Doppler effects and consequently spectral detection techniques exo-planetary microbial life.
