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It has long been known that small amounts of impurities are the ultimate cause of the brilliant color of many mineral specimens.  However, the trace elements contained in gemmy minerals provide much more than just beautiful colors for a particular mineral specimen.  The trace constituents record the physical and chemical conditions that were present as the mineral was growing.  In fact, the detailed variations in impurity content recorded in individual crystals provide a detailed record of the changes that occurred in the host fluid while the system was undergoing crystallization (Ihinger and Zink, 2000).  In fact, the only insights we have into the character of the elusive hydrothermal fluids are the clues that were left behind in their crystalline byproducts.  Hydrothermal fluid systems govern the mobility of organic compounds related to oil and natural gas formation and important dissolved species related to ore formation.  Unfortunately, the timing and evolution of natural hydrothermal channels in the crust are poorly known.  During the first two years of PRF funding, we have explored the potential of micro-FTIR analyses for revealing the growth evolution of individual quartz crystals within veins of variable metamorphic grade, and thus for offering insights into the thermal and chemical evolution of natural hydrothermal systems.  These results have proven enormously helpful toward constraining the timing of growth for individual crystals in fluid-filled fractures.  During this past year, we have extended our methods and techniques to analyze the evolution of organic-bearing fluids in the Cave-In-Rock Fluorspar District in Illinois, which contains world-class economic fluorite mines.

The Cave-in-Rock Fluorspar District in Illinois has been the most productive fluorine-bearing ore body in the United States.  Fluorite crystals grew in the fluid-filled veins at peak thermal conditions within a large hydrothermal system that was related to nearby magmatic intrusions during the Jurassic Era.  Initial fluids in the system were hot and acidic and dissolved away a large volume of host carbonate rock to provide decimeter-sized vugs and fractures.  Subsequent C-bearing fluids, saturated with CaF₂, deposited large fluorite, calcite and quartz crystals in the veins that now host the ore body.  Banded fluorite sampled from the Cave-In-Rock Fluorspar District, Illinois offer insights into the evolving hydrothermal fluids that hosted sulfide ore deposition.  Earlier studies have shown that fluids responsible for the Cave-In-Rock deposits followed similar trends in crystallization temperature, salinity, and isotope systematics.  However, these studies failed to identify chemical and/or environmental factors that correlate with the observed prominent color banding. 

The development of the high-resolution micro-FTIR instrument and its application to detailed studies of single crystals of quartz can potentially revolutionize our understanding of hydrothermal systems (Ihinger and Zink, 2000; Ihinger et al., 2009).  However, the application of the high-resolution IR technique to crystals other than quartz from hydrothermal solutions has not been attempted.  We have acquired a thick section of a world-class fluorite specimen sampled from the Hill Mine of the Cave-in-Rock Fluorspar District from my collaborator, Professor Paul Spry, an economic geologist at Iowa State University, in order to gather new information regarding the evolution of the fluorine ore-bearing fluid system.  The specimen consists of a core of brilliant yellow color, with alternating dark purple, light purple, and colorless bands in the outer regions of the crystal.  We have performed detailed micro-FTIR analyses of hydrous species within the gemmy specimen.  Our analyses represent the first high-resolution (100 µ) IR analyses on fluorite, and they indicate that significant variations in absorption characteristics exist between the different colored regions of the crystal.  For example, the yellow regions exhibit large broadband absorption near 3400 wavenumbers (cm^-1) and three sharp bands at 1400, 1550, and 1650 cm^-1.  Each absorption band reflects the presence of a different species.  In other crystals, the broadband absorption at 3400 cm^-1 has been shown to reflect fundamental stretching of the O-H hydroxyl in small clusters of molecular water (HOH).  The broadband at 3400 cm^-1 is not Gaussian in shape, however, and two additional peaks can be assigned to this region (perhaps representing the fundamental OH stretch attached to LiOH and NaOH molecules within the clusters).   A sharp band at 1650 cm^-1 represents the bending mode of molecular water.  However, the other two bands in this region have not been reported in other crystals, and may represent the bending modes of LiOH and NaOH molecular species, a hypothesis that we aim to test in future funding cycles.  Purple regions of the specimen show significantly smaller absorption at 3400 and 1650 cm^-1, and none at 1550 and 1400 cm^-1, indicating that the chemical constituents creating these species were not present in the host fluid while this portion of the crystal was growing.  We are looking for correlations between the infrared spectral features and other available information including temperature of formation, pH, salinity, isotopic composition, and speciation of organic compounds as measured in adjacent fluid inclusions.  Our results continue to shed new light on the evolving hydrothermal system responsible for ore deposition and organic carbon mobility, and we are looking forward to extend our analyses to other crystals from the District.