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The link between amphibole aluminum content and reaction rim mineralogy Public Deposited

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  • During volcanic eruptions ascending magma undergoes decompression, resulting in a decrease of dissolved H2O. This creates unstable conditions for the hydrous mineral amphibole. As a result a reaction rim forms from anhydrous minerals such as pyroxene, plagioclase, and Fe-Ti oxides (Rutherford and Hill, 1993). There have been numerous studies done on this breakdown reaction but none have drawn a connection between amphibole Al content (which broadly correlate to the pressure of amphibole crystallization) and the mineralogy and texture of the reaction rim. In this study hornblende crystals were examined from magma erupted during the two most recent eruptions of Mount Hood, OR. Two different populations of amphibole occur in these magmas, and these have distinctly different Al contents (range from 6-9 wt.%, 10-13 wt.%). Crystal sizes and reaction rim widths were measured under a petrographic microscope and mineral compositions and textures were analyzed using an electron microprobe with backscatter electron imaging. After analysis it was determined that there was no significant connection apparent between the Al content of the hornblende crystal and the mineralogy and texture of the reaction rim.
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  • Corvallis, Oregon, USA
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