INVESTIGATING AN UNKNOWN MINERAL WITHIN A TEXAN PEGMATITE

Thirteen miles outside the city of Llano Texas, there is an abandoned quarry on private property named Badu Hill. Exposed within the quarry are plutonic dikes filled with different minerals including silicates such as quartz, feldspars, and micas as well as other minerals such as fluorite, uraninite, and pyrite.. Found within the mix is a pegmatitic granite covered in a strange and unique phyllosilicate that has been altered due to the regional and local metamorphism from the protozoic era. With similar hand sample properties of biotite, the phyllosilicate in question has a unique cross-hatched and stellate crystal habit. The golden bladed mineral stands out compared to its silicate family members but cannot be identified without further examining the mineral’s chemical make-up and other physical properties. Five thin sections were made of the unknown mineral and examined using a petrographic microscope. Overall, the unknown mineral displays low relief, is slightly pleochroic, has an extinction angle of 45 degrees, contains one plane of cleavage, and exhibits interference colors of middle second order yellow to orange. Two more thin sections were made of the biotite found within the same pegmatite hand samples to compare. The properties of the biotite are typical and include bird’s eye extinction, one excellent plane of cleavage, and has an extinction angle of 45 degrees. Also visible are elongated flakes with moderate relief and interference colors in higher second-order oranges and reds within the thin sections. Major elements of both the unknown mineral and the biotite will be collected via electron microprobe and trace elements via laser ablation ICP-MS. The main goal of this research project is to identify the unknown mineral and determine how the mineral was formed in the unique stellate pattern. Lastly, the formation and subsequent alterations could be help understand the tectonics and metamorphism that took place during the Llano Uplift.