SCANNING ELECTRON MICROSCOPE AND OPTICAL MINERALOGICAL STUDY OF INCLUSIONS WITHIN TOPAZ CRYSTALS FROM THE TOPAZ RHYOLITE OF TOPAZ MOUNTAIN, JUAB COUNTY, UTAH

Topaz Mountain located in the Thomas Range of Juab County, Utah contains extensive F-rich rhyolite lava flows formed approximately 6.5 million years ago. The flow banding of the rhyolite provided pathways for the transportation of vapors and the crystallization of vapor phase minerals within vugs located along the distinctive bands. Over the years, several specimens were collected for the purpose of research at the University of West Georgia. From the samples several topaz crystals were collected and analyzed. The topaz ranges from clear gem quality to “sand topaz” that contains extensive quartz inclusions. Excellent crystals, sometimes associated with the topaz are the oxides hematite Fe2O3, bixbyite (Mn,Fe)2O3, and psuedobrookite, Fe2(Ti,Fe)O3. A series of orthogonal thin sections were created, examined under a polarizing microscope, and optical properties were recorded for the initial mineral identification. The orthogonal thin sections were then placed on a scanning electron microscope to create a detailed elemental map of the inclusions within the topaz. The elemental maps were examined and compared in order to verify mineral identification of the inclusions. Once the minerals were identified, quantitative elemental fingerprints were recorded as a means to compare elemental compositions in future samples. During this process identified mineral inclusions found within the topaz crystals include quartz, bixbyite, and psuedobrookite. It is believed that a correlation exists between the time of crystallization of quartz and the crystallization of bixbyite inside the topaz crystal. A quantitative x-ray diffraction using the reference intensity ratio method (RIM) will be utilized in order to identify elemental compositions of the minerals in question. The additional creation of topaz crystal thin sections of crystals will aid in the attempt to verify this correlation and establish a more detailed crystallization sequence. The predicted sequence is the crystallization of the quartz crystal with bixbyite crystals, either simultaneously with the quartz or after the quartz had completed the crystallization process. This was then followed by the crystallization of the topaz and psuedobrookite, and succeeded by late stage hematite crystallization.