SEM AND THIN SECTION STUDY OF TOPAZ, BIXBYITE, PSEUDOBROOKITE AND HEMATITE FROM THE THOMAS RANGE, JUAB COUNTY, UTAH

The Thomas Range is a world famous collecting site for a wide variety of minerals, some of which are almost exclusive to the area. Located in western Utah, the collecting site is comprised of five separate units of rhyolite. The Thomas Range topaz rhyolites are 6-7 million years old. A variety of minerals are present within the units, but the focus of this research is predominantly on crystals of topaz and the minerals that are most commonly found in association with topaz. Topaz can be seen in combination with bixbyite, red beryl, pseudobrookite, garnet, hematite, quartz, fluorite, orthoclase, calcite, and opal. Our focus is on topaz that is commonly associated with bixbyite, pseudobrookite and hematite.

The topaz found throughout the Thomas Range is usually a sherry wine color; however it will turn colorless when exposed to sunlight. Topaz crystals can occur alone or within clusters and are often times terminated. Crystals of topaz are found attached to the rhyolite in a number of ways, but attachments are commonly associated with quartz inclusions which typically produce a lower quality crystal, depending on the number of inclusions. The best quality topaz crystals, which contain little to zero inclusions, usually occur within the vugs which are found among the fractures in the rhyolite. The objective of this study is to understand the relationship between the topaz and the minerals that are frequently associated with it.

Samples were collected that included topaz rich vugs. These specimens have been cut into three dimensions extending from vug to rhyolite to observe differences or similarities that are present throughout each dimension. The examination of thin sections with a petrographic microscope will help comprehend relationships between minerals, establish a working knowledge of the sequence of crystal growth, and determine how such growths began. Thin sections of individual crystals of topaz both parallel and perpendicular c-axis were done examine any inclusions of foreign matter (rhyolite) or minerals. A scanning electron microscope is utilized for further analysis of specimens and elemental mapping.