TOPAZ AT HONEYCOMB HILLS, JUAB COUNTY, UTAH: EVIDENCE OF PEGMATITIC TOPAZ FROM THE 4.7MA ERUPTION AT HONEYCOMB HILLS

As part of ongoing research, the Geosciences Department at the University of West Georgia took experiential learning trips to Colorado and Utah in 2011 and 2012. One sample collection site was to the 4.7 ma topaz rhyolite at Honeycomb Hills, Juab County, Utah. It has been described by Nash and Gordon as ‘the volcanic eruption of a pegmatitic magma’ (Gongdon and Nash, 1988, 1991). In 2012 an unusual crystal was found lying on the ground, an opaque yellow granular crystal with perfect cleavage and interesting growth striations. The ground was searched for more crystals. Two more crystals were found with similar physical appearances, in a dry stream bed. These crystals were a light green color and weathered. A small clear topaz crystal, like the topaz found at Topaz Mountain, was also found at the Honeycomb Hills site.

The four crystals were determined to be topaz. They are different when compared to the vapor phase topaz from Topaz Mountain, also in Juab County, Utah. Topaz Mountain, a 6-7 ma rhyolite, yields beautiful vapor phase, transparent amber colored topaz with perfect crystal faces, unlike the crystals found at Honeycomb Hills. The amber color in these crystals is sensitive and the crystals will turn clear when exposed to sunlight, whereas, the crystals at Honeycomb in the stream bed were constantly being exposed to sunlight but they still retain their color.

The three crystals ranged in size from 8mm to 15mm across at the widest section. The largest crystal is translucent and displays perfect cleavage, and although it’s been etched and weathered, the crystal faces and its orthorhombic shape are still present; the termination of this crystal is covered with tiny crystals. The yellow and green topaz do not look like the vapor phase Topaz Mountain crystals; their appearance is similar to topaz found in pegmatites.

The smallest of the three crystals found at Honeycomb Hills was made into a thin section and analyzed by the scanning electron microscope (SEM) where many trace elements were found in this specimen including a few rare earth elements (REEs), Ho, Y, and U, and other transition metals Zr, Nb, Hf, Ta, Cr, and Te. Further analysis of these specimens includes more thin sections, SEM analyses, and x-ray diffraction. The purpose of this research is to better understand the topaz found at the Honeycomb Hills site in Juab County, Utah.