Topaz on Rhyolite (#60)

Lustrous light sherry colored crystals to 1" scattered on matrix. (Light sensitive)
North America Topza Mountain, Juab County, Utah 6" x 4" x 3 ½"


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Specimen #49 and this specimen come from the same locality, Topaz Mountain in the Thomas Range, Juab County, Utah. Specimen #49 is colorless, because the crystals have been exposed for a long time to sunlight. The topaz crystals in this specimen were formed in this chunk of rock (a volcanic rock called rhyolite) that was hammered out of the surrounding mass, and the crystals had not been bleached by sunlight. These "sherry-colored," unbleached crystals are prized by collectors.

Topaz is Al2SiO4(OH,F), where (OH,F) means that hydroxyl ions (OH) and fluorine (F) can substitute for one another without disrupting the crystal structure. Because these two are not exactly the same size, there are minor effects on the size of the unit cell caused by the substitution. These effects can be measured by routine procedures and used to estimate the amount of F in any given topaz specimen. The topaz from Topaz Mountain contains about as much fluorine as has been observed in topaz from any other locality. This may be the maximum amount Mother Nature allows, or it may just be that the volcanic rocks of the Thomas Range formed in as fluorine-rich an environment as occurs.

In addition to topaz, the Thomas Range also contains minerals called bixbyite, garnet, red beryl, and pseudobrookite, although they are not nearly as common as topaz there.