Bixbyite on Rhyolite (#80)

A single sharp twinned black lustrous crystal 5/16" on edge with minor micro topaz and pseudobrookite associated on grey rhyolitic matrix, a classic for the locality.
North America Thomas Mountains, Juab County, Utah 2" x 2" x 1 ¾"


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Bixbyite is the metallic looking cube. The long, thin black crystals are pseudobrookite, and rhyolite is the light colored rock in which they are embedded. Rhyolite is a volcanic rock, which means that it was erupted onto the surface and cooled relatively quickly, resulting in the average crystal size being quite small. Rhyolite is approximately equivalent to granite, which cooled beneath the surface, resulting in crystals large enough to be easily seen. So to a first approximation, the difference between rhyolite and granite is crystal size.

Bixbyite has the formula (Mn,Fe)2O3, and manganese is the predominant metal - some specimens contain no iron. There are a couple of specimens of hematite, Fe2O3, in this display, and although the chemical formulas look similar, the crystal structures (atomic arrangements) of the two are nothing alike.

The Thomas Range in Juab County, Utah, is the type locality for bixbyite. Whenever a new mineral species is discovered, it goes through a lengthy vetting process, which is followed to insure that it is adequately described physically and chemically, and that it is indeed a new mineral. The locality at which it was found becomes the type locality, and a type specimen is deposited for reference in some known collection. Bixbyite was discovered in 1897 by Maynard Bixby. The type locality is given as Maynard's Claim, Pismire Knolls, Thomas Range, Juab County, Utah, USA, and the type specimen is at Yale University in New Haven, Connecticut.