|FIELD SPECIMEN OF TOPAZ-BEARING RHYOLITE FROM THOMAS RANGE, UTAH, WITH CRYSTALS OF PSEUDOBROOKITE. ENLARGEMENTS BELOW. WIDTH OF SPECIMEN ~7 CM.|
Thursday, March 1, 2012
Last fall I was pounding around on some rhyolite brought home from a 2010 field trip to the Thomas Range in western Utah (see April 3, 2011 posting). A few members of CSMS had visited the Range in hopes of collecting topaz crystals (that was accomplished). And, we also had aspirations of collecting giant red beryl (“bixbite”) specimens since this is one of the rarest and most expensive gemstones in the U S. Current prices for good faceted stones range up to ~$10,000 caret. Alas, Jerry found a few tiny hexagonal flakes (non-gemmy) and we were not to become wealthy. But, even those tiny crystals are prized by collectors. I picked up a single cube of bixbyite and later found a second specimen attached to a topaz crystal while sorting rocks at home. Thus, last fall I dug out the rocks and thought perhaps a new bixbyite would show up or, never giving up hope, a crystal of red beryl. But that was not to be.
In a serendipitous moment of breaking rocks, I did locate a few small crystals of pseudobrookite--- not a common mineral and an exciting discovery. I noted to the field trip group back in 2010 that pseudobrookite might be present at the topaz diggings and so watch out for small sprays of black minerals. In fact, that was about all that I knew about the mineral—it was dark colored, some sort of a titanium mineral, occurred in tiny needle-like crystals (often), and was not related to the mineral brookite. I thought that amount of information was not too bad for an ole paleontologist!
So, in whapping the rhyolite I noticed some small dark minerals in the cavities and reached for my hand lens. It appeared that the elusive pseudobrookite had been found and this discovery was confirmed when I examined the specimen under a binocular microscope. Not quite red beryl, but good enough.
Pseudobrookite is an iron-titanium oxide, Fe2TiO5, and has both needle-like and tabular crystals. I presume the name comes from its non-relationship with Brookite, a titanium oxide (TiO2) mineral often with tabular crystals. It does occur in colors other than “dark”, such as black, reddish-brown, and brownish-black. The crystals have a hardness of 6 and a “shiny” (metallic or adamantine) luster (due to the titanium). It usually occurs in younger volcanic rocks, especially rhyolite, in lithophysal (vugs caused by expanding gases) cavities. According to Minerals of Colorado, pseudobrookite is found in the rocks at Ruby Mountain in Chafee County. This seems reasonable since the Ruby Mountain rocks are mostly a topaz-bearing rhyolite.
Now, here is an interesting tidbit. There is a solid solution series between pseudobrookite and armalcolite with magnesium substituting for part of the iron. The “interesting” point behind this is that armalcolite, (MgFe2+)Ti2O5, was first identified from samples collected by Neil ARMstrong, Edwin ALdrin, and Michael COLins (Apollo 11) on the “Moon” at Tranquility Base. Since then the mineral has been identified worldwide from several localities, including Utah, Wyoming, Montana and Texas. Now, there is a fact for a trivia game!