Life in the USA is not normal. It feels pointless and trivial to be talking about small looks at the fascinating natural world when the country is being dismantled. But these posts will continue, as a statement of resistance. I hope you continue to enjoy and learn from them. Stand Up For Science!
Pseudobrookite isn’t brookite, which is titanium dioxide. Pseudobrookite sometimes has a similar appearance to brookite, but it is iron titanate, Fe2TiO5.
These long black needles (the jargon word for this crystal habit is “acicular,” from Latin for “small needle”) have grown in a cavity in rhyolite at Topaz Mountain in the Thomas Range of west-central Utah. Those rhyolites are more famous for topaz, which is abundant there, but other minerals are also well known from the locality, including rare red beryl and bixbyite (a cubic black manganese oxide), as well as the pseudobrookite.
In the photo at top, the longest cluster, at far left, is glued on (not by me!), which probably explains why the specimen was cheap. But the others are naturally attached.
The pseudobrookite may represent vapor-phase crystallization, which is to say it crystallized directly from a hot gas rather than from an aqueous solution (Staatz and Carr, 1964, Geology and mineral deposits of the Thomas and Dugway Ranges, Utah: USGS Prof. Paper 415). The rhyolitic rocks are mostly volcanic ash falls and ash flows, solidified into solid rocks called welded tuff, or ignimbrite. Much of the ash probably erupted at the time of the collapse of the Thomas Caldera about 38 million years ago, but there were volcanic episodes as long ago as 42 million years and as recently as 6-7 million years ago. (Lindsey, 1982, Tertiary volcanic rocks and uranium in the Thomas Range and northern Drum Mountains, Juab County, Utah: USGS Prof. Paper 1221).
The Topaz Mountain rhyolite in the Thomas Range is dominantly quartz and alkali feldspar (largely sanidine) but includes anomalous amounts of fluorine, beryllium, lithium, titanium, niobium, uranium, thorium, and yttrium, all of which contribute to the unusual minerals found in the cavities in the rhyolite. Much of the mineralization appears to be related to the more recent activity and probably to fractures and faults associated with relatively recent basin-and-range faulting (Lindsey, 1982, USGS Prof. Paper 1221).
The volcanism over a vast area of eastern Nevada, western Utah, and southwestern Colorado was greatest about 40 to 25 million years ago, an “event” called the Mid-Tertiary Ignimbrite Flare-Up, which we’ve encountered before in these posts. It represents a truly vast amount of volcanic ash, enough to cover the state of Colorado with a pile close to a mile thick.
The volcanism is attributed to a complex process including low-angle subduction of the Farallon Plate beneath western North America, which, when it more or less ended, allowed the top of a mantle wedge (a triangular zone caught between the subduction oceanic crust and the bottom of the overlying continental crust) to reach levels where magma could form. That zone is typically enriched in water from the subducting slab, which contributes to the explosive, ash-forming volcanic activity, and that zone of upper mantle may also have a role in providing the more exotic elements (Best and others, 2016, Slab-rollback ignimbrite flareups in the southern Great Basin and other Cenozoic American arcs: A distinct style of arc volcanism: Geosphere 12:4, p. 1097–1135). That’s not the only possible tectonic explanation, and research continues; I don’t really understand why so much rhyolitic volcanism (highly silicic) should be related to mantle sources (expected to be more iron- and magnesium-rich), even if it did result largely from melting silicic continental crust.
Almost all of the United States’ production of beryllium comes from Spor Mountain, not far from Topaz Mountain in the Thomas Range. It doesn’t come from beryl, but from the more unusual mineral bertrandite, a hydrated beryllium silicate. Even though it’s just one location, it’s enough to make the U.S. the world’s leading producer of beryllium with 50% of the total. Brazil and China are a distant second and third with about 22% of world production each. But even as the world production leader (and recycling accounts for around 20% of U.S. consumption), until recently the U.S. still relied on imports for about 6% to 16% of its beryllium, imported from Kazakhstan (39%), Latvia (23%), Japan (17%), and Canada (6%). But in 2023 and 2024, the US was a net exporter of beryllium.
You use beryllium and beryllium alloys every day, in things such as computers and cell phones, automotive and airplane components including brake shoes (copper-beryllium alloys resist the tendency for metals to make friction-generated sparks), and in telecommunications infrastructure including satellites. It’s also part of medical x-ray technology.
Brookite was named in 1825 by Serve-Dieu Abailard "Armand" Lévy to honor Henry James Brooke (1771-1857), an amateur mineralogist who discovered 12 minerals including annabergite, autunite, arfvedsonite, caledonite, childrenite, linarite, nitronatrite, susannite, thomsonite, and whewellite. Bertrandite was named for French mineralogist Émile Bertrand (1844-1909).
There is some thought that decompression melting could generate silica rich melts from mafic sources. Recent (relatively) work on the Long Valley Caldera system shows that decompression melting is responsible for most of the large volume volcanics from it's eruptions. The decompression melting there related to structural "pull-apart". I don't know to what extent this process can be expanded to the massive mid-tertary ignibrite flare-up because it predates most of the B&R extension, but that puzzle has perplexed me since I first learned of it when I started doing minerals exploration in the early 1980's.