Smithsonite

From Gold Hill

I like big, flashy, gemmy minerals as much as the next guy, but since I can’t afford them, I settle for things like this rock from Utah that cost me $3.50 last year.

“Settle for” is certainly meant sarcastically: I really enjoy such things. This one was sold as “Talmessite with Aragonite” from Gold Hill, Utah. Talmessite is unusual enough, a calcium-magnesium arsenate, while aragonite, calcium carbonate, is pretty common. There’s certainly no aragonite in this piece; I’m pretty sure it’s a mistaken labeling for austinite, calcium-zinc arsenate whose type locality is Gold Hill.

But it’s the microscopic cluster of what look like insect egg cases (in the upper two photos) that got my attention. It took some effort looking at lots of photos of minerals (rough duty) but I’m sure these are (or were) crystals of smithsonite, zinc carbonate, in an unusual but well-known habit called “rice-grain smithsonite” for obvious reasons. The smithsonite has been replaced, or at least coated, by brown iron oxide/hydroxides (probably goethite), and in some parts of the specimen by black manganese oxides. So technically this is a goethite pseudomorph (“false form”) after smithsonite. Pseudomorphs are compositionally one mineral (in this case goethite) that has the crystal form of an earlier mineral (smithsonite).

In the overviews of two sides of the 45-mm specimen in the lower part of the photo montage, the snow-white material is the talmessite and the sparkly colorless stuff is the austinite, though the two have quite similar habits and appearances so I’m not absolutely sure which is which. The brown is goethite and the black is manganese oxide. The photo at right center was probably a cluster of cubic pyrite crystals, now altered to goethite and/or other iron oxides.

Gold Hill, Utah, USA, is a famous deposit, discovered in 1857. It contains a recorded 174 different mineral species. It was initially exploited for its silver and lead, with associated gold and copper, but in the 1920s it became a major producer of arsenic. Demand for arsenic then was driven partly as a chemical for treating wood, but even more as an insecticide for cotton fields in the U.S. southern states. Tungsten from Gold Hill also became important during and after World War II.

Most of the mineralization appears to be related to a Jurassic (152-million-year-old) granitic intrusion, with much of the ore in pipes in the igneous rock itself. There are also skarns, veins, and replacements in the adjacent Mississippian Ochre Mountain Limestone. There were likely two later mineralizing events, one about 42 million years ago (Eocene) that probably introduced much of the tungsten, and another about 8 million years ago (Miocene) related to some gold deposition and probably related to the basin-and-range normal faulting that pulled much of Nevada and western Utah apart. Gold Hill (or Clifton Mining District) is in the Deep Creek Range, near the Nevada border in west-central Utah.

Talmessite was described in 1960 and named for the place it was first found, the Talmessi Mine in Iran, and austinite was named in 1935 for Austin Flint Rogers (1877-1957), a mineralogy professor at Stanford University.

1816 portrait of James Smithson (ca. 1765-1829), by Henri-Joseph Johns (1761-1843).

Smithsonite honors James Smithson (1754-1829), British chemist, mineralogist, and benefactor of the Smithsonian Institution. Smithson was born James Macie, the illegitimate son of the Duke of Northumberland. In part because of his illegitimacy, rules of inheritance were complex, but Smithson not only had a fortune when he died, but also had no surviving blood relatives with heirs, so the money, as gold bars, went to establish an institution in Washington, D.C., “for the increase and diffusion of knowledge.” To receive the bequest of more than $500,000 in dollars of the day (about $2 billion today), it took an act of Congress; the bill was introduced by U.S. Representative from Indiana Robert Dale Owen, brother of David Dale Owen, one of the most prominent and pioneering geologists of the U.S. Midwest in the mid-1800s. Smithson, the Owen family, their father-in-law David Dale, James Watt of steam engine fame, Charles Macintosh (among whose lesser accomplishments was the invention of the rubberized cloth raincoat that bears his name in Britain), and Henry Cavendish, the discoverer of oxygen, are all complexly connected in history; for the full story, see The Knowledge Web by James Burke (Simon & Schuster, 1999).

For me, investigating the cool minerals and learning all that was well worth $3.50.