Hutchinsonite

Yes, I washed my hands

There are at least 89 minerals that contain essential thallium (symbol Tl) but the only one I’ve ever heard of (or have) is hutchinsonite. Thallium is the 60^th most abundant element in the Earth’s crust, ahead of mercury, silver, platinum, and gold, but it’s not a common mineral-maker. Hutchinsonite’s formula is TlPbAs₅S₉, thallium-lead-arsenic sulfide.

The only locality on MinDat in the western hemisphere for hutchinsonite is the Quiruvilca Mine, Quiruvilca District, Santiago de Chuco Province, La Libertad, Peru, where it occurs with orpiment (arsenic sulfide), enargite (copper-arsenic sulfide) and many other minerals. Extensive mining by ASARCO (American Smelting and Refining Company) for copper, lead, and zinc, beginning in 1924 and especially in the 1970s and 1980s, produced more than 30 km of underground workings. Mining continues today.

The ores are hosted mostly in breccias (highly fractured rocks) in andesitic (intermediate composition) and basaltic volcanics (Lewis, 1956, The geology and ore deposits of the Quiruvilca District, Peru: Economic Geology, 51, p. 41-63) of Eocene to Miocene age called the Calipuy Formation, deposited about 33 to 10 million years ago as an aspect of the subduction that is raising the Andes Mountains (Hollister and Sirvas, 1978, The Calipuy Formation of Northern Peru, and its relation to volcanism in the Northern Andes: J. Volcanology and Geothermal Research, 4:1-2, p. 89-98).

I have a flat (12 specimens) of orpiment, realgar, and barite, that is clearly labeled “Palomo Mine, Huancavelica Department, Peru.” But based on the associations and the nature of the minerals, I’m 99% certain that this is mislabeled and that the material is really from the Quiruvilca Mine. The hutchinsonite was not indicated on the label; I discovered it in the specimens microscopically.

Thallium in soluble salts is highly toxic, used historically in rat poison and murder. In the 1940s and 1950s serial killer Caroline Grills (who became known as “Aunt Thally”) used thallium-based rat poison to poison as many as 46 people in Sydney, Australia, 10 of whom died.

In the US the EPA’s Maximum Contaminant Level for thallium is set at two parts per billion in drinking water. It is used today in infrared sensors and other electronic devices where its sensitivity to infrared light is reflected in changing electrical conductivity.

Spray of red hutchinsonite about 1 mm across, within a colorless crystal of barite, surrounded by yellow orpiment.

Thallium also has medical uses (the thallium stress test) and is used in low-temperature thermometers and switches. As a glass additive, it increases the index of refraction. These applications call for only tiny amounts of thallium, and in 2023 total US consumption was estimated by the USGS at just 13 kilograms (29 pounds), and world production was probably about 10,000 kg (22,000 pounds), mostly from Russia, Kazakhstan, and China. In 2023 thallium’s estimated price was $8,800 per kilogram.

Hutchinsonite was named in 1904 in honor of Dr. Arthur Hutchinson (1866–1937), Professor of Mineralogy at the University of Cambridge, England.

Thallium was discovered by chemists William Crookes and Claude-Auguste Lamy independently in 1861 and named for its prominent green line in flame spectroscopy; the word is from Greek θαλλός, thallós, meaning "green shoot" or "twig."

“Quiruvilca” means “sacred tooth” in Quechua, an indigenous language of Peru, and refers to a sharp jagged outcrop in the mountains near the city and mine.