75-year-old coal mine owner’s letter leads to the discovery of one of Earth’s rarest minerals

A letter discovered in 2023 during the digitization of the archive of the Bavarian Environment Agency, or LFU, has rewritten a chapter of mineral history. The 1949 letter was written by a coal mine owner and sent to the agency; it mentions the Humboldtine, one of Earth's rarest minerals. The discovery of the misplaced letter in the government archive led the experts toward a shoebox of lemon‑yellow fragments that remained unnoticed for decades, according to Earth.com.

German geologist Roland Eichhorn of the Bavarian State Office for the Environment (LfU) led the modern hunt that followed. Soon, his team learned they were staring at one of the scarcest minerals on the planet, Humboldtine. The letter stated that Humboldtine was found in coal seams at Matthiaszeche, near Schwandorf, a town on the Naab River in Upper Palatinate. The agency requested samples to check the discovery. But there don't seem to be any more records about it.

According to Forbes, Humboldtine has only been found in about 30 places around the world, including some quarries and mines in Germany, Brazil, the UK, Canada, the US, Hungary, the Czech Republic, and Italy. It is most commonly found as a yellow, amorphous mass, and it rarely forms tiny crystals.

What makes Humboldtian special

The mineral is organic, a rare class whose crystal lattice holds carbon, hydrogen, and oxygen bound to metals, according to Earth.com. It is formed when carbon compounds and iron oxide react with water and is one of the few "organic minerals" containing carbon-oxygen-hydrogen groups in their crystalline structure.

The mineral was first found in a weathered brown coal deposit near the municipality of Korozluky by German mineralogist August Breithaupt in the Okres Most in the Czech Republic. It was first described in 1821 by Peruvian geologist Mariano Eduardo de Rivero y Ustariz, who named it after Alexander von Humboldt, a German naturalist and explorer from the 19th century. Von Humboldt had worked as a mining engineer and loved collecting minerals.

Humboldtine hunt

The 75-year-old letter written by a coal mine owner intrigued the archivists. While scanning shelves in Hof, they found the coal‑mine owner’s note and the phrase “Humboldtine from the Mathias mine near Schwandorf,” according to Earth.com. The team led by Eichhorn, head of the geological department at the LfU, decided to check the vast historic mineral collection comprising over 130,000 rock and mineral samples hosted in the agency's basement.

If any samples were ever sent in, they still could be here. In one drawer of the systematic mineral collection, where minerals are ordered according to their chemical composition, they found some fragments of a yellow mineral labeled "Oxalit," German for organic minerals, still inside an old cardboard box. The label also showed that the samples came from the locality mentioned in the letter.

If any samples were ever sent, they might still be present here. In a drawer of the systematic mineral collection, where minerals are sorted based on their chemical composition, they found pieces of a yellow mineral called "Oxalit," which is the German word for organic minerals, still in an old cardboard box.

The label showed the samples were from the same place mentioned in the letter. The discovery, which was made 75 years ago, was confirmed by modern chemical analysis. The six fragments, the largest almost the size of a nut, are indeed Humboldtine.

The Matthiaszeche, a former open-pit mine for brown coal, was closed in 1966 and subsequently flooded. There is no chance of getting any more Humboldtine from this locality.