How A 75-Year-Old Letter Leads To Rare Mineral Discovery

A long-lost letter, tucked away for 75 years in a government archive, has led to the accidental rediscovery of humboldtine, an exceptionally rare mineral. The 1949 letter, unearthed during a 2023 digitisation project in Bavaria, Germany, pointed curators to a shoebox containing forgotten lemon-yellow fragments.

Roland Eichhorn of the Bavarian State Office for the Environment (LfU) spearheaded the investigation. His team confirmed the fragments were indeed humboldtine, a mineral so scarce that it has been documented at only 30 locations worldwide. The discovery of these hazelnut-sized pieces instantly doubled Germany's documented reserves of the mineral.

Humboldtine is an organic mineral, a unique class where carbon, hydrogen, and oxygen are bound to metals within its crystal lattice. Its chemical formula combines iron with oxalate, resulting in a soft, yellow substance with a resinous sheen. The mineral's formation is a rare geological fluke, occurring when iron-rich rocks interact with specific natural acids under damp conditions.

The study is published in Clays and Clay Minerals.

This Bavarian trove is particularly significant, as the fragments are much larger than the typical millimetre-scale grains found elsewhere. While the exact conditions that led to their formation in brown coal remain a mystery, the find highlights the importance of historical records.

Beyond its rarity, humboldtine is attracting interest from materials scientists. Its iron oxalate composition makes it efficient at shuttling electrons, suggesting potential applications in future green technologies, such as safer, high-capacity cathodes for lithium-ion batteries.

The discovery underscores the value of digitising historical collections. As more archives transition to digital formats, experts anticipate further hidden gems and scientific breakthroughs from overlooked documents and specimens.