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

In 2023, a group of archivists in Bavaria was working on digitizing old records, completely unaware that this very project would eventually lead them to discover humboldtine — one of the rarest minerals on the planet, Earth.com reported. Well, it all began when archivists stumbled upon a dusty envelope while scanning through the shelves. Inside, they found a 75-year-old letter that read, "Humboldtine from the Mathias mine near Schwandorf."

Curious, the archivists, led by Roland Eichhorn of the Bavarian State Office for the Environment (LfU), began with their investigation, only to realize they were in fact dealing with humboldtine. The team went through more than 130,000 recorded mineral samples stored in the basement of the LfU headquarters. Luckily, the team found the first milestone in their investigation — yellow fragments and a handwritten note inside a small box that matched the location mentioned in the letter. With their first major finding, the team then shifted their focus to identifying if the samples found were truly humboldtine. 

The technicians carried the newly-discovered samples to the LfU laboratory. They crushed a tiny piece of the mineral and crushed it into powder to study its structure through X-ray diffraction. The result, called "diffractogram," matched with monoclinic humboldtine, hence confirming the authenticity of the old handwritten label. But the technicians, perhaps, didn't want to jump to conclusions without solid proof, so they decided to run another test. The second test further verified that the sample had identical chemical ingredients in the right proportion that is found in humboldtine. "We are legally obligated to make geological collection pieces accessible to the public," Eichhorn said once both tests gave positive results. 

But what's so special about the discovery of humboldtine? Notably, the crystal structure or the atomic lattice of the organic mineral holds carbon, hydrogen, and oxygen bound to metals. Besides, the report explains that it's formula pairs iron with oxalate, resulting in a soft hardness of 1.5-2 and a resinous yellow sheen. Moreover, the mineral is too pricey, particularly because humboldtine has been found only in 30 places in the whole world (across 8 countries). Now, the formation of the mineral is also very rare, forming only under very specific conditions — only when iron-rich rocks come in contact with natural acids in damp environments. In 1821, humboldtine first appeared when Peruvian geologist Mariano de Rivero named it for the Prussian explorer Alexander von Humboldt.

Remember, scientists are interested in Humboldtine not only because it's rare, but also because they believe it may pave the way for better energy storage. Basically, as Earth.com explained, the iron oxalate structure can efficiently move electrons for storing energy. Further, in 2021, Paula Vehmaanperä and colleagues described how humboldtine precipitated when oxalic acid dissolved hematite at modest temperatures, suggesting a low-energy synthesis route. As a matter of fact, the engineers are now considering using the mineral for making iron-based battery anodes that avoid cobalt and nickel supply risks. But for such industrial dreams to come true, we need a steady supply of humboldtine if not in abundance, but unfortunately, the mineral is just too rare to use for any experiments.