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🧵 2. Humboldtine, known from only 30 locations worldwide, rarely forms tiny crystals and is mainly found as a yellow, amorphous mass.

🧵 3. A 75-year-old letter led experts to the mineral's existence. The letter mentioned the presence of Humboldtine in coal seams in Germany.

🧵 4. After searching the historic mineral collection, six fragments were confirmed to be Humboldtine, doubling the total known amount.