Mellite

Mellite is a rare and beautiful mineraloid, a hydrated aluminum oxalate. Its chemical formula is Al2(C2O4)3·18H2O. Mellite is typically found in coal deposits, particularly in association with brown coal seams. It is formed through the reaction of aluminum-rich minerals with oxalic acid, which is often produced by the decomposition of organic matter in the surrounding coal.

The mineral is known for its distinctive color, ranging from honey-yellow to orange-red, which gives it its name, derived from the Greek word "meli" meaning honey. Mellite crystals are usually small and can be transparent to translucent. They often occur as prismatic or tabular crystals, sometimes as botryoidal or reniform aggregates.

One of the most notable occurrences of mellite is in Saxony, Germany, where it was first discovered. Other significant localities include Hungary, Poland, and various sites in the United States. Due to its rarity and attractive appearance, mellite is a sought-after specimen for mineral collectors.

While not a commercially important mineral, mellite is of interest to geologists and mineralogists for understanding the complex chemical processes that occur in sedimentary environments, especially those involving organic matter and aluminum. Its hydrated nature means it can dehydrate when heated, changing its optical properties.