Coal mines are very rarely considered promising mineral localities, and they are usually not indeed. However, in the Csordakút coal mine belonging to Bicske, world-record-sized mellite crystals were discovered in the early 1980s. The largest single crystals were over 10 cm in size, and the crystal groups could be several decimetres. Mellite is an organic mineral, chemically aluminium mellitate hydrate [formula: Al2C6(COO)6 • 16H2O], so it is perhaps not surprising that it appears in association with coal deposits. Its common name is honey-stone, referring to the honey-yellow colour of some of its specimens.
In the picture: Mellite, Bicske-Csordakút (Hungary). Size: 16 cm. Photo by László Tóth.