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Cumengeite
Boleo cumengeite occurs frequently in a star shape, unique in the mineral world. This shape was extensively studied at the end of the XIXth century by the crystallographers: In theory, the shape of a crystal of any mineral species has to be a convex polyhedron, i.e. without points. They thus came to the conclusion that it resulted from a grouping of oriented crystals of two different species, resulting from a highly unlikely sequence of events: Initially, a cube of boleite (silver bearing mineral) crystallizes from a silver-rich solution; as the mineral solution becomes depleted in silver, a new mineral, cumengeite, crystallizes and forms a 4-sided pyramid on each of the 6 faces of the cube of boleite. This phenomenon of oriented growth of one mineral onto another having a relatively close crystallographical structure is called an epitaxy. It is very important as it is the basis of the manufacture of modern electronic components. Epitaxies in nature are rarely perfect. The specimen presented is one of the largest groups known of epitaxial cumengeites.
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