Crookesite Crookesite Crookesite is a selenide mineral composed of copper and selenium with variable thallium and silver. Its chemical formula is reported either as Cu(Tl,Ag)Se or (Cu,Tl,Ag)Se. It is formed by precipitation from hydrothermal fluids, and contains by mass: 16.3% Tl, 47.3% Cu, 2.9% Ag, and 33.6% Se. Crookesite is an opaque, bluish grey to pink toned brown metallic mineral crystallizing in the tetragonal system. It has a Mohs hardness of 2.5 to 3 and a specific gravity of 6.9. It was discovered in 1866 in Skrikerum, Sweden and named for Sir William Crookes (1832–1919), the discoverer of the element thallium.
Chemical element more than one way, allowing the pure element to exist in multiple chemical structures (spatial arrangements of atoms), known as allotropes, which differ in their properties. For example, carbon can be found as diamond, which has a tetrahedral structure around each carbon atom; graphite, which has layers of carbon atoms with a hexagonal structure stacked on top of each other; graphene, which is a single layer of graphite that is very strong; fullerenes, which have nearly spherical shapes; and carbon nanotubes, which are tubes with a hexagonal structure (even these may differ from each other in electrical properties). The ability