A Rieke metal is a highly reactive metal powder generated by reduction of a metal salt with an alkali metal. These materials are named after Reuben D. Rieke, who first described the recipes for their preparation. Among the many metals that have been generated by this method are Mg, Ca, Ti, Fe, Co, Ni, Cu, Zn, and In, which in turn are called Rieke-magnesium, Rieke-calcium, etc.
Rieke metals are highly reactive because they have high surface area and lack surface oxides that can retard reaction of bulk materials.  The particles are very small, ranging from 1-2 μm down to 0.1 μm or less.  Some metals like nickel and copper give black colloidal suspensions that do not settle, even with centrifugation, and cannot be filtered.  Other metals such as magnesium and cobalt give larger particles, but these are found to be composed mainly of the alkali salt by-product, with the metal dispersed in them as much finer particles or even as an amorphous phase.