Element with atomic number 50, symbol Sn, atomic weight 118.71, mp 231.8ºC, specific gravity (grey) 5.75, (white) 7.31. A soft white lustrous metal obtained mainly from the mineral cassiterite, SnO2, but stannite (Cu2SnFeS4) occasionally occurs in ore grade concentrations. Tin is not affected on exposure to air at ordinary temperatures. At temperatures above 13.2ºC the white tetragonal allotropic form is stable and below this the grey cubic form may exist. Above 170ºC, tin is rhombic in crystal structure. The metal has low tensile strength and hardness but good ductility. Mechanical deformation of white tin makes a distinct sound – tin cry – due to the formation of deformation twins.