Terbium

In nature, terbium is found in the Earth's crust, 0.91-1.2 ppm, in minerals such as monazite, cerite, gadolinite and other rare earth minerals [4]. The isotope found in nature is ¹⁵⁹Tb (100 %, stable) [4].

In pure solid form, terbium exist in a hexagonal close-packed crystal form [4]