Gadolinium

In nature, gadolinium is found in the Earth's crust, 6.1-6.36 ppm, in minerals such as samarskite, gadolinite (ytterbite), xenotime and other rare earth minerals [4]. The isotopes found in nature are ¹⁵²Gd (0.20 %, T_½ = 1.08 · 10¹⁴ years), ¹⁵⁴Gd (2.18 %, stable), ¹⁵⁵Gd (14.80 %, stable), ¹⁵⁶Gd (20.47 %, stable), ¹⁵⁷Gd (15.65 %, stable), ¹⁵⁸Gd (24.84 %, stable) and ¹⁶⁰Gd (21.86 %, stable) [4].

In pure form, gadolinium exist in two forms. An α-form (hexagonal close-packed crystal) which changes to a β-form (body-centered cubic crystal) at >1262 °C [4].