Lanthanum is a white mallable metal, that tarnishes in air .
In nature, lanthanum is found in the Earth's crust, 18-35 ppm, in minerals such as monazite, bastnaesite and cerite, of which the first two are the commercially important sources. Often found with cerium and other light lanthanides . The isotopes found in nature are 139La (99.91 %, stable) and 138La (0.09 %, T½ = 1.05 · 1011 years).
In pure form lanthanum is found in three crystalline forms, hexagonal α-form, which transforms into a face-centered cubic β-form at 310 °C, and a body-centered cubic γ-form at 864 °C .