Fergusonite is a mineral comprising a complex oxide of various rare earth elements. Its formula is given basically as (Ce,La,Nd)NbO₄. However other analyses also include yttrium, erbium, and tantalum. It is found as small (less than 100 micrometres), needle-like crystals in pegmatite. General Name, Symbol, Number yttrium, Y, 39 Chemical series transition metals Group, Period, Block 3, 5, d Appearance silvery white Atomic mass 88. ... General Name, Symbol, Number Erbium, Er, 68 Chemical series Lanthanides Group, Period, Block NA, 6, f Density, Hardness 9066 kg/m3, ND Appearance silvery white Atomic properties Atomic weight 167. ... General Name, Symbol, Number tantalum, Ta, 73 Chemical series transition metals Group, Period, Block 5, 6, d Appearance gray blue Atomic mass 180. ... A micrometre (American spelling: micrometer), symbol µm, is an SI unit of length. ... Pegmatite is a very coarse-grained igneous rock that has a grain size of 20 mm or more; such rocks are referred to as pegmatitic. ...