Abundances In this table values are given in units of ppb (parts per billion; 1 billion = 109), both in terms of weight and in terms of numbers of atoms. Values for abundances are difficult to determine with much certainty, so all values should be treated with some caution, especially so for the less common elements. Local concentrations of any element can vary from those given here by orders of magnitude.
ppb by weight
ppb by atoms
Neodymium is never found in nature as the free element. Neodymium is found in the ores monazite sand [(Ce, La, etc.)PO4] and bastn°site [(Ce, La, etc.)(CO3)F], ores containing small amounts of all the rare earth metals. It is difficult to separate from other rare earth elements.