Naturally occurring solids, formed by geological processes, that contain the rare-earth elements—the lanthanides (atomic numbers 57–71) and yttrium (atomic number 39)—as essential constituents. In a rare-earth mineral, at least one crystallographic site contains a total atomic ratio of lanthanides and yttrium that is greater than that of any other element. The mineral name generally has a suffix, called a Levinson modifier, indicating the dominant rare-earth element; for example, monazite-(La) (LaPO₄) contains predominantly lanthanum, and monazite-(Ce) (CePO₄) contains predominantly cerium.