Monazite is a phosphate mineral containing rare earth metals (Ce,La,Th)PO4. These rare earths or lanthanides can substitute each other in the crystal structure. There are more lanthanides that can enter the lattice, these three are the most common.
This mineral mostly crystallizes out of magma (granite, syenite, pegmatite, carbonatite), but it occurs in several metamorphic rocks as well. It is resistant to weathering and is therefore common mineral in sand. But its grains are usually very small and therefore difficult to spot and identify. Why are they small? The reason is the chemical composition. Lanthanides are chemical elements that are required for the monazite to form. These elements are not wanted by more common minerals because lanthanides do not fit into their crystal structure. Hence, there is little competition and the crystallization centers of monazite form in many places. However, these elements are still pretty rare and there simply is not enough material for the large crystals to form.
Its grains in sand are usually yellow, reddish, or brown. They are mostly rounded and slightly elongated.
Monazite is an important mineral resource, it is one of two minerals that are mined for their rare earth content, the other being bastnäsite which is even more important as a source of lanthanides but not as common in sand. It is mined from sand, mostly beach sand.
Monazite concentrate from North Carolina, USA. Width of view 3 mm.