In geology, the mineral monazite is a reddish-brown phosphate containing rare earth metals and an important source of thorium, lanthanum, and cerium. It occurs usually in small isolated crystals. There are actually at least four different kinds of monazite, depending on relative elemental composition of the mineral:

monazite-Ce (Ce, La, Pr, Nd, Th, Y)PO4

monazite-La (La, Ce, Nd, Pr)PO4

monazite-Nd (Nd, La, Ce, Pr)PO4

monazite-Pr (Pr, Nd, Ce, La)PO2

The elements in parentheses are listed in the order in which they are in relative proportion within the mineral, so that lanthanum is the most common rare earth in monazite-La, and so forth. Silica, SiO2, will be present in trace amounts, as will small amounts of uranium. Due to the alpha decay of thorium and uranium, monazite contains significant amount of helium, which can be extracted by heating.

Monazite is an important ore for thorium, lanthanum, and cerium. It is often found in placer deposits. The deposits in India are particularly rich in monazite. It has a hardness of 5.0 - 5.5 and is relatively dense, about 4.6 to 5.7 g/cm3.

Because of the presence of thorium within monazite, it can be radioactive. If samples are kept, they should be placed away from minerals that can be damaged by radiation.

The name monazite comes from the Greek mona`zein (to be solitary), in allusion to its isolated crystals.