Monazite is a reddish-brown phosphate mineral that contains rare-earth elements. Due to variability in composition, monazite is considered a group of minerals. The most common species of the group is monazite-(Ce), that is, the cerium-dominant member of the group. It occurs usually in small isolated crystals. It has a hardness of 5.0 to 5.5 on the Mohs scale of mineral hardness and is relatively dense , about 4.6 to 5.7 g/cm3. There are at least four different "kinds" (actually separate species) of monazite, depending on relative elemental composition of the mineral: * monazite-(Ce), (Ce,La,Nd,Th)PO4 (the most common member), * monazite-(La), (La,Ce,Nd)PO4, * monazite-(Nd), (Nd,La,Ce)PO4, * monazite-(Sm), (Sm,Gd,Ce,Th)PO4.
The elements in parentheses are listed in the order of their relative proportion within the mineral: lanthanum is the most common rare-earth element in monazite-(La), and so forth. Silica (SiO2) is present in trace amounts, as well as small amounts of uranium and thorium of thorium and uranium, monazite contains a significant amount of helium , which can be extracted by heating.
Monazite is an important ore It is often found in placer deposits, and South Africa have large deposits of monazite sands. The deposits in India are particularly rich in monazite.
Monazite is radioactive due to the presence of thorium and, less commonly, uranium. The radiogenic decay of uranium and thorium to lead enables monazite to be dated through monazite geochronology. Monazite crystals often have multiple distinct zones that formed through successive geologic events that lead to monazite crystallization. These domains can be dated to gain insight into the geologic history of its host rocks.
The name monazite comes from the Greek μονάζειν (to be solitary), via German Monazit, in allusion to its isolated crystals.