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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 three different kinds of monazite, depending on relative elemental composition of the mineral:

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, SiO4, will be present in trace amounts, as will small amounts of uranium.

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 grams/cubic centimeter.

Because of the presence of thorium within monazite, it can be highly 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.

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