MonaziteLa

Monazite-(La) is the lanthanum-dominant variety of the rare-earth phosphate mineral monazite. Like other forms in the monazite group, its general composition is (REE)PO4, with lanthanum (La) as the principal rare earth element, accompanied by smaller amounts of cerium, neodymium, praseodymium, and samarium; thorium and uranium may occur as trace constituents. The exact composition varies among deposits.

Crystallography and appearance: Monazite-(La) crystallizes in the monoclinic system and is typically found as prismatic crystals or as granular to microcrystalline grains within rocks. It often has a brown to yellow-brown color and a resinous luster. The hardness is about 5–5.5 on the Mohs scale, and the mineral has a relatively high specific gravity, typically around 4.7–5.0.

Geological occurrence: Monazite-(La) forms as an accessory mineral in granitic and metamorphic rocks and is common in pegmatites and hydrothermal veins. It is a constituent of heavy-mineral sands along coastlines, where currents concentrate light REE minerals. In such environments it occurs with related REE phosphates such as xenotime-(Y) and other monazite varieties, as well as with apatite, zircon, and thorite.

Economic and safety considerations: Monazite-(La) represents a potential source of lanthanum and other light rare earth elements, contributing to REE inventories in deposits rich in La. Processing and handling are often subject to radiation safety considerations because monazite minerals can contain thorium and uranium.

See also: monazite-(Ce), xenotime, rare-earth phosphate minerals.