Monazite mineral. Monazite is a phosphate mineral belonging to the rare earth oxides, containing the rare earth metals thorium, lanthanum and cerium in varying proportions. Four types of monazite are distinguished depending on the relative content of these metals. Monazite is monoclinic, and most often occurs as small well developed prismatic or tabular crystals in dark to light shades of honey-brown colour. Due to the presence of thorium it may be radioactive. This sample is a 20-centimetre-high crystal aggregate interspersed with quartz. It was found in pegmatite rocks near Amelia in Virginia, USA.