'Monazite' is an important ore for 'thorium'. It is a reddish-brown phosphate mineral usually occurs in small isolated crystals. Due to the 'alpha' decay of 'thorium', 'monazite' contains a significant amount of 'helium', which can be extracted by heating. It is radioactive due to the presence of 'thorium'. This radiogenic decay lead enable 'monazite' to be dated through 'monazite' geochronology, which is a dating technique to study geological history.

'Monazite' sand from Brazil was noticed in sand carried in ship's ballast by Carl Aver von Welsbach, who was an Austrian scientist and inventor in 1880s. Von Welsbach was looking for 'thorium' for his newly invented incandescent mantles. 'Monazite' sand was quickly adopted as the 'thorium' source.

'Monazite' sand was briefly mined in North Carolina, but, shortly thereafter, extensive deposits in Southern India were found. There were also large 'monazite' deposits in Australia. The 'bastnäsite' came to displace 'monazite' in the production of 'lanthanides' in the 1960s due to its much lower 'thorium' content, as there was much concern over the disposal of the radioactive daughter products of 'thorium' present in significant amount in 'monazite'. The commercial 'monazite' sands typically contain between 6-12 % 'thorium' oxide. The 'thorium' has high melting point, 1750 deg C and is quite electropositive behavior.

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