Thulium
Occurrence

The element is never found in nature in pure form, but it is found in small quantities in minerals with other rare earths. Thulium is often found with minerals containing yttrium and gadolinium. In particular, thulium occurs in the mineral gadolinite. However, like many other lanthanides, thulium also occurs in the minerals monazite, xenotime, and euxenite. Thulium has not been found in prevalence over the other rare earths in any mineral yet. Its abundance in the Earth's crust is 0.5 mg/kg by weight. Thulium makes up approximately 0.5 parts per million of soil, although this value can range from 0.4 to 0.8 parts per million. Thulium makes up 250 parts per quadrillion of seawater. In the Solar System, thulium exists in concentrations of 200 parts per trillion by weight and 1 part per trillion by moles. Thulium ore occurs most commonly in China. However, Australia, Brazil, Greenland, India, Tanzania, and the United States also have large reserves of thulium. Total reserves of thulium are approximately 100,000 tonnes. Thulium is the least abundant lanthanide on Earth except for the radioactive promethium.
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