by solvent extraction from the mixed lanthanides isolated from bastnäsite (or monazite). Since heavier lanthanides have more affinity for the solvent used, they are easily extracted from the bulk using relatively small proportions of solvent. Not all rare-earth producers who process bastnäsite do so on a large enough scale to continue by separating the components of SEG, which typically makes up only 12% of the original ore. Such producers therefore make SEG with a view to marketing it to the specialized processors. In this manner, the valuable europium in the ore is rescued for use in making phosphor. Samarium purification follows the removal of the europium. , being in oversupply, samarium oxide is cheaper on a commercial scale than its relative abundance in the ore might suggest. Occurrence and production With the average concentration of about 8 parts per million (ppm), samarium is the 40th most abundant element in the Earth's crust. It is the fifth most abundant lanthanide and has a higher concentration than many other elements such as tin (which has an average concentration of 2 ppm). Samarium concentration in soils varies between 2 and 23 ppm, and oceans contain about 0.5–0.8 parts per trillion. Distribution of samarium in soils strongly depends on its chemical state and is very inhomogeneous: in sandy soils, samarium concentration is about 200 times higher at the surface of soil particles than in the water trapped between them, and this ratio can exceed 1,000 in clays. Samarium is not found free in nature, but, like other rare earth elements, is contained in many minerals, including monazite, bastnäsite, cerite, gadolinite and samarskite; monazite (in which samarium occurs at concentrations of up to 2.8%) and bastnäsite are mostly used as commercial sources. World resources of samarium are estimated at two million tonnes; they are mostly located in China, US, Brazil, India, Sri Lanka and Australia, and the annual production is about 700 tonnes. Country production reports are usually given for all rare-earth metals combined. By far, China has the largest production with 120,000 tonnes mined per year; it is followed by the US (about 5,000 tonnes) and India (2,700 tonnes). Samarium is usually sold as oxide, which at the price of about US$30/kg is one of the cheapest lanthanide oxides. Whereas mischmetal – a mixture of rare earth metals containing about 1% of samarium – has long been used, relatively pure samarium has been isolated