<P> Before the time that ion - exchange methods and elution were available, the separation of the rare earths was primarily achieved by repeated precipitation or crystallisation . In those days, the first separation was into two main groups, the cerium - group earths (scandium, lanthanum, cerium, praseodymium, neodymium, and samarium) and the yttrium - group earths (yttrium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium). Europium, gadolinium, and terbium were either considered as a separate group of rare - earth elements (the terbium group), or europium was included in the cerium group, and gadolinium and terbium were included in the yttrium group . The reason for this division arose from the difference in solubility of rare - earth double sulfates with sodium and potassium . The sodium double sulfates of the cerium group are difficultly soluble, those of the terbium group slightly, and those of the yttrium group are very soluble . Sometimes, the yttrium group was further split into the erbium group (dysprosium, holmium, erbium, and thulium) and the ytterbium group (ytterbium and lutetium), but today the main grouping is between the cerium and the yttrium groups . </P> <P> The classification of rare - earth elements is inconsistent between authors . The most common distinction between rare - earth elements is made by atomic numbers; those with low atomic numbers are referred to as light rare - earth elements, those with high atomic numbers are the heavy rare - earth elements, and those that fall in between are typically referred to as the middle rare - earth elements . Commonly, rare - earth elements with atomic numbers 57 to 61 are classified as light and those with atomic numbers greater than 62 (corresponding to Eu) are classified as heavy - rare earth elements . Increasing atomic numbers between light and heavy rare - earth elements and decreasing atomic radii throughout the series causes chemical variations . Eu is exempt of this classification as it has two valance states: Eu + 2 and Eu + 3 . Y is grouped as heavy rare - earth element due to chemical similarities . </P> <P> Rare - earth elements, except scandium, are heavier than iron and thus are produced by supernova nucleosynthesis or the s - process in asymptotic giant branch stars . In nature, spontaneous fission of uranium - 238 produces trace amounts of radioactive promethium, but most promethium is synthetically produced in nuclear reactors . </P> <P> Due to their chemical similarity, the concentrations of rare earths in rocks are only slowly changed by geochemical processes, making their proportions useful for geochronology and dating fossils . </P>

Where do most rare earth elements come from