| Like all other rare earths, holmium is not naturally found as a free element. It does occur combined with other elements in the minerals gadolinite,
monazite, and in other rare-earth minerals. It is commercially extracted via ion-exchange from monazite sand (0.05% holmium)
but is still difficult to separate from other rare earths. The element has been isolated through the reduction of its anhydrous chloride or fluoride with metallic calcium. Its estimated abundance in the Earth's crust is 1.3 milligrams per kilogram. Holmium obeys the Oddo-Harkins rule: as an
odd-numbered element, it is less abundant than its immediate even numbered neighbors, dysprosium and erbium. However, it is the most abundant of the odd-numbered heavy lanthanoids. The principal current source are some of the ion-adsorption
clays of southern China. Some of these have a rare earths composition similar to that found in xenotime or gadolinite. Yttrium makes up about two-thirds of the total by weight; holmium is around 1.5%. The original ores themselves are very lean, maybe
only 0.1% total lanthanoid, but are easily extracted.
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Occurrence