The rare earths are not really rare in that they are found in the parts per million range widely distributed throughout the earth's crust. They always occur as mixtures of all the rare earths, and these mixtures are frequently found concentrated in numerous minerals. The relative abundance of the rare earths varies from mineral to mineral, but in general the heavy rare earths are always present in minor amounts. It is estimated that erbium occurs in the earth's crust to about 24 parts in ten million. Erbium is more abundant, therefore, than bromine, iodine, antimony, mercury and cadmium.