Version 4.0 of this data release provides descriptions of more than 200 mineral districts, mines, and mineral occurrences (deposits, prospects, and showings) within the United States that are reported to contain substantial enrichments of the rare earth elements (REEs). These mineral occurrences include mined deposits, exploration prospects, and other occurrences with notable concentrations of the REEs. The inclusion of a particular mineral occurrence in this database is not meant to imply that it has economic potential. Rather, these occurrences were included to capture the distribution and characteristics of the known, reported REEs deposits in the United States, which are diverse in their geology and resource potential.

Concentrated, mineable deposits of the REEs are rare, such that most of the sites within this data release are for unmined locations where the published information may not contain thorough descriptions (Van Gosen and others, 2014). Therefore, decisions had to be made by the authors regarding the addition or exclusion of specific REE occurrences in the dataset, based principally on the available descriptions of the REE concentrations and the apparent size of the mineralized body. The level of detail of this type of information varied widely amongst the occurrences, ranging from general descriptions to detailed sampling and analysis of some deposits.