↑ Haxel G.; Hedrick J.; Orris J. (2002). “Rare Earth Elements—Critical Resources for High Technology” (PDF). Edited by Peter H. Stauffer and James W. Hendley II; Graphic design by Gordon B. Haxel, Sara Boore, and Susan Mayfield. United States Geological Survey. USGS Fact Sheet: 087‐02. Truy cập ngày 13 tháng 3 năm 2012. However, in contrast to ordinary base and precious metals, REE have very little tendency to become concentrated in exploitable ore deposits. Consequently, most of the world's supply of REE comes from only a handful of sources.
↑ “1787–1987 Two hundred Years of Rare Earths”. Rare Earth Information Center, IPRT, North-Holland. IS-RIC 10. 1987.
