United States. Bureau of Mines.. Mineral Facts and Problems: 1960 Edition. [Washington D.C.]. UNT Digital Library. http://digital.library.unt.edu/ark:/67531/metadc38790/. Accessed May 23, 2013.
United States. Bureau of Mines.. Mineral Facts and Problems: 1960 Edition. [Washington D.C.]. UNT Digital Library. http://digital.library.unt.edu/ark:/67531/metadc38790/. Accessed May 23, 2013.