1. Maignen, Compte Rendu de Recherches sur les laterites, UNESCO, Paris, 1966.
2. J.J. García, R.E. Galiasso, M.M. Ramirez de Agudelo, L. Rivas, J. Hurtado, US Patent 4701435 (1987).
3. F. Rueda, J. Mendialdua, Final report Project European Community CI1-0573-F(CD), 1994.
4. G. Thornton, R. Casanova, Third year and final report. European Community on contract no. CI1-CI92-0056.