1. Frondel, C. (Geol. Surv. Bull. [U.S.] No. 1064 [1958) 1/400).
2. Chukhrov, F. V.; Bonshtedt-Kupletskaya, E. M. (Mineraly Spravochnik, Vol. 2, Pt. 2, Nauka, Moscow 1965, pp. 1/341).
3. Goldschmidt, V. M.; Thomassen, L. (Skr. Nor. Vidensk. Akad. Kl. I Mat. Naturvidensk. Kl. 1923 No. 2, pp. 1/48).
4. Frondel, C. (Geol. Surv. Profess. Paper [U.S.] No. 300 [1956) 567/79; Proc. Intern. Conf. Peaceful Uses At. Energy, Geneva 1955 [1956), Vol. 6, pp. 568/77, 572/4).
5. Bayer, G. (in: Wedepohl, K. H.; Handbook of Geochemistry II-5, Springer, Berlin 1974, p. 90-A-1).