1. Bellati and Finazzi..Atti. 1st. Veneto, 1910,69, 1151.
2. Edwards, D. A...Zeits. f. Krist., 1931,80, 154.
3. Groth, P... “Chemische Kristallographie, 1910,2, 63.
4. Kracek, F. C., Barth, T. F. W. and Kosanda, C.J. Phys. Rev., 1932,40, 1034.
5. Leonhardt, L. and Borchert, W.Naturwiss., 1936,24, 412.