1. Eipeltauer, E., Zement-Kalk-Gips, 11, 264 (1958).
2. Kelley, K. K., Southard, J. C., and Anderson, C. T., US Bur. Mines Tech. Paper No. 625 (1941).
3. Powell, D. A., Nature, 182, 792 (1958).
4. Morris, R. J., Nature, 198, 1298 (1963).
5. Gay, P., Min. Mag., 35, 354 (1965).