No abstract

Samples of CalSiOr were prepared by three different methods. These samples were examined by differential thermal analysis, hightemperature X-ray diffraction, and air quenching of pellets. It was found that the @ modification, during cooling, converted completely and rapidly to the y modification and "dusted" only if it had been heated above 1420" f 10°C. If the sample was not heated above this critical temperature, a mixture of the 0 and y forms always resulted at room temperature. The addition of 0.5 weight yo of CaO, SiO,, &03, Fe203, or MgO had no effect on the sequence of phases for samples cooled from above the critical temperature. When, however, these same samples were cooled from below 141O0C., the final room-temperature product was not the same for all samples. On the other hand, the additional 0.5% of Na20, K 2 0 , Cr203, or B203 prevented the formation of the fl modification regardless of the temperature from which the sample was cooled. The critical heating temperature correlated with the transformation temperature for the a-a' inversion. A theory relating this inversion to the completeness of the @ -y transformation is presented.

An a pparat us is described for growing single cr ystals in a s mall droplet of melt at temperatures u p to 1,600° C. The droplet, held b y cal?illarity at ~he jun ction o,f a. ther~oco.uple, can be observed during the process through a blllocular mICroscope. 'rhe heatwg IS by means of a hi gh-frequency current that can be separated by a filter from th~ ther moco~pl e electromot ive force allowing the electromotive force to be measured contwuously. The apparatus is theref~re suitable a lso for determining meltin g point~.. . The process of mounting and grinding t he single-crystal sreclmens for X-ray dl~ractlOn and a simple but versatile mi cromanipu lator to facili tate these procedures are descnbed.

Wire models of tetrahedral networks have been built up by a process representing condensation from a fluid, which yields amorphous structures unless a nucleus of crystalline structure is present initially. A random tetrahedral network of 300 units has been constructed for direct measurement of coordinates; the construction process is uniquely defined so that it may be possible to synthesize larger networks by use of a computer.