40 Buddington — Natural and Synthetic Melilites. 



The quenching method was employed throughout the 

 research. Details of this method may be found in many 

 of the previous publications of this laboratory. A dia- 

 gram of the type of furnace used in this work may be 

 found in the paper by Ferguson and Merwin 8 on the 

 ternary system CaO-MgO-Si0 2 . The melting points 

 used in the calibration of the thermoelements were those 

 of: gold 1062.4% diopside 1391.5°, akermanite 1458°, 

 anorthite 1550°, and gehlenite 1590°. 



In most cases, crystals used for the determination of 

 their optical properties were prepared by holding the 

 proper glass over night at a temperature just below the 

 dissociation point, or the beginning of melting. A very 

 great difference was found in the character of the crystal- 

 lization. Mixtures with 50 percent or more of the 3CaO. 

 Al 2 3 .3Si0 2 compound crystallized in fibrous aggregates 

 which did not grow appreciably with continued heating. 

 If these charges are heated above the dissociation point, 

 so that a little liquid is obtained to facilitate recrystal- 

 lization, the resulting aggregates are mottled throughout 

 with the dissociation products and are unsatisfactory, 

 unless the material is crystallized.so far above the solidus 

 that a change in their composition results and the opti- 

 cal properties of the crystals obtained are materially 

 changed. The best crystallized preparations of such com- 

 positions were obtained by heating the glasses 20 hours 

 or longer somewhat above the lowest temperature at 

 which the glass will recrystallize. The optical determi- 

 nations of the indices of refraction made on mixtures 

 rich in the 3CaO.Al 2 3 .3Si0 2 compound may be in error 

 as much as ± 0.005. Glasses with less than 50 percent of 

 the 3CaO.Al 2 3 .3Si0 2 compound and as little as 20 per- 

 cent of the 3CaO.Fe 2 3 .3Si0 2 compound give very good 

 crystalline material if held for fifteen minutes to an hour 

 at a temperature a little below the solidus. The glass of 

 composition containing 90 molecular percent of 3CaO. 

 Al 2 3 .3Si0 2 and 10 percent of 3Na 2 O.Al 2 3 .3Si0 2 was 

 held for one week at a point just above the solidus to 

 obtain crystals free from air for the determination of the 

 specific gravity. Some glass and dissociation products 

 were present in this material. 



* J. B. Ferguson and H. E. Merwin, op. cit., p. 88. 



