2 Allen, etc. — D topside and its Relations 



curves, relying on very careful microscopic examinations to 

 make sure of the solid phases which separated from the molten 

 solutions. Since we are concerned here with equilibrium 

 conditions, it will be evident that the monotropic forms of 

 magnesium silicate do not enter into the problem.* In addi- 

 tion to this specific problem, viz., the relations existing between 

 the calcium and magnesium silicates, we planned to study the 

 most important properties of diopside and its transformations 

 on heating, if any. 



Many of the methods used in this investigation have been 

 already described in previous papers from this laboratory ;f 

 others have been devised in the course of the work. 



Preparation of the Mixtures. — Quartz, magnesia and cal- 

 cium carbonate were mixed in the proper proportions and 

 melted in large covered platinum crucibles. The molten 

 charges were chilled by dipping the crucibles into water, 

 cracked out of the crucibles, crushed fine, and remelted to 

 ensure thorough mixing. All the materials were carefully 

 examined for impurities, and we are able to state with cer- 

 tainty that the products contained no more than Ol per cent to 

 0'2 per cent of foreign matter, except in certain cases where 

 the same preparation was repeatedly melted and crushed, when 

 as much as 0*3 per cent impurity was sometimes found. 



We had considerable trouble in getting magnesia sufficiently 

 pure for our purposes. The chief impurity is generally lime, 

 of which all the samples examined by us contained 0'5 per cent 

 or more. At our request, the firm of Baker & Adamson under- 

 took the problem of preparing pure magnesia in quantity, and 

 succeeded in making a " basic carbonate" in which the mini- 

 mum quantity of lime amounted to about "07 per cent, or 015 

 per cent of the calcined oxide. A sample of magnesium 

 metasilicate prepared from one lot of this carbonate gave by 

 direct test '12 per cent lime. In mixtures very high in mag- 

 nesia a correction was generally made for this impurity, i. e., 

 the proper deduction was made from the quantity of lime 

 required by the particular mixture in hand. For the sake of 

 control we analyzed two of the preparations and the results 

 are given below. 



At one time during the investigation, it was suspected that 

 some loss might be caused by the strong blast of the gas fur- 

 nace in which the constituent powders were first melted together 

 in spite of the fact that the crucible was carefully covered. 

 The magnesia being by far the lightest of the constituents 



* Viktor Poschl incorrectly designates the series " ensftxfo'te-diopside," 

 Centr. Min., 1906, p. 572. 



fThis Journal, xix, 125, 1905; xxi, 89, 1905; Tschermak ; s Mitth., xxvi, 

 169, 1907. 



