166 Ferguson and Menvin — The Ternary System. 



sluggishness of inversion; and the presence of unstable 

 phases. 



The optical properties of these phases have been given 

 in the previous paper and will be given only incidentally 

 here, but a resume of other previous work is as follows : 

 According to Rankin and Wright 2 pure wollastonite 

 (calcium metasilicate, CaO.Si0 2 ) inverts at 1200 °C to a 

 high-temperature form, pseudowollastonite. Excess of 

 silica in solid solution raises the inversion temperature 

 10 c and an excess of tricalcium disilicate lowers this 

 temperature 10° (or more). According to Allen and 

 others, 3 wollastonite forms solid solutions containing a 

 maximum of 17 per cent of diopside (CaO.Mg0.2Si0 2 ) 

 with a maximum rise of the inversion temperature to 

 about 1345 ± 10 °C. 4 



The liquidus-solidus relations found by us in the ter- 

 nary system do not agree with the rather simple solidus 

 relations hitherto inferred; furthermore, subsequent 

 study has shown that the solidus relations also are very 

 complex. 5 For reference these relations are given in 

 fig. 1. 



Experimental Part. 



Methods. — The study of the purely solidus relations 

 was made by first preparing glasses of the desired com- 

 positions and then crystallizing these glasses at low 

 temperatures. In general, the glasses usually gave some 

 unstable pseudowollastonite when crystallized at a tem- 

 perature just below the stability region of this form of 

 calcium metasilicate. To avoid this difficulty, the glasses 

 were first heated over night at 800 to 900 °C which 

 caused them to crystallize, often not completely, and then 

 heated for some hours at a temperature just below the 

 inversion temperature of the particular charge in ques- 

 tion. This subsequent heat treatment caused the crys- 

 tals to grow somewhat and made possible a selection of 

 material nearly free from pseudowollastonite. 6 The 

 need for such care in the selection of material arose from 



1 Rankin and Wright, this Journal (4), 39, 1, 1915. 



8 Allen, White, Wright, and Larsen, this Journal (4), 27, 19, 1909. 



4 Grundlagen der physikalisch-chemischen Petrographie, 182, 1915. 



5 It is hardly necessary to point out that different forms of the same 

 compound are distinct phases, and their ranges of solid solution are not 

 necessarily the same. 



6 Charges containing disturbing amounts of pseudowollastonite were dis- 

 carded and when necessary the heat treatments were repeated with a fresh 

 sample of glass. 



