90 Allen and White — Polymorphic Forms of 



is known in nature nor any paramorpb of wollastonite after 

 pseudo-wbllastonite. On the other hand, pseudo-wollastonite 

 in crystals no less than a centimeter in width has been observed 

 in artificial slags by Vogt and others. 



Had the schists containing wollastonite crystallized at tem- 

 peratures above 1180°, it would seem from the following inves- 

 tigation that the calcium silicate must have separated out as 

 pseudo-wollastonite, and that even had it afterwards gone 

 over by paramorphism into wollastonite, the original form 

 must have left its traces. Such, too, must have been the case 

 had the contact metamorphosis of limestones taken place at a 

 higher temperature or had the fragments of limestones, often 

 very small, which are included in effusive rocks, been heated 

 above this critical temperature. Such included fragments are 

 often sharply angular, and there is no probability that they 

 were ever melted. Of course, the character of a melt is 

 greatly modified by the constituents present, so that the admix- 

 ture of other substances than calcium silicate must be taken 

 into account ; but it would appear that other substances, such 

 as iron compounds, could only reduce the melting temperature 

 of the mixture, making it safe to conclude that wollastonite 

 can under no circumstances have formed above 1180°. The 

 effect of pressure is probably without influence upon this con- 

 clusion in view of the very low vapor tension of the mineral at 

 its inversion temperature. The present investigation there- 

 fore adds a very considerable amount of exactness to our 

 knowledge of the temperatures at which the metamorphism of 

 limestone has gone on. If the wollastonite of the nepheline 

 syenite is primitive, at least one family of deep-seated intru- 

 sives has also been injected at temperatures lower than 1180°. 



Messrs. Day and Shepherd have shown that wollastonite is 

 very generally stable, and apt to make its appearance under a 

 wide range of circumstances from melts of very different com- 

 positions. This, too, is in entire accord with the geological 

 evidence afforded by contacts along which the chemical con- 

 ditions vary greatly and rapidly, while wollastonite, through 

 its frequency, exhibits, in nature as well as in the laboratory, 

 a strong tendency to form under varying conditions. 



Preparation of Wollastonite. — Attempts to synthesize the 

 mineral wollastonite (CaSi0 3 ) generally result in the formation 

 of a substance, apparently uniaxial, which has never been 

 found in nature. Even when the natural wollastonite is fused 

 and cooled again, it is almost invariably the uniaxial form 

 which crystallizes. The genuine wollastonite has been observed 

 as an accidental product of slow cooling in glasses, and crystals 



