Calcium Metasilicate. 93 



The Inversion of Wollastonite to Pseudo- Wollastonite. — 

 It is sufficiently well known that the polymorphic forms of 

 solid crystalline bodies are divided into two well-marked classes, 

 the monotropic and the enantiotropic. Substances which, 

 like phosphorus, belong to the first class, possess one form 

 which is more stable than the other at all temperatures below 

 the melting point. The unstable form may therefore change 

 directly into the other over a considerable range of tempera- 

 ture, but the change in the opposite direction cannot be 

 brought about. In substances of the second class, the enantio- 

 tropic bodies, the transformation from one to the other takes 

 ]Dlace at a characteristic temperature point, known as the 

 inversion point, and is reversible. Each form of a monotropic 

 substance possesses its owb melting point, while in enantio- 

 tropic substances only one form melts in the region of its 

 stable existence, viz : that form which is stable at the higher 

 temperature.* 



In calcium silicate we have unquestionably a case of enantio- 

 tixvpy, for one form changes to the other at a definite tempera- 

 ture, and the change is, under proper conditions, reversible. 



When wollastonite, either natural or artificial, is heated to 

 about 1180°, it passes entirely into the hexagonal form. This 

 change of state occurs without melting, for, in every case, the 

 coarsely-powdered mineral which we employed in experiment 

 was found, after the inversion, to have shrunk away from the 

 walls of the crucible, forming a sintered cake, which, of course, 

 a liquid could never have done. Moreover, the separate grains 

 of the substance always preserved their sharp edges. 



Brunf states that natural wollastonite from Auerbach melts 

 at about 1350° and then quickly solidifies. There is, of course, 

 the possibility that a rapidly heated charge might pass the 

 inversion point without change and melt in the metastable 

 region, as sulphur is known to do. A special effort was made 

 to test this possibility in the following manner: a 50 gram 

 charge of the j^urest natural wollastonite obtainable (from 

 Diana, 1ST. Y.) was heated in the electric furnace past the 

 inversion point at the fastest rate consistent with the safety of 

 the heating coil, i. e., about 16° per minute. To hinder inver- 

 sion as much as possible, we selected rather large fragments of 

 the mineral with comparatively little surface. At 1260° a 

 perceptible absorption of heat was observed, immediately after 

 which the temperature was lowered and the charge examined. 

 Some fragments had undergone a slight local vitrification 

 which gave the effect of glazing, but the original form of each 



* Koozeboom, Heterogene Gleichgewichte, vol. i, pp. 109, 110. Findlay, 

 The Phase Rule, 1904, p. 42. 



f Archives des Sciences phys. et nat., serie 4 e , tome 18, p. 551. 



