Day and Shepherd — Lime-Silica Series of Minerals. 293 



The scattered points lying below 1300° (fig. 3) occur only 

 in the compositions in which the metasilicate is present, and 

 correspond, as a microscopic examination at once shows, to the 

 change from wollastonite to the pseudo-hexagonal form. 

 With falling temperature, the points occur very much lower 

 or are lost, since the inversion does not occur quite as readily 

 on cooling. Allen and White observed that this inversion 

 could be brought about only with great difficulty with the 

 pure metasilicate, but in the presence of an excess of either 

 lime or silica, we found it to occur with comparative readiness 

 (Table Y) in many compositions. 



The line QR is the temperature at which the reaction j3 into 

 a takes place. As observed in the discussion of the proper- 

 ties of the orthosilicate, this reaction does not occur promptly, 

 but is liable to very serious superheating or undercooling. 



Table V. 

 Inversion (3 -Metasilicate to a- Metasilicate. 

 Percentage CaO 30 32-5 



Temperature of 1273° 1274 

 inversion. 1263 



35 



40 



45 



57 



1257° 



1288° 



1254° 



1286 



1399 



1266 







1323 









1328 









Part III. Optical Study. 



The different members of the lime-silica series are well 

 characterized optically and can generally be distinguished 

 under the microscope without difficulty. Occasionally, how- 

 ever, the preparations are extremely fine-grained and require 

 repeated examination before the minute details of each sub- 

 stance in the product are fully appreciated. In actual practice, 

 experience has shown that the best results can be obtained by 

 examining the preparations in powdered form rather than in 

 thin sections cut from larger fragments. The chief advantage 

 of thin sections over the powdered material is a textural one, 

 since by breaking any given preparation into small particles 

 its original texture is practically destroyed. Although prob- 

 lems of textural differences in artificial products are not to be 

 disregarded, actual determinations can' best be made with the 

 powder, since with it the crystallites can usually be examined 

 separately and the optical phenomena of a single individual 

 observed rather than those of an aggregate of overlapping 

 and interlacing crystals, as is often the case in slides ; further- 

 more, by embedding the fine grains in a viscous liquid, such 

 as Canada balsam, they can be rolled about in the liquid and 

 their optic properties in any desired direction ascertained ; the 

 refractive indices of a substance can also be determined directly 



