Day and Shepherd — Lime-Silica Series of Minerals. 299 



mite were obtained by allowing large melts of quartz glass to 

 cool slowly. On the surface of these melts radial spherulites 

 of tridymite frequently formed and in one instance coated the 

 entire charge with a film l mm thick of crystalline material. 

 The preparations, however, were without exception line- 

 grained and so intricately intergrown and affected by optical 

 anomalies that the exact determination of its optical constants is 

 not possible. The same conditions obtain in the mineral tridy- 

 mite and appear to be characteristic of this phase of silicon 

 dioxide. 



Intermediate mixtures. — The preparations from mixtures 

 between pure silica and wollastonite varied greatly in texture 

 and, with the exception of those mixtures which approached 

 the metasilicate in composition, were found to be in homogene- 

 ous and to consist of tridjmite and one of the forms of the 

 metasilicate. In products with less than 37 per cent of cal- 

 cium oxide, — the eutectic composition of silica and calcium 

 metasilicate — crystallites of tridymite were observed, often 

 arranged in systems of lines intersecting at 90°, 60° and less 

 angles or in rosettes and radial spherulites. The rosettes are 

 finer grained than the crystallites and frequently appear as 

 mere dust particles. The metasilicate is also fine-grained, 

 without crystal outline, and includes the larger phenocrysts of 

 tridymite. It can be distinguished from the latter most read- 

 ily by its higher refractive index and stronger birefringence. 

 The eutectic itself is extremely fine-grained and tridymite 

 occurs then only in minute dust-like particles without discern- 

 ible crystal outline. 



Preparations ranging in composition between the eutectic 

 and the metasilicate contain large, clear, lath-shaped crystal- 

 lites of the metasilicate. often in parallel orientation and inter- 

 rupted by a finely crystalline, less clear mixture of tridymite 

 and the metasilicate. The optical properties of the metasili- 

 cate in these intermediate products differ slightly from those of 

 the pure mineral, a condition which is due undoubtedly to the 

 presence of silicon dioxide taken up in solid solution by the 

 metasilicate. In these products the form equivalent to the 

 mineral wollastonite was found to differ from true wollastonite 

 chiefly in its lower refractive index and smaller optic axial 

 angle ; a measured in one instance 1*485 instead of 1*521, the 

 a of pure wollastonite. The smallest value for the optic axial 

 angle in air was found to be about 30° in air in place of the 

 70° of pure wollastonite. 



The second form of the metasilicate corresponding to 

 pseudo-wollastonite showed similar variations ; its refractive 

 indices were also lower, a — 1*490 having been measured in 

 one instance, a value '025 lower than a for pure pseudo-wol- 



