I) topside — Forsterite — Silica. 245 



Optical Study 



Throughout the investigation the products of the various 

 operations were examined under the microscope in order to 

 identify the phases present. In the ordinary course of the 

 work it was necessary to observe only a sufficient number of 

 the properties of any phase to make its identification certain, 

 but, whenever specially suitable material was obtained, deter- 

 minations of all the measurable optical properties were made. 

 In the following the values found for the various phases are 

 recorded. 



Forsterite. 



The properties of pure forsterite have been given in former 

 publications from this laboratory and will simply be listed here 

 without discussion. 



Crystal system-orthorhombic ; a : b : c = 0*463 : 1 : 0*584 



Cleavage 010, 001 distinct. G = 3*216 



Refractive indices, y= 1*670 4 (3= l*651 a= 1*635 1 



y-a = *03 5 3 2V Na = 85° 16'. Orientation c = ft and a = c = Bx a 



Cristobalite and Tridymite. 



Silica was found in the form of cristobalite in mixtures 

 quenched from 1500° or higher and as tridymite in those 

 quenched from about 1400° or lower. At intermediate 

 temperatures it was usually possible to prove that both were 

 present. These observations are in accord with Fenner's con- 

 clusion that tridymite and cristobalite are enantiotropic and 

 the inversion point about 1470°, the change being decidedly 

 sluggish.* 



Cristobalite occurred in the glass sometimes as sharply out- 

 lined octahedra, slightly modified by the cube. The cube was, 

 however, usually the more prominent form and, by distortion, 

 the crystals usually appeared as stout prisms. The double 

 refraction was barely discernible with the aid of the sensitive 

 tint plate. 



Tridymite was in the form of broad thin plates which 

 appeared as needles when turned on edge. The needle-shaped 

 sections had quite marked double refraction, parallel extinction 

 and negative elongation. 



The Diopside-Clino-enstatite Series of Pyroxenes. 



The compounds MgSi0 3 and CaMgSi 2 6 form a complete 

 series of mix-crystals or solid solutions, crystallizing in forms 

 corresponding with monoclinic pyroxenes. The existence of 



*Fenner, C. N.: The Stability Eelations of the Silica Minerals, this 

 Journal (4), xxxvi, 337, 1913. 



