498 JBoioen and Andersen — Binary System. MgO-SiO^ 



Cristobalite. — Silica occurs in our melts only as cristobalite. 

 As viewed under the microscope at room temperature the crys- 

 tals are birefringent, a-form, but have the crystal form of the 

 isotropic /3-form in which they crystallized at the temperature 

 of the furnace. When embedded in glass the crystals are some- 

 times sharply outlined octahedra, modified by the cube, but 

 usually the cube is the more prominent form and the crystals 

 are, by distortion, stout prisms due to slight elongation parallel 

 to one of the cubic axes. The birefringence is barely discern- 

 ible with the aid of the sensitive-tint plate, this apparently low 

 value being probably due to the mutual compensation of a 

 number of twinning lamellae. Occasionally polysynthetic twin- 

 ning was observed. The mean refractive index, 1*485 ± -001, 

 agrees with the values found by Fenner.* 



Forsterite. — We have found Mg s Si0 4 only in the form cor- 

 responding with the natural mineral forsterite. Measured arti- 

 ficial crystals of this mineral are figured in a former publication 

 from this laboratory. f Crystals from this same stock, prepared 

 by fusing together magnesia, silica and magnesium chloride in an 

 atmosphere of HC1, were used to determine the optical con- 

 stants of forsterite more accurately. The acute angle of the 

 unit prism was used as a wedge and the refractive indices a 

 and /3 determined for sodium light on the goniometer, by the 

 method of minimum deviation. The optic axial angle was then 

 measured directly on the Wulfing apparatus in sodium light, 

 the crystal being immersed in a liquid of index /3. From these 

 values the index y was calculated. The results are given below: 



/8 = 1-651 2V = 85° 16' ± 4' 



a = 1-635, y = 1-670, 



y — a = -035 3 



The cleavage is distinct after (010) and (001). 

 The optical orientation is c = b and a — c. 



A determination of the density of these crystals was made in 

 Rohrbach's solution by the flotation-refractive index method of 

 Merwin4 The crystals used were small and were examined 

 under the microscope and found free from inclusions of any 

 kind. The density found was 3-216 ± -002. 



Clino-enstatite. — With the exception of the occasional appear- 

 ance of the monotropic am pinhole form of MgSiO, in quickly 

 cooled melts, the stable monoclinic pyroxene, clino-enstatite, 

 was the only form of the metasilicate observed in our quenches. 



* The Stability Relations of the Silica Minerals, this Journal, (4), xxxvi, 

 1913, 354. 



f Allen, Wright and Clement, Minerals of the Composition MgSi0 3 , this 

 Journal. (4), xxii, 1906, 391. 



% This Journal, (4), xxxi, 425, 1911. 



