JBowen and Andersen — Binary System MgO-SiO^. 493 



The cleavage noted as parallel to (100) becomes, on rotation 

 of the crystal, parallel to (001), which in natural forsterites, 

 unlike other olivines, is the second best cleavage.* To this 

 cleavage (//001) the plane of the optic axes is parallel in the 

 measured crystals, and here again there is correspondence with 

 forsterite. In crystallographic properties, including the axial 

 ratios themselves, in refractive indices, and in the relation 

 between the position of the optical ellipsoid and crystallo- 

 graphic directions, the crystals called a-MgSi0 3 correspond 

 with forsterite. The only supposed difference remaining is in 

 the measured value of the optic axial angle. In view of the 

 possible disturbance due to a slight strain in the crystals this 

 minor difference noted in the former measurements cannot 

 be regarded as establishing a species, in the face of the identity 

 of the more distinctive properties with those of forsterite and 

 of the proof by means of quenching that material so obtained 

 must contain forsterite. 



In the table below, the degree of correspondence is shown 

 between the constants given for the crystals which were 

 called a-MgSiO s , after rotation through 90° on the b axis, and 

 the values noted for artificial crystals of forsterite in an earlier 

 publication from this laboratory, already referred to. 



It was a difficult matter to identify the clear grains in the 

 dusty material as originally prepared, but, by quenching, 

 identical material can be prepared in two steps which make 





Table II. 





Crystal system 

 Axial ratios 



Forsterite 



orthorhombic 



0-463 : 1 : 0-584 



"a-MgSi0 3 " 

 orthorhombic 

 0-47 : 1 : 059 



Forms observed 



010, 021, 110 and others 



010, 021, 110 



Cleavages 



II 010 



not noted (crystals 

 tabular// 010) 



Axial plane 



//001 

 //001 



//001 

 //001 



Refractive indices 



a = 1-645 ± -003 



a = 1-641 ± -003 





y= 1-668 ± -003 



y = 1-663 ± -003 



Optic axial angle 



86° 



60° 



the relations clear. A mixture of composition MgSi0 3 may be 

 held at the temperature 1560°, a little above the dissociation 

 point, and then quenched. The product consists of clear grains 

 embedded in glass and the properties of these grains are 

 definitely those of forsterite. Moreover, corresponding with 



* It appears that in artificial forsterite cleavage after (001) may sometimes 

 be as good as after (010), for Wright notes "cleavage perfect after (001) and 

 (010) " for crystals of forsterite made in a flux. 



