492 Bowen and Andersen — Binary System MgO-8iO n _. 



tions, forsterite 5*5 per cent, liquid 94*5 per cent. The 

 temperature must then be raised to 1577° before the solution 

 of forsterite is complete and the whole becomes liquid. 



Magnesium metasilicate was studied formerly at this labora- 

 tory and four forms described, the stable monoclinic pyroxene 

 clino-enstatite, and three monotropic forms described as ortho- 

 rhombic pyroxene, orthorhombic and monoclinic amphibole.* 

 Later another orthorhombic form termed a-MgSi0 3 was added 

 and described as enantiotropic with clino-enstatite (/3-MgSi0 3 ), 

 the latter being the low temperature form and the inversion 

 point abont 1365°. f The present work does not add any new 

 information concerning the monotropic forms mentioned, but, 

 concerning the material called a-MgSi0 3 a revision of the 

 former statements is necessary in the light of the completed 

 results of the binary system. 



If a mixture of composition MgSi0 3 is cooled from the 

 liquid condition, forsterite first separates out and with further 

 cooling the remaining liquid crystallizes to a mixture of silica 

 and MgSi0 3 . The product then consists of clear, strongly- 

 birefringent grains of forsterite embedded in a dusty matrix of 

 indefinite properties, the dusty appearance being caused by the 

 tiny specks of silica of very low refraction dotted through the 

 MgSi0 3 . It was these clear, strongly-birefringent grains of 

 forsterite which were described as a-MgSiO s and were con- 

 sidered to be the product of inversion of clino-enstatite at a 

 high temperature. They were really the product of dissocia- 

 tion of clino-enstatite at a high temperature and were not, as 

 we have pointed out, a form of the metasilicate, but were the 

 orthosilicate, forsterite. 



The measured crystals described as a-MgSi0 3 J and showing 

 the forms (010), (110) and (Oil), referred to the axes a : h : c = 

 1*19 : 1 : - 47 should have been termed forsterite. A rotation of 

 the crystals through 90° on the h axis will give the forms (010), 



(021) and (110) referred to the axes a : b : o = 0-47 : 1 : --£ • 



These latter ratios are those of forsterite, within the errors of 

 measurement, and the forms are common in that mineral. § 

 The form (010) is more prominent than is usually the case in 

 larger crystals, but there is a well-recognized tendency in small 

 and quickly formed crystals toward special development 

 parallel to the plane of the best cleavage and we have observed 

 this tendency in crystals of artificial forsterite. 



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

 Journal (4), xxii, 388, 1906. 



f Allen, White. Wright and Larsen, Diopside and its Eelations to Calcium 

 and Magnesium Metasilicates, this Journal (4), xxvii, 1, 1909. 



% Allen, White, Wright and Larsen, this Journal (4), xxvii, 31, 1909. 



§ See artificial crystals of forsterite, Allen, Wright and. Clement, this 

 Journal (4), xxii, 391, 1906. 



