to Calcium and Magnesium Metasilicatex. 45 



that instead of a single crystal or a regularly twinned crystal, 

 the aggregate of irregularly oriented individuals would result. 

 In case, however, the molecular redistribution was slight, the 

 inversion might proceed in regular fashion throughout the 

 entire crystal and the effect of inversion and reversion be 

 chiefly one of shifting of the ever present twinning lamellae. 

 And this is the exact state of change in the /3-.MgSi0 3 crystals. 

 Rarely the subdivision of a crystal into several irregularly 

 bounded parts was noted and usually only the shifting of the 

 lamellae. It was also of interest to observe that occasionally 

 a shifting of the twinning lamellae took place in crystals heated 

 to temperatures slightly below the inversion point. Because 

 of this property, no decisive determinations of the inversion 

 point could be made and recourse was taken to sudden chilling 

 experiments — the preparations being first melted, the tempera- 

 ture then lowered and kept at a specified point for one hour, 

 after which the preparation was dropped into cold water and 

 chilled almost instantly. The a-MgSi0 3 thus obtained clearly 

 showed the effect of incipient change even under these condi- 

 tions, the major part of the powder being full of minute 

 dustlike particles or cavities and as a result was semi- or sub- 

 transparent, and only now and then were clear portions of the 

 a-form observed. Whenever the a-MgSi0 3 was held at tem- 

 peratures slightly below the inversion point and then quenched, 

 the entire preparation consisted essentially of the twinned 

 /3-form alone in clear transparent individuals, the dusty effect 

 as well as the a-MgSi0 3 aggregates having practically disap- 

 peared except for an occasional clear crystal of the same. 



Summary. 



1. The end members of the system CaSi0 3 -MgSi0 3 both 

 exhibit enantiotropy. The inversion point in the former is 

 about 1190°. The a-form, pseudo-woUastonite, is unknown 

 in nature. The /3-form is the mineral wollastonite. The 

 /3-form of magnesium silicate is the magnesian pyroxene occur- 

 ring in meteorites and in intergrowths with enstatite and has 

 recently been called clino-enstatite.* At about 1365° it is 

 transformed into an orthorhombic form quite distinct from 

 enstatite and unknown in nature. 



2. Only one stable compound appears, viz., CaSiG 3 .MgSi0 3 , 

 identical with diopside. It melts at 1380° and has a specific 

 gravity of 3*275. It was obtained in well-formed, measurable 

 crystals extremely pure, when crystallized from molten calcium 

 chloride. 



3. A eutectic occurs between diopside and pseudo-wollas- 

 tonite at the composition 60 per cent diopside : 40 per cent 

 calcium silicate. It melts at 1348°. A second eutectic occurs 

 at about 63 per cent ]\IgSi0 3 : 32 per cent CaSi0 3 . It is com- 



* W. Wall], Die Eustatit-augite, Tscherinak's Mitth., xxvi, 1-131, 1907. 



