4 Rankin and Wright— Ternary System CaO-Al 2 0-SiO, 



Inversion 



a quartz — P quartz 

 P quartz— /3 2 tridymite 

 /?a tridymite — P cristobalite 

 a tridymite — pi tridymite 

 8i tridymite— y3 2 tridymite 

 a cristobalite — P cristobalite 



Temperature 



575° 

 870° 

 1470° 

 117° 

 163' 

 220' 



10° 

 10 c 



-275 c 



Eemarks 



Eapid, reversible. 



Very sluggish, reversible. 



Very sluggish, reversible. 



Rapid, reversible. 



Rapid, reversible. 



Rapid, reversible. Tempera- 

 ture depends on previous 

 heat treatment. Probably 

 monotropic. Changes slow- 

 ly to chalcedony, tridymite 

 or quartz in the presence of 

 a flux at relatively low tem- 

 peratures. 



The melting point of cristobalite is about 1625°, that of 

 inetastable quartz 1470°. Kecent investigations by K L. 

 Bowen* of this laboratory have shown that the melting temper- 

 ature of cristobalite under certain conditions is considerably 

 above 1625° and that probably the melting temperature, like 

 the inversion temperature, is variable, the cristobalite behav- 

 ing in these respects as a solid solution of two components. 



The forms which exist at low temperature are : a quartz, 

 a tridymite, a cristobalite and chalcedony. 



a quartz crystals obtained in sodium tungstate melts^ rarely 

 exceed 0*l mm in length ; they are doubly terminated with the 

 unit prism and rhombohedrons as bounding faces. The crys- 

 tals are hexagonal and resemble in every respect natural quartz 

 crystals. Kefractive indices for sodium light : o> Na = 1*544 ; 

 e Na = 1*553; birefringence about 0*01. Uniaxial, optically 



positive. 



a "Tridymite crystals occur in cryptocrystalline aggregates ot 

 hexagonal habit. 'Refractive indices approximately a Na = 1*469, 

 ry Na = 1-473. Birefringence extremely weak. Optic axial 

 angle, large. The a form is probably orthorhombic, while the 

 P forms, stable above 117°, are hexagonal. 



a cristobalite appears also in cryptocrystalline aggregates. 

 Refractive indices: a Na = 1,484, 7Na - 1*487 ; birefringence 

 weak. Crystal system probably tetragonal or orthorhombic. 

 The /3-form crystallizes in the isometric system. 



Chalcedony was not produced artificially ; the optical char- 

 acters of the natural mineral are given in the standard text 

 books on microscopical petrography. 



THE LIME-SILICA SYSTEM. 



Lime and silica when heated together can unite to form four 

 distinct compounds: (1) the metasilicate CaO.SiO, ; (2) the 

 compound 3Ca0.2Si0 2 ; (3) the orthosilicate 2CaO.Si0 2 ; (4) 



* This Journal (4), xxxviii, 218, 1914. 



