Rankin and Wright — Ternary Sy 'stem CaO-AlJJ \-SiO \. 27 



colors are confined usually to the first order, a pronounced 

 yellow hue being noticeably common. Uniaxial, optically 

 negative. 



The compound 3CaO.Al,0,.SiO, (CaO 50*9, A1 2 0, 30'9, 

 Si0 2 18*2) is unstable at its melting point and in the ternary 

 system there is no field. Its position in the ternary system is 

 shown in fig. 6. It dissociates at 1335° ± 5° into 2CaO.Si0 2 

 and CaO.Al 2 3 , and hence is best formed by crystallization 

 from glass of its own composition at a temperature below 

 1335°. Even then the preparation appears under the micro- 

 scope dusty and so unsatisfactory that it is not possible to state 

 definitely that it is homogeneous. The crystalline mass is 

 usually crypto- to microcrystalline and consists of overlapping 

 fibers and grains. The larger grains show under high powers 

 the ribbed structure common to crystal skeletons. Crystal 

 system possibly orthorhombic. Refractive indices, a Na =1*675 

 ± 0*005, 7 Na = 1*685 db 0*005. Birefringence medium, <y-a 

 about 0*01. Optic axial angle medium to fairly large. 

 Optical character positive. Plane of optic axes is apparently 

 parallel with positive elongation of fibers. 



The Stability Fields. 



In the complete CaO-Al 2 3 -Si0 2 diagram there proved 

 to be 14 separate fields of stability ; that is there are 14 

 separate regions — one for each separate chemical compound 

 (including the original components) which occurs in the system 

 — within which a single particular compound is in equilibrium 

 with liquid and vapor. Each field may be regarded as the 

 solubility region of a single compound in solutions of two other 

 definite compounds or the melting region of a single compound 

 in solutions of three definite compounds. 



The melting temperatures within each field are obtained 

 either from heating curves, which serve to record the tempera- 

 ture at which the energy change accompanying melting occurs 

 (a melting temperature within a field is the last absorption of 

 heat recorded on a heating curve) or by quenching to deter- 

 mine the temperature above which a substance is obtained as 

 a glass (liquid) and below which a single crystalline phase is 

 obtained imbedded in glass, as determined by optical examina- 

 tion with the microscope. Both methods are used, but if the 

 energy change is not evident, owing to sluggishness of the 

 change or to confusion with other energy changes, then the 

 quenching method is generally the more reliable. If, however, 

 the quenching method can not be applied, because of difficulty 

 of distinguishing the solid phases, then the heating curve 

 method is preferable. For some points both methods could be 



