12 Rankin and Wright — Ternary System CaO-Alfi-SiO^ 



The refractive index of glass of the same composition is 

 approximately 1*662. 



II. The unstable form has neither a definite melting point 

 nor any temperature range of real stability. It is obtained 

 only under special conditions of cooling and is usually not well 

 developed in single crystallites, but occurs as radial spherulites 

 or aggregates of overlapping, and often approximately parallel, 

 fibers. Crystal habit, fibrous and prismatic ; luster, vitreous ; 

 color usually pale green ; hardness about 5 ; crystal system, 

 probably orthorhombic. Refractive indices : a Na = 1*687 db 

 0002, 7 Na = 1*692 db 0*002 ; birefringence weak. Optic axial 

 angle apparently large ; plane of optic axes and extinction are 

 parallel with the fiber direction • optical character apparently 

 negative but difficult to determine because of overlapping 

 fibers. The more deeply colored grains are pleochroic ; 

 a = blue-green, 7 = olive-green ; absorption a < 7. 



(3) Calcium aluminate, OaO.Al 2 3 (CaO 35*4, Al 2 3 64*6) 

 melts at 1600° ± 5°. It forms with the compound 5Ca0.3Al 2 3 

 the eutectic mixture noted above ; and with the compound 

 3Ca0.5Al Q 3 a eutectic mixture of the composition CaO 33*5, 

 A1 2 3 66*5, which melts at 1590° ± 5°. 



It crystallizes readily in irregular grains and often in pris- 

 matic to fibrous aggregates. Twinning is a characteristic 

 feature and is especially prominent on sections nearly normal 

 to the acute bisectrix. Such sections are intricately divided 

 into a hexagonal mesh work of interlocking sextants which 

 extinguish in different positions. Cleavage parallel with the 

 fiber direction is occasionally indicated. Hardness about 6*5 ; 

 luster vitreous. Crystal system, monoclinic or triclinic, prob- 

 ably monoclinic. Refractive indices: a Na = 1*643 dr 0*002, 

 £ Na = 1-655 d- 0*002 ; 7 Na = 1*663 d= 0'003 ; birefringence fairly 

 strong, about 0*02. Optic axial angle 2Y = 36° ± 4°. Optical 

 character, negative. 



(4) The compound 3Ca0.5Al,0 2 (CaO 24*8, Al.O, 75*2) 

 crystallizes in two forms, (I) stable, (II) unstable and apparently 

 monotropic with respect to (I). 



(I) This form melts at 1720° db 10°. It forms with calcium 

 aluminate the eutectic mixture noted above and with alumina 

 a eutectic of weight percentage composition CaO 24, A1 3 3 76, 

 which melts at 1700° ± 10°. 



It crystallizes readily in rounded colorless grains, often with 

 indications of rectangular outline. Cleavage poor if present. 

 Hardness about 6*5 ; luster vitreous. Crystal system, 

 possibly tetragonal. Refractive indices, co N& = 1*617 db 0*002 

 € Na = 1-652 d= 0*002. Birefringence strong, approximately 

 0*035 ; uniaxial, optically positive. Indications of biaxial char- 

 acter with small 2E were noted occasionally. The interference 



