320 /Shepherd, Rankin, Wright — Binary Systems of 



some of the sections a slight opening of the interference cross 

 was observed as though the substance were biaxial with small 

 2E, but so many of the sections were perfectly uniaxial that there 

 is little doubt of the uniaxial character of the substance. This 

 compound is readily distinguished from the CaO.Al 2 3 com- 

 pound by its interference figure, optical character and constant 

 refractive index, o?=l'617. On practically every basal section 

 thin threadlike inclusions of a higher refracting, weakly bire- 

 fracting to isotropic substance, were observed and although small 

 in actual quantity they are nevertheless present and may be free 

 A1 2 3 or the unstable 3Ca0.5Al 2 3 compound. They are too 

 fine for satisfactory identification by optical methods. 



In several of the preparations the crystallographic habit 

 of the compound was entirely different from the small gran- 

 ular type. The individuals were elongated, lath-shaped and 

 intricately intergrown and resembled in aspect 7Ca 2 Si0 4 after 

 inversion from the /3-form."* The optic properties, refractive 

 indices, birefringence, uniaxial optical character, proved to 

 be identical with the normal 3Ca0.5Al 2 3 compound and the 

 peculiar appearance is due in fact to inversion from a high 

 unstable form, just as in the case of calcium orthosilicate. 



The unstable 30aO.SAl 2 3 compound: CaO ^^"]8,Alfi % 

 75 ,( 2 ( 2. — This compound was obtained only after consider- 

 able experimentation with preparations in the iridium furnace. 

 Its presence was surmised first from the difference in crystal- 

 lographic habit of different preparations of the optically 

 positive form of this composition. In no case was it obtained 

 in pure state but invariably showed more or less alteration to 

 the optically positive form, and could not therefore be used 

 for density determinations. 



No crystals of this phase were obtained and the determina- 

 tion of its crystal system rests entirely on the optical evidence. 

 The crystalline melts are colorless and often porcelain-like in 

 appearance. Here and there minute cleavage faces of lath- 

 shaped individuals glisten in strong light. On the whole, the 

 melts were well crystallized, some of the grains under the 

 microscope measuring as much as -5 mm in length. The crystals 

 are usually prismatic in habit and show under the microscope 

 fairly well-marked prismatic cleavage. Their luster is vitreous 

 and hardness about 5*5 to 6. The refractive indices were 

 measured by the immersion method <y — 1*674 rfc *002, fi= 

 1-671 ± -002; a = 1*662 ± *002. A direct determination of 

 .the birefringence was made and 7 — a found to be approximately 

 •013. The interference colors in ordinary powder sections 

 rarely exceed the second order blue and are usually gray to 

 * This Journal (4), xxii, 296, 1906. 



