10 Rankin and Wright — Ternary System CaO-AL % % -SiO ¥ 



sections of the fibers are square in outline, weakly birefracting, 

 and extinguish parallel with the diagonals of the rectangles. 

 In the center of nearly every section a minute inclusion is pres- 

 ent, often in the shape of a cross, the arms of which are paral- 

 lel with the sides. Eefractive indices : a Na = 1*638 ±0*003, 

 0^= 1-64:2 ±0-003, 7*a= 1*653 ± 0*003,— which are notice- 

 ably lower than those of the natural mineral ; birefringence 

 medium, y-a about 0*014. The optic axial angle is not large ; 

 optical character positive ; the acute bisectrix lies in the direc- 

 tion of the elongation of the fibers, along which the cleavage is 

 distinct. Minute inclusions of a lower refracting isotropic 

 substance (n about 1*530) are almost invariably present in the 

 artificial sillimanite preparations and are apparently glass. 

 No satisfactory explanation has been found for their occur- 

 rence unless sillimanite is unstable at its melting point and 

 dissociates into A1 2 3 and glass. This might also explain the 

 fine cross-shaped inclusions noted above. The temperature of 

 possible dissociation is, however, so high (above 1800°) that 

 no satisfactory methods are available to test this hypothesis. 



THE BINARY SYSTEM : LIME- ALUMINA. 



In this system four distinct compounds occur, namely (1) 

 tricalcic aluminate (3CaO.Al 2 3 ) ; (2) the compound 5CaO. 

 3A1 2 3 ; (3) calcium aluminate (CaO. A1 2 3 ) and (4) the com- 

 pound 3Ca0.5Al 2 3 '. Several of these compounds are dimor- 

 phous. The concentration-temperature diagram is given in 

 fig. 3. 



(1) Tricalcic-aluminate, 3CaO.Alfi % (CaO 62*2, A1 2 3 

 37*8) is unstable at its melting point, but occurs as primary 

 phase on the liquidus curve of mixtures of composition rang- 

 ing from CaO 50 to 59, A1 2 3 50 to 41. It dissociates^ at 

 1535°±5° into CaO and liquid ; it is, for this reason, best 

 obtained by crystallization from glass of its own composition 

 at a temperature below 1535°. It forms with the compound 

 5Ca0.3Al 2 3 a eutectic mixture of the composition CaO 50, 

 A1 S 3 50, which melts at 1395 & ±5°. It does not form a 

 eutectic with CaO ; but the composition CaO 59, A1 2 3 41 is 

 the quadruple (invariant) point at which these two compounds 

 are stable in contact with liquid and vapor, the equilibrium 

 temperature being 1535°. 



This compound occurs in equant colorless grains 0*l mm and 

 less in diameter, often hexagonal or rectangular in outline with 

 indications of imperfect cleavage after the octahedron or 

 rhombicdodecahedron. Crystal system, isometric. Refractive 

 index, w Na — 1*710 ± 0*00i. Hardness, 6. Fracture, con- 

 choidal. Luster, vitreous. Occasionally faint gray interfer- 

 ence colors were observed and were evidently due to strain. 



