SOSMAN AND MERWIN: LIME : FERRIC OXIDE 535 



or lower. The crystallized product then shows traces of free 

 CaO and CaO . Fe 2 3 , which are due to dissociation, and which 

 have not combined during cooling. The compound forms from 

 a finely powdered mixture of CaC0 3 and Fe 2 3 at a temperature 

 considerably below its liquefying point; under our conditions a 

 considerable quantity of the compound formed during 30 min- 

 utes heating at 1000°, although none was visible in a mixture 

 heated rapidly to 950°, at which temperature most of the carbon 

 dioxide had been expelled. 



The optical properties of the compound 2CaO . Fe 2 3 are : biax- 

 ial, positive, with a moderate optic axial angle. a Li = 2.200 

 ± 0.005, |8 Li = 2.220 ± 0.005, y Li = 2.290 ±0.005. The opti- 

 cal dispersion of a is the same as of the immersion medium, 

 therefore a Na is about 2.25; /3 and y have lower dispersion, a is 

 absorbed considerably through the red as well as for shorter 

 wave-lengths. /3 and y are absorbed little at wave-lengths longer 

 than 620mm, and are transmitted considerably at 550mm- 



There is no optical evidence of solid solution either of CaO or 

 of CaO.Fe 2 3 in the compound. 



The transition temperature at which the compound CaO . Fe 2 3 

 dissociates, and is in equilibrium with liquid and with 2CaO . Fe 2 3 

 is 1216°. All mixtures between about 48 and 65 molecular per 

 cent of CaO (about 24.5 to 38 weight per cent) liquefy in part 

 at this temperature, forming liquid and leaving excess of 

 2CaO . Fe 2 3 as the crystalline phase, which dissolves with rising 

 temperature and disappears at the liquidus curve. 



The compound CaO.Fe 2 3 , after dissociating and nearly all 

 liquefying at 1216°, becomes completely liquid at about 1250° 

 by the disappearance of the solid 2CaO.Fe 2 3 formed by dis- 

 sociation. The liquid crystallizes with less undercooling than 

 does the compound 2CaO.Fe 2 3 , and the temperature of the 

 thermoelement sometimes rises to within a few degrees of 1216°. 

 The crystallized product contains an appreciable amount of 

 2CaO.Fe 2 3 , and dark dusty ferric oxide. When powdered and 

 reheated at 1190°, these dissociation products recombine com- 

 pletely, forming a homogeneous product of red CaO.Fe 2 3 ; the 

 powder also sinters together. 



