536 sosman and merwin: lime: ferric oxide 



The compound forms from a finely powdered mixture of CaC0 3 

 and Fe 2 3 at a temperature well below the melting point, just 

 as the 2 : 1 compound does. It crystallizes well, giving black 

 crystals. The crystals are frequently in the form of long needles, 4 

 but no evidence of prismatic habit is visible in the powdered 

 preparation under the microscope. 



The optical properties of the compound CaO.Fe 2 3 are as 

 follows: Color: deep red, about like hematite; optical character: 

 nearly or quite uniaxial, negative; indexes of refraction: co Li = 

 2.465 ± 0.005, e Li = 2.345 =±= 0.005. For sodium light co is about 

 2.58 and c about 2.43. These values were determined by observ- 

 ing that throughout the red and orange the dispersion of the 

 compound was not noticeably different from that of the immer- 

 sion medium, e is absorbed slightly more than co in the orange 

 and at shorter wave-lengths, but not noticeably more in the red. 

 Grains 0.01 mm. in thickness show scarcely any absorption at 

 wave lengths longer than 610mm; at wave lengths shorter than 

 580mm they are very dark. 



There is no optical evidence of solid solution either of 

 2CaO . Fe 2 3 or of Fe 2 3 in the compound. 



There is a eutectic at 1203° between CaO.Fe 2 3 on the one 

 hand, and ferric oxide (hematite) on the other. The mixture 

 containing 10 molecular per cent CaO shows clearly an excess 

 of hematite. The mixture with 25 molecular per cent CaO con- 

 sists almost entirely of a deep red, apparently homogeneous, 

 material, which is probably an intimately intergrown eutectic of 

 CaO.Fe 2 3 and Fe 2 3 ; the mixture contains also a little excess 

 of red CaO . Fe 2 3 . The eutectic composition is therefore between 

 10 and 25 per cent CaO, and probably near the latter. 



The melting point of pure Fe 2 O s is unknown, as it dissociates 

 under atmospheric pressure of oxygen before the melting point 

 is reached into oxygen and a solid solution of Fe 3 4 in Fe 2 3 . 5 

 Its optical properties have been published elsewhere. 6 



4 See photographs by Hofman and Mostowitsch, Bull. Am. Inst. Min. Eng. 

 39: 628-653. 1909. 



5 Sosman and Hostetter. Jour. Am. Chem. Soc, 38: 807-833. 1916. 

 'Merwin. Jour. Am. Chem. Soc, 38: 830. 1916. 



