Ferguson and Merwin — The Ternary System. S3 



balite and tridymite 13 takes place in this system below 

 1500°C. ; for the pure substances Fenner's value is 

 1470 ± 10° C. 



The System lime-magnesia, CaO-MgO. 



In this system a simple eutectic relation exists. The 

 eutectic composition is CaO 67, MgO 33, and its tempera- 

 ture about 2300 °C. The diagram given by Rankin and 

 Merwin 14 is partially reproduced in fig. 1. 



The System lime-silica, CaO-Si0 2 . 



This system is somewhat more complicated. Four 

 compounds are known to exist. They are the calcium 

 metasilicate, the tricalcium disilicate, the calcium ortho- 

 silicate and the tricalcium silicate. The meta- and ortho- 

 silicates only are stable at their melting points and the 

 tricalcium silicate does not even occur as a primary 

 phase in this binary system. Two forms of the metasili- 

 cate, wollastonite and pseudowollastonite are known and 

 also three forms of the orthosilicate. The temperature 

 relations existing in this system are given in fig. 2 which 

 is a corrected reproduction of the major part of the dia- 

 gram given by Rankin and Wright. 15 The corrections 

 deal with the melting point of cristobalite and the extent 

 of the metasilicate solid solution. 



The following are the optical properties observed for 

 the phases in this binary system which occur in the ter- 

 nary system. 



Pseudowollastonite, 18 aCaO.Si0 2 ; pseudohexagonal 

 equant grains, poly synthetic twinning common; nearly 

 uniaxial +, a = 1610, /? = 1-611, y = 1-654; extinction 

 angles small. 



Wollastonite, 16 ' 17 /3CaO.Si0 2 ; monoclinic, lath-shaped; 

 cleavage parallel to elongation; <* = 1-616, /? = 1-629, 

 y = 1-631, 2E about 70° ; extinction parallel, optic plane 

 normal to cleavage lines. 



3Ca0.2SiO<>; probablv orthorhombic, equant grains; 

 a = 1-641, y = 1-650 ; + 2V large. 



a calcium orthosilicate, 2CaO.Si0 2 , is stable from its 



13 This inversion is discussed on page 118 of this paper. 



14 G. A. Eankin and EL E. Merwin, J. Am. Chem Soc, 38, 568, 1916. 



15 This Journal, 39, 1, 1915. 



16 For later observations, see pp. 91, 92. 



17 Day, Shepherd, Wright, this Journal 22, 290-291, 1906. 



