Ferguson and Merwin — The Ternary System. 85 



melting point to 1420 °. 1S The grains tend toward pris- 

 matic habit, are characteristically twinned, have + 2V 

 large, a = 1-715, p = 1-720, and y = 1-737. 19 ' 20 



P calcium orthosilicate is stable from 1420° to about 

 675 °. 18 ' 23 The habit and optical properties are practi- 

 cally the same as for the a-form except that twinning is 

 seldom present (a = 1717, y = 1735 ). 19 ' 21 



In the binary system CaO-Si0 2 , the inversion a to p 

 orthosilicate takes place promptly if the inversion tem- 

 perature is passed through not very rapidly. 22 



y calcium orthosilicate is stable below 675 °. 23 It has 

 prismatic habit, a = 1642, p = 1-645, y = 1-654, + 2V = 

 about 60°. 24 



The System magnesia-silica, MgO-Si0 2 . 



Two binary compounds occur in this system, the meta- 

 silicate and the orthosilicate. The former is unstable at 

 its melting point, while the latter is stable. The temper- 



Fig. 4. 



1600 

 1550 

 1500 

 1450 

 MOO 

 1350 

 1300 



Cristobalitz * 7{<dt 



TridtJ/ruJte, -rffeLt 



CaO J%pO-Z5,iO z -*TrudyrrwU 



SiO z CaO '/fyO-Z5AO e 



Fig. 4.— The binary system Si0 2 -CaO.Mg0.2Si0 2 . wt. per cent. 



ls Eankin and Wright, this Journal, 39, 76, 1915. 



19 Ibid., p. 7. 



20 Observations during the present study on crystals from various parts of 

 the a orthosilicate field, confirm these data except that p is about 0003 

 larger, and in some quenches scarcely a twinned grain could be found (see 

 note 22), and the elongation may be either positive or negative. 



21 No accurate measurements of refractive index of this form were made 

 during the present study. 



22 In most if not all of the quenches of the ternary system here considered 

 which contained the a form as a primary phase, the a form has apparently 

 persisted for many months. This is true unless either the criterion of 

 twinning is not sufficient for distinguishing this form; or the inversion is 

 like that of quartz at 575° and leaves no distinguishable optical effects. 

 (See note 20.) 



23 Day, Shepherd and Wright, this Journal, 22, 281, 1906. 

 "Corrected values (see note 19). 



