92 Ferguson and Merwin — The Ternary System. 



solid state, and the characteristic poly synthetic twinning 

 with extinctions of 3° on either side is present. Solid 

 solution represented by about 3 per cent magnesia does 

 not appreciably change its optical properties, except to 

 lower y about 006. 



Wollastonite : Crystals containing the maximum 

 amount of diopside at the high temperatures (about 17 

 per cent) in solid solution had the following observed 

 optical properties : a = 1-619, ft = 1-631, y = 1-634, 

 — 2V = 40°-65°. Mixcrystals of wollastonite and 5CaO. 

 2Mg0.6Si0 2 , and also 2CaO.Mg0.2Si0 2 , have optical 

 properties which are intermediate, so far as they have 

 been determined. Crystals in the middle of the latter 

 series are optically positive with large axial angle. 



5CaO.2MgO.68iO 2 : Irregular, elongated grains, (3 par- 

 allel to elongation; a = 1-621, p = 1-627, y = 1-635, + 

 2V = about 80°. 



2CaO.MgO.28iO 2*. Appears in stubby prisms occasion- 

 ally have definite octagonal cross-section. Crystals 

 from melts of various compositions have o> = 1-631 ± 

 •002, e = 1-638 ± -002. Frequently the crystals did not 

 appear unless the melt was considerably undercooled. 

 The relation of this compound to akermanite is discussed 

 later. 



Monticellite: The crystals of monticellite appeared as 

 equant grains without facets, and only optical and chem- 

 ical relationships have been established between the crys- 

 tals and the natural mineral. The observed values of 

 a were 1-638 to 1-640, of p 1-646, and of y 1-651 to 1-655 ; 

 + 2V = 85° to 90°. These were obtained from five 

 quenches at temperatures between 1400° and 1500°, hav- 

 ing compositions ranging from 33 to 37 CaO, 21 to 28 

 MgO, and 39 to 42 Si0 2 . 



Diopside: The refractive indices of pure diopside were 

 observed as follows : a = 1-666, y = 1-695. 



Diopside-clinoenstatite solid solutions have not been 

 studied further, but in the application of Mallard's 

 formula to their extinction-angles volume per cent not 

 mol. per cent should have been used. 35 



Limits of the Fields of Stability. 



Any charge with a composition in a ternary system 

 similar to the one under investigation may be heated 



33 This Journal (4), 38, 248, 1914. 



