18 
CHEMISTRY: FERGUSON AND MERWIN 
The binary compounds which do not have fields in the ternary system are : 
(1) Tricalcium sihcate, 3CaO.Si02, (2) Chno-enstatite, MgO.Si02. 
The ternary compounds which have fields in the ternary system are: (1) 
Diopside, CaO.Mg0.2Si02, (2) 2CaO.Mg0.2Si02. 
The additional phases which have fields in the ternary system are not of 
constant composition but are ternary in character. They are: 
1. The wollastonite (jSCaO.Si02) solid solutions. These consist of either 
two series of solid solutions or an area of solid solution. The one series may 
contain up to 18 mols. per cent of diopside, CaO.Mg0.2Si02 and the other 
up to 44 mols. per cent of the compound 2CaO.Mg0.2Si02. If an area of 
solid solution exists, it will probably extend from the one series to the other 
forming a triangular area with the apex at the CaO . Si02 composition. The 
wollastonite-pseudowoUastonite inversion, i8CaO.Si02^o:CaO.Si02 normally 
occurs at 1200°C. but this inversion temperature is raised by the addi- 
tion of the dissolved substances reaching a maximum of 1343°C. with the 
diopside series and of 1365°C. with the 2CaO.Mg0.2Si02 series. Only those 
solid solutions which invert at the higher temperatures occur as primary 
phases since the liquidus at no point falls below 1320°C. 
2. The pyroxene solid solutions which form a continuous series with end 
members, diopside, CaO.Mg0.2Si02 and clino-enstatite, MgO.Si02, all 
occur as primary phases. 
3. The monticellite (CaO.MgO.SiO2) solid solutions which may contain 
up to 11% of forsterite, 2MgO.Si02, are partly represented as primary 
phases. The pure compound monticellite probably does not occur, as a 
phase, stable in contact with a liquid. 
Of the phases which have just been enumerated the jSCaO.Si02 — 2Ca0.- 
Mg0.2Si02 solid solutions, and the compound 2CaO.Mg0.2Si02 have not 
been previously noted. This latter compound has practically identical 
optical properties with the mineral akermanite for which the formula 
8Ca0.4Mg0.9Si02 has been proposed, by Schaller.^ 
The summarized temperature and concentration relations are shown in 
figure 1. The compositions are herein represented on an equilateral triangle 
in mol-per cent, and the diagram includes (1) the location of the fields, with 
their boundary curves, (2) the location of the invariant points, (3) the temper- 
atures which correspond to the fixed points, and (4) the isotherms which 
indicate the slopes which the various fields have on a solid model upon which 
the temperatures of complete melting are represented as vertical distances 
above a triangular concentration diagram similar to the one shown. 
1 Rankin, G. A., and Wright, F. E., Amer. J. Sci., New Haven, 39, 1915, (1). Rankin, G. 
A., and Merwin, H. E., /. Amer. Chem. Soc, Easton, Pa., 38, 1916, (568). Rankin, G. A., 
and Merwin, H. E., Amer. J. Sci., 45, 1918, (301). Bowen, N. L., Ibid., 38, 1914, (207). 
Allen, E. T., White, W. P., Wright, F. E., Larsen, E. S., Ibid., 27, 1909, (1). 
2 Schaller, W. T., Bull. U. S. Geol. Survey, Washington, Nq. 610, 1916, 
