THE MINERALOGRAPHY OF THE FELDSPARS 285 
(eb, easy, Tnxrés, melting). The line PC,E£,F,L is the other curve 
of the diagram, the solidus. 
Now having described the manner in which an equilibrium 
diagram is constructed let us see what it can tell us of the process 
of solidification of an isomorphous series. Figure 16 is a diagram 
of such a system (Type I of Roozeboom’s classification), the 
time-temperature construction curves not being shown. The 
temperatures form the ordinate and the composition the abscissa. 
Above the liquidus line TyD,C,B,ATy the solution of the two 
components as a liquid is a mutual one, that is they mix in all 
proportions. Between the liquidus and the solidus—the lens- 
shaped area—is the area in which solidification is going on and is 
occupied by both liquid and solid phases. Below the solidus the 
system is solid. Now let us trace in detail what happens during 
the freezing of a melt composed of 60 per cent of WV and 40 per cent 
of M as is indicated by the vertical line XY. Above A the system 
is liquid, but as the temperature, falling, reaches the liquidus line, 
crystallization commences, precipitating crystals of the composi- 
tion A’. The composition of such a crystal is obtained by con- 
structing the horizontal dash line from A to the solidus. Thus 
the first crystal formed has a composition of A’. The remaining 
liquid has a composition of A. With continued lowering of the 
temperature the point represented by B is reached. The composi- 
tion of the crystal is B’ and that of the liquid is B,. 
Temperature Composition of Crystal Composition of Liquid 
IPs AW A 
T, IBY JB 
iP Cc’ CG 
ay, D All liquid frozen 
It will readily be seen that in the phenomenon above, where 
we are assuming, for the time being, that no adjustment of the 
crystals between themselves or between the crystals and the liquid 
takes place, the resulting crystals will have a wide range in 
composition, or a single crystal will be built up concentrically 
of zones of variable composition. It follows from the examina- 
tion of the diagram that the center of such crystals will have a 
