220 HAROLD L. ALLING 
it is of the general character shown in Figure 4 with the two com- 
ponents so regarded for the present only partially soluble in each 
other in the solid state, that it is a eutectiferous series. ‘The 
first attempts to represent the diagram of the potash-soda feldspars 
were based upon the assumption that each component maintained 
uniform physical and chemical properties throughout heating and 
cooling. This assumption of course eliminated the possibility of 
their possessing dimorphous modifications. Consequently Vogt’s 
diagram is extremely simple, and some of the lines are only approxi- 
mate. Warren however suggested that the solubility lines should 
be drawn somewhat inclined instead of vertical as shown by Vogt. 
Harker illustrates by a diagram a eutectiferous system of which 
one component is dimorphous but did not apply it directly to the 
potash-soda feldspars. Warren stated the probability of the 
dimorphism of the potash component, orthoclase to microcline, 
and constructed his diagram with this in mind. Marc* has a 
different conception, which, although it possesses considerable 
merit, cannot be discussed in detail here. For the present we can 
derive considerable light upon the nature of the potash-soda 
feldspars by considering the diagram as given by Warren (Fig. 4). 
In spite of the fact that the locations of the various points and 
the slopes of the curves of Warren’s diagram are only approximate 
at the best it is believed that they are sufficiently accurate for our 
purpose. Warren has emphasized the truth that the two crystal- 
line phases that make up perthitic intergrowths are solid solutions 
and not pure components. ‘That is if we define orthoclase as pure 
KAISi,Os and albite as pure NaAlSi,O; then the usual definition of 
perthite is not entirely satisfactory. Before the application of 
the phase rule to silicate systems the theory that the imbedded 
spindles had been introduced from without, subsequent to the 
solidification of the feldspar, could be regarded as plausible.* But 
today such an idea is abandoned by most workers in the field. In 
Figure 4 the composition of the original feldspar melt with an 
assumed value of 60 per cent K-feldspar and 4o per cent Na- 
feldspar is represented by the vertical line NOC. Now following 
t Robert Marc, Chemische Gleichgewichtslehre (1911), p. 102. 
20. Wenglein, Ing. Diss. Kiel, 1903. 
