THE MINERALOGRAPHY OF THE FELDSPARS 203 
feldspar is inferred from the average of the chemical analyses and 
the lime component is the remainder of the 100 per cent. 
23. Many zoned plagioclase feldspars are to be explained by 
the process of normal crystallization under rapid chill instead of 
““magmatic corrosion.”” Homogeneous crystals of plagioclase are 
probably due to readjustment between crystal phases, or between 
them and the surrounding unfrozen liquid, in a slowly cooling 
magma. ‘The degree of homogeneity is therefore a function of the 
rate of chill. Some zoned feldspars are undoubtedly the result of 
more complex processes in which the phenomenon of undercooling 
plays an important rdle. ° 
24. The physical properties of a series of solid solutions are 
direct functions of the composition. If the properties, such as 
specific gravity, indices of refraction, extinction angles, etc., are 
plotted in conjunction with the thermo-equilibrium diagram they 
assume the form of smooth curves which rise and fall with the 
freezing (liquidus) curve. A break, a cusp, or a sharp change in 
direction in these curves at least suggests a discontinuity in the 
chemical properties of the system. Many involved formulas of 
minerals will probably be abandoned when they are shown to be 
solid solutions and mixtures of solid solutions of simple end mem-— 
bers. The formula for hyalophane, “(K,,Ba)Al,(SiO,),,” cannot 
be entertained as possessing any true value. 
| 25. In attempting to classify the feldspars on a basis of their 
true composition several new names have been proposed. See 
Figure 13 for schemes of classification. ‘‘ Anorthoclase”’ is defined 
in paragraph 13 and its range delimited as Or,Ab,o-Or,;Abes. 
The term “soda microcline” is confined to microclinic feldspars 
containing 10 to 30 per cent of the soda component. For albitic 
feldspars containing from 5 to 20 per cent of the potash component 
the term ‘‘potash albite”’ is proposed. 
26. Because there are more contours of equal extinctions 
(isogonic lines) cutting the plagioclase side of the triangle (see 
Fig. 12, facing p. 251) than the number of those cutting the 
potash-soda side, the identification of the soda-lime feldspars can 
be accomplished with comparative ease. Nevertheless proper 
identification of the subspecies of the potash-soda series can be 
