194 HAROLD L. ALLING 
Three-Component Systems 
Ternary Diagrams 
The Potash-Soda-Lime Feldspars 
Physical Properties of the Potash-Soda-Lime Feldspars 
Extinction Angles 
Specific Gravities 
Classification — 
Examination of Chemical Analyses of Feldspars - 
Microscopic Examination of Natural Feldspars 
Examples of Plagioclase Feldspars 
Examples of Potash-Soda Feldspars 
Applications of the Mineralography of the Feldspars to Geological Problems 
Case One—Location of a Fault 
Case Two—Ortho-amphibolites versus Paramphibolites 
Appendix 
The Solubilities of the Feldspar Components 
Conclusions 
INTRODUCTION 
The rock-forming minerals can be studied (1) as solids capable 
of assuming definite geometrical forms—crystallography; (2) as 
optical media that affect light in characteristic ways—determina- 
tive petrography; (3) as chemical substances—the chemistry of 
silicon compounds; and (4) as the end products of crystallization 
of melts—geophysical chemistry, or as here proposed—mineralog- 
raphy. 
This last classification embodies the point of view which has 
been adopted in this paper for the study of the feldspars, treating 
them as chemical substances formed by the solidification of melts. 
When the important mineral groups are fully examined, and their 
various thermo-equilibrium diagrams published, then the interpre- 
tative petrographer will be able to unravel the life-histories of rocks 
with much greater accuracy and in finer detail than is possible at 
present. 
Appreciating the value of metallographic methods as applied 
to the elucidation of the feldspar system, it is therefore proposed 
that these silicates be examined with the aid of the phase rule. 
The phase rule has shed a flood of light upon the nature and con- 
stitution of alloys, and promises to be of equal value in the study 
of magmas and their crystallization into igneous rocks. 
