THE MINERALOGRA PHY OF THE FELDSPARS 213 
The method of approach here is the application of the phase 
rule to the feldspar system. The original presentation of the 
phase rule was garbed in mathematical terms. For the involved 
equations as set forth by Gibbs the thermo-equilibrium diagram has 
been substituted. ‘To the metallographer the construction and inter- 
pretation of these diagrams present no great difficulties, but to the 
average geologist they are conventions that possess little or no 
meaning. As an understanding of the diagrams of the feldspar 
system is essential to what follows, a section at the end of the paper 
in the Appendix is introduced in which their construction and 
interpretation are discussed. 
For most purposes it is sufficient to consider that the binary 
systems, soda-lime and potash-barium feldspars, constitute a 
series of solid solutions, the thermo-equilibrium diagrams of which 
are classified by Roozeboom as Type I. On the other hand the 
binaries, orthoclase-albite and orthoclase-anorthite, are repre- 
sented by the eutectiferous type of diagram. ‘Thus the feldspar 
binaries may be classified as follows: 
Limited Solubility 
Aaa ae \ Ee eRSEnY. Type 
SIStelle arise per mare Soda-lime feldspars Potash-soda feldspars 
INIDISTS 5 sas cota eee ae Plagioclase series | Perthite series 
_ SYSUC RRS aoe eee Potash-barium feldspars Potash- lime feldspars 
INAIND SS elt Hig eee Re Cee Hyalophane series No name* 
* The writer proposes on a later page the term ‘‘oranite.” 
TWO-COMPONENT SYSTEMS 
THE SODA-LIME FELDSPARS—-PLAGIOCLASE SERIES 
The soda-lime feldspars or the plagioclase series is the best 
known isomorphous series in the mineralogy of the rock-forming 
minerals. Tschermak in 1864 propounded the theory that the 
plagioclase feldspars are isomorphous mixtures of albite and anor- 
thite as is indicated by the formula: m(NaAISi,Os) +2(CaAL,Si,Os). 
Vogt developed this theory, and established it from indirect evi- 
dence. It was experimentally demonstrated by the classical work 
