196 HAROLD L. ALLING 
of certain assumptions. .... The belief that true equilibria are 
attained in these solid mixtures . . . . lacks support.” 
This quotation is quite typical of such criticisms: it gives the 
impression that in studying a system, such as the feldspars, by 
means of the phase rule the laws of physical chemistry are violated. 
Metallographers such as Howe, for example, are recognizing the 
metastability of many systems. Many feldspars are metastable 
solid solutions. 
It would appear that the most satisfactory method of attack 
would be a laboratory study of the melting and freezing phenomena 
of the feldspars. ‘The work of Allen and Day’ has shown, however, 
that little information can be secured through this means because 
of the high viscosity of the alkali feldspars when above their fusion 
temperatures. Even when in a molten condition it is impossible 
to effect crystallization. Watts? using pyrometric cones in his 
work upon artificial mixtures of natural potash-soda feldspars has 
found it very difficult if not impossible to secure reliable thermal 
data. 
Thus the method of attack here is in part a reverse process. 
By a study of textures and properties of various specimens differ- 
ing in their chemical composition, the thermo-equilibrium diagrams 
of the binary systems can be inferred by analogy with the diagrams 
of other systems and by means of suggestions found in the metal- 
lurgical and mineralogical literature. The writer suggests that 
the application of the methods of the metallographer to the study 
of silicate systems for the purpose of increasing our knowledge 
concerning them may properly be called the science of mineralog- 
raphy. 
Murdoch? has already proposed the use of this term in a slightly 
different sense to cover the employment of the metallurgical (metal- 
lographic) microscope in the study of non-transparent minerals in 
the same way that it has been used for years in the study of metals. 
tF—. T. Allen and A. L. Day, “The Isomorphism and Thermal Properties of 
the [Plagioclase] Feldspars,”’ Carnegie Inst. of Wash. Pub. 31, 1905. 
2A.S. Watts, ‘‘The Feldspars of the New England and North Appalachian 
States,” U.S. Bur. Mines Bull. 92, 1916. 
3 Joseph Murdoch, Microscopial Determination of the Opaque Minerals, 1916, p. iii. 
