222 HAROLD L. ALLING 
the melt, each increment of which is in various degrees of satura- 
tion in respect to the sodium component. Then the phenomenon 
of diffusion (due to osmotic pressure) should take place tending to 
bring about a homogeneous mass. Let us suppose this diffusion 
takes place to completion while the temperature remains con- 
stant at J,. A homogeneous solid solution nearly but not abso- 
lutely saturated with the sodium component results. We observe 
that the line AL is inclined, a fact which informs us that as the tem- 
perature falls the solubility likewise decreases. A temperature will 
soon be reached where saturation is complete. As “‘exsolution’’? 
in solids is up to this point excessively slow, due to the high vis- 
cosity of that state, a condition of a supersaturation will occur. 
Assume that the feldspar is at normal temperature (HL/JKM1), 
then in the course of geologic time this supersaturated crystalline 
mass will gradually separate into the two phases and give perthitic 
intergrowth. The perfect orientation of the crystal units in the 
host mineral would be maintained and hence we would expect to 
find that albitic phase “in irregularly lenticular layers .... in 
planes parallel to (801) or (100) and that both feldspars could 
have (oro) in common.’? This is probably the common form of 
perthite, due to exsolution of a supersaturated solid solution. As 
this is not a eutectic mixture it is well to be cautious in applying 
the term to all intergrowths found in rock sections. In fact many 
intergrowths that have the appearance of being eutectics may 
be due to secondary or subsequent processes. Whitehead? has 
emphasized this and illustrates the striking similarity in appear- 
ance between a true eutectic of 70 per cent of silver and 30 per cent 
of copper, and certain intergrowths which are found in sulphide 
« There is no satisfactory word to express the phenomenon of the separation of 
two crystal phases due to supersaturation. “Precipitation”? cannot be used as it 
designates a chemical relation; there is no chemical reaction taking place; it is merely 
a physical change. Warren uses the term “‘unmixing.”’ This is not satisfactory for 
we are not dealing with original mixtures but with solutions. If we were observing 
a liquid solution we could say “crystallizes out of solution” but the homogeneous 
mass is already crystalline and such an expression would be unfortunate. It is 
really the opposite of ‘‘passing into solution,’ hence the term ‘“‘exsolution” is pro- 
posed. The German term is “entmischung.” 
2J. P. Iddings, Rock Minerals (1911), p. 239. 
3 W. L. Whitehead, Econ. Geol., XI, 1-13. 
cee eae = 
a 
