THE MINERALOGRAPHY OF THE FELDSPARS 291 
natural inorganic substance of definite chemical composition needs 
revision. 
9. That the plagioclase feldspars are not ‘“molecules,’’ portions 
of which are “replaceable” by analogous units. This statement 
applies with almost equal force to the potash-soda series as 
well. 
to. While the term ‘“‘mixed crystals” is frequently used to 
signify “‘solid solutions,” yet it should be avoided for the sake of 
clearness. 
t1. Perthite is an intergrowth of two (or more) solid solutions, 
and not an intergrowth of the simple components, microcline (or 
sometimes orthoclase) and acid plagioclase. The two phases are 
solid solutions, one rich in potash and the other rich in soda. 
12. That most perthites are not the direct result of the freezing 
of a magma, but are the result of subsequent processes, where the 
decrease in solubility of one phase for the other with falling 
temperature is the principal factor. The inversion of orthoclase 
to microcline is regarded by some as also a contributing cause. 
Perthites are commonly the result of the process here called 
“‘exsolution.”” Such perthites (or ‘‘perthoids’”’) are analogous to 
pearlite in steels. 
13. That many anorthoclase specimens are supersaturated, 
undercooled metastable solid solutions, potentially perthite through 
the intermediate stages of crypto- and microperthite. 
14. That intergrowths of potash-rich and lime-rich solid solu- 
tions occasionally are found. To such intergrowths the term 
oranite (the first two letters of orthoclase and anorthite and the 
ending -ite) has been applied. 
15. That the feldspars of many basic monzonites and the 
granodiorites are approaching, as the limit, the potash-lime binary 
system. But because such feldspars are not as viscous at their 
melting temperatures as the potash-soda series, they separate 
more completely into definite identities, and consequently the 
orantitc feldspars are not usually recognized as such. 
16. The significance of the process of exsolution is that many 
so-called inclusions in mineral grains are due to secondary processes, 
and consequently are of late rather than of early development. 
