218 HAROLD L, ALLING 
It will be observed that the term “‘albite”’ is used in two senses: 
as a component of the system, that is pure NaAlSi,O,; and as a 
mineral capable of a range in composition within definite limits. 
All mineralogists, consciously or unconsciously, use this dual 
nomenclature: (1) albite as a component; (2) albite as a mineral 
found in nature; a distinction necessary to make. No modifica- 
tion of the classification limits of the plagioclase series can reduce 
these two uses to one. The metallographer has a similar problem. 
His components are metals, his solid solutions and definite chemical 
compounds are ‘‘meterals.”* Meterals are strictly analogous to 
minerals. Consequently we see that in mineralogy there is no 
term corresponding to the metallographer’s term “metal.” There 
would be a distinct gain if we possessed a name for the components 
of a mineralogical system. The writer is using the word “‘minal”’ 
to convey the meaning here expressed. Las there is albite as a 
minal, and albite as a mineral. 
The theoretical percentages given above are based upon the 
assumption that the plagioclase series is a simple binary system. 
While such a conception is sufficient for most cases, a little study 
shows that natural specimens almost invariably. contain: some 
potash feldspar, the maximum being 10 to 12 per cent of the 
total in specimens near the albite end of the series, and decreasing 
as the percentage of anorthite increases. The potash component 
does not enter into the system in quite the same manner as the other 
two members for it is not a completely isomorphous component. 
Thus in classifying natural specimens one is compelled to consider 
the plagioclase series as a three-component rather than a two- 
component system. 
Now the question arises what is the particular form of the potash 
feldspar which is found in the plagioclase feldspars ? Miers’ says 
since most of the plagioclase [feldspars] contain potash, -we have to suppose 
either that monoclinic orthoclase can form [imperfect] isomorphous mixtures 
with triclinic plagioclase, or that the potassium feldspar is dimorphous, and 
that a modification exists belonging to the triclinic system and capable of 
entering into these mixtures. 
tH. M. Howe, Metallography of Steel and Cast Iron, p. 232; Albert Sauveur, 
Metallography and Heat Treatment of Iron and Steel, pp. 293-94. 
2H. A. Miers, Mineralogy (Macmillan, 1902), p. 452. 
