204 HAROLD L. ALLING 
by ore” due to metasomatism. The term, however, is used in 
a very different sense by mineralogists. Is it chemically correct 
to speak of an element in a formula as “partially replaced” or 
“replaceable” by another? A replacement in a chemical sense 
means double decomposition taking place through chemical 
reaction, involving a thermal change; chemical reaction and ther- 
mal change being functions of each other. Thus if the potassium 
of orthoclase is ‘‘replaced”’ by sodium by chemical reaction then 
there should necessarily be a change in the thermal state of the 
system, the absorption or liberation of heat. While it cannot be 
stated positively that there is no change in the thermal state when 
liquid albite is added to liquid orthoclase at the same temperature, 
it seems more than likely that the albite will pass into solution 
without greatly disturbing the thermal equilibrium. The same 
principle probably applies to solid minerals, although it would be 
very difficult to prove it experimentally. 
Of all the mineralogical systems studied up to the present time 
none is better known and understood than that of the plagioclase 
feldspars. It can be asserted with considerable emphasis that the 
variations in composition in the series are not due to chemical 
replacement but that they constitute a series of solid solutions. 
Moreover the plagioclase feldspars are not unique in nature, for 
many similar systems undoubtedly exist to which the same prin- 
ciple applies. 
The reader should not overlook the fact that in this discussion 
of the feldspars we are dealing with them from the point of view 
which considers them solid solutions, and mixtures of solid solu- 
tions, and not “mixtures, admixtures, and mixed crystals” nor 
“molecules” which have certain portions “‘replaceable” by analo- 
gous units. 
Thus the evidence that can be brought to bear upon this difficult 
problem points to the conclusion that NaAISi,Os, when it exists as 
albite, and CaAl,Si,0s as anorthite probably possess similar struc- 
tures. The suggestion of Bayley that the NaAISi,O3 in orthoclase 
is barbierite (see later under ‘‘Feldspar Components’’) may be 
true but in view of the fact that other evidence points to a possible 
inversion of albite to barbierite (in the same way that orthoclase 
oe 
