THE MINERALOGRAPHY OF THE FELDSPARS 205 
probably changes to microcline) at temperatures below the freezing- 
point of any melt in the system, it does not seem very likely that 
the soda feldspar in a magma slowly cooling at great depths could 
take the form of barbierite. It is very likely that barbierite and 
microcline, when found, are usually not in their original form, but 
are the result of inversion, or transition. 
The possible inversion of the potash and soda feldspars intro- 
duces a new element into the subject. Microscopic evidence does 
not seem to verify the supposition that microcline and barbierite 
are high-temperature forms, but rather that they are the result 
of inversion or that the presence of mineralizers has lowered 
the temperature of freezing below the inversion range of these 
minerals. 
From the foregoing we conclude that orthoclase and albite have 
dissimilar structures, while barbierite and orthoclase possess 
similar atomic groupings. In an analogous way albite and micro- 
cline are probably alike. 
FELDSPAR COMPONENTS 
Modern textbooks on mineralogy list the following feldspars: 
(1) orthoclase, (2) microcline, (3) soda-orthoclase, (4) soda 
microcline, (5) anorthoclase, (6) albite, (7) oligoclase, (8) andesine, 
(9) labradorite, (10) bytownite, (11) anorthite, (12) plagioclase, 
(13) perthite, (14) celsian, (15) hyalophane, (16) carnegieite, 
and (17) anemousite. All the feldspars from orthoclase to and 
including perthite (1 to 13 inclusive) belong to a three-component 
system where the three units are: (1) potassium feldspar, 
(2) sodium feldspar, (3) calcium feldspar. Or if celsian, hyalo- 
phane, carnegieite, and anemousite be included then all the 17 mem- 
bers fit into a five-component system, of which the following are 
the components: 
Type Empirical Formula 
Potassium Feldspar KAISi,Og 
Sodium Feldspar NaAlSi,Os 
Calcium Feldspar CaAl,Si,08 
Barium Feldspar BaAl,S1,03 
Carnegieite Na,ALSi.O0s 
