THE MINERALOGRAPHY OF THE FELDSPARS 233 
conception, such as this, seems necessary to supplant the present 
nomenclature which is very unsatisfactory due to its indefinite- 
ness. 
The question arises whether it is best to restrict ‘“‘anorthoclase”’ 
to the central portion of the equilibrium diagram and to under- 
cooled homogeneous crystals of perthitic composition, or to limit 
it to albitic feldspars containing the potash component up to a 
maximum of 20 per cent. After due consideration the former 
proposal is the one here adopted. ‘The average of the 47 available 
analyses of this mineral is higher than 20 per cent in the potash 
component and is therefore within the perthitic range. 
If the term “‘soda orthoclase” is used for that portion of the 
potash-soda series indicated by the range Or,,Ab,,-Or,,.Ab,. and the 
term “‘anorthoclase’’ for the range in the center of the equilibrium 
diagram, then it is necessary to supply a term for albitic feldspars 
containing the potash component up to 20 percent. For this range 
of solid solutions, K-feldspar 20, Na-feldspar 80- K-feldspar 5, 
Na-feldspar 95, the term “potash albite”’ is proposed. 
Anorthoclase is here used as the name to designate the range, 
K-feldspar 70, Na-feldspar 30- K-feldspar 20, Na-feldspar 80, when 
it is a supersaturated undercooled metastable homogeneous solid 
solution, potentially perthite, through the intermediate stages of 
cryptoperthite and microperthite. 
The emphasis that mineralogists place upon the distinction 
between monoclinic and triclinic crystals has resulted in consider- 
able confusion about the distinction between soda orthoclase, 
anorthoclase, and soda microcline. For example, the interesting 
classification of Klockmann’ will illustrate this point: 
Monoctinic TRICLINIC 
FORMULA 
Component* Isomorphous Mixture Component* Tsomorphous Mixture 
KAISi,0g....| Orthoclase | Microcline Anorthoclase 
WaAlsO:...| Unknown} {| -00? Orthoclase’ || Aihite 
PaANSrOn) Unknowne (ee... Anorthite | Plagioclase 
* Free translation of ‘‘Selbstandig.”” 
{ For this modification of the sodium component the term barbierite has been proposed. 
™Klockmann, Lehrbuch der Mineralogie (1912), p. 488. 
