THE MINERALOGRAPHY OF THE FELDSPARS 27 
Groth’ says that the physical properties of a completely isomor- 
phous series “are continuous functions of their composition.” In 
fact it may be safe to regard a binary system to be isomorphous 
from a knowledge of these physical curves alone even though the 
system has not been subject to thermal investigation. Thus for 
the plagioclase series we can draw the following curves: (1) specific 
gravity, which in case of artificial feldspars is a straight line, 
(2) indices of refraction, alpha, beta, and gamma values, and 
(3) extinction angles on the (oro) and (001) crystallographic faces. 
Although these functions have long been known to petrographers 
it may be that their presentation in graphic form, as shown in 
Figure 3, demonstrates relationships which hitherto have not been 
sufficiently emphasized. 
Classification of the Plagioclase Series —The plagioclase series 
is classified into six subdivisions, albite, oligoclase, andesine, 
labradorite, bytownite, and anorthite. These subdivisions are not 
a random group of minerals with definite composition arbitrarily 
classified into a group but constitute a series of steps or gradation 
from one end member of the series, albite, to the other end member, 
anorthite, and consist of these end members in reasonably definite 
but differing proportions. There is, however, a lack of agreement 
among petrographers as to the limits of the variations that may 
occur in these subdivisions without altering the mineral names 
which have been assigned to them respectively. F. C. Calkins? 
has lessened the difficulty by suggesting a decimal standard of 
composition as a basis of classification. This method, adopted 
by the writer, gives the following proportions as applied to the 
members of this series. It will be noticed that each of the minerals 
varies within definite limits as to the percentage of the two con- 
stituents which it may contain without losing its identifying name. 
Albite, from Abyoo Any to Abgo Anio 
Oligoclase, from Aboo Ani to Abyo Anjo 
Andesine, from Abjyo Anjo to Abso Anso 
Labradorite, from Abs. Ango to Ab3o Anyo 
Bytownite, from Abjo Anjo to Abyo Ango 
Anorthite, from Aby Ang to Aby Anioo 
t Paul Groth, Chemical Crystallography (1906), p. 96. 
2F. C. Calkins, Jour. Geol., XXV, 157-59. 
