THE MINERALOGRAPHY OF THE FELDSPARS 220 
The Optical Properties of the Potash-Soda Feldspars.—Iit is a 
‘surprise, even to an experienced petrographer, that no adequate 
tabulation of the optical properties of the potash-soda series 
exists. What information there is in the literature is fragmentary 
and unrelated to the actual chemical composition. The writer 
has attempted to synthesize the data so given and has carried out 
sufficient microscopic examination of thin sections and crushed 
fragments to lead him to believe that it is possible to identify 
the subspecies of the potash-soda series with an accuracy approach- | 
ing that obtained in the plagioclase feldspars. 
Although the ease of distinguishing and identifying the dite 
ent ranges in the plagioclase series is not duplicated in the potash- 
soda series, yet the maximum extinction angles on the (oro) and 
(oo1) faces appear to offer fairly reliable criteria. In most text- 
books on optical mineralogy the extinction angles for orthoclase on 
the (cor) and on the (oro) faces are given as 0°, and 4°30’ to 5°, 
respectively. As most, if not all, natural orthoclase contains 
some sodium feldspar in solution, the angular value for the pure 
substance is perhaps a little lower than the last named. As the 
amount of the sodium component increases to about 25 per cent 
the extinction angles on the (oro) face increase from 5° to about 
12° while the (oor) extinction angle changes from zero to about 3°. 
Tn the form of crushed fragments this test becomes a simple though 
not a quick method of determination. A similar relation exists 
for microcline and soda microcline. ‘The extinction angle on the 
(oo1) face is slightly less than 15° for nearly pure material but 
becomes 17° to 18° with increased soda content; likewise the (o10) 
face extinction increases from 4°30’ to nearly 10°. 
Potash albite is usually distinguished by the fine albite twin- 
ning, although this may be replaced by fine microclinic twinning 
instead, or no twinning may appear at all even under high 
magnifications, and reliance upon the extinction angles from 
cleavage faces must be substituted. The extinctions on the (cor) 
face are about 5° and on the (oro) face about 10°. With increasing 
soda content pure albite is reached when 21° on the (oro) is found. 
The great difficulty is in distinguishing by extinctions alone satu- 
rated (or supersaturated) soda orthoclase from potash albite 
