OBSERVATIONS ON THE FELDSPARS 
AUSTIN F. ROGERS 
Leland Stanford Junior University 
A PECULIAR PERTHITE FROM PORT HENRY, N.Y." 
An interesting variation of the albite-microcline intergrowth 
known as perthite was noticed among some specimens obtained 
from a mineral dealer. The body of the specimen is pale flesh-red 
microcline. On the basal pinacoidal {oro} cleavage there are two 
intersecting sets of narrow, colorless albite strips, which are plates 
inserted parallel to the unit prism {1ro} (Fig. 1). The peculiarity 
is that the two sets of albite plates are twinned with respect to 
each other on the albite law. This can be demonstrated by holding 
Fic. 1.—Peculiar perthite Fic. 2.—Albite from perthite 
(3 natural size). (magnified). 
a specimen with the basal cleavage close to the eye. A reflection 
being obtained from the microcline cleavage, on revolving the 
specimen about the a-axis a little toward one, there is a reflection 
from the albite plates parallel to the (110) face, and on revolving 
it a little in the opposite direction, there is a reflection from the 
albite plates parallel to the (110) face. In each case the reflection 
is from the basal {oor} cleavage. The angle between the (oor) 
1 This perthite, as well as the albite described in the next item, are evidently from 
pegmatites. See Kemp. Trans. Am. Inst. Min. Eng., XXVI, 195 (1807). 
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