Penfield and Sperry — Mineralogical Notes. 325 



found on either fragment, a cleavage piece showing one square 

 inch of surface on the basal plane, being perfectly flat and free 

 from striations. The specific gravity is 2 '651. 

 The following analysis was made by Mr. Sperry : 



Eatio. Anorthite. Albite. 



Si0 2 62-60 1-043 -160(2) -883(6-00) 



A1 2 3 23-52 -228 -080(1) -148(1-00) 



Fe 2 3 -08 



CaO _ 4-47 -080 -080(1) 



Na 2 8-62 -139) -145 (-99) 



K 2 -56 006 J 



Ign -10 



99-95 



Disregarding the trace of Fe 2 3 and H 2 0, and calculating 

 the Si0 2 and A1 2 3 which would unite with the CaO to form 

 anorthite (ratio Si0 2 : A1 2 3 : CaO = 2 : 1 : 1), we find that the 

 remaining constituents are in the right proportion to form 

 albite (ratio Si0 2 : A1 2 3 : JSTa 2 = 6 : 1 : 1). The ratio of anor- 

 thite molecule to albite molecule is 1 : 3*6, the per cent being : 



Anorthite Ca 2 Al 4 Si 4 Oi 6 =22-32 



Albite... Na 2 Al 2 Si 6 16 =n-45 



The above chemical results are sufficient to classify this mineral 

 as oligoclase. 



The most interesting and important thing in connection with 

 this mineral is that its optical properties are very far from those 

 of oligoclase. Sections parallel to the basal plane do not show 

 the extinction of oligoclase +1°, but the very large angle 

 + 39° to 40°. The positive nature of this angle was determined 

 very carefully on the two fragments whose cleavage angles 

 were measured. The extinctions were measured with a Fuess 

 microscope, and in the two sections varied from 38° 30' to 40°. 

 Sections parallel to the br achy pin acoid did not show any 

 change from light to dark in ordinary polarized light when the 

 sections were turned, but in convergent light showed an optic 

 axis, as nearly as one could tell, exactly in the center of the 

 field. When the cleavage lines are parallel to the principal 

 planes of the polarizer and analyzer, the dark bar running 

 through the axis is almost parallel to the cleavage lines. A 

 similar section of oligoclase should show an extinction of +5°, 

 and in convergent light a bisectrix almost in the center of the 

 field. 



We regret that we cannot give any facts regarding the mode 

 of occurrence and associations of this interesting feldspar, but 

 must leave that for others who may be so fortunate as to study 

 this mineral in the field. A study of the crystals, if any are to 

 be found, is also to be desired. 



